Introduction to Evolution

Supplemental Lecture (98/06/08 update) by Stephen T. Abedon (abedon.1@osu.edu)

Chapter title: Introduction to Evolution

A list of vocabulary words is found toward the end of this document
When one speaks of biological evolution, one is implicitely referring to a number of mechanisms all of which either do or can result in the occurrence of change in allelic frequencies with time. Recall that an allele is a variation in the nucleotide sequence associated with a particular loci (or gene) on a chromosome of a particular organism. By means of mutation (i.e., changes in nucleotide sequences), new alleles may be formed. The ratio of the absolute number of a given allele to the total number of alleles found at a given loci is called the frequency of that allele. Any mechanism which results in a change in the frequency of any allele is called biological evolution (or simply evolution if it is understood that one is speaking of a biological system). Since all such mechanisms either do not occur instantaneously or tend to be not limited to single events, one speaks of changes in allele frequency occurring as time advances. Thus, evolution is change in allele frequency over time.
At its basis, defined thus, evolution is a simple enough concept. Nevertheless, evolution is anything but simple in practice. This is because life itself is not simple. Instead, individual organisms consist of a huge number of variables (e.g., individual genes) each of which is capable of varying in typically a huge number of ways (i.e., many alleles are possible at any given locus). Organisms, additionally, interact with organisms of their type, organisms of other types, and their physical environment. Furthermore, with time all of these interactors can change. Consequently, the number of possible permutations avaiable to life at any given moment can be huge. Another way of stating this is that it is difficult to predict the future, expect when dealing with very simple systems. Evolution, consequently, tends to occur down very complicated paths and very unpredicatably (at least when and as viewed by small minds such as our own).
Evolution may be divided into two likely highly interelated types termed microevolution and macroevolution. The basic differences between these two terms is that the former involves evolutionary change (i.e., change in allele frequency) which occurs within species (i.e., between individual members of individual species, or populations) while the latter occurs between species. At the deviding line between these two concepts is the idea of speciation. That is, the changing of one kind of species into another kind of species. Our understanding of the mechanisms of speciation is not enormously robust, partly because our understanding of just what a species is is not enormously robust. Nevertheless, it is abundantly clear that species change over time (microevolution), one species can give rise to one or more additional species (speciation), and then individual species can show differential propensities to survive and prosper (macroevolution).
As noted, there exist mechanisms by which change in allele frequency (microevolution) is thought to occur (e.g., selection, mutation, migration, and genetic drift). Prominant among these mechanisms is the theory of evolution by natural selection (Darwin, 1859). We will dwell deeply upon these mechanisms in subsequent lectures. However, here for our purposes note that these various ideas of evolutionary mechanisms are built upon a number of often quite dramatic evolutionary "facts." Particularly, in addition to the various ideas by which evolution is thought to proceed, there exists an entire body of knowledge simply associated with a cataloging of the occurrence of evolution; and the two bodies of knowledge, one evidencing the fact of evolution and the other evidencing the theory of evolution by, for example, natural selection, do not entirely overlap. Indeed, the former tends to be more of a record of progressive change while the latter by necessity is a record of directed change for which key environmental correlates are known. In addition, the evidence for the fact of evolution tends to be macroevolutionary (evolution occurring above the level of the species) while the evidence for the theory of evolution by natural selection tends to be microevolutionary (evolution occurring below the level of the species). Or to put it another way, speciation and macroevolution are tough to catch in the act, but relatively facile to draw from the fossil record, while microevolutionary change is difficult to catalog in the fossil record but relatively easy to observe in extant populations.
It is thought that macroevolutionary change occurs due to forces similar to those acting within species (mutation, selection, migration, and drift) with a particularly big dollop of chance (being, for example, the right population in the right place at the right time). Evolution ultimately acts on variation and any variation acted upon in a macroevolutionary process must have its genesis in a microevolutionary process (e.g., selection acting on random mutational variation). Here we discuss evidence for the existence of macroevolutionary processes, microevolutionary processes, and natural selection.

Overview, introduction to evolution

Evidence for an evolutionary genesis of the diversity of life: Extant organisms look and behave in a manner which is exactly what one would expect given an evolutionary genesis.
Organisms bear numerous shared characteristics that in total are undeniably strong evidence for the existence of thematic relatedness. The scientific explantion for such relatedness of structure is relatedness by blood, just as you probably look and act more like your parents and siblings than like your spouse and his/her family (or me, for that matter).
Organisms are distributed about this planet in a manner that would appear to directly reflect restrictions on travel. That is, similar niches in similar ecosystems tend to be filled with the same organism if the two ecosystems are spatially near one another, but different organisms if the two ecosystems are far apart. Particularly, it is clear from distribution data that organisms do not simply fill a niche in a specific local because they are the best available (in the world which could fill that niche), but instead because they are the best of those available locally to fill that niche. In other words, the distribution of organisms conforms, to a large degree, to what one would expect based on the evolutionary processes constrained by distance as well as geologic, oceanographic, and meteorological processes.
Bottom line: all evidence suggests that organisms look like they do in large part because of what lineage they evolved from. In addition, organisms live where they do as a consequence of where the lineage which they evolved from lived, plus some function of mobility.

Why care about evolution

Self evident:

To an evolutionary biologist, the question of why one should study evolutionary processes is self evident.
Indeed, it is difficult to imagine oneself (or anyone for that matter) not studying evolutionary processes.
Of course, not everyone agrees with this assessment. Particularly included among those individuals who fail to agree are many of the non-biologists in the world.

Evolutionary biology is complex and difficult:

To gain a robust understanding of biological evolution (i.e., that evolution which occurs within populations of organisms) requires broad knowledge of both biology and evolutionary processes. In addition, it is necessary to understand numerous physical processes including, particularly, geological processes.
This contrasts starkly with the level of understanding necessary to gain significant insights into such things as, for example, cell or molecular biology (though, of course, knowledge of chemistry can be of significant help).
These latter disciplines may be complex, but their complexity actually serves as only one component of the complex understanding which is necessary to gain significant understanding of evolutionary processes.
In short, to know evolution you first have to know biology, and then you have to know how evolution works, and still you won't really understand evolution until you've managed to synthesize together all of this (and other) information and understanding.

So why bother? Basically the bottom line is that an understanding of evolutionary biology provides unparalleled insights into the functioning of biological systems, ranging from the function of the smallest proteins to the largest ecosystems.

Biology’s defining metaphor:

Biology simply fails to make sense without some underlying metaphor.
Evolution, particularly by the process known as natural selection, provides that metaphor.

Example: behavioral ecology:

If you want to predict how an organism might behave in a novel situation, you may be well advised to first gain an understanding of the behavior of that organism in not novel situations, and then to attempt to gain some idea of why that organism behaves the way it does in these not novel sitations.
Attempting to understand why organisms behave the way they do is one facet of evolutionary biology.

Example: human behavioral ecology:

If you are trying to figure out, for example, how to stop a population of organisms from driving themselves to extinction in response to environmental benignness (humans for example), you would be hard pressed to come up with a solution unless you had some clue as to why individuals of that population choose to drive themselves to extinction rather than live sustainably.
An understanding of evolutionary biology is necessary to answer questions such as this.

Typically an unquestioned assumption:

Below we will discuss numerous evidences for the existence of evolution particularly as it occurs via a process known as natural selection.
Please remember as we walk through these ideas that sitting around attempting to come up with generalizable evidence for the existence of basic evolutionary processes is not what an evolutionary biologist generally spends her time doing.
Instead, rightly or wrongly the existance of evolution (often by natural selection) is usually taken as a given, either explicitely or implicitely, by almost all biologists as they embark on their study of biological systems.
Evolution is an extremely useful metaphor, a tool, that is wielded by biologists with the same confidence and assurance that you have when, for example, you take pen to paper to write.

Microevolution

Within species:

Microevolution is evolution which occurs below the level of species.
That is, it is changes in allele frequency with time, where the allele frequencies that occur over that within some reasonably well defined population.

Standard mechanisms:

Microevolution proceeds by a variety of mechanisms, all of which impact on allele frequency.
These include such straightforward things as:

mutation
migration
selection
genetic drift

Macroevolution

Between species:

Macroevolution is evolution which occurs above the level of species.
It is evolution which is marked by the differential survival of populations of organisms.
That is, macroevolution is differential changes in population or taxon sizes over time.

Mechanisms:

The mechanisms which drive macroevolution may at least in part result from the same mechanisms which drive microevolution.
However, macroevolutionary change additionally is driven by such things as the ease with which one species is capable of founding additional species (a function equivalent to that of mutation in microevolution).
In addition, macroevolution tends to be driven much more profoundly by immensly disruptive random events than typically are encountered during microevolutionary processes. This is true, however, in part simply because macroevolution occurs over much longer periods than microevolution while typically encompassing larger geographical areas, and thus is much more likely to encounter dramatic environmental changes
Furthermore, species tend to exist over larger geographical areas than do individuals so disasters capable of wiping out whole species must be larger than those capable of wiping out single individuals.

Speciation

Formation of new species:

Speciation is the formation of a new species from older, parental species.
The processes of speciation are complex and not fully understood. Neverthless, they are straightforward in the sense that they are mechanisms which result, basically, in a dramatic decline in the sexual reproduction occurring between two populations.
We will discuss speciation in much greater detail as a lecture unto itself.

Transmutation of species:

Simply another, older name for speciation.

Bridge between microevolution and macroevolution:

Macroevolution represents, at least in part, the differential survival of species.
One key product of microevolutionary processes is the creation of new species. Speciation, therefore, serves as a bridge of sorts between macroevolutionary processes and microevolutionary processes.

Difficult to catch:

Speciation appears to be a fluid process that is either difficult to observe over short periods (because it takes much longer than merely short spans of time for speciation to occur) or occurs in populations so small and remote that, again, they are difficult to observe (e.g., find/be aware of).
Thus, while different stages of speciation are not difficult to infer, typically it simply is not possible to observe speciation in real time over standard periods of scientific investigation such as the length of one individual's scientific career.
Geologically, however, speciation events are likely sufficiently short lived and local that observation via the fossil record is rare (though not unheard of).

Evidence for the occurrence of macroevolutionary change

Shared characteristics:

Evidence that living things are products of evolutionary divergence (i.e., we are related macroevolutionarily) basically come down to observations of shared characteristics.
A shared characteristic can be an aspect of genotype or phenotype, though generally only those characteristics that have a genotypic origin are relevant to establishing evolutionary relatedness. Recall that phenotype is a product of both genotype and environment, so only that portion which is a product of genotype is pertinent here.

Statistical similarity suggests relatedness:

Generally a degree of chance is involved in the occurrence of structures (loosely defined) such that it is always at least possible that any one structure will have arisen independently (i.e., in an evolutionarily unrelated manner) despite an appearance to the contrary. Nevertheless, the more two structures resemble eachother, the more likely that those two structures are similar as a consequence of similar decent (i.e., from some common ancestor), so long as those similarities are a consequence of specific gene expression.
Numerous structures may thus be employed to infer evolutionary relatedness.
Successful comparisons of structures (i.e., the finding of profound similarity) may be employed as evidence of evolutionary relatedness.
Evidence of evolutionary relatedness may be employed as evidence for the occurrence of evolution in general and macroevolution in particular.

Similarity without evolutionary relatedness:

Similarities can also result without close evolutionary relatedness.
Though for now it might appear anti-intuitive, such similarities may also be employed as evidence for evolutionary decent, though not necessarily of close evolutionary relatedness.

Multifaceted evidence:

Evidence in general for the occurrence of evolutionary relatedness or just plain macroevolution specifically may be found among studies of:

homologies (in general)
molecular evolution
biochemical similarities
comparative embryology
comparative anatomy
convergent evolution
vestigular structures
biogeography
the fossil record

Most or all of these approaches work basically on the premise that similarity is suggestive of the occurrence of evolutionary decent.

Homology [homologous trait; homologous structures/traits]

Identical by descent:

A homology is a characteristic which is similar if not identical as a consequence of common evolutionary descent.
Only structures present in common ancestors are considered homologies.
Homologous structures abound among extant organisms.

Homologies predict homologies:

Similarities can exist by chance, because of developmental constraints combined with similar selective pressures, or due to evolutionary relatedness.
If a similarity exists due to chance or due to constraints, then one would not expect that the existence of the similarity would be predictive of additional similarities, especially in apparently unrelated structures.
The existence of homologies, however, since they are, by definition, a consequence of shared evolutionary descent, predict the occurrence of additional homologies, particularly in unrelated structures.

Example: homologies are predictive:

An example of the predictive power of homologies can be seen in the description of a mammal: If she's got hair you can be awfully sure she also has breasts and reasonably confident she bears live young.
Or, if she's got feathers, you can bet she lays eggs and doesn't have teeth.

Evidence for evolution:

Since homologies are the existence of similar structures among somewhat dissimilar organisms (i.e., difference species), they suggest a shared descent, and one which is more closely shared than with less similar organisms.
In other words, your relatives (especially close relatives such as parents, children, and siblings) statistically tend to look more like you than do non-relatives. Keeping in mind, of course, that your distant cousins, the chimpanzees, look a lot more like you than your even more distant cousins, for example, the domestic pig.

Homologies thus serve as evidence for the existence of evolution as well as of evolutionary relationships (e.g., all birds share numerous homologies that mammals do not share, thereby suggesting that birds are more closely related to each other than any are related to mammals).

"The forelimb of various mammals exemplify homology. Forelimbs can perform many different functions: Human hands can manipulate different-sized objects, a cheetah's forelimb permits rapid running, a whale's flippers allow efficient swimming, a bat's wings enable it to fly. Yet each type of limb is composed of the same skeletal elements. Fossil evidence supports the hypothesis that the forelimbs, including the digits, of various mammals arose from the forelegs of ancestral five-fingered reptiles and became modified by natural selection during the decent of these animals in ways that facilitated different tasks. The alternative hypothesis, that these bones--one in the upper part of the limb, two in the lower part, and five digits--are the very best for manipulation and running and swimming and flying seems unreasonable to most anatomists." (p. 370-371, Postlethwait and Hopson, 1995)

Molecular evolution

Neutral mutations:

Mutations can and do accumulate in genetic material.
Often such mutations have essentially no effect on the fitness of the harboring organism.
Operationally, this latter statement means that the effect of the mutation on fitness is no greater than the effect of genetic drift on the frequency of the allele.

Robust mutational history:

The genomes of organisms consequently display a robust mutational history that appears to be independent of the environment in which that organism and its ancestors lived.
Independent of the genetic background within which these mutations are found (i.e., their presence is not highly constrained by forces of natural selection).

Greater similarity suggests greater relatedness:

If two organisms are related, then one would predict that they would harbor more alleles (and, especially, nucleotides in their DNA sequences) which are identical by decent than would two less closely related organisms.
If the two organisms are not even of the same species, they will still display genome similarities, though less similarity than two individuals chosen from the same species.
By extrapolation, the greater the extent the genomes of two organisms differ, the less likely those two organisms are closely related.
Thus, the degree of relatedness two organisms exhibit may be readily inferred from nucleotide sequence homologies displayed by the two organisms.

Molecular clock:

The retention of some similarity suggests commonality by decent and therefore the existence of a common ancestor.
However, the less similarity, the more generations likely have passed since the most recent common ancestor lived.
Generations and time are roughly convertible (especially since the number of generations a lineage goes through is a function of cell replication rather than individual replication, and a lot of replication goes on prior to the production of sperm) and thus to some degree it is possible to estimate the length of time since any two organisms shared a common ancestor.
In fact, employing molecular data (16S rRNA gene sequences), an enormous variety of organisms from bacteria to man have been linked together evolutionarily and their lines traced back to a hypothetically most recent common ancestor existing billions of years ago (see universal tree).

Biochemical similarities

A generalization of the concept of molecular evolution is the observation that the basic biochemical processes of all organisms are very similar, despite the apparently arbitrary nature of many of these processes.

Example: codons:

As we discussed when considering translation, codons, with only a few minor exceptions, are consistent for all extant organisms.
There is no reason this should be except as a consequence of all organisms being derived evolutionarily from a common ancestor.
Indeed, for example, codon usage (how often certain codons are employed) varies widely among organisms, thus implying a relative lack of constraints on codon evolution at least in principle.
However, in practice one would predict that any redesignations of codons would be very costly to organisms since it would result in a potentially severe loss genetic information. Redesignation of codons is thus unlikely to occur (i.e., one codon would not be properly translated thus altering on average one-twentieth of all amino acids found in the proteins of an organism).
There nevertheless is no known reason to expect that any two organisms would choose the same set of codons de novo. Thus, the employment of the same codon set is taken as strong evidence for the universal relatedness of extant organisms.
Only if all ancestral organisms started out with the same set of codons would one expect the similarity in codon designation observed today. The simplest exmplanation for why codon designation is so constant is that, indeed, a single organism probably served as the ancestor of all extant organisms.

Example: endosymbiotic bacteria:

How is it that mitochondria and chloroplasts could so significantly resemble bacteria except that they in all probability share a common ancestor (a more recent ancestor than the universal ancestor, of course) from which each has evolutionarily diverged?
Thus, either certain free living bacteria evolved from mitochondria, or mitochondria evolved from free living bacteria (much, much more likely, the latter).
Either way, the implication is one of profound past macroevolutionary change in organismal lifestyles and biochemistries.

Example: model systems:

Model systems are routinely employed in biological systems. When a model system is employed for the study of molecular processes, in most instances an assumption is made that experimental results obtained from one system may at least potentially be applied to the second system, for example from mice to men.
An expectation of applicability results from a prediction that the existence of observed homologies between systems is predictive for the existence of the specific additional homologies whose presence is necessary to allow the two systems to act in a biochemically similar manner.
Thus, mice are employed in drug studies rather than, for example, cockroaches. And this is simply because those similarities between humans and mice, that are not found to the same extent between humans and cockroaches, we use as a predictor for further similarities between mice and men, ones which are similarly absent between men and cockroaches.
That is, biochemical research employs model systems either implicitely or explicitely based on assumptions evolutionary relatedness between organisms.

Example: sequence comparisons:

One of the first things a molecular biologist does upon sequencing a novel gene is to compare its sequence with that of genes that have been previously sequenced and with luck better characterized.
An assumption is made that sequence similarity can supply information on shared function.
Additionally, it is possible to key in on which amino acids are more relevant to a protein's function by comparing the sequences of proteins having similar structures but which are found in more distant evolutionarily related individuals.
The explicit assumption in this case is that only those nucleotides (or, particularly, amino acids) which are selectively constrained will retain similarity.
An understanding of the tenets of molecular evolution (e.g., silent mutations) thus forms a key foundation to understanding this common form of biomolecular analysis.

Embryology

No entry!

Convergence [parallel evolution; convergent evolution; analogous structures/traits]

Similarity without common evolutionary origin:
Often two organisms will possess strikingly similar adaptations, but the embryology of the associated structures clearly indicate that there is little or no underlying structural similarity.
Such adaptations are said to occur in parallel.
They are a situation where superficial morphological or functional similarity evolved independently in two distinct lineages.

Analogous trait:

An analogous trait is one common to two species but which has an independent evolutionary origin in the two lineages, i.e., a common characteristic which is not an example of homology.
Analogous traits are products of convergent evolution.

The implication is that natural selection has operated on two completely different structures in order to come to a similar solution to a similar environmental problem.

Similar solutions to similar problems:

Convergent evolution is a product of directional selection, where the selective pressures are similar for different systems, thus resulting in similar (i.e., convergent) solutions.
This contrasts with homology where instead either selection is stabilizing, or insufficient time has lapsed for structures to significantly change as a consequence of drift alone.

Convergent evolution, thus, represents essentially a macroevolutionary process which has clear roots in microevolution (i.e., local adaptation to local problems).

Vestigal structure

Unused structures:

Often evolutionary change involves the loss of structures that served an organism's ancestors well, but in the present environment or organismal body plan (biochemistry, whatever) and no longer useful.
Such structures, should they simply serve no purpose but are otherwise not harmful, will slowly be lost over evolutionary time as neutral mutations accumulate (i.e., mutations which lead to a lack of successful development of the structure). Prior to their loss, these unused structures are considered vestigial.

Such structures serve as examples of homology, but ones which clearly (i.e., by definition) are not a product of stabilizing selection (i.e., selection for their continued existence) but instead result as a consequence simply of the inefficiencies of genetic drift.
Thus, whales and some snakes have pelvises and undeveloped hind leg bones, but no obvious use for these structures, i.e., no hind limbs, or, minimally, no use which is terribly similar to the means by which these structures are employed in, for example, walking land mammals and reptiles.
A human has an appendix, despite the infrequent danger they can pose to their owners, but which serve no apparent positive purpose. Humans also have tail bones and unusuable but nevertheless present muscles for wiggling ears.

Evidence for the occurrence of profound morphological change:

The presence of vestigial structures is additional evidence for both profound morphological/evolutionary change and for the transmutation of species (speciation).
That is, if the ancestor of snakes lacked legs, then why should some snakes nevertheless have vestigial legs? The answer is that ancestral snakes actually possessed legs, and actually were lizards rather than snakes.

Biogeography

Similarity by geography:

Though organisms tend to be similar by environment, even more so they tend to be similar by geography.
Thus, two organisms living similar lives in similar environments, one in South America, the other in Australia, will far more likely possess features homologous with those found in organisms sharing their continent than with matched organisms living in similar environments but different locals.

Similarly, island residents tend to resemble the organisms present on the nearest body of land, and do so far more than they resemble organisms living on similar islands having similar climates but located far, far away.

A triumph of laziness:

Clearly convenience has far more to do with what organisms live where than does the degree to which they might fit a given environment.
This truism is dangerously exemplified by the consequence of the introduction of certain exotic species into new environments, e.g., the zerbra mussel.

This tendency for organisms to geographically stratify before ecologically stratifying is further evidence of the power of microevolutionary forces, the occurrence of speciation, and the significance of geographical barriers to organismal dispersion.

Macroevolutionary change in the fossil record

Natural selection among natural populations unaffected by the activities of man is difficult to convincingly observe. This is particularly because lives and careers are sufficiently short that direct observation of even microevolutionary events by a single individual is difficult. Macroevolutionary events are far worse in this regard.

Remnants of past glories:

Fortunately, the past often fails to wipe the slate clean and there exists significant and numerous evidence for macroevolutionary change in the fossil record.
In fact, there exists profound evidence in the fossil record for the existence of a diversity of life (far above and beyond the diversity currently in existence today).
Additionaly, the fossil record, despite the odds against the occurrence of such, is replete with examples of transitional forms, thus providing additional evidence for the occurrence of speciation.

Change in structures over time:

Thus, while study of extant organisms can suggest the past occurrence of evolutionary change, the fossil record has actually preserverd an impressive sampling of ancestral species and structures, albeit with detail mostly limited to hard part (e.g., bones and shells) anatomy.
From observation of present day organisms we can predict past change, while via observation of the fossil record, though usually with limited resolution, we nevertheless often can directly observe that change.

Evidence for the occurrence of microevolutionary change

Evolution running in real time:

Macroevolution occurs over long periods and consequently is ideally followed by a medium capable of recording slow changes, e.g., DNA, morphology, and the fossil record.
Microevolutionary processes, on the other hand, much more readily occur here and right now, so may in theory be measured in real time.
Unfortunately, most microevolutionary processes are subtle, meaning that while they can be incredibly meaningful and significant only over relatively long periods. Over short periods instead they are difficult to detect over random noise, either in allele frequency itself (i.e., drift) or, more likely, as a consequence of sampling procedures and measurement error.

Directional selection is key:

The least subtle of evolutionary processes, other than the incredibly easy to account for mutation, migration, and even genetic drift, is directional selection. That is, selection which results in a measurable, non-random change.
This contrasts with stabilizing selection, which simply is selection for more of the same.
However, does the feature remain the same because drift has not had time to tear the feature apart, or because the feature is being actively maintained? Over relatively short periods, except in very rapidly replicating organisms, this question is impossible to answer.
Thus, evidence of that key component of microevolutionary processes, selection, tends mostly to be documented in artificial situations, or during relatively natural episodes of rapid, dramatic change.
As follows we discuss evidence for the occurrence of selection in the guise of:

artificial selection
the development of resistance to antibiotics
the development of resistance to pesticides
the development of resistance to high levels of cadmium
sickle cell anemia
industrial melanism
cancer
guppy body size
sex ratios

Natural selection in the wild

Nothing but problems:

It is, unfortunately, not a trivial exercise to simply go out into the wild and measure natural selection. This is because, in order to call something natural selection in a wild population, one must first know that allele frequency is changing, then actually have a sufficient understanding of both the organism and the environment in order to predict how natural selection may be occurring, and finally actually have evolution that occurs to a significant extent over reasonably short periods (the longer the period, and smaller the effect, the harder it is to make a case and/or sustain an investigation).
Thus, successful observation of natural selection in the wild, which is simultaneously both subtle and very convincing, is very difficult.

The most powerful manner in which natural selection may be observed is as directional selection and therefore in response to environmental change. Since the vast majority of profound environmental changes are caused by man, and since these changes predominate around the areas where man exists and therefore is in a position to observe, the vast majority of the more subtle observations of natural selection ultimately find there way back to some sort of evolutionary response to man-inflicted events.
Also, many of the adaptations we hold near and dear came into being in the distant past (e.g., eyes) and won't likely evolve again because a holder of such proto-adaptions would today have to compete with organisms in which that adaptation was already fully developed--not a recipe for success, nor easily tracked natural selection.

Artificial selection

Proof of the power of selection:

The existence of artificial selection is an indisputable fact.

Artificial selection is also far easier to demonstrate than natural selection simply because a scientist may control (and even understand) all of the relevant variables in an artificial selection scheme:

the mechanism of selection
the population of organisms
the presence or absence of selection
etc.

Thus, the concept that evolutionary change can occur through selection is indisputable.
In addition, artificial selection provides evidence for the power of evolution-driven morphological change even given short periods of action on limited pools of variation (e.g., domestic dogs).

The many permutations of the domestic dog, all of which together form only a single species, exist today as consequences of artificial, often directional selection.

Unintentional selection upon domestication

Natural seleciton in artificial situations:

Upon domestication it is possible to observe all sorts of adaptations to the new environment now occupied by an organism.
Often these adaptations are not a response to anything the domesticator did intentionally (i.e., are not a consequence of artificial selection). Instead they tend to be responses to a relaxation of selective pressures, for example, as seen in increases in fecundity at the expense of adaptations which in turn had evolved in response to these fomerly present selective pressures.
In a very real sense these results constitute natural selection, though not natural selection occurring on natural populations existing in the wild. Instead they occur on domesticated populations existing in captivity.
Note that unless one can assume that environmental change does not occur in the wild, then there exists no fundmental difference between this artificial situation and more natural examples of adaptation by populations to newly formed, novel environments.

Example: smaller brains in cats
Example: larger litter sizes among the domesticated

Resistance to antibiotics, pesticides, etc.

Artificial interventions select for unintended evolution:

Closer to the wild situation is the effect various man-made interventions have on natural systems.
Particularly it is a very real truism that a large fraction of the attempts man has made to eradicate various pest species has resulted in the evolution, presumably via natural selection, of pests resistant to whatever agent is employed.

Thus, treatment of human immunodeficiency virus (HIV; the cause of AIDS) with AZT (an anti-HIV anti-viral) results in AZT-resistant HIV.

DDT resistance in mosquitos:

Populations of mosquitoes rapidly "acquire" resistant to DDT given exposure to this substance.
Note, though, that many organisms with much longer generation times and smaller populations are not nearly as well equipped to adapt to the presence of DDT--for example, birds tend to be severely affected when this poison is used for mosquito control, thus leading to the paradoxical situation of having resistant pests while simultaneously having killed the natural predators of these pests--another prediction, I might add, of the concept of evolution by natural selection.

Thwarting evolution first by understanding evolution:

If eradication at any cost is an object, then these problems of pest evolution of resistance may be overcome simply by employing multiple irradication schemes in parallel rather than serially (i.e., simultaneously rather than one after another--nuclear weapons work best).
This is because the way evolution works best is by solving one problem (such as coming up with an adaptation conferring resistance to DDT), reestablishing a large population possessing both a large amounts of allelic variation and the problem-solving adaptation, and then taking on the next problem (such as a second pesticide).
The way evolution works worst, on the other hand, is when insults strike small, stuggling populations. Thus, if DDT were applied at the same time that, for example, swamps were drained (i.e., destruction of the mosquito source habitat), then the number of mosquitos in the population resistant to DDT could very well be zero because the population was simultaneously reduced in size due to habitat detruction (i.e., there is only some more or less constant fraction of the mosquitos which are DDT resistant at any one time; reducing the population size reduces this fraction, ideally, to absolute numbers of less than 1), thus resulting in at least the potential for driving the local mosquito population to extinction, the goal in this case.
Such approaches are equally relevant when employing antibiotics, antivirals, and even anti-cancer chemotherapeutics (one chemotherapeutic reduces pest load on top of the other chemotherapeutic, ideally reducing the load to less than one). Consequently, an understanding of microevolutionary processes is absolutely essential to gaining long term, consistent success (i.e., extinction of the pest population).

Resistance to cadmium

The metal cadmium constitutes a potent poison (e.g., don't throw out rechargeable batteries and devices---the cadmium of nickel-cadmium, NiCad, batteries is considered a toxic waste!).

Toxic dump/Foundry Cove:

In Foundry Cove on the Hudson River (about an hour's drive from where I grew up), there exists an immense toxic waste problem.
More than 100 tons of nickel-cadmium waste was dumped there by a battery manufacturer.
In places the cadmium content of the bottom mud is as much as 25%.

Unusual cadmium-resistance in local fauna:

Local mud-dwelling fauna taken from a different cove were not able to tolerate living in the cadmium-rich mud of Foundry Cove.
Nevertheless, Foundry Cove fauna is nearly as rich, abundant, and healthy than that found in other, unpolluted coves.

Ecological mechanism for the development of cadmium resistance:

If clean-cove living fauna cannot survive in Foundry Cove, and if Foundry Cove fauna are genetically adapted to high cadmium concentrations (as experimentation has documented in both cases), then the most parsimonious explanation is as follows:
Foundry Cove, following contamination, was an environment devoid of life.
Those variants of organisms which exhibited some cadmium tolerance could exploit the margins of the cove, an area free from competition from their less resistant con-specifics due to the relatively high levels of cadmium.
Over time, and with input of mutational variation, members of populations growing at the margins of the cadmium rich area presumably displayed enhanced cadmium resistance, thus allowing those individuals and their progeny to invade further into the cove and settle in regions exhibiting ever higher cadmium concentrations.

In this way, the presence of the cadmium poison served as a selective agent in Foundry Cove leading to the re-colonization of the cove by cadmium-tolerant individuals.
Note, however, that one would predict that mostly it would be high population number, rapidly reproducing organisms which would evolve most easily to high cadnium levels, thus predictably biasing this disturbed habitat toward population by these more weed-like species.

Sickle cell anemia

Codominance:

On the surface, sickle cell anemia appears to be caused by a simple autosomal recessive allele. Thus, those homozygous for the sickle cell allele are stricken with sickle cell anemia while heterozygotes and homozygous dominant individuals are not.
However, the number of those stricken with sickle cell anemia is actually greater than would be expected given the severity of this disease in the homozygous state. Consequently, there exists the possibility that something more is going on than first meets the eye and that something might be of interest.
As it turns out, what is going on is that the sickle cell heterozygote, though otherwise pretty much normal, is simultaneously more resistant to malaria than homozygous, dominant individuals. This is an example of codominance.

Malaria selected overdominance:

In regions in which malaria causes significant disease and mortality, an advantage is accrued by the heterozygote even after factoring in the likelihood that some fraction of that individual's progeny may be afflicted by sickle cell anemia (i.e., will be homozygous for the sickle cell allele).
Sickle cell anemia consequently appears to be an example of a case of overdominance (a.k.a., heterozygote advantage) maintaining a polymorphism in populations living in malaria-prone areas (that is, the greater evolutionary fitness of the heterozygote maintains the frequency of the deleterious allele at unexpectedly high levels).

Testing this hypothesis:

How might one test this hypothesis? If the presence of malaria selects for an over representation of the sickle cell allele in populations, then removal of those populations to a malaria-free region should result in a steady decline the sickle cell allele frequency.
That is, in the absence of good effects on fitness, on balance the fitness lowering effects will predominate thus leading to a relative fitness of less than one associated with the sickle cell allele.
This would result in the deterministic elimination of the allele from a population.
In fact, just such a predicted decline in the frequency of the sickle cell allele is observed among such populations, e.g., slave descendants living in the U.S.

Note, for the record, even the existence of endemic malaria in the case of sickle cell anemia (discussed below) is a consequence of man's activity. That is, endemic malaria is probably not sustainable in small hunter-gatherer populations and thus is likely an evolutionary response to the invention of agriculture (another probable example of evolution, this time of the malaria parasite, again through natural selection). Indeed, even the testing of hypotheses concerning sickle cell anemia using the descendants of slaves shipped over from Africa was obviously only possible as a consequence of a man-inflicted event (this time one inflicted directly on man).

Industrial melanism

Effects of environmental change:

If a population of individuals is well adapted to its environment, then one might expect a change in environment to result in a change in allele frequency.
In a well defined species, a well defined environmental change might lead to a relatively predictable change in allelic frequency.
On the other hand, as with the study of sickle cell anemia, it is also possible to observe a change in allelic frequency along with an apparently associated environmental change, then to work backward to determine if what is being observed really is evolution by natural selection.
In practice such determinations are often not straight forward. However, it takes a jaded individual indeed to consider industrial melanism, discussed below, without reacting with at least some delight to the power of evolution by natural selection.

Selection for dark moths:

One celebrated example of natural selection in response to environmental change is that referred to as industrial melanism, a change in frequency of body color alleles in peppered moths:
Pre-industrial populations of peppered moths were predominantly light in color. This presumably was an adaptation to their habit of resting on the trunks of trees. According to experimentation, a light color on predominantly lightly colored trees allowed the peppered moth to avoid predation. In other words, they are camouflaged.
Upon industrialization, severe pollution led to an accumulation of soot on trees and death of lichen thus resulting in the significant darkening of what were previously light colored trees.
An observed correlate to the darkening of trees was a dramatic increase in the abundance of dark peppered moths, which previously had only a very minor frequency.
Given that this is a one loci, two allele system (i.e., one allele, recessive, codes for light color while the second allele, dominant, codes for the dark color), it is clear that the frequency of the allele associated with dark coloration showed a dramatic increase.
Through experimentation and observation it was possible to show that light colored moths predominated around light colored trees because they suffered significantly less predation (by birds) than dark moths. However, just the opposite was true when trees were dark. The increase in the frequency of the allele associated with dark coloration thus appears to have occurred as a result of a change in a key environmental variable, the color of trees, coupled with the a selective agent, predation by birds: evolution by natural selection.

Cancer

A microevolutionary disease:

Cancer is probably the most popularly familiar example of natural selection in action.
That is, a cancer is simply a group of cells which grows better (e.g., faster or in different contexts) than does the rest of the population of cells making up an individual.

Regulation of replication:

This occurs as a consequence of normal cells being strongly regulated to not replicate indiscriminately.
Such regulation benefits the overall organism, and since ultimately the immortality of the genotype of an organism is tied to the health of the organism, those highly differentiated, multi-celled organisms who could not bring the division of their component cells under control essentially went extinct.
However, increasing control over one's cells is a diminishing returns situation whereby increasing control requires more and more energy to accomplish (i.e., just as DNA replication fidelity is not infinitely good but instead represents a ballance between energetic costs and mutational costs).
Thus, control over cell replication may be lost as a consequence of the aberant expression of only a handful of genes.

Such mutant cells are selected relative to other cells in the body simply because these cancer cells display differential reproductive success (i.e., they make more baby cells) and thus cancer alleles tend to increase in frequency within the individual, which is simply another way of saying that microevolution is occurring.

Over population, environmental crashes, and extinction:

This state cannot be maintained forever since the continued success of these cancer cells is absolutely dependent on the continuation of a healthy growth environment (i.e., a healthy body).
However, the very growth of these cells contributes to a decline in environmental quality ultimately resulting in a complete environmental crash (i.e., the death of the body) and the consequent extinction of all cell lines present in the individual.

Guppy body size

Predator selection for resistant guppies:

Researcher David Reznick "scooped the lucky fish from a waterfall pool brimming with predators, then released them upstream in a pool where only one enemy species lurked. The guppies adjusted to the perks of life with few predators by growing bigger, living longer, and having fewer and bigger offspring." (here and below all from Morell, 1997 which reviews Reznick et al., 1997)
". . . guppies evolve in size, reproductive strategies, and other traits in response to predators in the wild. For example, in Trinidad's Airpo River, a species of cichlid fish feeds primarily on relatively large---2 to 3 centimeter---sexually mature guppies; in nearby tributaries, killifish prefer tender young fish. In response to these different pressures, the guppies have evolved two different life-history strategies. Those in the Aripo River reach sexual maturity at an early age and bear many young, while the guppies in the tributaries do just the opposite."

Rapid morphological change:

"The guppies adapted to their new environment in a mere 4 years---a rate of change some 10,000 to 10 million times faster than the average rates determined from the fossil record. Although lab studies have shown similarly fast rates of natural selection, this is one of the few examlpes from a natural environment."
The researchers "proved that predation caused this pattern . . . by transplanting guppies from the Aripo River to a tributary that happened to be empty of guppies and where killifish were the only predators. By 11 years later, the transplanted guppies had switched strategies, delaying their sexual maturity and living longer."
"In a little as 4 years, male guppies in the predator-free tributary were already detectably larger and older at maturity when compared with the control population; 7 years later, females were also noticeably larger and older."
". . . analysis showed that in only 4 years, the male guppies increased 15% in weight---roughly, from that of a dime to that of a penny. Biologists estimate the speed of evolutionary shange in darwins, or the proportional amount of change per unit of time, and the guppies evolved at a rate between 3700 and 45,000 darwins. For comparison, artificial selection experiments on mice show rates of up to 200,000 darwins, while most rates measured over millions of years in the fossil record ar only one-tenth to 1.0 darwin. 'It's further proof that evolution can be very, very fast and dynamic. . . It can happen on a time scale that's as short as one generation---from us to our kids.'"

Stabilizing selection:

In an earlier study of evolution in action Peter and Rosemary Grant "found that Galapagos Island finches also adapt rapidly to sudden changes in their environment, such as variations in rainfall. But because years of drought faver larger, big-billed finches while rainy weather favors smaller, small-beaked finches, the net long-term effect was that 'nothing seemed to change.'"
"The guppies also quickly entered a new period of stasis, but in a different way: Because their new environment was constant, they stopped evolving when the reached a new optimum size and age. Unless faced with some fresh selective pressure, the transplanted guppy population will remain as it is now---in stasis."

Comments/criticisms:

"'Can we get some inkling about what happened in the past by observing what's going on today? Microevolutionary studies such as this show that we can.'"
"'His study is fine, but his conclusions drive me crazy. He's overstepped the boundaries of what it means.' Evolutionary rates from the fossil record are necessarily lowered because they average periods of rapid change with periods of slow change or stasis."
"The data also provide little understanding of such phenomenon as bursts of new species . . . That would take studies of speciation itself, rather than research on change within an interbreeding group of guppies."

Sex ratio evolution

A sex ratio is the number of male individuals in a population relative to the number of female individuals (particularly sexually mature individuals).

Products of selection:

Theory suggests that evolution should adjust sex ratios to one-to-one at sexual maturity and this is what is usually seen in the wild.
The reason given for this result goes as follows:

sex ratio is assumed to be controlled genetically (and this appears to be the case whether gender is genetically or environmentally determined---if the latter it is the response to the environmental determinate which is under genetic control)
an excess of one gender results in maximum reproductive success being achieved by the other gender (if there are more girls than guys, lucky guys; if there are more guys than girls, lucky girls).

Thus sex ratios tend to be under stabilizing selection for a one to one ratio.

Experimental verification:

This has actually been experimentally verified using fish for which water temperature determines gender.
Raising the water temperature in a captive population from normal results in a perturbation of gender ratios.
However, the population quickly adjusts to this perturbation by evolving a lower sensitivity to this higher water temperatures, thus resulting in an evolutionary return to a one to one sex ratio.

How eyes evolve

Evolution of complex structures:

Often the argument is made that evolution by natural selection cannot explain the occurrence of complex structures since the removal of just one part, as with a house of cards, could result in the complete absence of employable function.
Thus, a complex organ, such as an eye, would have to form preexisting in order to function, an unlikely occurrence, and only once fully formed would be subject to natural selection.

The eye as metaphor:

Evidence suggests that this improbable event has occurred at least 40 independent times in the earth's history (though this claim seems to be rapidly losing ground, see below).
The steps necessary to create an eye have been suggested by computer simulation to be neither necessarily large, unlikely, simultaneous, nor maladaptive.
It has been conservatively estimated that the time from the development of light sensitive cells to the level of complexity of a fish eye might take only on the order of a half million years (a mere geological blink).
In short, the existence of eyes is not a robust argument against the existence of either evolution or the utility of natural selection.

Evidence that all eyes are homologous structures:

"Human eyes, fly eyes, and horseshoe crab eyes, to name a few, differ so greatly that it would seem nature invented eyes dozens of times in the course of evolution. A blow to this argument came 2 years ago when a mouse eye gene spliced into fruit flies prompted them to form extra fly eyes on their bodies, suggesting that a single ancient genetic program kicks off eye development throughout the animal kingdom.
"Now this feat has been duplicated in fruit flies, using a gene from an animal even more evolutionarily remote from a fly that is a mouse: a squid. The finding, reported in the 18 March issue of the Proceedings of the National Academy of Sciences, is the strongest evidence yet that all animals with eyes inherited them from a common ancestor." (though keep in mind that these common eyes must have been pretty primitive given the extreme divergence of subsequent eye structures seen among extant animals.)
". . . both the mouse and the squid Loligo opalescens carry a gene called Pax-6. In squid embryos, they found the gene is active in the brain, olfactory organs, and the iris and lens of the eye---just where it's active in mice. What's more, when the researchers engineered Drosophila fruit flies to express squid Pax-6, extraneous fly eyes sprouted in locations where the flies' wings, legs, and antennae normally grow. That's just what happens when the fly version of Pax-6, called eyeless, is activated in these areas.
"The results strengthen the idea that eyes evolved only once, and suggest that Pax-6/eyeless has acted as a key regulator of eye development since before vertebrates and invertebrates parted ways some 500 million to 600 million years ago . . .
"'If [the gene] were active only in flies and mammals, then you could suggest there was independent recruitment of the same gene to make eyes' in different species . . . 'but when it happens many times in many organisms, that's harder to imagine." (p. 1885, Holden (ed.), 1997)

As follows is a plausible scenario for the evolution of a fish eye, the forerunner of the amphibian eye (which begot the reptilian eye, which begot the mammalian eye and, via the dinosaurs, which begot the bird eye as well):

"Light sensitivity must be as ancient as life itself, having developed before the first photosynthetic bacterium absorbed its first proton nearly four billion years ago. Furthermore, any particularly light-sensitive surface spot might bring real advantage to bacteria, protozoa, algae or multicellular organisms seeking to remain near the water's surface where food was most plentiful. And if such a photosensitive spot happened to be located in a surface indentation, that dimple might sufficiently improve information about light direction to help "Dimples" become queen of all the beasts (because of her increased efficiency in moving toward the surface and avoiding predation). Of course, her advantage would only last until some more progressive multicellular descendant came up with an even deeper dimple. Thus deeper-dimple-with-a-photosensitive-bottom probably ruled until almost-covered-deeper-dimple, having accidentally invented the pinhole camera concept, became first to clearly image the world. Her reaction was not recorded but we can hope she found it good. In any case, clear-covering-over-pinhole was perhaps the next big advance, a covered pinhole being less easily obstructed by debris. Sudden advantage then came to those with a larger aperture (opening) when an unusually concentrated protein solution enclosed within that primitive orb just happened to crystallize. The resulting clear glob of protein (consequent to accidental local overproduction of some easily crystallized enzyme) most likely took on a relatively spherical shape as it rolled about within the eye, bringing the world in and out of focus. Presumably relatively spherical pre-lens with focus dependent upon position eventually gave rise to the modern spherical fish-eye lens with its attached fibers that move the lens back and forth to focus light at the photosensitive retina. There must have been an endless succession of other helpful mini-modifications before your own wonderful eye lens finally became available with its perfect shape and appropriate index of refraction. So it came to pass that those creatures with minimal chromatic aberration saw the light most clearly and ruled more or less wisely over the rest." (pp. 280-281, von Hippel, 1994)
"Eye evolution probably began with small groups of light-sensitive cells like those in limpets. Mutations that caused such cells to sink into cup-like depressions benefited their bearer by enabling it to detect the direction of light sources and hence to find food and avoid predators more efficiently. Today's nautilus, a mollusk, possesses such an eye. ¶Further random mutations could cause the edges of the cup to join and a cellular fluid to accumulate in the cavity. If the fluid focused light on the sensitive cells, it would be easier for the organisms to detect specific images. Modern scallops have this type of eye. . . Finally, if mutations arose that caused skin to grow over the eye, the cellular fluid to harden into an effective lens, and become connected to eye muscles, a complex eye is capable of forming tightly focussed images would have evolved. Some modern complex mollusks--the squid and octopus--have such eyes, which are very similar to our own. ¶(These) eyes do not form an evolutionary series because all the organisms (described) are alive today and none was a descendant of another. What the group of eyes does show is that an organ doesn't need to be totally evolved to work well for a organism in its environment. In addition, the series shows that eyes exist today that are predicted by the hypothesis of a stepwise accumulation of mutations occurring gradually over many generations altering eye shape and visual acuity." (p. 375, Postlethwait and Hopson, 1995)

Evolutionary pondering

"Imagine two men competing with each other to be chosen by a mate. One has a naturally healthy complexion. The other is actually terminally ill but has recourse to bright ochers. He uses some belladonna to dilate his pupils and dabs oil of muskrat behind his ear. Through this clever sexual culture, it is possible that the ill man can outdo the healthy one in the display-and-attraction stakes and be chosen as the woman's mate. Of course, this may turn out to be bad news for the women, as she may have to raise the child alone, but her consolation is that the child will have clever, culture-using genes on board." (quote from Timothy Taylor, in The Prehistory of Sex in Science (1997) 275:1894)
"In human social life quite different levels and criteria of success come to the fore; these cannot be represented by a single set of numbers in computer games. There is always variety, and seldom extinction. No doubt dominant members of a society have more chances to raise their children successfully; but it appears that again and again special elites rose to power who produced fewer children but, through an elaborate culture, kept control over their inferiors who produced more children. Should this be called a lack of fitness of the ruling class." (Or yet another example of K vs. r selection?) (quote from Walter Burket, in Creation of the Sacred: Tracks of Biology in Early Religions in Science (1997) 275:1894)

Scientific creationism

Objection to the concept of biological evolution has become a time-honored tradition within many circles in this country, particularly among biblical literalists.
While certainly an individual's right to believe whatever he or she chooses is central to U.S. citizenship (so long as others, ones own offspring often excepted, are not harmed by whatever shape these beliefs might take), science and biblical literalism have clashed numerous times since the publication of Darwin's On the Origin of Species. These clashes have traditionally involved biblical literalists arguing that biological evolution not only has not occurred but cannot occur within this universe.
Recently these arguments have become institutionalized with the founding of alternative hypothesis to evolution by natural selection going under the rubric, scientific creationsim. Almost without fail the various the various tenets of scientific creationism collapse under even casual inspection so we will not consider these here.
Despite the obvious faults associated with the scientific creationism hypotheses, scientific creationism is a politically powerful explanation for the origin of the diversity of life on our planet.
Recognizing the reality that political power can be a profound distorter of truth, scientists who study evolution have become more and more vocal in defending evolutionary theory from this political, quasi-religious, scientifically (and morally?) bankrupt movement. In this lecture presented above we carry on this tradition by walking through various considerations that scientists consider to be strong evidence for the existence and occurrence of evolution.

Vocabulary

Analogous trait
Convergence
Convergent evolution
Homologous trait
Homology
Evidence: biochemical uniformity
Evidence: convergent evolution
Evidence: embryology
Evidence: biogeography
Evidence: industrial melanism
Evidence: molecular evolution
Evidence: sickle cell anemia
Evidence: vestigial structures
How eyes evolve
Macroevolutionary change in the fossil record

Practice questions

Bats, birds, and mosquitoes all have wings. Were I to lie and tell you that the wings of all three organisms are identical structurally, embryologically, and likely arose because their common ancestor had such wings (i.e., all of the previous statements in this sentence are complete garbage but assume in answering this question that they're not), what would you nevertheless conclude about the wing adaptation in these organisms?[PEEK]
If I told you that bat, bird, and mosquito wings are not structurally similar, not embryologically distinct, and that the common ancestor to these three organisms did not have wings (which are all true), then what would you conclude about the wing adaptation in these organisms?[PEEK]
Why do whales and snakes have vestigial legs?[PEEK]
Why do you suppose the sickle cell allele is highly prevalent in some but not other human populations? (note that the answer to this question has two aspects) [PEEK]
Why do you suppose it is easier to demonstrate artificial selection than it is to demonstrate natural selection?[PEEK]
In the peppered moth example of natural selection, what was the selecting agent? [PEEK]
Name a homologous feature associated with chimpanzees, gorillas, and humans. [PEEK]

Practice question answers

You would conclude that it is an example of a homology.
You would conclude that they are examples of convergent or parallel evolution.
Because though these organisms have no need to employ legs (thus resulting in the accumulation of selectively neutral but nevertheless debilitating mutations affecting the development of these structures), their ancestors had and used legs.
(i) The heterozygote confers resistance to malaria and therefore serves to increase the prevalence of the sickle cell allele in populations exposed to significant levels of parasitism by the malaria protozoa. (ii) In other populations, strong selection against the sickle cell homozygote is sufficient to drive the allele to extinction.
Artificial selection occurs in a highly controlled environment in which all relevant variables may be controlled and understood.
predation by birds.
Anything they all have in common that presumably is a consequence of common evolutionary decent such as the fact that all four exhibit four limbs, demonstrate bilateral symmetry, etc.

References

Anonymous (1997). Vignettes: Human Fitness. Science 275:1894.
Darwin, C. (1859). On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. John Murray, London.
Dawkins, R. (1995). Where'd you get those peepers? New Statesman and Society June 16, 1995:28-29.
Holden, C. (1997). On the path of the primordial eye. Science 275:1885.
Levinton, J.S. (1992). The Big Bang of Animal Evolution. Scientific American November:84-91.
Morell, V. (1997). Predator-free guppies take an evolutionary leap forward. Science 275:1880
Postlethwait, J.H., Hopson, J.L. (1995). The Nature of Life Third Edition. McGraw-Hill, Inc., New York. pp. 364-385.
Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 385-403.
Reznick, D.N., Shaw, F.H., Rodd, F.H., Shaw, R.G. (1997). Evaluation of the rate of evolution in natural populations of guppies (Poecilia reticulata). Science 275:1934-1937.
von Hippel, Arndt (1994). Human Evolutionary Biology. Stone Age Press, Anchorage, AK. pp. 280-281.