Supplemental Lecture (97/06/10 update) by Stephen T. Abedon (email@example.com)
Wallace, Alfred Russel (1823-1913)
- Chapter title: Evolutionary Biology
- A list of vocabulary words is found toward the end of this document
- Biological evolution (evolution) is a phenomenon that occurs within populations of individuals, rather than to individual organisms. At its simplest, evolution may be defined as progressive change within populations of organisms. Such a definition, however, could use a bit of modification lest we think that spending a week spray painting pink a herd of zebras is evolution. Such phenotypic modification, in the absence of an underlying genotypic modification, is not evolution. Instead, biological evolution involves progressive changes to populations of genotypes.
- Progressive change can occur only if information may be carried, not unchangingly, through time. As we know, genotypic information is carried through time in genomes which are replicated in the course of cell division. We have also encountered the idea of change to genetic information. Specifically, such things as mutation, translocation, nondisjunction, and recombination all constitute genotypic change.
- The idea of progress in evolution is a tricky concept if defined in terms of net improvement. However, progress may be defined instead simply as partial change built upon previous partial changes. Thus, a statue chipped from stone is progressively worked whether it becomes slowly recognizable or ultimately is best discarded. Evolutionary change therefore need not be synonymous with improvement (as so deemed from a given vantage).
- Nevertheless, evolution tends to refine information content (statues in progress ideally tend to become more, not less recognizable with time). Let's say that units of genetic information may be viewed as environmental descriptors. Since we know that genotypes interact with their environment through associated phenotypes and that evolution operates on populations rather than on individuals, then we might say that those individuals who are better adapted possess a more refined genotypic description of their environment.
- In this lecture we will consider the basic terms and concepts of evolutionary biology. In a subsequent lecture we will delve into populations genetics, the discipline within which one studies the genetic changes that occur as populations evolve, particularly considering the Hardy-Weinberg equation and the effect of natural selection on populations which otherwise are found in Hardy-Weinberg equilibrium.
- Evolution, summary
- The following is quoted from p. 387 of Raven and Johnson, 1996 as quoted from Mayr, 1992:
- All species have such great potential fertility that their population size would increase exponentially if all individuals born would again reproduce successfully
- Except for minor annual fluctuations and occasional major fluctuations, populations normally display stability.
- Natural resources are limited. In a stable environment they remain relatively constant.
- Since more individuals are produced than can be supported by the available resources but population size remains stable, it means that there must be a fierce struggle for existence among individuals of a population, resulting in the survival of a part, often a very small part, of the progeny of each generation.
- No two individuals are exactly the same; rather, every population displays enormous variability.
- Much of this variation is heritable.
- Survival in the struggle for existence is not random but depends in part on the hereditary constitution of the surviving individuals. The unequal survival constitutes the process of natural selection.
- Over the generations this process of natural selection will lead to a continuing gradual change of populations, this is, evolution, and to the production of new species.
- Jar of marbles evolution analogy
- Consider a finite jar of marbles as a metaphor for a gene pool, i.e., a pool of haploid gametes pontially able to fertilized one another.
- There are red marbles and there are blue marbles. The frequency of red marbles, for example, is equal to the number of red marbles divided by the total number of marbles.
- By how many mechanisms might the frequency of red marbles change?
- Red marbles could spontaneously turn into blue marbles and vice versa (mutation analog).
- Red marbles could spawn new red marbles faster than blue marbles can spawn blue marbles (effect of selection on replication analog).
- Some force could remove red marbles from the jar faster or slower than the same force removes blue marbles (effect of selection on survival analog).
- The contents of a jar of blue-only (or red-only) marbles could be poured into the mixed jar (migration analog).
- All but a handful of marbles from the mixed jar could be removed, i.e., the likelihood that the frequency of red marbles in the handful is the same as the frequency of red marbles in the jar as a whole is smaller the smaller the handful size (genetic drift analog).
- Malthus, Thomas (1766-1834)
- Limits to population growth:
- Argued in an essay, Essay on the principles of populations, first published in 1798, that:
- population size tends to increase geometrically
- food supplies tend to increase only linearly
- this leads ultimately to inevitable population crashes (or, at the very least, limits on growth)
- Thus, Malthus argued that populations reach limits on growth and that these limits are not exceeded due to the occurrence of the death of individual members of populations.
- In other words, through their potential for growth in numbers alone populations posses a capacity to exceed the carrying capacity of their environment.
- Darwin, Charles Robert (1809-1882)
- Argued, most completely in the book On the Origin of Species by Means of Natural Selection (1859) that the inevitable trimming of geometric population growth through death was actually likely differential death, whereby the "weaker" members of a population died preferentially to the "stronger."
- Heritable differential death:
- Darwin also argued, without the luxury of modern Mendelian genetic theory, that at least some of the differences between the weak and the strong are heritable.
- Thus, evolutionary change occurs, at least in part, as a consequence of differential death.
- This simple idea is one of the most powerful and predictive of any in the entire history of our species.
- Independent co-discoverer of natural selection:
- Alfred Russel Wallace is credited as the independent co-discoverer of the natural selection.
- It's one of the great stories of biology that Wallace came to Darwin with this wonderful idea which Darwin, actually, had been working on, but not publishing, for years previously (Darwin was delaying publication until he had as air-tight a case for evolution by natural selection as possible). Wallace, through his generosity, agreed to delay publication so that Darwin could present the theory publicly on the same occasion.
- Interbreeding group:
- A group of potentially interbreeding individuals, which does not exclude other individuals which have at least the same potential for interbreeding.
- Note that I'm using the term "interbreeding" here, not "inbreeding." They are not synonimous.
- Gamete frequency = allele frequency:
- How can it be that Hardy-Weinberg equilibriums are so easily set up and then remain indefinitely?
- Recall that in the process of random mating, it is not diploids that are mixing but their haploid germ cells. Thus, at the point of fertilization, it doesn't matter what the parental genotypes were, only what the germ line genotypes are.
- Germ line genotype frequency is equivalent to allelic frequency.
- Thus, the genotype frequency of the parental generation influences the genotype frequency of progeny generations only to the extent that the parental genotype frequencies defines the parental allelic frequency.
- This mixture of gametes is known as a gene pool (the imagery of a slimy mess of sperm and egg floating and swimming about your favorite swimming spot is almost too wonderful to ignore-think fish people!).
- A gene pool in a genetic sense, is all of the alleles present in a potentially interbreeding population.
Change in gene frequency with time:
- The basic definition of biological evolution, also known simply as evolution, is: "Occurrence of change in the genetic endowment of populations of organisms."
- Incorporation of novel genetic changes:
- A narrower definition, but nevertheless one that embodies an idea of evolution as progressive improvement is:
- "Incorporation of novel beneficial genetic changes (e.g., mutations) into populations of organisms."
- Thus, evolution can, though not necessarily does, lead to greater adaptation of organisms to their environments.
The population genetics definition of evolution is a restatement of the first definition, above. The population genetics definition of evolution may be stated as::
"Change in the gene frequency of populations with time."
This definition specifically refers to changes in the frequency of the genetic material found at specific loci. Thus, a more precise restating of definition (iii) might be: "Change in the allele frequencies within populations as a function of time."
Forces of evolution:
How might allelic frequency change with time?
Thus, re: this latter mechanism, allelic frequencies in a finite population will rise and fall even in the absence of mutation, natural selection, and migration. This fourth mechanism of allelic change reflects this random variation over time and is called drift, or, more precisely, genetic drift.
- mutation (and other changes in genetic material) introduces new alleles into populations (or adds minusculely to the number of a type of already existing alleles).
- natural selection removes alleles from populations, some faster than others, either through outright killing (differential survival) or through differential reproductive success in absence of premature death
- migration results in the mixing of one population having a given frequency of alleles with a second population potentially possessing different allelic frequencies, thus resulting in a combined population possessing different allelic frequencies from those found in the two original populations
- unless populations are infinite (or, at least, very large), simple statistical variation will result in some alleles surviving and reproducing to a greater extent than other alleles
Very common/often not limiting:
- Genetic polymorphism describes the existence of more than one allele at a given loci.
- Raw material of evolution:
- The existence of genetic polymorphisms is the raw material of evolution.
- As defined above, evolution as a change in allele frequencies cannot even occur without genetic polymorphisms since allelic frequency is, and only can be, equal to 1.0 unless more than one allele exists at any given loci.
In fact, genetic polymorphism is very (very, very) common at many loci within wild populations of organisms.
Thus, the raw material of evolution, very often, is not limiting.
Small but real change in allele frequency:
- A physical change in hereditary material resulting from some process other than genetic recombination.
- Mutations, with minor exceptions, are by definition replicable.
- Because mutations are random changes to highly evolved structures (i.e., loaded with information), most mutations are detrimental to the organism possessing the mutation.
- Sole source of beneficial variation:
- Nevertheless, mutations are the sole source of novel genetic information.
- Thus, on rare occasions mutations can lead to a selective advantages to the organisms carrying the mutation.
- More frequently, mutations, at least iniitally, are selectively neutral.
Note that under normal circumstances, organisms generally will acquire on the order of one mutation per round of genome replication.
Though low in frequency, mutations nevertheless constitute one method by which allelic frequency may change. That is, a population lacking a given allele may gain that allele through mutation, while the number of alleles of a given type (already existing) may increase or decrease as a consequence of mutation.
Note that for a common mutation it is far more likely that a mutation will decrease its allelic frequency (slightly but nevertheless in the negative direction) while for a very rare allele a mutation will more likely increase its allelic frequency (with the extreme example being an allele which does not currently exist in a population).
Selection [natural selection, Darwinian evolution; Darwinism]
Darwinian evolution/non-random evolution:
- Selection is the non-random, negative effect an organism's environment has on that organism's survival and/or potential for replication.
- Replication and survival are interchangable:
- Algebraically, replication and survival are pretty much interchangeable.
- Selection is a force that acts to reduce these characters.
- Selection results in there being less replication per unit time along with shorter survival).
When one considers evolution as a non-random phenomenon (i.e., one which acts to increase the content of relevant information found in organism's genomes), one is considering an evolution whose major driving force is selection.
Such evolution is termed Darwinian evolution or Darwinism.
Action on genotype through phenotype:
Selection acts on an organism's genotype indirectly through an organism's phenotype.
Genotypes that have associated phenotypes which are better adapted to a given environment will, on average, produce more surviving offspring than genotypes associated with less well adapted phenotypes.
Recall that especially in diploid organisms phenotypes don't directly map onto genotypes (due, for example, to dominance-recessive relationships as well as to interactions between loci, or interactions between genotypes and very local environments).
Selection can only act when genotype differences result in differences in phenotype.
Selection cannot act directly on recessive alleles unless and until they are found in the heterozygous state.
Selection consequently can result in fairly complex changes in allelic frequencies.
Allele frequency dependent selection:
Selection can act to an extent which is dependent upon genotype frequency.
Thus, for example, selection may act strongly against common genotypes, while weakly against rare genotypes (or vice versa).
This is one explanation for why human major histocompatibilty alleles (which in novel forms may be better suited to resisting communicable disease) exist in many numbers, each at low frequencies.
Selection's effect on gene pools
Fitness [absolute fitness]
- Biased input:
- In terms of gene pools and Hardy-Weinberg principles, selection acts by decreasing the number haploid germ cells a given genotype enters, on average, into the gene pool.
- That is, those genotypes for whom the fewer germ cells make it into the gene pool have been affected to a greater extent by selection than those genotypes which, on average, are able to supply comparatively greater numbers of germ cells to the gene pool.
Prediction of next generation contribution:
- Absolute fitness is the potential for individuals of a given genotype to survive and reproduce in the face of natural selection.
- Measure of potential natural selection:
- The effect of natural selection is usually considered algebraically in terms of effects on fitness.
- If a given genotype has a tendency to increase in number given a certain degree of natural selection, they have an absolute fitness in excess of 1; if a genotype tends to decrease in number then they have an absolute fitness of less than 1.
Multiplication of absolute genotype prevalence by absolute fitness, per generation, gives a predicted new absolute prevalence.
Thus, if there are five individuals displaying an absolute fitness of 1.4, then after one generation we would predict that here would be 7 such individuals.
- The mean absolute fitness value exhibited by all the members of a population.
- A not growing, not shrinking population has an average fitness of 1.0.
- The potential for individuals of a given genotype to survive and reproduce in the face of natural selection, as compared to the average fitness exhibited by the population in which this individual is a member.
- In a population which is neither increasing nor decreasing overall in number, a genotype with a relative fitness of less than one would (on average) be decreasing in frequency while a genotype with a relative fitness of greater than one would, on average, be increasing in frequency.
- A phenotypic change that results in an organism or part of an organism being better able to accomplish a given task in a given environment, often resulting in the organism being better suited to surviving and reproducing in that environment.
- Increase in information:
- If the phenotypic change is a reflection of an underlying genotypic change, then its occurrence may be considered an evolutionary refinement of an organism's genotype: An increase in the content of relevant information.
- The key to the occurrence of adaptation is the positive input into a population of variation via mutation and migration. This variation is heritable.
- Selection is a negative effect which prunes those genotypes less well adapted to the environment.
- "Multiple, recursive cycles of selection from small pools can 'compute' complex answers that far exceed the capacity of any single library of sequences. . . Recursive means that the best (partial) solutions selected from one pool are amplified and mutated to form the next pool." (Stemmer, 1995)
Adaptive change is dependent on the existence of mutational variation.
Such variation can be temporally used up after especially strong bouts of selection or other mechanisms that signficantly reduce population sizes.
Future adaptations can be constrained by past adaptation (e.g., the loss of air worthy wings in penguins diminishes the potential for penguins to evolve to fly).
- Loss of one extreme:
- Directional selection is natural selection against extremes, especially against one particular extreme.
- That is, the population as a whole tends to lose those alleles associated with one extreme phenotype and that phenotype's underlying genotype.
- When you imagine selection as directional (e.g., toward an ever increasing giraffe's neck), you are imagining (not surprisingly) directional selection.
- Populations can exist for long periods without any major phenotypic changes.
- The fossil record shows many such cases (leading, I might add, to the false claim by the ignorant and misinformed that the fossil record lacks intermediate forms---what then is an intermediate form?).
- Loss of many extremes:
- A population which shows no change over time is clearly not undergoing directional selection. Does this mean that selection is not operating in such populations or that variation upon which selection might act does not exist?
- Not likely on either count. Instead, it is likely that selection acts in such populations by weeding out extreme forms.
Thus, in such populations selection is very much at work but has a stabilizing effect (about an intermediate form).
Such selection is called, again not surprisingly, stabilizing selection.
As a working hypothesis one might assume that the intermediate form in such a population is well adapted to its environment.
- Selection against the intermediate form:
- In a situation when it is the intermediate form, rather than any one or more extremes, that is selected against, one ends up with a population of genotypes representing some degree of more than one extreme form.
- Such selection is termed disruptive.
- Extreme phenotypes occupy different niches:
- The occurrence of disruptive selection often can be interpreted as the intermediate form lacking in some manner relative to the extreme forms.
- For example, the extreme forms may each be well adapted to specific, differing environments, and the intermediate form may be well adapted to neither.
- Heavy hand of man:
- Selection achieved by the artificial manipulation of a population, as by a breeder on domesticated animals or plants.
- Artificial selection tends to be away from that typically achieved through natural selection thus resulting in organisms that are both novel (interesting) and less fit than their ancestors to the environment in which their ancestor's evolved.
- However, the very fact that, more than anything else, it is the whim of their breeder which controls their domesticated fate, the fitness achieved through artificial selection can be quite high within the artificial environment in which they are kept.
Migration [gene flow, gene exchange]
- Competition for mates:
- Sexual selection is a situation in which reproductive advantage is gained or lost in the competition for mates.
- Particularly, males are under strong selection to appeal to females and vice versa.
- Not necessarily non-sexually adaptive:
- Sexual selection can lead to adaptation in organisms which does not increase survival value or, given the occurrence of reproduction, the number of offspring that are produced and raised to adulthood per mating.
- However, the adaptation may either allow the organism to mate more often with different individuals (in the case of males for whom sperm is cheap and therefore worth spreading around) or more selectively (in the case of females for whom eggs are expensive---note that this metaphor only goes so far since a male that participates heavily in raising young certainly can invest something approaching that invested by the female, while in some species the males are vested entirely with the caring for juveniles, e.g., sea horses, and therefore actually have more reason to be selective than the females in that species).
- Note that the advantage of being able to chose your mate (for example, in the case of female choice) is to make sure that matings are made with particularly fit individuals. (Again, males who don't participate in a mating much beyond the donation of their sperm don't have much of an incentive to care about degrees of evolutionary fitness displayed by the genotype of their partners).
- The movement of alleles (as contained in individuals) from one population to a second.
- Sampling error:
- A consequence of populations not being infinitely large.
- Genetic drift is sampling error--random effects on the survival and reproduction of individual alleles.
- Sampling error has an increasingly large effect on allelic frequency as populations become small.
Bottleneck [genetic bottleneck]
- Drift upon population founding:
- When populations are founded by a small number of individuals, we can expect genetic drift to be operating.
- Thus, small, isolated subpopulations may differ from their parental population in terms of allelic frequency due simply to the subpopulation being founded by a small group of individuals who were not representative of the allelic frequency of the parental population.
- Drift due to decline in population size:
- A small population that is forced to exist as a small population over time will experience a decline in allelic variability as well as a likely change in allelic frequency (the former, actually, is simply an extreme example of the latter: one where the frequency of one or more alleles is reduced to zero).
- This phenomenon is known as an evolutionary bottleneck (i.e., a bottle's neck has a smaller cross section than the rest of the bottle thus reducing the rate of flow---the number of individual, in this case molecules, present in the cross section at any given time).
- Evolutionary bottlenecks can have devastating effects on the variability, and fitness, of a population.
Non-random mating in gene pools
- Any situation which increases or decreases the likelihood that any two gametes will meet, relative to any two gametes picked at random within the population, is non-random mating.
- Common factors lead to non-random mating are
- geographic (you are far likelier to mate with the person living next to you than a person living a thousand miles away)
- temporal (you are far more likely to mate with a person of similar age than having a significantly different age)
- cultural (you are far more likely to mate with a person sharing your values---who is of your group---than a person who does not)
- phenotypic (you are far more likely to mate with a person sharing your phenotype---tall with tall, short with short, within ancestral grouping, etc.)
To the extent that random mating represents a structureless, non-biased situation, non-random mating is a structured and biased situation. Hardy-Weinberg does not occur given non-random mating patterns. For example, if AA individual mate preferentially with AA and Aa individuals while aa mate preferentially with aa individuals, you will observe an overabundance of AA and aa individuals at the expense of Aa individuals---a deviation from Hardy-Weinberg ratios.
Inbreeding [identical by descent]
- Note that though Hardy-Weinberg equilibrium requires random mating, the concept of a gene pool does not itself imply random mating among gametes, only that the probability that two given alleles sourced from any two parents (of differing gender) may end up in the same individual (progeny) is greater than zero.
- A gene pool may also be considered to be a group in which the probability of alleles found within that group will end up in the same individual is greater than the probability that two randomly chosen individuals from a larger population will mate.
- Thus, gene pools can exist as relatively distinct sub-pools within a larger pool, implying some degree of impediment to random mating (i.e., some kind of structure exists).
Prediction of evolutionary outcomes
- Identical by descent:
- An increased likelihood that the two alleles at any randomly picked loci within an individual are identical by descent.
- That is, that both alleles are the replicative products (going back however many generation) of a single allele existing within a single individual.
- In other words, inbreeding occurs as a consequence of a mating between individuals who can claim to have at least one ancestor in common.
- Inbreeding naturally tends to increase the likelihood of homozygosity, and is especially dramatic when it is an otherwise typically rare, recessive allele which is achieving homozygosity.
- Note that as the shared ancestor becomes more and more distant, the degree of inbreeding and therefore the effects become less and less significant.
- Not good long-term predictor:
- Any good theory allows prediction, and therefore experimental testing. Evolutionary theory, for example, is capable of explaining and even predicting a few evolutionary trends in natural populations.
- Nevertheless, evolutionary theory is not very good at predicting real world outcomes.
- As an extreme example, starting with an inflating universe, evolutionary theory could not have predicted the evolution of guinea pigs.
- Why not? Does this mean that evolutionary theory isn't a good, robust theory after all? Beyond all the pontification, is evolutionary theory just a lot of hot air? Not at all. This is because accurate prediction is only facile in very simple systems.
- A problem of complexity:
- As complexity builds, non-linearities in systems rapidly become intractable.
- Furthermore, certain aspects of our physical world are literally not predictable (for example, the radioactive decay of a single atom). Any before the fact prediction of the direction of evolution therefore is doomed to failure, even given absolute knowledge of predictable consequences, the moment a non-predictable phenomenon is encountered.
- As Stephen Jay Gould has put it, "Webs and chains of historical events are so intricate, so imbued with random and chaotic elements, so unrepeatable in encompassing such a multitude of unique (and uniquely interacting) objects, that standard models of simple prediction and replication do not apply. . . The Darwinian revolution remains woefully incomplete because, even though thinking humanity accepts the fact of evolution, most of us are still unwilling to abandon the comforting view that evolution means (or at least embodies a central principle of) progress defined to render the appearance of something like human consciousness either virtually inevitable or at least predictable." (p. 91, Gould, 1994)
- In short, for similar reasons evolutionary theory, though a far more robust explanation of the world around us, often can predict outcomes with no more reliably than historical theory.
- Absolute fitness
- Artificial selection
- Average fitness
- Darwinian evolution
- Directional selection
- Disruptive selection
- Founder event
- Gene exchange
- Gene flow
- Gene pool
- Genetic drift
- Genetic polymorphism
- Identical by decent
- Jar of marbles analogy
- Prediction of evolutionary outcomes
- Natural selection
- Natural selection (summary)
- Non-random mating
- Random genetic drift
- Relative fitness
- Sexual selection
- Stabilizing selection
Practice question answers
- Who was Alfred Russel Wallace?[PEEK]
- What is a population? [PEEK]
- Name four ways in which allelic frequency might change with time? [PEEK]
- Describe a circumstance (real or imaginary) in which sexual selection is occurring or has occurred. The easiest way to answer this question is with a specific example (e.g., "the elephant's trunk"). [PEEK]
- What is a founder effect?[PEEK]
- Stabilizing selection appears to be occurring within a population. From general principles, why might you suppose this so?[PEEK]
- Define absolute fitness.[PEEK]
- What is non-random mating? [PEEK]
- True or false, the effect of mutation on relative allele frequency (i.e., fractional/percentage change in frequency) is constant regardless of allele frequency. (circle one correct answer) [PEEK]
- (i) What is the general effect of selection on the majority of new mutations. (ii) In terms of information content, justify your answer. [PEEK]
- Generally speaking, what fraction of new mutations would you expect to be adaptive? (circle one correct answer) [PEEK]
- very few
- about half
- it depends on the organism
- Given is a very large, randomly mating, highly remote population which is otherwise stable (i.e., constant) in size. A genotype is decreasing in frequency within this population. Describe this genotype in terms of its absolute fitness.[PEEK]
- Prior to the application of selection, three genotypes, AA, Aa, and aa, are in Hardy-Weinberg equilibrium with the frequency of the A allele equal to 0.1. This translates to genotype frequencies 0.01, 0.18, and 0.81, respectively. If the A allele is dominant to the a allele, the aa genotype has a relative fitness of 0.5, and standard conventions apply, then what are the genotype frequencies after one round of selection (and prior to mating)?[PEEK]
- In terms of their effects on a normal distribution of characters, contrast disruptive and stabilizing selection. Be sure to indicate what is the effect of selection. [PEEK]
- A population is founded by a small number of organisms. This population is completely isolated from its parental population and is found in a habitat that prevents its numbers from growing to greater than a dozen or so individuals at any one time. In terms of alleles and allele frequencies, name three characteristics of this populations. [PEEK]
- If otherwise random mating is assumed, then why are rare alleles more likely to occur in the homozygous state as a result of matings which occur within subpopulations (especially long established subpopulations) and less likely to occur as a result of matings which occur between subpopulations.[PEEK]
- codiscoverer of Darwinian evolution (evolution by natural selection)
- A group of potentially interbreeding individuals, which does not exclude other individuals which have at least the same potential for interbreeding.
- mutation, selection, migration, drift.
- e.g., the peacock's tail. Females choose to mate with the males with the flashiest tales. There consequently is selection for flashy tails and the tails of males become ever more flashy through time.
- When a population is founded by a small number of individuals therefore likely representing an erroneous sampling of the parental population's allelic frequencies.
- The population is well adapted to its environment.
- The potential for individuals of a given genotype to survive and reproduce in the face of natural selection.
- Note that absolute fitness can be a value of greater than one. If a genotype has a tendency to increase in number, it has an absolute fitness of greater than 1.0. If the genotype tends to double in number each generation, then it has an absolute fitness of 2.0. If the genotype tends to stay exactly the same over time, then it has an absolute fitness of 1.0. If the genotype is losing ground then it will have an absolute fitness which is somewhere between 0.0 and 1.0. Finally, a dominant early in life lethal mutation has, by definition, an absolute fitness of 0.0.
- In contrast, the average fitness of a not growing, not shrinking population is 1.0 (i.e., each individual, on average, manages to replace herself).
- For relative fitness the genotype with the highest fitness is often assigned a value of 1.0 and all lower fitnesses therefore a value of less than 1.0. However, this designation is nonetheless arbitrary and one could, if one wanted to, just as reasonably assign a value of 1.0 to the lowest fitness genotype and therefore present the relative fitness associated with all other genotypes in terms of values in excess of 1.0.
- Any situation which increases or decreases the likelihood that any two gametes will meet, relative to any two gametes picked at random within a population.
- False, the effect is greater with the formation of rare alleles and less with the loss of common alleles, especially in large populations, though generally it takes large populations just to generate mutational variation at an appreciable rate.
- (i) Negative or, at most, neutral. (ii) Mutations generally represent a loss of information and genomic information exists particular in proportion to its positive effect on organismal survival and reproduction. Thus, a loss of information generally would result in a decline in the potential for a organism to survive/reproduce. That is, there would be a tendency toward a negative effect on new mutations by selection.
- ii, very few.
- The absolute fitness associated with this genotype is less than one.
- The relative fitness of AA and Aa must be the same and must both be equal to 1, given standard conventions (i.e., when dealing with relative fitness, one genotype is designated as having a fitness of 1.0, the rest fitnesses relative to this number---often the one genotype so designated is the one with the greatest absolute fitness). With Hardy-Weinberg equilibria the frequencies of AA, Aa, and aa are 0.01, 0.18, and 0.81, respectively. Multiplying these frequencies by the appropriate selection coefficients yields values of 0.01, 0.18, and 0.405. These total to 0.595. Dividing these frequencies by these numbers establishes the genotype frequencies following selection: 0.017, 0.303, and 0.681 for AA, Aa, and aa, respectively. Finally, for whatever it is worth, the A allele frequency can be recalculated: 0.017 + 0.303/2 = 0.169.
- Disruptive selection removes all but the extremes while stabilizing selection removes the extremes from this normal distribution. The effect of selection is the differential removal of genotypes.
- (i) Right from the start the population will differ in allele frequencies from that of the parental population. (ii) The population will tend to have fewer alleles than the parental population, especially as time goes on. (iii) alleles will not be exchanged with the parent population at any point following the founding event. Thus, the parental population will have no influence on allele frequencies past this founding event.
- Matings within subpopulations results in a higher degree of inbreeding than do matings between subpopulations. This is a consequence of the definition of subpopulations, which essentially is equivalent to the definition of inbreeding, i.e., matings between individuals who have more closely related ancestors than do two randomly chosen individuals from the larger population.
Gould, S.J. (1994). The evolution of life on earth. Scientific American October:85-91.
Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 7-14, 370-384, 419.
Stemmer, W.P.C. (1995). The evolution of molecular computation. Science 270:1510.