Supplemental Lecture (97/04/13 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Speciation
    1. A list of vocabulary words is found toward the end of this document
    2. Speciation is the occurrence of more than one species (or, at least, a single different species) where previously there was only one.
    3. Implied in this concept is the transformation of that one species into the mentioned others. The processes by which speciation occurs are difficult to pin down experimentally. This is due, as is often the case in evolutionary biology, to the events themselves occurring with apparent durations typically in excess of evolutionary biologist's grants, careers, or even lives.
    4. Observation of existing populations either prior to, during, or following speciation events, however, has led to extensive insight into the process. In short, a speciation event is a process whereby a population within a single species becomes indefinitely and irretrievably reproductively isolated from other populations of that same species.
    5. The interesting steps are those which lead to reproductive isolation as well as those which maintain it, not always the same thing. In this lecture we first consisder just what a species is before develing into mechanisms of isolation and then further consideration of the process of speciation.
  2. Species
    1. A fuzzy concept:
      1. Part of the problem of understanding speciation is understanding just what a species is.
      2. Unfortunately, just what is a species is not nearly as solid a concept as one might hope.
    2. Distinct populations:
      1. Minimally, a species represents some sort of population which is both continuous through time.
      2. Additionally, a species has a within-population similarity that distinguishes it from other populations which we would consider to be different species.
    3. Genetic differences (as opposed to or in addition to simple phenotypic differences) must exist between two populations considered to be different species.
  3. Species concepts
    1. What a species is seems to be based to some degree on what you might want a species to be.
    2. Three species concepts:
      1. There exist at least three common species concepts that distinguish one species from other species by different criteria.
      2. These include the:
        1. the biological species concept
        2. the phylogenetic species concept
        3. the ecological species concept
    3. Lack of supremacy:
      1. Arguments exist both for and against the primacy of each of these three species concepts.
      2. Consequently, just what a species often does not lack of ambiguity.
  4. Biological species concept
    1. The biological species concept considers a species to be defined solely in terms of reproductive isolation (or, more specifically, sexual isolation).
    2. The biological species concept is perhaps the most famous of species concepts and is commonly stated as a quote from Ernst Mayr:
      1. "Groups of populations that can actually or potentially exchange genes with one another and that are reproductively isolated from other such groups" (or, "Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.")
    3. Despite its fame, the biological species concept runs into problems for a number of reasons, however.
    4. It is very often difficult to actually test whether individuals from two populations are capable of reproducing together, much less whether they have any tendency to reproduce in the wild.
    5. Sometimes two species look and behave for all the world like different species but still retain some degree of gene flow (coyotes, wolves, and domestic dogs, for example).
    6. A definition based on the occurrence of sex is only relevant to species that have sex. (i.e., is not terribly applicable to species which reproduce solely by asexual means).
  5. Phylogenetic species concept [fossil species, bacterial species, and viral species]
    1. Common sense approach:
      1. If two populations "look" like different species but either retain some low level of gene flow or it is difficult to tell whether sex is occurring or could occur, then what?
      2. Well, the next step is to distinguish two populations in terms of how they "look" (defined very broadly, e.g., could include DNA sequence information) and then come up with some sort of criteria that distinguishes two divergent populations which have not speciated from two which have.
      3. Distinguishing species in this manner represents the phylogenetic species concept.
      4. A less sophisticated version of this phylogenetic species concept is what we as lay persons use all the time to distinguish the types of organisms around us.
    2. Caveat: what if reproductively isolated populations look the same?
      1. Clearly in many cases it is very easy to distinguish two species using their "looks" (your dog and your cat, say, though it is usually very clear these examples also are not interbreeding), or to say that there clearly exists only a single species based on a biological species concept (a herd of apparently interbreeding zebra, for example).
      2. However, once "looks" are relied upon in difficult cases, distinctions can look for all the world (especially, no doubt, to the completely uninformed) to be arbitrary.
      3. Consider the extreme range in morphology displayed by the single species of domestic dog.
    3. Fossil species:
      1. With fossil species it is obviously impossible to apply the biological species concept to extinct organisms.
      2. Fossil species must thus be differentiated solely by a phylogenetic species concept approach.
    4. Bacterial and viral species are often distinguished using a phylogenetic species concept.
  6. Ecological species concept
    1. Based on ecological competition:
      1. "A species is a number of related populations the members of which compete more with their own kind than with members of other species." (p. 152, Colinvaux, 1986)
      2. The more similar two organisms are, the more likely their needs will overlap, the more likely they will compete, and therefore the more likely that they are of the same species.
    2. Caveat: intraspecific life history divergence:
      1. Even the ecological species concept has problems since it requires that members of individual species not have divergent life histories (which, in practice, is not always the case).
      2. It also runs into the same problem as the phylogenetic species concept: At what point does one stop the process of splitting divergent forms into new species?
    3. It is not necessarily trivial to determine the degree to which two or more individuals are compete ecologically.
  7. Splitting [splitter]
    1. Reading too much into subtle differences:
      1. Splitting is the process of seeing a bunch of populations and declaring that there are more species there than would generally be agreed upon (as in to split things apart).
      2. A spitter is an individual who tends to split.
    2. Only slightly tongue in cheek, a spitter is an individual who might spend hours pondering whether a glass is half empty or half full with water.
  8. Lumping [lumper]
    1. Reading too little into subtle differences:
      1. Lumping is he process of seeing a bunch of populations and declaring that there are fewer species there than would generally be agreed upon (as in to lump things together).
      2. A lumper is an individual who tends to lump.
    2. Only slightly toungue in cheek, a lumper is an individual who observes a glass which is either half empty or half full with water, and declares that it is a glass of water.
  9. Subspecies
    1. Morphologically but not reproductively distinct:
      1. A subspecies is a morphologically distinct population which is not reproductively isolated from other, also morphologically distinct populations.
      2. What a splitter might call a separate species, the lumper would suggest is just a distinct population within a larger species population, i.e., one with which true reproductive isolation is far from established and with which profound characteristics remain shared.
    2. Requirement for minimal range overlap:
      1. For two populations to be considered different subspecies of the same species, minimally they cannot have extensively overlapping ranges.
      2. Otherwise you would suspect that a strong mechanism of reproductive isolation must exist and therefore that they are actually two separate species.
    3. Differences at more than just a few loci:
      1. What appears to be two distinct populations could in fact extensively interbreed but nevertheless maintain two distinct morphotypes.
      2. In this case the morphotype must be controlled by one or, at most, a few tightly linked loci.
    4. More than just developmental stage differences:
      1. Alternatively, juvenile and adult forms or even males and females could differ sufficiently to fool you into thinking that they are unrelated.
      2. In this case the morphotypes would be considered neither separate species nor separate subspecies.
  10. Hybridization [hybrid]
    1. Some interbreeding allowed:
      1. Another thing that is certain about species is that if interbreeding between two populations becomes too extensive, both genetic differences and any pretension to retaining separate species-hood will be lost.
      2. Thus, while two species might display some interbreeding and still not completely violate all the various species concepts, that interbreeding mustn't get too out of hand.
      3. Nevertheless, the mere fact that two distinct species may nonetheless interbreed without severely damaging their speciesship provides an example example, again, of just how shaky are the various species concept.
    2. Consequence of overlapping ranges:
      1. The product of the interbreeding of two distinct populations is called a hybrid and the process hybridization.
      2. For two separate species to share extensively overlapping ranges there must be some sort of mechanism by which hybridization is avoided.
    3. Evolutionary biologists suspect that such mechanisms will evolve and/or at least be strengthened whenever range overlap occurs.
  11. Mechanisms of isolation [reproductive isolating mechanisms]
    1. Barriers to hybridization:
    2. Various mechanisms have been documented and/or hypothesized whereby one population can prevent hybridization with a second population.
    3. The existence of all such barriers is predicated upon hybrid progeny displaying a lower fitness than non-hybrid progeny of either parental type.
    4. An organism which has the potential to devote time and energy to the production of an inferior hybrid thus will itself be under strong selective pressure to not make that kind of mistake.
    5. Various reproductive isolating mechanisms serve as barriers to hybridization.
    6. Isolating mechanisms are divided into those which act prior to the fertilization and those which act afterward.
    7. Note that the concept of isolating mechanisms, biological species concept, and mechanisms of speciation are deeply intertwined, as will be considered in some detail below.
  12. Postzygotic isolating mechanisms
    1. Postzygotic isolating mechanisms are more expensive than prezygotic isolating mechanisms since, minimally, they entail the loss of an egg, by a female, to hybridization.
    2. Extreme cost: rearing of sterile hybrid:
      1. The most expensive postzygotic isolating mechanism (and, indeed, of any isolating mechanism) is the energy intensive rearing of a sterile hybrid (unable to reproduce such as the mule, a horse-donkey hybrid).
      2. In such a case true gene flow between populations does not actually occur (since offspring represent gene flow dead ends), but depending upon the amount of effort typically put forth per offspring, the cost to the parents of such a mistake can be enormous.
  13. Prezygotic isolating mechanisms
    1. A prezygotic isolating mechanism is a reproductive isolating mechanism that acts prior to fertilization.
    2. Cost minimization:
      1. The point of fertilization is considered significant in terms of isolating mechanisms because, while the act of sex may itself be relatively expensive (and/or dangerous), wastage of gametes and time and energy devoted to progeny development and rearing is even more expensive.
      2. This is especially so for females.
      3. Thus, hybridization is best avoided and such avoidance may be accomplished when prezygotic isolating mechanisms are employed.
    3. Examples of prezygotic isolating mechanisms include:
      1. geographical isolation
      2. prevention of gamete fusion
      3. mechanical isolation
      4. behavioral isolation
      5. temporal isolation
  14. Geographical isolation
    1. Geographical isolation is perhaps the simplest mechanism of isolation to achieve.
    2. It is also the least robust should populations ever again come to share ranges.
    3. Lack of range overlap:
      1. Geographical isolation simply means that mating does not occur because of a separation of egg and sperm by distance (i.e., ranges do not overlap).
      2. This lack of overlap can be a consequence of ecological stratification or as a consequence of physical barriers to migration (e.g., mountains, oceans, etc.).
  15. Prevention of gamete fusion
    1. Should geographical isolation end, but two populations have nevertheless sufficiently genetically diverged during their isolation that hybrids display reduced fitness, then strong selection will exist for the prevention of hybridization.
    2. Sperm-egg incompatibility:
      1. One of the simplest of genetically-based mechanisms which can lead to an avoidance of costs hybridization is some kind of sperm-egg incompatibility.
      2. For sperm-egg incompatibility to be a true block of hybridization, it must be an incompatibility which results in a prevention of gamete fusion.
      3. Note that alternatively sperm-egg incompatibility could result in a lack of hybrid gestation and rearing, though at the cost of one or more of the female's eggs.
    3. If the sperm-egg incompatibility prevents gamete fusion, then this would represent the simplest of the genetically-based prezygotic isolating mechanisms.
  16. Mechanical isolation
    1. Copulation incompatibility:
      1. Should sex occur but gamete fusion fail to occur, such is a great advance since prevention of hybridization is achieved.
      2. A perhaps simpler way to achieve such a result is to simply make the act of sex impossible.
      3. This may particularly be achieved by the occurrence of mechanically incompatible sexual organs.
    2. Lack of gamete wastage:
      1. Especially given internal fertilization, mechanical isolation can act to limit gamete wastage, especially the wasting of eggs to hybridization.
      2. However, given the previous existence of sperm-egg incompatibility, mechanical isolation additionally can prevent the wastage of sperm, thus giving such mechanisms at least one advantage over sperm-egg incompatibilities.
  17. Behavioral isolation
    1. Avoidance of mating:
      1. While mechanical isolation and other means by which gamete fusion is prevented may appear to be reasonable responses to a high cost of hybridization, one should not discount the additional costs and dangers associated simply with the finding of a mate and mating (for example, both are times of great distraction in which it is all too easy to let one's guard down to predators).
      2. Consequently, a mechanism which allows two organisms to very rapidly distinguish that right organism, from one which is sufficiently different that mating is best avoided, would be expected to leave the possessor with a selective advantage.
      3. One such category of mechanisms is termed behavioral isolation.
    2. Courtship behavior:
      1. Courtship behavior is a behavioral mechanism of isolation.
      2. Minimally, a male (and usually it's a male since a male usually has much less to lose) must act like a member of a female's species before she will allow him to mate with her.
      3. Often the behaviors a male must engage in to convince the female (that he desires) of his proper species-hood are quite elaborate.
      4. Bottom line, various behavioral cues allow males and females to recognize their own species and thus avoid wasting a great deal of time and energy.
    3. Dual role/female choice:
      1. Efforts by males are often not limited to demonstration of proper species-hood.
      2. A male very often also must rather quickly convince a given female why he rather than some other equally suitable male should be her mate.
    4. Females too:
      1. Though courtship behavior and appearance is often seen in the animal world as various cases of males trying to convince females that the latter wants to mate with the former, in less promiscuous mating systems, especially where males must contribute significantly to the rearing of young, the onus is on females as well to convince males that they are worth mating with.
      2. This would be a situation where while sperm may still be cheap, the cost of raising young requires more from males than just their sperm, thus raising the stakes for males considerably.
      3. In humans, of course, both males and females go through elaborate rituals to demonstrate to their opposite genders the particular advantages which may be accrued should the former allow the latter to mate with them.
  18. Temporal isolation
    1. Gamete exchange during different seasons:
      1. A truly effective prezygotic mechanism of isolation is simply to have mating and gamete exchange occurring at different times of the year.
      2. This prezygotic isolating mechanism is termed temporal isolation.
    2. This is especially useful to very promiscuous species which participate in multiple matings without much consideration of partner choice (e.g., plants pollinated on the wind).
  19. Scenario for speciation
    1. Incomplete though sufficient mixing:
      1. Consider a single population of organisms which inhabit a single, large range and has done so for thousands of years.
      2. Mating that occur within this range occur somewhat randomly though there is a strong bias for mating between individuals which are not significantly geographically separated.
      3. In this manner gene flow occurs throughout the entire range, thus leading to a cohesive population, one in which complete interbreeding is at least theoretically plausible.
    2. Geographical isolation:
      1. Continents collide, mountains are uplifted (or other mechanisms of geographical isolation), and suddenly this once vast and homogeneous population finds that it is splintered into a large population as well a number of geographically isolated smaller populations.
      2. These smaller populations were founded by a small number of individuals thus resulting in a founder's effect.
      3. In addition, these smaller populations remain small, in their splintered ranges, thus displaying significant genetic drift.
      4. They are also may be in somewhat different environments compared to that in which the parent population evolved and prospered.
      5. Most of these small populations eventually go extinct (i.e., all of their members die).
    3. Marginalization of parental population:
      1. Significant climate change occurs. This affects the parent population leading to a further shrinking of its range.
      2. What was once a vast and robust parental population surrounded by a few barely holding on, geographically isolated populations, now consists of a significantly smaller, somewhat impoverished parental population and one or a few smaller populations which are holding their own.
      3. (My reason for shrinking the parental population is because too large a parental population will tend to swamp out distinctions among subpopulations or even destroy such populations should gene flow stop being rare, even if hybrids are inviable---such is the premise, by the way, behind releasing sterile males into the environment to control insect populations.)
      4. Note that, in the biological species concept sense, none of these populations necessarily yet constitutes a separate species.
    4. Should one of the progeny populations find its range expanding to now include that occupied by the parental population (or vice versa or both), then hybridization may occur.
    5. Testing/forcing robustness of speciation:
      1. Should hybrids be selectively inferior to parents, then there will be selection to establish various isolating mechanism: two populations and one species will thus evolve into two populations and two species.
      2. Should hybrids not be inferior or one population greatly outnumber the other, then the two populations will in all likelihood merge. Two populations and one species will become one population and one species.
    6. Replacement of parental species:
      1. Should what was initially one species end up as two species (as discussed above), and then the original parental species dies out (for whatever reason), one would have what appears to be, over long times and geographic resolution, a situation in which one species was replaced by a second, progeny species.
      2. This is, in fact, how such a speciation event might look in the fossil record, as discussed below (i.e., under punctuated equilibrium).
  20. Punctuated equilibrium
    1. Dominant populations dominate the fossil record:
      1. The fossil record is biased toward the preservation of dominant forms.
      2. This makes great intuitive sense. If 10 million individuals exist of one group and only 1,000 of another, then all things being equal, one should expect 10,000 more fossils from the former group than from the latter.
      3. If our knowledge of the former group is based on 100 discovered fossils, then chances are our knowledge of the latter group is based on no fossils, i.e., we have no knowledge.
      4. Dominant (i.e., big numbers), long-lasting groups should therefore dominate the fossil record while small, short lived species should be at best rare.
      5. Thus, we might expect gaps in the fossil record associated with small, short lived species.
    2. Speciation genesis in small groups:
      1. If speciation and significant amounts of morphological experimentation occur in small, short lived groups, and if such groups show up in the fossil record only once they become a long lasting dominant group, then one might predict that the fossil record would show a comparative dearth of intermediate forms.
      2. In other words, if species A lasts for 100,000 years in large numbers, then there is an environmental upheaval which leads to a decline in species A. "Suddenly" 10,000 years later species A has been replaced by species B, does this mean that intermediate forms don't (can't?) exist? Or was speciation and morphological innovation occurring in small populations which consequently did not fossilize with high probability?
    3. Large populations/benign environment = stasis:
      1. Paleontologists Niles Eldridge and Stephen Jay Gould together argue (convincingly, I think) the latter possibility.
      2. They refer to the long periods in which large populations fail to undergo morphological innovation as stasis (or periods of equilibrium), and describe the rapid periods of speciation and innovation separating periods of stasis as punctuation.
    4. Thus, they propose a mechanism of punctuated equilibrium to explain why the fossil record has a tendency to contain gaps, a mechanism quite consistent (especially the punctuated part) with our discussions of mechanisms speciation in this lecture.
  21. Vocabulary
    1. Bacterial species
    2. Behavioral isolation
    3. Biological species concept
    4. Ecological species concept
    5. Fossil species
    6. Geographical isolation
    7. Hybrid
    8. Hybridization
    9. Isolating mechanisms
    10. Lumper
    11. Lumping
    12. Mechanical isolation
    13. Mechanisms of isolation
    14. Phylogenetic species concept
    15. Postzygotic isolating mechanisms
    16. Prevention of gamete fusion
    17. Prezygotic isolating mechanisms
    18. Punctuated equilibrium
    19. Scenario for speciation
    20. Splitter
    21. Splitting
    22. Subspecies
    23. Species
    24. Temporal isolation
    25. Viral species
  22. Practice questions
    1. Under what circumstances would the existence of a prezygotic isolating mechanism be advantageous to a population? [PEEK]
    2. Which of the following is the simplest to achieve but is also the least robust isolating mechanisms? (circle best answer) [PEEK]
      1. geographical isolation
      2. mechanical isolation
      3. behavioral isolation
      4. temporal isolation
      5. all of the above
      6. none of the above
    3. "A species is a number of related populations the members of which compete more with their own kind than with members of other species." This statement defines the . . . (circle best answer) [PEEK]
      1. biological species concept
      2. phylogenetic species concept
      3. ecological species concept
      4. relational species concept
      5. all of the above
      6. none of the above
    4. Which species concept is not applicable to species which do not have sex? (circle best answer) [PEEK]
      1. biological species concept
      2. phylogenetic species concept
      3. ecological species concept
      4. relational species concept
      5. all of the above
      6. none of the above
    5. Which species concept is applicable to fossil species? (circle best answer) [PEEK]
      1. biological species concept
      2. phylogenetic species concept
      3. ecological species concept
      4. relational species concept
      5. all of the above
      6. none of the above
    6. Which of the following does not play a significant role in speciation? (circle best answer) [PEEK]
      1. climate change
      2. geographical isolation
      3. genetic drift
      4. hybrids exhibit lower fitness than parents
      5. all of the above
      6. none of the above
    7. One would expect the evolution of ________ particularly between two populations (i) whose ranges overlap, (ii) who are capable of hybridizing, and (iii) whose hybrids display a lower average fitness than that of either parent population. [PEEK]
    8. What does the equilibrium part of the concept punctuated equilibrium refer to?[PEEK]
    9. A morphologically distinct population which is not reproductively isolated from other, also morphologically distinct populations is a ___________. [PEEK]
  23. Practice question answers
    1. a population's range overlaps that of a second population in which hybridization would be otherwise possible, and with which hybrids display a lower average fitness than the parent population.
    2. i, geographical isolation
    3. iii, ecological species concept
    4. i, biological species concept
    5. i, phylogenetic species concept
    6. vi, none of the above
    7. prezygotic isolating mechanisms
    8. long periods of morphological stasis in the fossil record
    9. subspecies
  24. References
    1. Colinvaux, P. (1986). Ecology. John Wiley & Sons. New York. p. 152.
    2. Mayr, E. (1942). Systematics and the Origin of Species. Columbia University Press, New York.
    3. Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 399, 404-422.
    4. Stiling, P.D. (1992). Ecology: Theories and Applications. Prentice Hall, Upper Saddle River, NJ. p. 62-66.