Zoology 510, Class Notes for Ridley, Chapter 16
Speciation.

No written assignment, but you should try to answer the Study and Review Questions at the end of the chapter (pp. 424).

Brief Outline

• Introduction to Chapter 16, Speciation.
• Checklist of important concepts and terms
• Section-by-section comments
  • 16.1. Reproductive isolation.
  • 16.2. Abstract events of speciation.
  • 16.3. Geographic variation.
  • 16.4. Allopatric speciation.
    • 16.4.1. Geographic barrier.
    • 16.4.2. Experimental isolation.
    • 16.4.3. Secondary reinforcement.
    • 16.4.4. Peripheral isolates.
    • 16.4.5. Conclusion.
  • 16.5. Parapatric speciation.
    • 16.5.1. Hybrid zone.
    • 16.5.2. Reinforcement.
  • 16.6. Sympatric speciation.
    • 16.6.1. Theoretical possibility.
    • 16.6.2. Polymorphism.
    • 16.6.3. Host shifts.
  • 16.7. Hybridization and polyploidy.
  • 16.8. Evidence for reinforcement.
  • 16.9. More evidence; genetic distance.
  • 16.10. Chromosomal changes.
  • 16.11. Conclusion.

• Recent references.

510 index page

Introduction to Chapter 16.

• Chapter 16, on speciation, should be studied together with Chapter 15, on species concepts.

• The key to this chapter is the first sentence:  "The crucial event, for the evolution of new species, is reproductive isolation."  Thus understanding this chapter will mean understanding how reproductive isolation begins and becomes strengthened until it is complete.

• Both speciation and reproductive isolation remain deeply problematic issues in evolutionary biology.  For example, a recent article in the journal Evolution says, "Despite more than a century of deliberation on the origin of species, evolutionary biologists remain undecided as to the mechanisms by which reproductive isolation is generated. Whether geographic isolation, or allopatry, is a prerequisite to speciation has been hotly debated ..., and there is is not even consensus as to the nature of the reproduction isolation that accumulates in allopatric populations" [Reference 1].

• Note that the discussion in this chapter (1) presumes the biological species concept, and (2) applies only to sexually reproducing populations.
• Speciation events are generally understood to involve several steps.
  • First, some predisposing circumstance.
  • Second, initial divergence.
  • Third, reinforcement of isolation.
  • Finally, complete isolation.
• Most of the modes of speciation described in this chapter can be summarized by the above steps.  In particular, allopatric speciation, parapatric speciation, and sympatric speciation all share similar processes.  
• Different modes of speciation may differ:
  • In the initial predisposing circumstance,
  • In the basis for initial divergence, and
  • In the importance of reinforcement.
• Note that any mode of speciation depends on the availability of appropriate genetic variation.

CHECK LIST of important TERMS

• speciation
• gene flow
• reproductive isolation
  • allopatric
  • parapatric
  • sympatric
• cline
• Bergman's rule
• reinforcement (of reproductive isolation)
  • primary reinforcement
  • secondary reinforcement
• peripheral isolate
• founder effect
• genetic revolution
• punctuated equilibrium (see p. 560)
• hybrid zone
• tension zone
• assortative mating
• habitat polymorphism
• host shift
• hybridization with polyploidy
• genetic distance

Chapter 16, Section-by-Section Comments

16.1. "How can one species split into two reproductively isolated groups of organisms?."

• Reproductive isolation is introduced as the essential process for understanding speciation.

16.2. "A newly evolving species could theoretically have an allopatric, parapatric, or sympatric geographical relation with its ancestor."

• The geography of speciation is introduced.  The key question, for each of the geographic distributions, is why and under what circumstances should reproductive isolation arise?

16.3. "Geographic variation is widespread and exists in all species."

• Existence of suitable variation is a precondition for any evolutionary process.
• Because extensive variation can be found in all sorts of traits in all sorts of populations, most discussions of speciation presume that appropriate variation will be available.  Thus most discussions (include this one) concentrate on how selection can cause or reinforce reproductive isolation.
• The repeated mention, by Ridley, that variation within a species can be almost as great as that between species is rather curious, just as if there were a universal, valid, and generally accepted phenetic definition of species.  Old ideas cast long shadows.

16.4. "Allopatric speciation."

• 16.4.1 "Allopatric speciation may occur when a barrier is intruded within the continuous geographic variation"
  • The intrusion of a barrier across a population that already shows strong geographic variation creates an ideal circumstance for speciation.  Reproductive isolation is established by the barrier, prior to any evolutionary change in the populations.
  • In the absence of gene flow, initial diverge can increase by selection and/or drift (with additional mutation).
  • Divergent populations may re-encounter one another at some future, an event which may reverse or reinforce divergence.

• 16.4.2 "Laboratory experiments illustrate how separately evolving populations of a species tend incidently to evolve reproductive isolation."
  • Even over quite short time spans, a tendency toward premating isolation can evolve in artificially separated populations.
  • This result confirms the theoretical expectation that such divergence is possible.

• 16.4.3 "When the diverged populations meet again, reproductive isolation may be reinforced by natural selection.."
  • Basically two different things can when diverged populations.
    • The populations may fuse.
    • The populations may remain separate.
  • Fusion occurs when gene flow (from hybridization between members of the two populations) and selective competition between alleles is not prevented by selection or by isolating mechanisms.
  • Consequent to selection against hybrids, selection can reinforce isolation by favoring assortative mating (premating isolating mechanisms).
  • Note stringent conditions for speciation to take place by reinforcement.
    • Selection must not eliminate variation too quickly.  Selection against hybrids amounts to selection favoring the most common alleles.  (Removal of heterozygotes removes equal numbers of each allele; after enough rounds of selection, only the more frequent allele will remain.)
    • Reinforcement cannot occur without genetic variation for mating preferences.
    • So, speciation can occur by reinforcement onlyif necessary variation is present and if selection acts quickly enough on that variation, before gene flow can equalize gene frequencies or selection remove the variation which distinguishes the populations.
    • Nevertheless, evidence (see 16.8 below) suggests that reinforcement can and does occur.
  • Allopatric speciation does not depend on reinforcement.  Only when previously separated populations meet again before reproductive isolation is complete will reinforcement be needed to complete the process of speciation.
  • In contrast, both parapatric and sympatric speciation require reinforcement to establishment reproductive isolation.

• 16.4.4 "Allopatric speciation may take place in peripherally isolated populations."
  • Small, isolated populations are commonly around the edge of a large ancestral population.
  • Because such peripheral isolates are small, they may diverge by founder effect and by drift.
  • Because of inbreeding, genetic rearrangements may be more likely to become fixed in small populations.
  • Whether by such special processes or simply by selection for marginal conditions, allopatric speciation involving peripheral isolates may be commonplace.
  • Allopatric speciation involving peripheral isolates provides the theoretical basis for the hypothesis of punctuated equilibrium, discussed in Chapter 20, p. 560.

• 16.4.5 "Allopatric speciation: conclusion."
  • Allopatric speciation is the least controversial mode of speciation.  It undoubtedly happens and is undoubtedly important.
  • Most other modes of speciation remain controversial.

16.5. "Parapatric speciation."

• The analysis of parapatric speciation is essentially equivalent to the analysis (above) of allopatric speciation with secondary contact after divergence has begun.
• Parapatric speciation presumes divergence across the range of a species, caused by selection operating differently across the range while gene flow is low enough that panmixis does not occur.
• The result of these of conditions is a transition, which may be broad or narrow, across which gene frequencies change from those which prevail on one side to those which prevail on the other.
• 16.5.1 "Parapatric speciation begins with the evolution of a hybrid zone."
  • The transition across which allele frequencies change may be broad or narrow.
    • A broad transition, extending across much of the total range, is called cline.
    • A narrow transition is called a hybrid zone or stepped cline.
  • The width of a hybrid zone depends on hybrid fitness and on gene flow.
    • The zone will be narrower with less migration (or lower rate of gene flow) and/or with lower fitness for heterozygotes.
    • The zone will be wider with more migration (higher rate of gene flow) or with higher fitness for heterozygotes.
  • With parapatric speciation, contact between diverging populations is primary.  The populations remain in contact while they diverge.
  • In contrast, with allopatric speciation, contact between diverging populations is secondary.  The populations diverged before coming into contact.

• 16.5.2 "Hybrid zones may evolve into species barriers by reinforcement."
  • In contrast with secondary contact following allopatric divergence, which may be limited in time before fusion of populations or speciation by secondary reinforcement, hybrid zones can be stable.
  • If, in a hybrid zone, hybrids suffer selective disadvantage and both races are adapted to conditions different from those in the hybrid zone, the region may be called a tension zone.
  • If a stable hybrid zone is a tension zone, conditions should favor reinforcement of reproductive isolation.  If variation in mate preferences is present, selection should favor reproductive isolation.
  • Hybrid zones are common.  Evidence for reinforcement is less so.

16.6. "Sympatric speciation."

• Both allopatric and paratric speciation are well supported by theory. Sympatric speciation has long been more controversial.
• 16.6.1 "Sympatric speciation is theoretically possible."
  • A circumstance predisposing to sympatric speciation is a niche resource which could be most effectively utilized by a polymorphic population (one with differing adaptive specializations).
  • Polymorphism can be supported by assortative mating.
  • If polymorphism resulting from assortative mating is advantageous, selection can reinforce the assorting until reproductive isolation is complete.
  • As in other modes of speciation, reinforcement depends on availability of variation affecting mate preference.
  • Compelling cases where sister species that can only be explained sympatrically are difficult to provide.  The text provides a couple of examples.

• 16.6.2 "Two species of green lacewing may have split sympatrically."
  • Green lacewings present divergent sister species occupying sympatric territories.
  • However, a history of continuous sympatry is not available.

• 16.6.3 "Phytophagous insects may split sympatrically by host shifts."
  • Tephritid fruit flies present recently split populations which are diverging sympatrically.
  • However, speciation is not complete.
  • The phylogenetic patterns of host-specific insect species (including parasites as well as phytophagous insects) suggests that speciation by host shift is not uncommon.

• The mechanism for some examples of sympatric speciation may be more similar to allopatric speciation, since populations can be physically separated within a share geographic territory.

16.7. "Some plant species have originated by hybridization and polyploidy."

• Many plant species can hybridize much more readily than most animal (you should be asking, what exactly is meant by "species").  Sometimes polyploid hybrids arise, with a full complement of chromosomes from both parental species.  Such hybrids can be both fertile and reproductively isolated from parental species.
• Establishing a new species by hybridization polyploidy has a couple requirements
  • The initial individuals must be able to find mates.  Asexual reproduction can enable one initial individual to propagate until a population becomes established.
  • The new species, which arises in territory already occupied by two related parent species, must be able to compete successfully.
• Speciation by hybridization polyploidy is quite common among plants.

16.8. "Reinforcement is suggested by greater sympatric than allopatric prezygotic isolation between a pair of related species."

• When two distinct species have ranges which partially overlap, premating reproductive isolation is generally greater between members of the overlapping populations than between members taken from non-overlapping regions of each range.
• This observation supports the idea that selection can and does produce reinforcement.

16.9. "A study of speciation in Drosophila ... provides evidence about reinforcement and other interesting results."

• Genetic distance (defined in Box 16.1, p. 453) provides a convenient statistic for measuring divergence between populations.
• Correlation of genetic distance with prezygotic isolation and with postzygotic isolation, for comparisons of many sympatric and allopatric species of Drosophila, shows that prezygotic isolation is higher among species with lower genetic distance.
• This correlation provides evidence for reinforcement operating in sympatric but not in allopatric species..
• Since genetic distance may be roughly proportional to divergence time, correlaton of genetic distance with prezygotic isolation can be used to estimate the time needed for speciation.  In the case of these Drosophila species, it is between 1.5 and 3.5 million years.

16.10. "Chromosomal changes could potentially lead to speciation."

• Chromosomal rearrangements can establish postzygote isolation in a single step.
• If chromosomal rearrangements could become established, they could provide a basis for rapid speciation.
• A rare rearrangement would be unlikely to be paired with a matching chromosome and would therefore be selected against.
• However, rearrangements could become common enough to yield a successfully interbreeding population, either by something like molecular drive or by inbreeding in which siblings would be likely to share the rearrangement.
• Inbreeding is more likely with subdivided population structure; therefore this mode of speciation might be more common in species with subdivided population structure.
• Chromosomal differences between species are fairly common, but they also occur within species.
• The importance of chromosomal changes for speciation remains unknown.

16.11. "Conclusion."

• Many hypotheses can account for speciation.   Each hypothesis requires some special conditions.  Which hypothesis applies in any particular case is difficult to determine, and which conditions are generally more important remain uncertain.

Recent References

1. Kruuk, L. (1999) Sticklers for Sympatry, Trends in Ecology and Evolution 14:465-466.  [Plausibility for sympatric speciation, with stickleback examples.]
2. Kruuk, L.E.B., J.S. Gilchrist and N.H. Barton (1999) Hybrid dysfunction in fire-bellied toads (Bombina) Evolution 53:1611.  [Abstract]

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