ZOOL 304

Class Notes

Chapter 11, Speciation

Text reading:  Chapter 11.

New reference to current literature:  Speciation, by Jerry A. Coyne and H. Allen Orr, Sinauer, 2004.

Evolution embraces several domains, from basic microevolutionary change in allele frequencies to major macroevolutionary tranformations.  Somewhere in the midst of that lies speciation, a fundamental requirement for diversification and adaptive radiation.

Although Darwin's famous book was titled Origin of Species, it is mostly about adaptation.  He concentrated his attention on how a species may transform over time and had much less to say about the process by which one species may split into two or more descendent species.

Understanding speciation involves addressing several related issues.

• First of all, What is a species?  (This issue is addressed on a separate webpage.)
• How can it happen, that one ancestral species splits into two or more distinct descendent species?
  • This question has several theoretically-possible answers which range from the demonstrably true to the unproven and dubious.  Which leads us to the next big question about speciation.
• Which of the possible answers are most relevant in nature?
  
• And, finally, how do processes of speciation in nature contribute to larger-scale patterns of evolution (such as adaptive radiation and phylogeny).

Discussion.  [The following notes (and the quoted headings) are adapted from Chapter 16 of Mark Ridley's textbook, EVOLUTION, 2nd ed. (1996), Blackwell Science, Inc., Cambridge MA. ISBN 0-86542-495-0.]

• The concept of species (separate webpage).
• Introduction to speciation.
• Reproductive isolation.
• Abstract events of speciation.
• Geographic variation.
• Allopatric speciation.
  • Geographic barrier.
  • Experimental isolation.
  • Secondary reinforcement.
  • Peripheral isolates.
  • Conclusion.
• Parapatric speciation.
  • Hybrid zone.
  • Reinforcement.
• Sympatric speciation.
  • Theoretical possibility.
  • Host shifts.
• Hybridization and polyploidy.
• Evidence for reinforcement.
• Chromosomal changes.
• Conclusion.

Introduction to speciation.

• "The crucial event, for the evolution of new species, is reproductive isolation."  Thus understanding speciation 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.  

  "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" [Kruuk, L. (1999) Sticklers for Sympatry, Trends in Ecology and Evolution 14:465-466.  [Plausibility for sympatric speciation, with stickleback examples.]

• Note that the following discussion (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.

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

• Reproductive isolation is the essential process for understanding speciation. Established species are often prevented from hybridizing by isolating mechanisms.

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

• The geography of speciation raises a key question.  For each of the geographic distributions, why and under what circumstances should reproductive isolation arise?

"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 (including this one) concentrate on how selection can cause or reinforce reproductive isolation.

"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 divergence can increase by selection and/or by drift (both supplied by ongoing mutation).
• Divergent populations may re-encounter one another at some future, an event which may reverse or reinforce divergence.

"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.

"When the diverged populations meet again, reproductive isolation may be reinforced by natural selection.."

• Basically two different things can occur when divergent populations come into contact.
  • 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 only if 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 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.

"Allopatric speciation may take place in peripherally isolated populations."

• Small, isolated populations are commonly found 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.
  

"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.

"Parapatric speciation."

• 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.
• The analysis of parapatric speciation is essentially equivalent to the analysis (above) of allopatric speciation with secondary contact after divergence has begun.

"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.
   

"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.

"Sympatric speciation."

• Both allopatric and paratric speciation are well supported by theory. Sympatric speciation has long been more controversial.
• "Sympatric speciation is theoretically possible."
  • A circumstance predisposing to sympatric speciation is a niche resource which could be more 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 of sister species that can only be explained sympatrically are difficult to provide.  

"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 shared geographic territory.

"Learning mechanisms might affect evolutionary processes" (TREE, 15:179-181).

• A host shift by an avian brood parasite (a species which lays its eggs in another species' nest, with hatchlings to be raised by the host parents) can produce a sudden (one generation) shifts in culturally-transmitted behavioral phenotype.  Both male and female hatchlings can learn the courtship song of the new host and then use their version of this song as their own mate-recognition signal.  This can lead to immediate reproductive isolation and (presumably) to subsequent sympatric speciation.  (See brief review in Trends in Ecology and Evolution, 15:179-181.)

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

• Many plant species can hybridize much more readily than most animals (you should be asking, "What exactly is meant, then, 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.

"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.

"Chromosomal changes could potentially lead to speciation."

• Chromosomal rearrangements can establish postzygotic 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.

"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.

Notes for chapter 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15 / 16 / 17

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