Speciation & hybridization
Taken from http://www.uwyo.edu/dbmcd/molmark/lect2a.html
Suggestd reading: Avise, J.C. 2004. Chapter 7: Speciation and hybridization. In Molecular Markers,
Natural History and Evolution (2nd edn.). Chapman and Hall, New York.
Having at least briefly considered the problems of phylogenetics and systematics using genetic markers,
we will turn to speciation. This is one of the fundamental problems of evolutionary biology -- how do
new species arise? What are the patterns and processes underlying the bewildering diversity of species
that we are unfortunately destroying at a rate that equals or exceeds the rate at which we uncover them?
From here I will go on to population genetics.
Some history of landmarks in the study of speciation:
This is a huge subject area. Many classics exist, perhaps the most famous of which is Darwin’s Origin of
Species, published in 1857. I have listed some of the classic books in a section titled "classics". Many of
these, though old, are very well worth reading.
Here are the topics I will cover:
I. Allopatric vs. sympatric speciation
II. Sexual selection and speciation
III. Islands as "natural laboratories" for exploring speciation
IV. Species concepts -- biological vs. phylogenetic
V. "Instant speciation" via polyploidy etc.
VI. Ecological vs. genetic speciation
VII. Punctuated evolution and speciation
VIII. Shifting balance theorem
IX. Haldane's rule
Some of the major issues:
I. Allopatric vs. sympatric speciation (+ parapatric)
Almost certainly most speciation occurs allopatrically (that is, when the forms occur in different places).
The main driving forces appear to be stochastic -- drift and/or mutation (both stochastic forces) drive the
divergence between the forms, although sometimes very different selection pressures (natural selection)
may be the dominant force. Reduced gene flow (lack of "migration" in the population genetics sense) is
also important. For example, the rise of the Andes appears to have been very important in producing
patterns of genetic differentiation in neotropical plants and animals (Gascon et al., 2000). The bestworked examples of sympatric speciation in animals used to be for Rhagoletis flies (Feder et al., 1988).
More recently, it has become a very active area for fish (Schliewen et al., 1994) and brood parasitic
birds (Sorenson et al., 2003). A major driving force here may be specialization on different host plants
or different habitats and resources (in the case of fish or birds). As the host plants diverge in phenology,
secondary compounds or other ecological features, it becomes more and more disadvantageous to fall
"somewhere in the middle". The eventual result may be the evolution of reproductive isolating
mechanisms that reduce or prevent successful offspring production across forms. Some of the
spectacular radiation of African cichlid fishes is probably also a result of sympatric speciation, perhaps
driven by a combination of ecological and sexual selection (Schliewen et al. 1994; but see Meyer et al.
1996). Recent work (Verheyen et al., 1996) on the cichlids also suggests that water level lowering
created separated lakes that further promoted speciation processes (i.e., more traditional allopatric
speciation). One of the "fly" guys reviewed sympatric speciation in Trends in Ecology and Evolution
(Bush, 1994). We will come back to an example of "ecological" sympatric speciation in a later section.
Sympatric speciation can occur more readily in plants (Savolainen et al., 2006), because plants can
change their chrosome number (polyploidy) and persist or spread by asexual reproduction. Parapatric
speciation occurs where the ranges of two forms abut but do not overlap extensively. In such cases,
lower fitness of hybrids drives increased differentiation, eventually resulting in premating isolation.
Fig. 1 Diagram of modes of speciation (from the Wikipedia page:
http://en.wikipedia.org/wiki/Allopatric_speciation)
II. Sexual selection and speciation
Ever since Darwin (1871) it has been apparent that sexual selection may be a potent force in speciation.
Perhaps the most succinct theoretical treatment of the idea is by Russ Lande (1981). Strong selection on
secondary sexual traits (ornaments such as plumes or behaviors such as courtship displays) can lead to
rapid divergence that produces reproductive isolation. See also the work by Schluter and Price (1993).
Mary Jane West-Eberhard (1988) produced a classic treatment of sexual selection and the intriguing
additional idea of social selection. Sexual selection produces the classic sexually dimorphic traits that
distinguish, for example, peacocks from peahens. Social selection may be equally intense but produces
monomorphic exaggerated traits. Good examples would include keel-billed toucans (Ramphastos
sulfuratus) and puffins (Fratercula spp.). In such species, both mates compete intensively for scarce
resources (e.g., nesting cavities for toucans and burrows for puffins) as well as for access to the best
mates of the opposite sex (both sexes compete for mates and both parents invest heavily in parental
care). Speciation here was probably NOT ecological -- the two sister species (horned and Atlantic) have
different breeding microhabitats (rock crevices and earth burrows respectively), whereas the two most
different species (Atlantic and tufted) are both burrowers. Whether the force is sexual or social selection
a common feature is a certain degree of arbitrariness (stochasticity). Toucans in one location may
become red-breasted and yellow-billed, while those elsewhere may become yellow-breasted and greenbilled. Natural selection, in contrast, usually has a certain directionality or even optimality attached to it.
John Endler has been a proponent of the idea that traits such as plumage color are predictable from
features of the environment -- in a paper on manakins, he and Marc Théry argued that the particular
colors of male plumage are a fine-tuned fit to the ambient light environment (Endler and Théry, 1996).
Sexual selection may not always be a diversifying force. An interesting example of how sexual selection
can help mask speciation is provided by my graduate school colleagues Dave and Jean Zeh (1994). They
studied several different species of pseudoscorpions (Cordylochernes) that ride on harlequin beetles.
Although the pseudoscorpions have undergone considerable genetic divergence, sexual selection has
maintained a consistent morphology across the set of different species. [See Turner and Burrows (1995)
for a model for sympatric speciation via sexual selection].
Behavior and hybrid zones: Two recent examples exist for how the dynamics of hybrid zones may be
affected by the mating system, sexual selection and behavioral differences among hybridizing forms.
Sievert Rohwer and his colleagues have studied the interaction between Townsend's Warblers
(Dendroica townsendi) and Hermit Warblers (Dendroica occidentalis). The Townsend's males are more
aggressive and the hybrid zone is moving rapidly into the range of the Hermit Warbler. Behavioral
asymmetries are therefore driving Townsend's genes into Hermit country -- Hermit females mate with
Townsend's males but not vice versa (Pearson and Rohwer, 2000; Rohwer and Wood, 1998). I worked
on a hybrid zone between two species of manakins in Panama. The Golden-collared Manakin (Manacus
vitellinus) and the White-collared Manakin (M. candei) come together in a narrow hybrid zone along the
Caribbean coast of Panama near the Costa Rican border. Beyond the hybrid zone is a zone of
introgression. Here the birds are genetically and morphologically indistinguishable from White-collared
Manakins (Parsons et al., 1993). One trait, though, the golden collar, has crossed the species boundary.
The clines for the genetic and morphological traits are narrow and coincident, but the cline for collar
color is displaced 50 km to the west (Brumfield et al. 2001). Using taxidermic mount experiments, my
colleagues and I were able to show that Golden-collared males are more aggressive and that the
introgression zone "lemon-collared" males resemble Golden-collared males not only in their collar color
but also in their behavior toward other males (McDonald et al., 2001). A recent study by Stein and Uy
(2006) updates that work by demonstrating that female choice may be the most important driving force
behind the advantage to the introgressive yellow plumage.
Fig. 2. Manakins (Pipridae) in the genus Chiroxiphia. Manakins have a lek mating system, with strong
sexual selection on males for mating success.
Has sexual selection promoted speciation in the family?
III. Islands as "natural laboratories" for exploring speciation
We all know how much of an influence the juxtaposition of variation and similarity among islands in the
Galapagos archipelago played in crystallizing Darwin’s thoughts on evolution, speciation and natural
selection. Perhaps less well known is the influence of the Indonesian archipelago on Alfred Russell
Wallace, whose development of ideas on evolution and speciation paralleled Darwin's. Islands have
continued to play an important role in shaping the thinking of biologists on speciation. Much of Ernst
Mayr’s (1942) classic work on speciation derived from his extensive knowledge of the birds of the
Indonesian archipelago. A recent classic on speciation returns to the Galapagos. Peter Grant and his wife
Rosemary have long studied the evolution of Galapagos finches (Grant, 1986; see particularly Chapters
10, 11, 13; Grant and Grant, 1996, 1997). Another classic example of the radiation of a group on an
island is the Hawaiian honeycreepers (Lovette et al., 2001).
Fig. 3. Hawaiian honeycreepers radiated from an ancestral founder that was a cardueline finch.
Two recent studies provide interesting insights into patterns of differentiation along the Hawaiian
archipelago. Roderick and Gillespie (1998) recently examined the patterns of speciation in arthropods of
the Hawaiian archipelago (Arthropods comprise over 75% of the endemic biota of the Hawaiian
Islands). Much of their study depended on compiling the results of many previous studies. Their major
conclusions included: classifying patterns of speciation within Hawaiian arthropod lineages into three
categories: (i) single representatives of a lineage throughout the islands; (ii) species radiations with
either (a) single endemic species on different volcanoes or islands, or (b) multiple species on each
volcano or island; and (iii) single widespread species within a radiation of species that exhibits local
endemism. A common pattern of phylogeography is that of repeated colonization of new island groups,
such that lineages progress down the island chain, with the most ancestral groups (populations or
species) on the oldest islands. While great dispersal ability and its subsequent loss are features of many
of these taxa, there are a number of mechanisms that underlie diversification. These mechanisms may be
genetic, including repeated founder events, hybridization, and sexual selection, or ecological, including
shifts in habitat and/or host affiliation. The majority of studies that Roderick and Gillespie reviewed
suggested that natural selection is a primary force of change during the initial diversification of taxa.
Fleischer et al. (1998) used the fact that the Hawaiian Islands are know to have arisen sequentially to
put a geologically-based time window on speciation events along the archipelago. Kauai is estimated to
have arisen 5.1 MYA (million years ago), compared to the big island of Hawaii at 0.43 MYA. The
sequential geological pattern provides a relatively rare opportunity to calibrate the molecular clock
against a well-documented pattern of geological change as well as phylogenetic trees.
Fig. 4. Map of the Hawaiian islands, showing the age-graded sequence from oldest in the NW to
youngest in the SE (from Fleischer et al. 1998).
IV. Species concepts -- biological vs. phylogenetic
Avise's Chapter 7 provides a useful overview of the several major species concepts that have been
proposed. Perhaps the two major contenders at this point are Mayr (1942) and Dobzhansky’s (1937)
biological species concept (BSC) based on the primacy of interbreeding as a criterion, versus the
phylogenetic species concept (PSC) in which the major criterion is that species must be monophyletic
units with a diagnosable shared character (synapopmorphy). Cracraft (1983) defined a phylogenetic
species as "the smallest diagnosable cluster of individual organisms within which there is a parental
pattern of ancestry and descent" -- where the diagnosis depends upon synapomorphies defining
monophyletic clades. Zink and McKitrick (1995) provide a well-developed argument for the adoption of
the PSC. Avise and Wollenberg (1997) argue that the two are not the mutually exclusive concepts that
many have considered them to be -- part of their argument is that the BSC can usefully incorporate some
population genetics principles that are ignored by the strictly phylogenetic PSC.
V. "Instant speciation" via polyploidy etc.
Plants can speciate almost instantaneously by changing the ploidy (number of sets) of their
chromosomes, accompanied by their ability to persist/disperse as "founders" via asexual reproduction.
They spread vegetatively and don't necessarily have to deal with the problem of sexual reproduction for
some time. In animals, less dramatically, a more rapid founder effect process (Mayr, 1942; Templeton,
1996) may sometimes be an alternative to the gradual process usually considered to dominate speciation
in animals. On population genetics grounds, Slatkin (1996) argued for the potential importance of what
is sometimes called "founder flush" speciation in the face of several dismissive reviews of the process.
For animals, the major interest in "instant" speciation involves rapid divergence due to a few changes in
major regulatory genes affecting development. Recently, however, a tetraploid rodent was discovered in
South America (Gallardo et al., 1999). I suspect that few biologists would have considered this a
possibility until it was reported.
VI. Ecological vs. "genetic" (drift and mutation) speciation
Dolph Schluter (2001) has recently championed the idea that speciation can occur as a result of local
adaptation (i.e., emphasizing adaptation over random genetic drift as a driving force). Sockeye
(anadromous) and kokanee (lake-spawners) salmon occur in the same lakes. Likewise two forms of
whitefish, with different morphologies and mtDNA haplotypes, coexist in Maine and E. Canada. Smith
et al. (1997) argue for a similar role of natural selection in the diversification of African passerine birds.
They found high levels of genetic divergence across ecotones (habitat transitions) despite high levels of
gene flow. The emphasis, in their eyes, lies in the relative importance of ecological selective forces over
the stochastic forces of drift and mutation (rather than an emphasis on allopatric vs. parapatric vs.
sympatric). Thus, ecology/natural selection, rather than pure genetic architecture, may drive speciation
in certain cases (see a recent review in TREE by Orr and Smith, 1998). Look back at the sexual/social
selection section to see how that has a stochastic flavor. A recent paper by Rieseberg et al. (2002) used
quantitative trait loci (QTLs) to argue that directional selection (natural selection that changes traits, as
opposed to blanacing selection which keeps traits at an intermediate level) is the primary force behind
diversification and speciation.
VII. Punctuated evolution and speciation
Stephen Jay Gould (1973), who died recently, was a major champion of the idea that speciation often
precedes as relatively short bursts of accelerated evolution followed by long periods of stasis (low rate
of change). "Punctuated equilibrium" is the term describing this paleontologically derived view of the
evolutionary process. Along with this idea goes the idea of evolutionary constraints. Gould argued that
many possible courses of evolution are simply not an option once certain paths have been followed.
Ardent adaptationists, such as Richard Dawkins (1976), feel that the idea of constraints is badly
overemphasized. For an amusing interchange see the back-to-back reviews of each other’s books by
Gould (1997) and Dawkins (1997) in the journal Evolution.
VIII. Shifting balance theorem of Wright vs. mass selection/large size theory of Fisher
Two of the founding giants of the theories of population genetics were American Sewall Wright (U. of
Chicago) and Briton R.A. Fisher. Although they agreed on most of the hard core of mathematical
principles underlying population genetics, they disagreed in important ways about the relative
importance of the three major forces (mutation, drift and selection) in producing the great diversity of
organic form. Fisher (1958) felt that natural selection played a large role and that it did so in the context
of large populations in which drift played a negligible role but rare advantageous alleles could spread by
"mass selection". Wright (1978), in contrast, felt that natural selection worked in a bumpy "adaptive
landscape"-- where the peaks represent gene combinations leading to high fitness and the valleys
represent combinations of low fitness. Moving to higher peaks might be difficult and might require
interdemic selection, migration between demes, periods of drift that could move populations across
adaptive valleys towards higher peaks. Populations might reach local peaks but be unable to reach
higher peaks because natural selection would keep them from moving down into the valleys. He termed
this complex interplay between local natural selection and drift the "shifting balance theorem". Recently,
Wade and Goodnight (1998) argued that the current balance of evidence makes Wright’s shifting
balance more plausible. Metapopulation dynamics, especially, may well be the heretofore-unconsidered
factor that makes Wright’s scheme work. Metapopulations involve blinking on and off, sinks and
sources and other features that seem much more compatible with Wright's shifting balance theorem than
with Fisher's "mass selection" view.
IX. Haldane's rule
In the event of hybridization, the heterogametic sex is the most likely to suffer reduced fertility (hybrid
sterility) or viability. In mammals, this means male sterility (e.g., mules); in birds, females are the
heterogametic sex. Haldane's rule can play an interesting, if somewhat minor role in the dynamics of
speciation [See Orr, 1997; Turelli, 1998; Tegelström 1987 provides a case history for Ficedula
flycatchers].
References: (* denotes references cited in the text above)
Some of the classics:
*Darwin, C. 1859. The Origin of Species by Means of Natural Selection or the Preservation of Favoured
Races in the Struggle for Life. 1962 edition by Collier Books, New York.
*Darwin, C. 1871. The Descent of Man, and Selection in Relation to Sex. 1981 edition by Princeton
University Press, Princeton, NJ.
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NJ.
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Evolution. Chapman and Hall, New York.
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