There are many definitions of species, but they all boil down to the fact that species are different at a number of gene loci. A pair of different species, even when in contact, usually form a bimodal distribution of phenotypes or genotypes (i.e. two clusters of individuals); they can clearly be distinguished from one another by some morphological, genetic, ecological or behavioural traits, though there may be some overlap and/or hybridization between them. In contrast, members of a single species when in contact present a unimodal distribution (only one group of organisms can be distinguished).
How are these bimodal distributions of genotypes and phenotypes caused?
Causes of speciation: The origins of species, (speciation), might be due to either random forces, such as mutation and drift, or deterministic forces, i.e. natural selection.
Geographical milieu of speciation: Speciation might also occur in various kinds of geographical situations: sympatric, parapatric, or allopatric. Whatever people may have told you before, these geographical situations are not "mechanisms of speciation", they simply tell you where, not how they occur.
We will here explore both the evolutionary
causes of speciation, and geographical milieu in which they can occur.
Here is a brief summary of what we know (this is a kind of lecture summary):
|Causes of speciation||Status (my opinion - you may disagree!)|
|Mutation + drift||Deeply suspect! If occurs, probably slow, needs allopatry|
|Chromosomal mutation (polyploidy)||Known|
|Environmental, pleiotropic, and disruptive||Known|
|Reinforcement||Known, but rare?|
|Random + selection|
|Shifting balance||Possible, contested|
|Founder event||Dubious, contested|
|Geographic milieu of speciation||Status (my opinion - you may disagree!)|
|Sympatric||Known, but rare? Maybe not!|
|Parapatric||Known to give partial isolation; complete would not be surprising|
The lecture will attempt to explain how
we know (or not!) these things.
Evidence so far (see Clines and hybrid zones, and Species and species differences lectures) tells us that:
1) Speciation is gradual (usually), and involves many loci. We know this because hybrid zones often separate forms that differ at many loci (allozyme and molecular differences, as well as chromosomal, morphological, ecological and behavioural differences), even though we do NOT consider these hybridizing forms to be separate species, but instead geographical races or subspecies. Species can overlap without losing their identity in a parapatric hybrid zone or in sympatry, whereas hybridizing races do not, it stands to reason that species should differ at even more loci than races. Ayala's work in the 1970s showed this to be true for enzyme proteins in Drosophila. Since then, many other molecular studies have shown the same thing.
A major exception to this "gradual speciation" rule is speciation via polyploidy, which is sudden (see Biodiversity and Species). See also the interesting website "Observed instances of speciation" for rapid examples that have been seen to occur recently. Most cases were plant polyploidy (see below), which is an exception to the general gradual rule. See also the case of the apple maggot fly, below.
2) Speciation involves epistasis. In order to maintain a bimodal distribution of phenotypes/genotypes of two species in sympatry, multilocus intermediates must be either unfit or never produced. For example, AABB and aabb have high fitness, whereas AaBb and AAbb genotypes are less fit. So A and B collaborate, or are epistatic in their effects on fitness.
3) A third general rule is that there is
clear geographic rule for speciation. Species differ
at loci that cause intrinsic
selection against hybrids, as well as at loci affecting
choice. Speciation must have required evolution at these same
loci. To remind you, extrinsic selection
is caused by geographic variation in the environment;
intrinsic selection may be caused by heterozygous
frequency dependent selection against rare forms,
and, perhaps most importantly,
many loci.Variation at mate choice loci may be affected by sexual selection
among other factors. These are the types of selection acting on loci that
affect reproductive isolation.
Cline theory predicts that loci under selection may diverge over quite short distances, small multiples of cline widths, w = 1.73/s where measures gene flow distance, and s measures the strength of selection. So, there is no particular a priori reason why these extrinsic isolation loci might not diverge in parapatry. This is also true in nature; although strong selection in N. Wales keeps peppered moths peppery-coloured, and strong selection in Liverpool keeps peppered moths melanic, no geographic isolation was required for divergence to take place. All that was required was that the spatial scale of selection was somewhat greater than w.
4) I have stressed repeatedly that species,
geographic races, and local morphs are part of a continuum.
There is no fundamental difference between species and races and morphs;
species just have more of the same type of genetic divergence we have already
encountered in this course. The only difference is that in species, genetic
divergence and selection is sufficiently intense so that two distinct populations
can be maintained in sympatry.
I would like here to mention four important potential causes of speciation, in addition to the ordinary forces already studied:
1) Speciation via polyploidy. We have already treated this in Species and species differences. Chromosomal doubling has the capability for instantaneously producing new species. The speciation is sympatric.
In polyploidy, a failure of cell division can lead, at a stroke, to a new species that produces sterile triploid hybrids with all diploids. The new species is, however, a hopeful monster. Without selfing, it has no-one with whom to mate. Plants and more amorphous animals are predominant among polyploids because their development is perhaps more resilient to disturbance, and because they can self. However, some sexual animals, such as most of the Salmonidae (trout and salmon family) are also polyploids.
Three additional potential causes of speciation we will treat here are:
2) Disruptive selection. We have already treated disruptive selection within populations. This type of selection against intermediates is normally thought of as a pre-requisite for gradual sympatric speciation.
3) The "shifting balance", where genetic drift and selection interact (in a shifting balance of evolutionary forces) to allow populations to move across an adaptive trough, and enable a transfer from a lower to a higher adaptive peak in a rugged adaptive landscape. We met the idea of the adaptive landscape in our discussion of intrinsic selection.
where direct selection for pre-mating isolation occurs to prevent wastage
of gametes on hybridization.
We have already dealt with disruptive selection in Evolution of quantitative traits, Evolution in space and time, and in Biodiversity and species. Disruptive selection is selection against intermediates. This has the peculiar effect of selecting FOR extremes. Thus, if disruptive selection actually worked, it would produce a bimodal distribution of phenotypes/genotypes. It is important for speciation because, essentially, two species that have just speciated are essentially ALSO a bimodal distribution of phenotypes. Thus, speciation could occur in sympatry if disruptive selection were successful.
However, you will also have learnt in your quantitative genetics lectures that when a lot of loci combine together to produce quantitative traits, they almost inevitably follow a normal distribution. Now you may not be a statistician, but you will undoubtedly remember that a normal distribution is not in the least bimodal; it is unimodal.
What's going on? Well, it turns out that the standard quantitative genetic assumptions don't work for strong disruptive selection. Assuming random mating, multiple genes of individually small effect, and a lack of correlation between characters (one cause of correlation is linkage disequilibria), then the distribution will be normal, provided the selection isn't too strong. But if disruptive selection is strong, and especially if mating is assortative (like mates with like), it is possible for a bimodal distribution of genotypes to result.
Disruptive selection is epistatic, rather than additive (Why? Because instead of genes adding together in their effects, the effects of genes depend on the other genes present in the individual. In a bimodal distribution of sizes, for example, genes for bigness will do better if there are lots of other bigness genes around, i.e. in big individuals; they won't be successful in small individuals).
This kind of disruptive selection, coupled with a pleiotropic "by-product" of assortative mating, is exactly what happened when the apple maggot shifted host-plants....
Example: Host races in the apple maggot. The apple maggot, Rhagoletis pomonella, is a "true fruit fly" (Tephritidae), native to North America, where its larvae normally feed in the fruits of hawthorns (Crataegus). (the work has been done by Guy Bush, Jeff Feder, Stewart Berlocher and others over the last 30 odd years (Thanks, Stewart Berlocher, for the photo). In the 1860s, this fly suddenly became a fruit pest in apple-growing regions in the northeast states, and quickly spread to all apple-growing regions of the USA. Apples had been introduced to North America by European colonists only a century or so earlier. It has been shown that:
The mate choice is thus assortative as a pleiotropic result of host choice behaviour (3,4,5). The strong advantage for specialization, and the weak gene flow opposing it enabled divergence to a bimodal genotypic distribution. It isn't completely clear that speciation occurred in sympatry (in the sense of in the same field), but it seems clear that the divergence and spread of the new form occurred in spite of the kind of gene flow that we see today in natural populations
With m = 6% gene flow between the host races of Rhagoletis every generation, many would deny that the apple and haw races have speciated. But given that typical hybridizing, but ecologically divergent species seem to be very much in the same boat (for example, the Darwin's finches, with different beak widths/seed foods), the importance for speciation is obvious. And if this kind of sympatric speciation (or almost-speciation) can occur in a few tens of years in the apple maggot, it could be an extremely important process over geological time.
Nonetheless, many people still think that
the process is unlikely to be very general, even if they agree that it
happened in Rhagoletis. Some even say that the Rhagoletis
data has been misinterpreted. I personally think that sympatric speciation
is the most likely explanation of this case.
This theory was proposed by Sewall Wright in 1931 and 1932, and has been hotly debated ever since. Fisher thought his rival's idea was daft. Only a couple of years ago, Coyne, Barton and Turelli published an enormously long paper arguing that there is no evidence that the shifting balance occurs. However, the great length of the Coyne et al. paper, and the need to shoot down multiple arguments, shows clearly that the idea is far from dead!
Wright suggested that drift, as well as fluctuating environmental selection might be important where multiple adaptive peaks existed because of epistasis or other intrinsic modes of selection. Populations otherwise would otherwise become trapped at local gene frequency optima or fitness peaks, even though fitter "adaptive peaks" exist elsewhere. These other adaptive peaks would be unattainable purely via natural selection because they are separated from the current populations via low fitness "adaptive troughs"
Phase I: Genetic drift in small local populations. When a rare variant occurs locally, it will usually disappear under natural (intrinsic) selection. Occasionally, it will occur in a low density population, and drift may allow some increase in the variant.
Phase II: If the variant increases sufficiently under drift, the local population may evolve over the selective trough towards the domain of attraction of the new adaptive peak. Natural selection within that population will now cause evolution towards a new adaptive peak.
Phase III: We now have the situation that a small local population or area fixed for the new adaptive peak. Even if this peak is superior, it will not necessarily spread to other subpopulations or areas, because emigrants are still selected against in those areas where they are rare. Only if there is some bias, or deterministic process which gives an advantage to populations with the more favourable adaptive peak will the new peak spread. Wright envisaged "interdemic selection". If fitter, populations in areas with the new adaptive peak will grow larger and send out more migrants than usual, and adjacent populations on the old adaptive peak may be progressively swamped.
Other processes might accomplish the same thing. An intrinsically selected cline will form between areas with the old and new adaptive peak. Even if the population density and migration remains constant, asymmetrical selection against heterozygotes across a cline will lead to cline movement, and this will be true for other types of intrinsic selection as well. Any of these mechanisms should eventually spread new, fitter adaptive peaks to the farthest reaches of the region.
Unfortunately, this is the weak part of the theory!
Chromosomal evolution seems, on the surface, well-explained by a variant of the shifting balance. Heterozygous disadvantage (usually caused by partial sterility) is common between chromosomal races, and for de novo chromosomal mutations (well known in human chromosomal abnormalities, for example). The patchwork of chromosomal races in many species seem a likely consequence of multiple shifting balances followed by spread from small populations.
But Coyne et al. argue, on the basis of experimental evidence from Drosophila, that actual chromosomal transitions that have occurred within species have very little heterozygous disadvantage. Chromosomal heterozygous disadvantage between older races and species could be due to the evolution of individual heterozygote disadvantage of alleles or to groups of epistatic genes that have diverged on each chromosome after separation. My own feeling is that it would be perverse, given what we know about meiosis and the effects of chromosomal evolution to explain away ALL chromosomal negative heterosis in this way, but Coyne's argument does have some force when you consider that there are many cases where chromosmal pairing, gamete production, and selective zygote abortion ensure that only balanced gametes end up in embryos. (See our Chromosomal evolution lecture)
The patchwork of warning colour races in many taxa, from coral snakes to Heliconius butterflies again suggests that the shifting balance may be operating. With Heliconius, it is very hard to argue that there is no selection against rare warning colours, since warning colour and mimicry prove that the selection against the rare forms maintains clines. Laboratory and limited field experiments prove that this is so.
But critics argue forcefully that this is circumstantial evidence at best. To really prove that the shifting balance explains a geographic pattern, we need to have direct evidence of shifts happening; and there is none yet. Maybe the environment has changed, making new chromosomal morphs or colour patterns more favourable locally, even from a very low frequency. For instance, suppose a very unpalatable species of butterfly became common in one area. A species of Heliconius might then mimic this new species. This is simple natural selection which does not require the complexities of genetic drift and interdemic selection to shift to a higher adaptive peak (see Warning colour lecture, coming soon).
Wright primarily focused his shifting balance idea on multilocus adaptive peaks generated by epistatic interactions between genes, rather than examples of heterozygote disadvantage and frequency-dependent selection that can work for a single or few loci. For polygenic traits, the evidence is even more murky. The whole point of quantitative genetics is that we don't know what the actual genes are, and that each gene is assumed to be under very weak selection. So how can we tell whether they are drifting or under selection? It is easy for critics to erect a "Fisherian" counter-argument to explain any evolution in an epistatic rugged, adaptive landscape.
So: the shifting balance is a nice idea (to me at least!), but has little direct evidence, and lots of critics. It is hard to prove or disprove.
for speciation. The
shifting balance shows how, in principle, evolution between adaptive peaks
could occur in parapatry, so generating intrinsic isolation, particularly
epistasis, between populations. However, we already know that most species
differ at many loci, and that many loci usually contribute to isolation
(Rule no. 1). Shifting balance relies on drift, which will mean that each
shift should create very little reproductive isolation (so that it will
not have to work against strong selection); it is unlikely that a single
shifting balance could lead to enough isolation to generate a new species.
For example, in Bombina toad hybrid zones, there are abundant hybrids,
so speciation has not really occurred, even though there are multiple gene
differences (see Evolution in Space and Time). If the shifting balance
is important, it must occur many, many times to generate enough isolation
to cause speciation. Fisherian polygenic evolution is hard to rule out
in this situation.
Suppose some adaptation has led to divergence which causes a reduction in fitness of hybrids. As we have seen from our discussion of intrinsic and extrinsic selection pressures, there are many ways in which this could happen. Now the two forms may either meet in secondary contact (if they were previously allopatric), or they may already be in contact (if divergence were parapatric or sympatric).
Regardless of how the contact happens, random mating between the divergent forms now creates unfit hybrids. Dobzhansky in 1940 postulated that, when the offspring produced will be of low fitness, hybridization should be opposed by natural selection: assortative mating is selected to reduce the time and effort put into mating and rearing of offspring. Essentially, this is the evolution adaptive mate recognition, and is now (perhaps confusingly) called reinforcement. Reinforcement is so called because populations can reinforce post-mating barriers by evolving pre-mating barriers. You will also recognize it as a kind of disruptive selection on mate choice, or a good genes mechanism, such as we have already dicussed in sexual selection. In Dobzhansky's view, reinforcement was able to take over after the evolution of some post-mating barriers, leading to a completion of speciation by the evolution of pre-mating barriers.
In Drosophila, too, there is evidence for reinforcement. Coyne and Orr surveyed post-mating isolation in 171 pairs of sympatric and allopatric closely related divergent forms, usually species but sometimes subspecies. Post-mating isolation was measured crudely as the proportion of crosses in which hybrids were either sterile or inviable in crosses between two species A and B. An isolation index of 1.0 would indicate a complete lack of hybrid adult progeny. Pre-mating isolation between pairs was measured similarly, as the proportion of enclosures of males and females of opposite species that produced mating.
If we plot pre-mating isolation versus genetic distance, a surrogate for time since divergence, we see that sympatric species evolve pre-mating isolation much faster than post-mating isolation. These two pieces of evidence are hard to explain in any other way: pre-mating isolation seems to have evolved to prevent hybridization between species that are incompatible.
Potential problems with reinforcement
For a long time, reinforcement was invoked as a likely factor in speciation. In the late 1980s, however, doubt was thrown on the whole idea. Roger Butlin has been a particularly influential critic. Butlin distinguished two different situations in which selection against cross-mating might arise:
However, where hybridization is successful, and gene flow occurs, it is much harder to imagine how reinforcement will work. Hybridization occurs freely, and hybrids quickly form a swarm in the centre of the hybrid zone. Because there are no "pure species", individuals don't know how they should mate even if genes for reinforcement were available. And even if they were able to choose optimally, genes for mate choice would themselves flow across the boundary between the forms, and as much inappropriate as appropriate mate choices could result.
Coyne and Orr's data seem to provide better evidence of reinforcement than the Litoria example because the process seems continuous, and because some hybrids do survive in crosses (isolation indices < 1.0). However, it is clear that the isolation indices used by Coyne and Orr are crude (although the best data we have). Possibly few of the sympatric species hybrids ever survive in the wild, so that gene flow is zero, even though hybridization is present; if so, many cases of apparent reinforcement could actually be examples of reproductive character displacement.
Recently, Mohamed Noor studied two sibling
species of Drosophila (D. pseudoobscura and
which do hybridize in the wild, and for which some hybrids are known to
be fertile. He clearly showed that, in areas of overlap, mating was more
assortative than in areas where one species was absent. We don't
yet know how common reinforcement is, but it does again seem likely that
it is important in speciation.
It has often been said that speciation can only happen in allopatry, but recent evidence suggests that sympatric and parapatric speciation is also possible. Here I run through these ideas in sequence.
The classical geographical mode of speciation is allopatric, when the range of a species is split in two by a mountain range or sea, and the remaining large populations either side evolve differences due to divergent drift or selection in different environments (or a bit of both, as in the shifting balance). The change could even be due to similar selection on either side of the barrier: if different mutational and drift events happen on each side, different alleles become fixed, again leading to pleiotropic incompatibilities. Eventually, the barrier may become eroded, and the two forms could come back together in secondary contact. Three outcomes are possible in this case:
1) They may not have diverged much genetically, in which case the two forms will fuse, or be recognizable only as geographic races across a broadly clinal, stable zone of intergradation.
2 a) If genetic differences have evolved which cause hybrid inviability and sterililty, a narrow hybrid zone may form at the zone of secondary contact. Sexual behaviour or habitat preferences are also likely to have changed during the period of geographic isolation, leading to a reduced propensity to cross-mating. These conditions could lead to a stable, narrow hybrid zones.
2 b) But these conditions are also ripe for reinforcement. If selection is strong enough, or linkage tight enough, linkage disequilibria between genes for mate choice and genes for inviability and sterility could lead to successful reinforcement and completion of speciation.
3) If the two forms have diverged still further, so that their mating and reproductive systems are completely incompatible, they may have already become separate species.
Clearly, vicariant speciation does eventually occur. To take a reductio ad absurdum, no-one doubts that marsupials in Australia are distinct from placentals in the rest of the world: separate evolution ultimately will lead to speciation, and there is now no potential for interbreeding or fusion between the two types of mammals.
However, vicariance speciation can be very slow. The London plane tree Platanus, is commonly planted in our parks and streets because it survives well anoxic root conditions under pavements and massive levels of urban smoke pollution, even in Victorian times of coal-burning. This species is in fact a hybrid between P. orientalis (the Oriental plane tree) and P. occidentalis, the American plane tree (known over there as "sycamore"). The Oriental plane is found in southeastern Europe and Western Asia, and we know from fossil records that it has not been in contact with the American species for over 20 million years. Yet the hybrid London plane gives abundant fertile seed, so the two isolated plane trees have not really "speciated" at all.
b) The founder effect - peripheral isolation
The slowness of vicariance speciation led to alternative models. Mayr in 1954 proposed an additional kind of allopatric speciation, in which a small group of founders migrates to a new habitat or island. His "founder effect" model capitalized on the general belief in the importance of genetic drift popularized by Wright at the time. Mayr suggested that the founder emigrants, being few, could only take a fraction of available genetic variation to their new home (genetic drift as in shifting balance phase I), and that these genes, "unused" to interacting under conditions of low genetic diversity, would undergo a selection-driven "genetic revolution" or reorganization of the genome (as in phase II). Mayr believed that a genetic revolution took place, in part, because the new population was also exposed to unusual environmental conditions absent in the centre of the species' range, causing rapid divergence and speciation. Therafter, the consequences of the founder effect would be similar to those of vicariance speciation if there was secondary contact.
Mayr produced this idea while thinking about the evolution of a group of spectacular New Guinea birds called the racket-tailed kingfishers, genus Tanysiptera. [SEE OVERHEAD] On mainland New Guinea, there are three widely distributed and similar subspecies of Tanysiptera galatea (nos. 1-3). On the islands, on the other hand, very divergent forms 4-7 are found. One of these, Tanysiptera hydrocharis (H1 & H2), has apparently come back into secondary contact with the mainland form, with which it overlaps without interbreeding.
There are many other examples of this kind of distribution, which suggested to Mayr that peripheral allopatric speciation is the most common form of speciation. However, Mayr did not exclude an alternative: that the mainland form has been evolving more rapidly, and that the island forms that have either remained constant or are responding directly to local conditions without an involvement of genetic drift. It is known that the islands off New Guinea were connected to the mainland less than 10,000 years ago during a sea level drop. Thus, vicariance speciation, or even parapatric speciation aided by habitat differences on the islands seem as likely. We know nothing about the genetics of the island Tanysiptera, for instance whether they are genetically depauperate, as expected if violent episodes of genetic drift have caused "genetic revolutions".
Other examples, for example, the Hawaiian Drosophila, which have produced a huge radiation of species in a few million years, are among the strongest examples that support the founder effect model. Genetic studies HAVE been done on the Hawaiian Drosophila, and there is no evidence that speciation involved a reduction in heterozygosity and genetic diversity at allozyme loci, as expected if genetic revolutions in founder populations during inter-island migrations were important. Some of the most closely related species on Hawaii are from the same island, and this is even more true for snails, crickets and other insects, suggesting that complete island allopatry is not necessary for speciation.
Worse still, laboratory population bottlenecks
have never successfully produced new species, as they ought if the founder
effect was a common and rapid means of speciation. Horrendous bottlenecking,
to say nothing of high doses of X-rays and chemical mutagens, have all
been applied to hundreds of unfortunate Drosophila melanogaster
stocks, both mutant and wild-type. Yet, provided they can walk unaided,
they can still (with very few exceptions) be easily crossed with wild members
of their own species.
Extrinsic selection plus reinforcement
John Endler was one of the most influential proponents of non-allopatric speciation. In a 1977 book, he pointed out that parapatric adaptation to different habitats was likely over very small distances. He suggested that extrinsic selection, followed by reinforcement in clines might lead to speciation.
Intrinsic selection plus pleiotropic evolution of mate choice
We might add that any of the processes involved in vicariance speciation should operate in parapatry as well as allopatry, because gene flow is almost never universal over the whole range of even a geographically isolated populations. A buildup of incompatible epistatic solutions to the same (or different) environmental problems is likely in different parts of the range of a species, provided that the distances apart of the different forms are greater than approximately w k/s', where k is usually in the range 2-4, and s' represents the selection pressure acting against inferior epistatic combinations. Since may often be tiny (in plants and some flightless insects, a few metres, and even in many birds and bats, just a few km), there is an enormous possibility for parapatric speciation.
We might also add that, while it might help, reinforcement is not actually necessary for speciation either. A change in habits means a change of potential mates one runs into, so many forms of disruptive selection may lead to assortative mating via pleiotropy (Rhagoletis, above). In addition, sexual selection and the evolution of mate choice is chaotic, and might result in absurd male "handicaps" or "runaway" male traits, again, potentially over small distance scales, leading to speciation.
Old thought patterns are hard to change!
The older allopatric ideas were never clearly framed to be testable. Allopatric
speciation COULD explain the data; but whether it DOES explain it better
than parapatric speciation is still a matter for discussion. In either
case, parapatric and allopatric distributions of species do suggest that
geographical speciation (either parapatric or allopatric) is very important.
I hope you can see my argument that allopatric speciation is only superficially
different from parapatric speciation. Both ideas rely on a reduction of
gene flow due to distance. Perhaps we shouldn't worry too much about whether
speciation is allopatric or parapatric.
Like parapatric speciation, sympatric speciation requires (a) disruptive selection towards divergent adaptive optima or (b) polyploidy to generate post-mating isolation, and ...
(c) reinforcement and/or pleiotropic changes in mate choice (to generate pre-mating isolation).
The main difference from parapatric
speciation is that selection must occur under very high levels of potential
gene flow within the normal "cruising range" of the species (i.e. within
a radius of about 2
dispersal distances). This means that the selection pressures must be very
strong, which makes sympatric speciation seem somewhat unlikely.
However, the rapidity of sympatric speciation means that it may be important
(recent estimates suggest that speciation due to polyploidy is about 3%-7%
of total speciation in flowering plants and ferns).
1) Sympatric speciation can and does occur in two ways: instantaneously (via chromosomal doubling, polyploidy), and gradually (e.g. Rhagoletis).
2) Both types of sympatric speciation are rapid enough to have actually been observed or very strongly inferred, in more or less natural conditions, as well as in the laboratory. In contrast, allopatric speciation is expected to be slower, and has never been observed.
3) Parapatry is very similar to allopatry, in that populations can diverge under selection or drift relatively easily over distances of a few multiples of the dispersal distance . Many strongly selected differences, which provide extrinsic (ecological) or intrinsic (genomic) isolation currently exist in parapatry, in spite of gene flow.
4) Yet many people still believe that sympatric
and parapatric speciation do not occur very often. Odd, isn't it?
FUTUYMA, DJ 1998.
Evolutionary Biology. Chapter 16 (pp. 481-516). Speciation.
COYNE, JA, BARTON, N, and TURELLI, M. 1997. A critique of Wright's shifting balance theory of evolution. Evolution 51: 643-671.
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