Suppose now that for some reason or other there is a complete geographic separation between parts of the species range. Gene flow from the other parts of the range stops, and the separated populations evolve to suit their new, restricted environment. This encourages the genetic divergence of the separated populations, and might become so great that if the two populations were rejoined they would no longer be successful at interbreeding. [Even if limited interbreeding were still possible, the local adaptations might have become so advantageous that there would be strong selection for individuals to discriminate in order to mate preferentially with members of their local population, rather than with immigrant individuals.] The two sets of organisms have become new species. This concept, in which physical and genetic separation of populations leads to speciation, describes allopatric speciation.
An extreme version of the allopatric speciation idea suggests that allopatric speciation only takes place when small, peripheral populations, on the edge of the species range, are separated. The theory behind this is that the gene flow that occurs between members of a species would normally prevent speciation within the main body of populations that make up the species, since gene flow would overwhelm the genetic divergence that tends to result from local selection and local adaptation.
This scenario is not always true, however, and it is not clear that it is the major mode of allopatric speciation. We know practically nothing about rates of gene flow within and between natural populations. It's easy to see that peripheral isolates from a species might evolve rapidly. But it's not clear that allopatric speciation always takes place in peripheral populations, in fact it's not clear whether speciation need be allopatric.
In contrast to times of rapid genetic change (in isolated peripheral populations), once a species is established, rapid change is prohibited because populations are large, and gene flow gives a good deal of genetic inertia that acts against extreme divergence.
This view of "quantum" or "punctuated" speciation has been the subject of a great deal of controversy, and as we've seen already, there are few data we can use to test the assumptions that underlie it.
The model of a punctuated speciation event explicitly calls for a small peripheral isolated population to evolve rapidly into a new species. If the new species re-invades the parental territory and displaces the parent species, the fossil record of the area will show no intermediate forms between the two species, because the transitional forms were always few, and were restricted geographically to a small region outside the parental range.
Theoretically, rapid evolution of an isolated population can lead to speciation, with no extraordinary process at work except that the environmental setting is appropriate. Thus there is nothing extraordinary in the concept of punctuated equilibrium except that it models a process that is at one end of the spectrum of possible evolutionary events. The spectrum is continuous, however, and punctuated equilibrium is not a challenge to our concepts of "normal" neo-Darwinian evolution.
The concept that a species is in morphological stasis is not extraordinary, either. One would expect peripheral populations that are somewhat different from the mean morphology in any wide-ranging species: examples might be Eskimo or Australian populations of humans. Peripheral populations may not become isolated, or may become extinct if they do, like the Norse Greenlanders. Their fate need not affect the fate of the species as a whole, and its average morphology will not change unless selective pressures induce change. What is unusual in the model of punctuated equilibrium is the idea that the species as a unit cannot change. There are counter examples: Homo erectus changed markedly between its first appearance 1.5 m.y. ago and the late populations 0.5 m.y. ago that had evolved a good deal, particularly in brain size. However, these changes in H. erectus are small compared with the changes that occurred in the eventual transition to H. sapiens.
In the Pleistocene evolution of Siberian woolly mammoths, from glacial to interglacial, the adult size of mammoths changed by about 25%, and then reversed with the climate. There were no peripheral isolates involved, no rapid transitions, only normal evolution throughout the range of the species (or subspecies). So morphological change is normal when environmental parameters are variable: morphological stasis may be normal when environmental parameters are stable.
Punctuated equilibrium can be accommodated within the classical neo-Darwinian synthesis as an end-member of the spectrum of possible evolutionary events that lead to the appearance of new species. But it's not the only possible mode of speciation: it is one possible mode. There's no intrinsic reason that evolutionary change should occur only in small populations, and there's no evidence that populations that have undergone marked evolutionary change are any smaller than those that are stable. Evidence from the deep-sea record of planktonic organisms such as radiolarians and foraminifera suggests that some species can and do evolve as units, gradually, even when their populations are well mixed and extremely large. Conclusion: speciation is likely to have happened in very different modes. The evidence also says that speciation events occur quickly in relation to the average life of a species.
This scenario for punctuated equilibrium is not always true, however (see below), so we have to ask how often it is true. It is not clear that it is even the major mode of allopatric speciation.
Could reproductive isolation originate in a continuously interbreeding population? A hybrid zone by definition is pretty close to reproductive isolation. Perhaps geographic isolation (allopatry) is not required for speciation: as long as interbreeding is small, on the same order as that of a hybrid zone, sympatric speciation could happen as sub-sections of the interbreeding population chose not to breed with other sub-sections.
In fact, it turns out that some of the best examples of speciation-that-has-almost-happened have been noticed where the two "proto-species" are living together in sympatry.
Example: the apple maggot fly
The apple maggot fly of Eastern North America lays its eggs, and its caterpillar feeds on, the leaves of the hawthorn. But when European farmers introduced apple trees, the flies laid their eggs on them too, becoming such a pest that the fly was called the apple maggot fly, and its scientific name reflects this: Rhagoletis pomonella. By now, it is clear that it there are two separate subgroups of flies, one that lives on hawthorn as before, and another that lives on apple trees. The two subgroups cluster and mate on their target plants, so rarely interbreed, even if they are flying around on the same farm. (Observations suggest perhaps 6% interbreeding, about the same as you might find in a hybrid zone between two species.)
But the reality is that the two subgroups of flies have to have different responses to temperature and day length. Newer apple varieties flower and fruit earlier than they once did, so that the optimum time for hatching, mating, and egg-laying for the two subgroups has moved apart (at least in Michigan). Hybrids will therefore be selected against, as they are timed poorly for either host plant. There will be increasing selection favoring those flies that choose mates from "their" subgroup.
So has Rhagoletis pomonella split into two species, sympatrically, just by clustering (and mating) in slightly different places? If it hasn't yet, it will soon: and I hope you have got the message that it can only be a matter of time before biologists grudgingly admit that these are two species, and that sympatric speciation has happened.
And note that it happened fast: about 150 years so far, 150 generations of flies.
References: you can get at the literature from this 2000 paper:
Filchak, K. E., et al. 2000. Natural selection and sympatric divergence in the apple maggot Rhagoletis pomonella. Nature 407: 739-742.
Hawaii is tropical and oceanic, but its climate is dominated by trade winds which create a wet windward microclimate contrasting sharply with a dry rainshadow semidesert only a few kilometers away. Occasional volcanic eruptions and typhoons can wreak extensive local damage, and erosion has created deep valleys separated by high exposed ridges which can act as severe barriers to migration for some organisms. The volcanic nature of the islands allows close control on the timing of evolutionary events, because volcanic rocks are easily dated in absolute numbers of years. Most important, the islands are young on geological and evolutionary time-scales.
Many Hawaiian species are endemic: this is true of plants and birds in particular. But fruit flies are the most species-rich, the most diverse, and the most endemic of all: more than 800 species have been described from Hawaii. Of these, there are 250 species in the genus Drosophila alone, all but 12 confined to the islands: this is 25% of all the species of Drosophila in the world. The flies were studied for years by Hampton Carson and his colleagues, and more recently by molecular geneticists from Berkeley.
On a larger scale, all fruit flies in the islands are more closely linked to one another than they are to any on the surrounding continents: this may imply that there was only one colonization. If the molecular data are reliable, the colonization took place perhaps 40 m.y. ago, as the geological hot spot of Hawaii lay beneath Midway, or even Koko Seamount, now eroded below sealevel. As the hot spot migrated, so the flies migrated, from older island to newer island along the Hawaiian chain.
The living species of fruit flies show a pattern of continuing changes from Kauai, the oldest of the larger islands, down the island chain to the youngest island, Hawaii itself. There are so many species that Carson concentrated on a subgroup, the "picture-winged" Drosophila. There are 17 species of this group on the island of Hawaii, none of them shared with Maui, only 50 km away. What's more, the 17 species will not hybridize with one another: they are truly reproductively isolated, and are good biological species.
The chromosome structure shows that seven of the Hawaiian species can be traced to three ancestral species that must have come from Maui on three separate occasions. But the ancestral species did not cross: immigration was accompanied by a speciation event, sometimes several speciation events. This in turn suggests that the founders were few, perhaps single fertilized females. The island of Hawaii is only about 700,000 years old, so speciation events were rapid.
Of course, there is no record of any speciation event, only evidence that it occurred. Carson believes that the founder event was followed by a "population flush" as the founders multiplied in the new environment. In the flush, genetic drift and the founder effect between them acted to cause a genetic divergence away from the ancestral species. Only then, Carson believes, would natural selection constrain the new population to adapt closely to its new habitat. As a result, Carson sees the Hawaiian speciation events as buffered from the process of natural selection, and speciation as preceding adaptation. He recognizes that this would only apply to cases where a few founders invaded a "permissive habitat": in cases, for example, where a large continental species was broken into demes by geographical events, speciation might follow that was entirely constrained by natural selection, and speciation would be accompanied by adaptation.
Carson interprets these events as quantum speciation: the extremely rapid and profuse proliferation of species in a short geological history. Each species has a very restricted geographic range, and a comparatively homogeneous population. Carson has suggested that the species are subject to rapid and severe challenges from the environment -volcanic eruptions and hurricanes come to mind. A few survivors, or a few immigrants, are most likely to have an incomplete or atypical representation of the genes of the parent species, and are likely to be subject to severe environmental selective pressures as they found a new or renewed population.
The plausible scenario thus suggests that new species can arise very quickly in some circumstances: but there is no appeal to any genetic or evolutionary processes that are unique, or different from those operating on "normal" populations. It is the environmental settings that are unusual, and they result in "bottlenecking" of the populations that are "founders": they are isolates in time and space that result from the environmental settings, and although there was no strong selection in the process of isolation, there is strong selection after a brief period of "population flush."
But both the founder event and the subsequent flush are events that are interpreted, not observed. Clearly the processes of quantum speciation would work most strongly on organisms that happen to be relatively immobile but very fecund. It is important, too, that the founders are a small sample of the pre-existing population.
In "Evolution Canyon", a study site on the slopes of Mount Carmel, Israel, two populations of one species of Drosophila, D. melanogaster, live on the canyon sides, one on the north and one on the south. These fruitflies can easily fly several kilometers, yet the canyon walls are much closer than that. Yet the north-wall and south-wall populations are physiologically and behaviorally different, and they are different genetically to an extent that would be called "separate species" in another geographic area. They could easily cross the canyon and interbreed, but they do not. They are driven apart because the northern canyon wall gets more sun, is hotter, and drier, and has different plants growing on it, than the southern wall. This has been enough to overcome the fact that the flies shared a species range, and to drive them, by natural selection, to their present situation. Are they separate species, or not?
Populations of Geospiza fortis, the medium ground finch, have been studied in detail. In 1977 the Galapagos had abnormally low rainfall, and the seed plants on which the finches depend produced small crops. The population of finches fell to 15% of its previous level, and the survivors were not a random sample of the original population. Large birds, especially males with strong beaks, survived preferentially because they alone were able to crack large seeds. The population as a whole was shifted toward larger body size. The level of selection measured in the drought and the drastic population crash was the highest yet recorded in a vertebrate population. However, a few years later there was a particularly wet year, and enormous quantitities of seeds were produced. The finches had multiple broods that year, and smaller individuals, having great numbers of small seeds available, and reaching maturity faster, raised more offspring. The effect of the wet year was to shift the average body size back toward smaller size. Similar major fluctuations in climate and food supply occur on approximately a 20-year spacing in the Galapagos as El Nino perturbs the local climate, and in any case there are important fluctuations each year. It's likely that such events, producing the oscillating selection described above, are the major controlling factors on the morphology of this finch. Thus selection pressure does not have to be constant to be effective: and the constraints on the morphology of a species, which tend to maintain its morphology in stasis, may not be obvious to a biologist without years of intensive study. Moreover, a species may change its morphology over much of its range in reponse to selective pressure: the relative frequency of such change may depend on the extent of the range and the selective pressure involved.
Older optional references:
Today in each of at least six lakes that have been studied, there are species pairs. In each lake, one species of stickleback swims close under the surface, snatching insects. This species is streamlined for rapid swimming darts (for feeding and for escape), and it has well-developed spines (probably for defense). The other species in each lake swims near the bottom, eating small crustaceans that live on and in the mud. It swims slowly, is not so streamlined, and has smaller spines. The sticklebacks in each lake are separate species.
Obviously, these pairs of species have evolved in less than 15,000 years, and they evolved sympatrically. They must have diverged genetically when one subgroup spent its time neat the surface and the other spent time on the bottom.
References: Many papers are referenced in this recent one:
Peichel, C. L., et al. 2001. The genetic architecture of divergence between threespine stickleback species. Nature 414: 901-905.
Instead, there are dozens of species, some of them restricted to a single bay. All of them must have evolved very quickly indeed, because sampling the floor of Lake Victoria went through lake-floor sediment and into salt-pan (desert) deposits. Dating the sediments revealed that the lake was DRY only a few thousand years ago, so all the cichlid species have evolved in that short time. This is even faster than the Hawaiian fruit-flies, probably.
Cichlids have an immense array of adaptations to different diets as well as rocky vs. muddy habitats. Some cichlids have mouth parts specifically adapted to crunch small crustaceans; others nibble algae. The most spectacular cichlids (in terms of diet) bite scales off the sides of other species of cichlids.
To do this, they sneak up from behind. Those that evolved a head twisted to one side had their mouths ready positioned to take a quick bite: for example, if the head is twisted to the right, the mouth is ready to take a quick bite at the left side of a prey. However, a few mutants may have their heads twisted to the left, and they would be best suited to biting into the right side of a prey.
Now here is natural selection caught in the act. If most bites are coming from the left, prey cichlids will pay more attention to that side. The few mutants, approaching from the right, will get more food and reproduce better than the majority, who have more failed attacks on vigilant prey. So over time, one would predict that the proportion of right-and left-twisted scale-eating cichlids will change, as more left-twisted mutants are born and are in turn successful parents. And a dedicated and astute Japanese ecologist in fact observed the cichlids over a few years and saw exactly that effect.
And of course, it will continue, with stabilizing selection acting always to cut the numbers of the majority twisted-headed cichlids. Fortunately for humans, selection for baseball alone is not likely to affect the proportion of right- and left-handed people.
[ASIDE: But all organisms may not perceive environmental changes in the same way. The Nile crocodile has inhabited the same waters as the cichlid fishes for the same time, but is a single species which ranges throughout the Nile basin and surrounding regions. The human species is another one that warns us that rapid speciation may not be a universal process. Despite the range of "races" that have evolved in a very broad range of climates and environments, humans remain a fully interbreeding biological species.]
Optional references on cichlid research:
News report, February 1999:
Human mutation rates are large
In the "classical" model of allopatric speciation, separated populations need not be small (Arctic gull populations are unlikely ever to have been small or isolated), although evolutionary change away from a common inheritance would surely be more rapid among small populations. Nor is it necessary that the isolated population(s) be peripheral: it's more likely that selection would diverge from the species average in a peripheral population, because the local environment might be more extreme, but the concept is not mandatory.
If the concept of punctuated equilibrium is correct, then evolutionary change can be so slow as to be practically negligible most of the time, but occasionally is very rapid. "Slow" and "rapid" are relative to the geological time frame. But how fast is evolutionary change to a geneticist, or a paleontologist, and is it fast enough to put a limit on some of the theories of evolutionary mechanisms? How can we attack the question of rapid evolutionary change, suddenly occurring within a species that had had a long period of morphological stability?
Major revision begun January 22, 2002.
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