In yesterday’s post, we made a number of points worth summarizing here:
New alleles arise as unique events in individuals, but may become common in their population through various processes, including genetic drift and natural selection.
New alleles, should they become common in a population, may shift the average characteristics of that population.
If exchange of alleles between two populations of the same species is blocked or reduced, then average characteristics of the two populations may diverge from each other.
Given enough time, these processes may lead to differences between the two groups that are significant enough to establish them as distinct species.
With these points in hand, we are now ready to have a closer look at the various ways that genetic exchange between populations can be reduced or eliminated. We’ll start by looking at the simplest case, that of complete geographic isolation.
Geographic separation of two populations of the same species is a rapid and effective way of stopping the exchange of alleles between them. At the point of separation, the two populations are, of course, fully capable of interbreeding biologically, but prevented from doing so by physical separation. One example of geographic isolation leading to speciation that we have discussed already is the various species of finches that Darwin observed on the Galapagos islands off the coast of South America. The original finch population of the Galapagos was founded by a small group of birds that arrived on the islands from the South American mainland, most likely blown there during a storm. These birds, as a population, were biologically cut off from the source population on the mainland, since the Galapagos are hundreds of miles offshore. Once separated from the larger population, the smaller “founding” group no longer received new alleles from it, nor passed new alleles that arose back to it. Despite being two populations of the same species, they were now genetically sealed off from one another, and differences in allele frequencies began to accrue between them. These differences lead to changes in average characteristics over time, and ultimately the formation of a new species.
The founder effect
In many cases, this process of accumulating differences gets a head start right at the point of separation, due to a phenomenon known as the “founder effect.” A small founding population is very often a non-representative sample of the genetic diversity of the source population. For example, consider a hypothetical population with 36 individuals. Each individual carries two alleles of a given gene, and there are four different alleles of this gene in the population (represented by the four colors):
Note that the yellow allele is the most common, followed by the blue allele. The purple and red alleles are comparatively uncommon in this population. In fact, their rarity means that it would be unlikely for this population to have an individual with two red alleles in the next generation, for example. In order to have such an individual, two parents who were “carriers” of the red allele would have to mate, and both pass the red allele on to their offspring. This is not impossible, but in this population it would be unlikely.
Now suppose that a few members of this population start a new population on an isolated island. Only six individuals start the new population, and the alleles that they carry are not a perfect representation of the allele frequencies of the larger, source population (approximate frequencies are shown for the source population and the new “founding” population):
We can see that for the common alleles, the yellow allele has increased in frequency, and the blue allele has decreased in frequency. Despite these differences, the common alleles are reasonably similar in frequency to the source population. The rare alleles, however, have had larger shifts: the red allele is now much more common in the newly founded population, and the purple allele has been lost entirely.
Now, these changes are subtle, and changes for one gene would not likely be enough to precipitate a speciation event between the two populations. These sorts of changes could, however, be significant in the long run. Consider the red allele in the newly founded population. As this population increases in number, it will be much more likely to have individuals with two red alleles crop up in this population than in the original source population. If this genetic combination has a selective advantage, then natural selection will be able to act on it in the new population. In the source population, however, this genetic combination is much more unlikely, largely preventing natural selection from acting on this allele combination. Over time, the red allele could come to dominate the new population, but remain rare in the source population. Additionally, it is likely that the environment will be somewhat different for these two populations, leading to differences in natural selection. What might be a winning allele combination for the mainland might not be as suitable for the island environment, and vice versa. A second issue is that the newly founded population, like any small population, is much more subject to genetic drift than the larger source population. The red allele might increase in frequency in the new population simply due to chance alone, and not due to the action of natural selection.
Taken together, these mechanisms can put the two populations onto different trajectories, and, over time, lead to significant differences between them. Given enough time, the differences that accrue may be enough to keep the populations separate even if they should come into contact again. If so, most biologists would classify the two populations as distinct species. While this is easier to do for species that have been separated for a long time and have accumulated significant differences (and as such no longer interbreed, or interbreed only rarely), it is notoriously difficult for more recently separated populations that are not yet fully reproductively isolated. As such, what constitutes a “true species” instead of merely a “subspecies” or “variety” is often a subject for discussion and debate between scientists, and indeed was a topic that Darwin devoted much time to in his works. The ambiguity arises out of the mechanism of slow, gradual divergence of species from a common ancestral population.
Not just differences
Given the foregoing conversation, you might be under the impression that the differences between species are the main issue. Certainly differences are vital, since ultimately it will be the accumulation of differences that will lead to new species being formed. It is important to remember, however, that for closely related species, these differences will be small in number compared to the features that remain unchanged for both groups. At the genetic level, we can illustrate this by considering a gene for which there is only one allele in the source population – perhaps an allele that has been under natural selection and has displaced all other alleles. The newly founded population will inherit only this allele, despite the small sample size of the founding group, since there are no other variants in the population. The result is that the island population will be identical to the mainland population for this trait until a mutation event (in either population) even allows for the possibility of change. For most traits, mutations will not arise, since the DNA copying mechanism is highly accurate. This will keep most traits between the two populations constant. The pattern we expect for recently diverged species, then, is one of mostly identical characteristics overlaid with only a smattering of differences. You might recall that it was exactly this pattern in its biogeographical context that caused Darwin to reflect on the possibility that species may not be stable:
“The most striking and important fact for us in regard to the inhabitants of islands, is their affinity to those of the nearest mainland, without being actually the same species. Numerous instances could be given of this fact. I will give only one, that of the Galapagos Archipelago, situated under the equator, between 500 and 600 miles from the shores of South America. Here almost every product of the land and water bears the unmistakeable stamp of the American continent. There are twenty-six land birds, and twenty-five of these are ranked by Mr. Gould as distinct species, supposed to have been created here; yet the close affinity of most of these birds to American species in every character, in their habits, gestures, and tones of voice, was manifest.”
Note that it was the combined pattern of overwhelming “affinities” (distinctive features in common) with subtle, but significant differences that Darwin observed. The birds in question were distinct species, but they retained the “unmistakeable stamp” of their heritage. It was these observations that led Darwin to hypothesize that these finch species were the product of a speciation event brought on through geographic isolation.
While geographic isolation is a straightforward situation that can lead to genetic barriers and the formation of new species, speciation can also occur without full separation. In the next post in this series, we’ll examine a case of speciation with only a partial geographic (and genetic) barrier – a case that will also demonstrate the “fuzziness” of what exactly constitutes a species.