In the last post in this series, we examined the relatively simple case of geographic separation of populations (such as a new population being founded on an island). Geographic separation is an effective barrier to what biologists call “gene flow” between populations – an effect more properly described as “allele flow”. As new alleles arise in separate populations, lack of interbreeding keeps each allele in the population where it arises. These new alleles may contribute to speciation over time if they affect the characteristics of the organism. If, on the other extreme, new alleles can pass freely between two populations, then they will not contribute to a speciation event, since they will not make the two populations become more different over time.
What goes around, comes around
While these two extremes (geographically separated populations and fully continuous populations) are straightforward to understand, it is possible to find situations that are shades of gray between them. For example, consider two populations (we’ll call them “A” and “B”) that are members of the same species. They are able to exchange alleles between them, but at a reduced rate compared to sharing within each population. This effect can arise due to the geographic shape of their habitat – if it is long and narrow, then the two populations may abut each other only along a small portion of their range. This means that, on average, an individual from population A is more likely to find a mate within population A than to mate with a member of population B in their small area of overlap. We can represent this with boxes representing the two populations, abutting each other along one of their narrow sides:
This arrangement thus restricts, but does not completely abolish, allele flow between the two populations. In effect, this is a partial barrier to allele flow. Populations A and B are members of the same species, but the two populations are not genetically identical. As new alleles arise in population A, they are not shared across to population B as often as they are shared within population A, and vice versa. As such, populations A and B may have different frequencies of any given allele, and may even have some alleles that the other population lacks all together. It is also possible that the two populations may experience differences in natural selection (since their environments are not identical), and/or differences in genetic drift, depending on the population size for each. The net result is a balance of forces acting on the two populations – some favoring differences (selection and/or drift) and another favoring similarities (limited flow of alleles through interbreeding).
In nature, this effect can extend to multiple populations in a “string” spread out across a ribbon of suitable habitat. Let’s add three more populations (C, D and E) to the above example to illustrate:
Once populations become spread out over a wide geographic area, the differences between the populations at the extremities (populations A and E in our diagram) can become quite significant. In some cases, interestingly enough, the populations on the ends of the string can be different enough that they do not recognize each other as members of the same species, despite the fact that they are genetically connected through a series of intermediate populations. In some cases, scientists need to bring members of the extreme populations together to see if they are able to interbreed (i.e. employing the biological species concept as a definition of species). In other cases, the topography of the habitat brings them together in nature, allowing the populations at the extremes of the string to meet each other around a ring, but with a natural barrier in the middle (such as a mountain or a valley of unsuitable habitat). The result is what are known as “ring species”:
You can see the inherent difficulty for defining which populations are separate species, (if indeed any are at all). There is allele flow between all populations, but only around the ring. The two populations at the (overlapping) ends, despite encountering each other in the same habitat, are different enough that they do not interbreed. Defining these populations as separate species (or not) is a fruitless attempt to draw a line of demarcation on a gradient. For those interested in a real-life example of a ring species, the subspecies of the salamander Ensatina eschscholtzii on the west coast of North America are both a textbook case and a subject of ongoing research.
Now, if we encountered the populations at the extremities in the wild without the intermediate, “bridging” populations, we would not hesitate to classify them as distinct species. It is also easy to see what would follow if any of the bridging populations were lost, or if changes in habitat severed the connection between any of them – the result would be a break in the chain of allele flow, cutting the terminal populations off from one another. What ring species illustrate is that though speciation is a slow process of accumulating differences between populations, it is possible even without a full barrier to allele flow.
Speciation without geographic separation
While ring species illustrate how species can form by partitioning variation out over a wide geographic area, it is also possible for barriers to allele flow to arise within a population in a more geographically compact location. All that is needed is a bias that promotes allele exchange within a subgroup of the population at the expense of exchange with the wider population – and as we have seen with ring species, this barrier need not be absolute to allow two subpopulations to accumulate differences and diverge from one another over time. One way for this to occur is for subpopulations to begin to exploit resources within a common geographic area differently – an effect known as resource partitioning. As subpopulations begin to specialize into slightly different “manners of life”, as Darwin put it, they become more likely to interbreed within their subpopulation than with the population as a whole. Since preferential breeding is a (partial) barrier to allele flow, this can place the two subpopulations on a genetic trajectory that reinforces their differences and leads to a speciation event. Resource partitioning is the likely mechanism that drives multiple, rapid speciation events that occur when a founding population reaches a new habitat where competitors are largely absent. The colonization of volcanic islands, a topic we have discussed previously, can lead to adaptive radiation. One example is the numerous species of Darwin’s finches on the Galapagos Islands that descend from one species of finch that originally colonized the archipelago, and subsequently diversified into numerous species that specialize in different food sources. In the absence of other birds on the islands, many “manners of life” (what we would now call niches) were available for different subpopulations of birds to occupy.
Summing up – speciation starts as barriers to allele flow
Full geographic separation, the partial geographic separation seen with ring species, and resource partitioning of subpopulations are all barriers to allele flow between (what starts as) members of the same species. This provides the opportunity for new alleles to arise that are not shared between two populations, and shift the average characteristics of the two groups away from each other. In the next post in this series, we’ll examine some of the traits that such alleles contribute to – traits that improve barriers to allele flow and thus promote speciation events.