Speciation in Galapagos Islands finches
We often think of as a slow process. All the available evidence supports the idea that different evolved from, and yet, new species don't pop up around us on a daily basis. For many biologists, this implies that speciation happens so slowly that it's hard to observe on human timescales that we'd need to track a for millennia or more to actually see it split into two separate species. However, new research suggests that speciation may be easier to observe than we thought. We just need to know where to look.
Where's the evolution?
For organisms that reproduce sexually, speciation begins when parts of a population differences from one another. Eventually, the two parts of the population may evolve so many differences that they can no longer interbreed. This can happen in many different ways through shifts in mate availability, mate choice, or simply the organisms' ability to mate successfully with one another. Now, two groups of researchers have shown exactly how these mating differences, the first steps in speciation, can evolve in bird populations in less than 50 years.
The Central European blackcap
The Central European blackcap spends its summers in Germany and Austria and, until the 1960s, had spent its winters in balmy Spain. About 50 years ago, however, backyard bird feeding became popular in Britain. With a ready supply of food waiting for them in Britain, blackcaps that happened to carry that caused them to migrate northwest, instead of southwest to Spain, were able to survive and return to their summer breeding grounds in central Europe. Over time, the proportion of the population carrying northwest-migrating genes has increased. Today, about 10% of the population winters in Britain instead of Spain.
This change in migration pattern has led to a shift in mate availability. The northwest route is shorter than the southwest route, so the northwest-migrating birds get back to Germany sooner each summer. Since blackcaps choose a mate for the season when they arrive at the breeding grounds, the birds tend to mate with others that follow the same migration route.
In December of 2009, researchers from Germany and Canada confirmed that these migration and mating shifts have led to subtle differences between the two parts of the population. The splinter group has evolved rounder wings and narrower, longer beaks than their southward-flying brethren. The researchers that both of these traits evolved via . Pointier wings are favored in birds that must travel longer distances, and rounder wings, which increase maneuverability, are favored when distance is less of an issue as it is for the northwest migrators. Changes in beak size may be related to the food available to each sub-population: fruit for birds wintering in Spain and seeds and suet from garden feeders for birds wintering in Britain. The northwest migrators' narrower, longer beaks may allow them to better take advantage of all the different sorts of foods they wind up eating in the course of a year. These differences have evolved in just 30 generations and could signify the beginning of a speciation event.
The Galapagos finches have been intensely studied by biologists Peter and Rosemary Grant since 1973. At that time, the Galapagos island Daphne Major was occupied by two finch species: the medium ground finch and the cactus finch. Then, in 1981, a finch arrived on Daphne Major from a neighboring island. It was part ground finch, part cactus finch, and quite large compared to the locals. It also happened to have an extra-wide beak and an unusual song a mash-up of the songs sung by ground finches in its birthplace and on Daphne Major. The immigrant paired up with a local female ground finch (who also happened to carry some cactus finch genes), and the Grants followed these birds' descendents for the next 28 years.
|The new immigrant finch (left), a cactus finch (middle), and a ground finch (right)|
Several factors probably contributed to the isolation of the new lineage. Since males mainly learn their songs as juveniles in the nest, the immigrant's male descendents also sang his strange, mixed song. This likely affected which females were willing to mate with them. In addition, female finches tend to choose mates with beak sizes similar to their own, so the extra-wide beaks of the new lineage probably also biased it towards within-group mating.
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These two examples make it clear that the division between species is not a black-and-white issue. Rather, speciation occurs as many different sorts of traits (physical, behavioral, and genetic) diverge from one another along a continuum. Because of this, biologists sometimes disagree about where to draw the line between incipient species about when a division has become deep enough to warrant a new species name. Whatever we choose to call them, these two cases clearly illustrate how a lineage can split and begin to make its way down two separate evolutionary paths.
Of course, there's no way to know if these paths will converge at some point in the future or are even completely distinct now. Another chance event on Daphne Major could cause the new finch lineage to begin interbreeding with the local population again. And blackcaps may never evolve differences beyond a slight change in wing and beak shape. While we can't know the fates of these lineages, directly observing such divergences in real time highlights the fact that we don't always need to look into the distant past or far off future to find examples of speciation in action. Evolution is occurring all around us. We just need to learn where and how to look for it.
Read more about it
- Grant, P. R., and Grant, B. R. (2009). The secondary contact phase of allopatric speciation in Darwin's finches. Proceedings of the National Academy of Sciences. 106(48): 8.
- Rolshausen, G., Segelbacher, G., Hobson, K. A., and Schaefer, H. M. (2009). Contemporary evolution of reproductive divergence in sympatry along a migratory divide. Current Biology. 19:2097-2101.