Evolution Within Limits: Nature Study Identifies Genetic Basis for Galápagos Finch Beak Variation
Casey Luskin February 17, 2015 11:51 AM
Decades ago researchers Peter and Rosemarie Grant conducted painstaking research on finches in the Galápagos Islands. They found that during a drought period the seeds eaten by the finches became tougher, and those birds with bigger beaks were better able to survive and reproduce. After the drought ended, however, the seeds returned to their normal state and beaks also returned to their pre-drought sizes. As the Grants explained:
Effects of the droughts of 1977 and 1982 were approximately offset by selection in the opposite direction -- toward smaller body size -- in 1984-85. A relative scarcity of large seeds, together with an ample supply of small ones, favored small finches. Because the food supply on this island changes in composition and size from year to year, the optimal beak form for a finch is shifting in position, and the population, subjected to natural selection, is oscillating back and forth with every shift. Whether or not there is a net directional arrow through the oscillations is unclear and could be determined by a much longer study.(Peter R. Grant, "Natural Selection and Darwin's Finches," Scientific American, pp. 82-82 (October, 1991).)
The finches thus provide us with an example of natural selection, specifically oscillating selection, but the example entails small-scale evolutionary change. Indeed, many of the finch species can interbreed, regardless of whether they have slightly different-sized beaks. As a paper in BioScience explains, the "finch species retain the ability to interbreed and produce viable, fertile hybrids." There is little doubt that these highly similar and genetically compatible finch species are closely related, but what is the genetic basis for the differences in the size and shape of finch beaks? A new paper in the journal Nature has identified a gene, ALX1, involved in helping to cause the different beak shapes. One variant of the ALX1 gene seems to be associated with pointed beaks, while the other variant is involved with blunted beaks.
But the Nature paper's authors also conducted a phylogenetic study that further confirms extensive interbreeding between the finch "species":
Phylogenetic analysis reveals important discrepancies with the phenotype-based taxonomy. We find extensive evidence for interspecific gene flow throughout the radiation. Hybridization has given rise to species of mixed ancestry. ... The discrepancies between phylogenies based on morphology and genome sequences may be due to convergent evolution and/or interspecies gene flow. ... [O]ur analysis of demographic history using the pairwise sequentially Markovian coalescent (PSMC) model was consistent with extensive interspecies gene flow among the ground finches, as they have maintained larger effective population sizes than the other species.
In other words, they had difficulty creating a gene-based phylogenetic tree because these species are so closely related that they still interbreed, obscuring the true phylogenetic signal. This evidence for interbreeding further reveals the small degree of evolutionary change that has taken place within the finches. As BBC News reports, some of these finch "species" really aren't even different species:
The most extensive genetic study ever conducted of Darwin's finches, from the Galápagos Islands, has revealed a messy family tree with a surprising level of interbreeding between species.[...]Prof Peter Keightley from the University of Edinburgh, though largely convinced by the results, was less surprised that the finches had interbred so extensively."These islands are pretty close together. So it's not surprising that they are flying from one island to the other," he said.Some of the traditional species might not, in fact, be genuinely distinct, he added.
It turns out that these interspecies hybrids may be responsible for mixing producing different sizes of beaks. According to an article in the Wall Street Journal, there are different alleles of ALX1 in the population, and heterozygosity for the gene may lead to intermediate-sized beaks:
A lot of evolution is driven by random mutations. The process of natural selection favors mutations that provide some advantage, and organisms evolve in particular directions. But in the Galápagos, another mechanism was at play as well.In their fieldwork, the Grants noticed that individuals of two different finch species would sometimes pair off, a process known as hybridization.In theory, that should transfer genes from one population to another. But the evolutionary consequences of this were unknown.The Nature study shows that the process of hybridization had indeed mixed the different variants of ALX1 and has thus played an important role in the evolution of Darwin's finches.
So can intelligent design make sense of this? What we may be looking at are variants of an allele that have been in this population of finches since long before they migrated to the Galápagos Islands. These variants can help cause pointed beaks, blunted beaks, or beaks of intermediate size and shape. Environmental conditions can cause the variants to increase or decrease their prevalence within the population of finches.
The bottom line is that we might be looking at variants of an allele that are designed to allow a population of finches to evolve different beak sizes and shapes within predefined limits. Of course work on the finches from an ID perspective could bear out this hypothesis, but one thing seems clear so far: the Galápagos finches don't show the unlimited creative capacity of the Darwinian mechanism, as popularly claimed, but instead only small-scale, very modest evolutionary change, the kind almost no one doubts.
