The deer mouse is described as one of the "most abundant and widespread mammals in North America" and normally has a dark coloured pelt. The Sand Hills of Nebraska are home to a sub-population of deer mice (Peromyscos maniculatus) with a light-coloured fur that matches the local habitat. The potential for understanding more about adaptive change and the genetics of hair colour has attracted the attention of researchers.
"To unravel evolutionary mechanisms in the wild, we must estimate the fitness advantage of adaptive alleles and infer their source, either as new or pre-existing variation."
Pale-coated deer mouse on a dark soil (Source here)
Based on what is already known about pigment-producing cells at the base of mammalian hair follicles, the gene Agouti was identified as responsible for the different phenotypes. In the house mouse (Mus musculus), it is known that knockouts of Agouti result in dark hairs, whereas overexpression of Agouti leads to light colours. It is also known that "light alleles are generally dominant to dark ones". Significantly, the crosses between dark and light coloured deer mice produced light-coloured offspring. Empirical work pointed unambiguously to Agouti expression as the underlying cause of the two phenotypes (referred to in the paper as wildtype and wideband phenotypes). Sequencing analysis led to the identification of 20 nucleotide differences between the dark and light mice. Of these, two stood out as prime candidates for a genetic mechanism: "a conservative amino acid substitution (Arg37Lys) and a serine deletion (residue 48), both in exon 2". After more detailed investigations, the authors report:
"On the basis of these results, we cannot determine whether the serine deletion, a linked mutation, or both cause wide bands and light coats. [. . .] Nonetheless, the haplotype containing the serine deletion[. . .] explains a substantial amount of ecologically relevant phenotypic variation."
Further research concluded that "selection is probably acting on the wideband Agouti haplotype". Also, "both population genetic data and predation experiments suggest that selection for light color is strong, and our estimates fall within the range of selection coefficients for other color polymorphisms, including beach mice, pocket mice, ladybirds, and land snails."
Allowing all this, what can be said of this example of adaptive change?
First, selection demonstrably acts on phenotypes that affect the ability of organisms to survive and reproduce. In this case, the phenotype relates to coat colour and its ability to provide camouflage, thus avoiding predation.
Second, the genetic variation involved is linked to a "single amino acid deletion" in the Agouti gene. Deletions are not infrequently found in cases of natural selection. By contrast, additions are rare. Information loss is the norm.
Third, both types of deer mouse interbreed readily. Coat colour is not a factor in mate selection. "Therefore, we hypothesized that light and dark mice interbreed with ample opportunity for recombination between the wideband and wild-type alleles." We are not looking at new species or sub-species here.
Fourth, the adaptive changes are inferred to have been rapid. The authors consider this worthy of comment because they consider the genetic changes to be de novo (i.e. natural selection was not acting on a pre-existing allele).
In the light of these comments, natural selection can be identified as a factor of relevance in ecological studies. Over generations, organisms adapt to their environment and become more optimised to local conditions.
What this research does not do is lend support to Charles Darwin's claim that evolution by natural selection is the key to understanding the development of biological complexity and the origin of species. the genetic changes associated with the modified coat colour are attributed to the deletion of a single amino acid in the Agouti gene - there is no increase in information and no move towards specified complexity. It is necessary to point this out because some (e.g. the BBC report) are suggesting this research is so important for evolutionary theory that it deserves iconic status.
"Rival icon. In some respects, the dune-living deer mice are similar to the famous peppered moths of northern England. For decades, the peppered moths (Biston betularia) have been heralded as one of the best-examples known of a wild animal adapting to its environment due to natural selection. [. . .] "In both species, changes in colour evolve rapidly due to selection by visually-hunting predators," says Prof Hoekstra. But the study by Dr Linnen and Prof Hoekstra's team takes our undertaking of natural selection to a much deeper level. The selection pressure on the moths was technically artificial, caused by pollution produced by people. Whereas the selection causing the pale mice is truly natural. What is more, the scientists have found the gene responsible, and worked out exactly how long it took to evolve and take hold in the population. "Despite the fact that the peppered has been an icon of 'evolution in action', we do not yet know the genetic changes involved," says Prof Hoekstra. "Once researchers find the pigmentation gene responsible for moth colour change, they can do the same types of analyses we have done. It will be really interesting to compare these estimates between mice and men."
Like so many other studies of natural selection, Darwinists think that if they can only demonstrate natural selection in action, their theory is verified. Of course, this is fallacious for the reasons pointed out above. The deer mice example demonstrates ecological adaptation, and does not provide any insight into the origin of complex specified information. To say that the deer mouse example is an icon of evolution is like saying 'if we can only generate amino acids in a reducing atmosphere, we will have shown that the first living cell could have arisen by natural processes'. It is like saying 'if we can show that oxygen levels elevated significantly in the Early Cambrian atmosphere, we will have explained the Cambrian Explosion'. We need these discussions to mature - there is no dispute over natural selection as a driver of adaptive change in ecological systems, but if it is to be presented as a key mechanisms for the creation of biological complexity, the quality of argument needs to be improved by many orders of magnitude!
On the Origin and Spread of an Adaptive Allele in Deer Mice
Catherine R. Linnen, Evan P. Kingsley, Jeffrey D. Jensen, and Hopi E. Hoekstra
Science, 28 August 2009, 325: 1095-1098 | DOI: 10.1126/science.1175826
Adaptation is a central focus of biology, although it can be difficult to identify both the strength and agent of selection and the underlying molecular mechanisms causing change. We studied cryptically colored deer mice living on the Nebraska Sand Hills and show that their light coloration stems from a novel banding pattern on individual hairs produced by an increase in Agouti expression caused by a cis-acting mutation (or mutations), which either is or is closely linked to a single amino acid deletion in Agouti that appears to be under selection. Furthermore, our data suggest that this derived Agouti allele arose de novo after the formation of the Sand Hills. These findings reveal one means by which genetic, developmental, and evolutionary mechanisms can drive rapid adaptation under ecological pressure.
Walker, M. Mouse set to be 'evolution icon', BBC News (27 August 2009)
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