In a recent study, nestling swamp sparrows of coastal and inland subspecies were raised in captivity. When they reached adulthood, their plumage and bill size differed, just as they do in wild populations; the coastal birds became larger-billed and had darker feathers.
Therefore, the differences between the subspecies are due to genetic differences. However, various comparisons of the DNA have not revealed substantial differences between them, their DNA cannot be told apart.
This is likely because they have only recently (perhaps 10,000 years) become separated from each other. Their different environments, coastal salt marshes versus inland bogs, exhibit strong selective pressures but the lack of time of their separation has not been enough for their DNA divergance to be detectable.
The birds involved in this experiment can be viewed at the National Zoo's Bird House in Washington, D.C.
This article summarizes the information in this publication:
Ballentine, B., and Greenberg, R. 2010. Common Garden Experiment Reveals Genetic Control of Phenotypic Divergence between Swamp Sparrow Subspecies That Lack Divergence in Neutral Genotypes. PLoS ONE 5(4): e10229. doi:10.1371/journal.pone.0010229
Adaptive divergence between populations in the face of strong selection on key traits can lead to morphological divergence between populations without concomitant divergence in neutral DNA. Thus, the practice of identifying genetically distinct populations based on divergence in neutral DNA may lead to a taxonomy that ignores evolutionarily important, rapidly evolving, locally-adapted populations. Providing evidence for a genetic basis of morphological divergence between rapidly evolving populations that lack divergence in selectively neutral DNA will not only inform conservation efforts but also provide insight into the mechanisms of the early processes of speciation. The coastal plain swamp sparrow, a recent colonist of tidal marsh habitat, differs from conspecific populations in a variety of phenotypic traits yet remains undifferentiated in neutral DNA. [Methods and Principal Findings] Here we use an experimental approach to demonstrate that phenotypic divergence between ecologically separated populations of swamp sparrows is the result of local adaptation despite the lack of divergence in neutral DNA. We find that morphological (bill size and plumage coloration) and life history (reproductive effort) differences observed between wild populations were maintained in laboratory raised individuals suggesting genetic divergence of fitness related traits. [Conclusions and Significance] Our results support the hypothesis that phenotypic divergence in swamps sparrows is the result of genetic differentiation, and demonstrate that adaptive traits have evolved more rapidly than neutral DNA in these ecologically divergent populations that may be in the early stages of speciation. Thus, identifying evolutionarily important populations based on divergence in selectively neutral DNA could miss an important level of biodiversity and mislead conservation efforts.
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