January 5, 2005 | David F. Coppedge

Bird Studies Overthrow Evolutionary Assumptions About Population Genetics

The assumption was that gene flow homogenized a population, and selection diversified it.  But now, two studies in Nature1,2 of an English songbird called the great tit, Parus major, carried on for decades, has shown that differences between closely-associated populations can persist in spite of homogenizing gene flow.  Garant et al. explain the significance of this to evolutionary theory:

Evolutionary theory predicts that local population divergence will depend on the balance between the diversifying effect of selection and the homogenizing effect of gene flow.  However, spatial variation in the expression of genetic variation will also generate differential evolutionary responses.  Furthermore, if dispersal is non-random it may actually reinforce, rather than counteract, evolutionary differentiation.  Here we document the evolution of differences in body mass within a population of great tits, Parus major, inhabiting a single continuous woodland, over a 36-year period.  We show that genetic variance for nestling body mass is spatially variable, that this generates different potential responses to selection, and that this diversifying effect is reinforced by non-random dispersal.  Matching the patterns of variation, selection and evolution with population ecological data, we argue that the small-scale differentiation is driven by density-related differences in habitat quality affecting settlement decisions.  Our data show that when gene flow is not homogeneous, evolutionary differentiation can be rapid and can occur over surprisingly small spatial scales.  Our findings have important implications for questions of the scale of adaptation and speciation, and challenge the usual treatment of dispersal as a force opposing evolutionary differentiation. (Emphasis added in all quotes.)

David W. Coltman (U of Alberta),3 commenting on these papers, summarized, “Gene flow between populations – caused by migration, for instance – is most often viewed as a homogenizing force in evolution.  But two studies of wild birds and non-random dispersal find otherwise.”  These long-term studies complicate theories.  He says: “we ought to be paying more attention to how quantitative genetic variation is spatially and temporally structured.”  But even that is not enough: “Indeed, a truly mechanistic understanding of microevolution requires an understanding of genetic architecture (the properties of the individual genes underlying variation).  One way of gaining such an understanding of microevolution in nature will be to apply genomics to ecological and evolutionary studies in non-model species, using comparative approaches.”  The fact that he speaks in future tense indicates this has never been done.

1Garant et al., “Evolution driven by differential dispersal within a wild bird population,” Nature 433, 60 – 65 (06 January 2005); doi:10.1038/nature03051.
2Postma and Van Noordwijk, “Gene flow maintains a large genetic difference in clutch size at a small spatial scale,” Nature 433, 60 – 65 (06 January 2005); doi:10.1038/nature03051.
3David W. Coltman, “Evolutionary genetics: Differentiation by dispersal,” Nature 433, 23 – 24 (06 January 2005); doi:10.1038/433023a.

On the surface, this looks like it could accelerate evolution by removing the homogenizing effect of gene flow.  But consider what these papers indicate.  First, assumptions can be flat wrong.  Many evolutionists assumed, with armchair modeling, that populations with many opportunities for interbreeding would become more homogeneous.  Both these studies, however, showed that slight differences in clutch size and body mass could be maintained in spite of shared habitat.  Second, they found that differentiation of two populations can be rapid and occur in a small area.
    Keep in mind that these studies involve only microevolution.  They are about one species of bird, Parus major, that were still the same species at the beginning and end of the observations.  They have nothing to say, therefore, about the origin of birds, the origin of flight, the origin of feathers, the origin of species or any other major change that would help Charlie feel gratified.  In fact, creationists could use these studies to support the idea that microevolution was rapid after the Flood.  The evolutionists themselves were surprised that their assumptions about population genetics were wrong, and admitted that these studies “challenge the usual treatment” of dispersal, as well as gene flow and selection, as agents of evolution.  If you cannot trust your assumptions, you cannot trust your models; and if you cannot trust your models, you cannot trust your perception of reality.
    Incidentally, despite its embarrassing name, the great tit is beautiful bird.  The picture accompanying David Coltman’s commentary shows the bird in flight, wings splayed in geometrical artistry, exquisitely-designed feathers extended, handsome black-capped head, food in its beak, alert eyes, displaying aerodynamic excellence.  Differences in clutch size and egg size say nothing about these examples of functional adaptation par excellence.  The caption reads, “The great tit: challenging assumptions about gene flow and genetic differentiation.”  Darwinists preach macroevolution, but cannot understand microevolution.  They want to explain the whole living world by their theory, and can’t even get the population genetics of one species of bird right.

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Categories: Birds

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