Thursday, August 4, 2011

Mendelian Inheritance and evolution. Part I

We recently discussed problems with the idea of Mendelian inheritance, and the way that assuming that was a mode of trait inheritance has been so misleading and, we think for that reason incorrect.  (Here's the first of the five posts in that series.)  We'd now like to consider some of the implications of our point of view, when it comes to evolution.  We think this in fact reinforces that point of view, as we'll try to show.

Some traits do seem clearly to 'segregate' (be distributed) among family members in a present or absent mode, associated with a single allele (variant) at a gene, and if one allele is more likely to prevail that is called 'dominant'.

We pointed out that Mendel chose traits that 'worked'--that provided clear-cut material that allowed him to understand the nature of 'inheritance.'  That word is actually somewhat off, because Mendel was interested in the nature of hybridization (inheritance in a mixed cross between strains), and there was the confusion that still widely reigns, between inheritance of traits and inheritance of genes.  By pragmatically equating the two, because that wasn't too far off in his chosen model situation, Mendel got his result.  Even he knew that most traits didn't work that way (and even some traits that should, in some species he later studied, didn't).

For many decades after Mendel, it was thought that such traits had two important properties:  they were stable, and they made clear-cut qualitative trait differences.  If either wasn't true, then Mendel's rules wouldn't work, hybridization wouldn't work as expected, and how inheritance worked would remain obscure.

And, when Darwin's and Wallace's ideas came along, it was clear that evolution wouldn't work, either!  The most one might expect would be that one of the alleles would replace the other over time, by natural selection.   But this was not satisfactory, because evolution clearly involved smaller, gradual changes more than big dramatic ones.  And evolution, even by Darwin's mistaken genetic ideas, would continually generate new variation for selection to screen.  That was a big problem, because the observed new alleles, traits in offspring that were not present in their ancestors, seemed mainly to be grotesquely harmful, not material that selection could work with to adapt to changing circumstances.

Darwin was aware of much of this, too, in fact.  He could not see how immutable traits, like the dominant/recessive ones, could evolve and also he felt that evolution simply had to be gradual and more or less continuous. Among his reasons were his erroneous largely Lamarckian notion of the physical basis of inheritance, and the fear that without gradualism traits or species might arise suddenly....requiring explanations for their origin.

As a result, for decades leading biologists, realizing that Darwin's gradual inheritance theory (that he called 'pangenesis') was fundamentally wrong, were not convinced that natural selection could account for adaptive evolution or, in particular, that this had anything to do with the known mechanisms of inheritance.

The solution that was widely accepted and prevails (not always accurately) to today, was worked out in the 1930s and later, based on some theory and data from earlier in the 1900s.  This solution was known by names like the 'modern evolutionary synthesis.'   It reconciled Mendelian inheritance with gradual evolution in ways that were consistent with variation within and between species, and with the fossil record.

There are many things to dispute about the modern synthesis, and we'll point them out (and have mentioned them before here on MT).  But one thing that is important, but so far as we know hardly if at all recognized, was that the modern synthesis was possible only if Mendel was wrong, in ways we tried to outline in our recent previous series.

We'll elaborate on these thoughts in the next stay tuned!

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