How Did Natural Selection Shape Human Genes?

A paper proposes that climate favored certain mitochondrial DNA
polymorphisms
By Douglas Steinberg

Many selective forces must have influenced human evolution, but the only one
that all population geneticists seem to agree upon is malaria. Time and
again, studies have identified certain DNA polymorphisms--most famously, the
ß-globin variant underlying red-cell sickling--that helped people resist
this mosquito- borne disease. The reproductive success of such individuals
spread these polymorphisms throughout regions where malaria is endemic.

Geneticists have been much more reluctant, in contrast, to conclude that
other selective forces favored or dis-favored particular polymorphisms. That
attitude is changing, however, as technological advances allow the rapid
sequencing and analysis of genomes. Researchers are increasingly seeking
evidence of natural selection, as they question whether many newfound
variants are as "neutral" as a theory dominant during the past 20 years
would suppose.

Promoted by the late Motoo Kimura, the neutral theory of molecular evolution
holds that most polymorphisms confer no fitness advantage or disadvantage,
and spread or disappear randomly. "The problem is that we've now saturated
the neutralist hypothesis," contends Douglas C. Wallace, a professor of
molecular medicine at the University of California, Irvine. "There are more
things going on than that, and we have to go back to Darwinian selection."

In a new paper, Wallace and colleagues at UC-Irvine attribute some
polymorphisms in human mitochondrial DNA (mtDNA) to climatic adaptation.
Other recent research suggests how selective forces might have affected
autosomal genes2 and casts doubt on the neutrality of Y-chromosome
variation. Besides illuminating evolution, these studies have a potential
clinical benefit: Knowledge of DNA variants' evolutionary fitness might
provide clues to developing better gene-based diagnostics and therapies.

Read the rest at The Scientist.com
http://www.the-scientist.com/yr2004/may/research2_040510.html



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