In the most recent Heredity Podcast, University of South Carolina evolutionary biologist Austin Hughes explains why he doesn’t believe that positive natural selection is always the primary factor driving the spread of adaptive traits in a population. When asked about the problem with positive Darwinian selection, he says, “The problem is there really isn’t all that much evidence that it actually happens to the extent to which it would be needed to explain all of the adaptive traits of organisms.”
In Hughes’s view, the evolutionary literature has “clogged up science” with purported examples of evidence of positive natural selection. But he thinks that “most of that literature is worthless” because the methods being used don’t work to discriminate between positive selection (selecting for beneficial mutations) from the relaxation of purifying/negative selection (e.g. selecting against deleterious mutations) which allows for random genetic drift. In other words, the evidence for positive selection that we see in the genome might just be a process of genetic drift rather than a process that is being driven by selection. This leads to the question: How, then, do new traits arise?
Rather than relying on positive selection, Hughes claims that one prevalent mechanism in producing new traits is the relaxation of purifying selection — i.e. random genetic drift. But genetic drift, of course, is essentially a random process where mutations not only arise without respect to the needs of the organism, but also are preserved (or lost) without regard for the needs of the organism. In other words, it would have no reason to build complex traits.
Instead, Hughes presents the phenotypic-plasticity model of evolution, which was well explained by P.J. Levi here. Hughes still believes that positive selection occurs, but it isn’t nearly as prevalent as is often thought, and taught.