If trait t1 and trait t2 are endogenously linked, then if an organism possesses one of these traits it must have the other. It cannot possess just one or the other. But if only one of the two traits is adaptive then the other will go along for the ride, even though it is neither adaptive or non-adaptive.
Percy writes:
Does this sound like Kimura to anyone?
No, it sounds like a basic explanation of pleiotropy.
Pleiotropy is when a single gene affects more than one "trait" (an ill-defined term to begin with, btw). If a gene affects more than one trait, and one is adaptive and the other is neutral, then of course, natural selection will increase the frequency of that gene in the population and both traits will go toward fixation. There is no new argument here, and the only reason Darwin didn't bring it up is because he didn't know DNA existed, let alone how it worked.
This is an old argument, adaptation vs. pluralists and it is really a straw man. "Pluralists" argue that "adaptationists" think that every trait that exists has some adaptive advantage. This hypothetical "adaptationist" says that the human nose sticks out from the face in order to hold his glasses. Or he might argue that
RAZD writes:
Crucially, however, the evolutionary process in such cases is not driven by a struggle for survival and/or for reproduction. Pigs don't have wings, but that's not because winged pigs once lost out to wingless ones. And it's not because the pigs that lacked wings were more fertile than the pigs that had them. There never were any winged pigs because there's no place on pigs for the wings to go. This isn't environmental filtering, it's just physiological and developmental mechanics.
No one is arguing that wingless pigs out-competed winged ones, nor that evolution is only caused by natural selection. The real argument is that ADAPTIVE evolution can only be brought about by natural selection. Evolution is a change in frequency of alleles in a population. Adaptive evolution is an increase in fitness of a population. Adaptive evolution will be most strongly influenced by natural selection. Genetic drift can also happen to produce it, but it is most likely incredibly rare (because it involves the random fixation of adaptive mutations which are rare). However, general evolution, i.e. simply change in a population, is probably mostly influenced by genetic drift (a la Kimura).
The argument proffered by these authors is aimed at a straw man. They say nothing new, nothing interesting, and their only stoking the anti-evolution fire. Shame on them.
We have many intuitions in our life and the point is that many of these intuitions are wrong. The question is, are we going to test those intuitions?
-Dan Ariely