Monday, February 21, 2005

Darwin and Natural Selection

Over on the Panda's Thumb I find the following comment"

"The evidence doesn’t support mutation/selection as the all powerful force of evolution. Darwin never subscribed to it. Anyone who’s actually read “The Origin of Species” knows Darwin believed that evolution was driven by the heritability of acquired characters."

Here's what Darwin had to say on natural selection:

"As natural selection acts soley by the preservation of profitable modifications, each new form will tend in a fully stocked country to take the place of, and finally exterminate, its own less improved parent form and other less favoured forms with wich it comes into competition. Thus extinction and natural selection go hand in hand (Origin of Species pg 159)."

"Can the principle of selection, which we have seen is so potent in the hands of man, apply under nature? I think that we shall see that it can act most efficiently. Let the endless number of slight variations and individual differences occuring in our domestic productions, and, in a lesser degree, in those under nature be borne in mind; as well as the strength of the hereditary tendency(Origin of Species pg 87)."

I could probably find more but these two will suffice. Let's examine all three quotations. I interpret the first quote to mean that Darwin did not believe natural selection acting on mutation was the driving force of evolution. Two things could be said about that. First, the concept of mutation, being a term related to genetics was unknown. This gets back to comments I made in a previous post. Darwin was trying to talk about variabilty using archaic and outmoded concepts and trying to create a new vocabulary. Since genes and chromosomes and whatnot had not been discovered yet. For Darwin, variability was casued by a wide variaty of phenomena, including atavism, effects of previous sire, use/disuse of parts, and condition of life (for example, domesticated animals were more variable than wild animals because they were attended to by humans, the concept also had to do with rearing animals in a different environment than what they were accustomed to). So to say that Darwin didn't think mutation/selection "drove' evolution was misleading at best. Second, the crucial idea is that for Darwin natural selection acts on variability (however introduced). "Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should occur in the course of many successive generations. If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind? On the other hand , we may feel sure that any variation in the least degree injurious would be rigidly destroyed (Origin of Species pg 87-88)." Later in the same chapter Darwin uses the concept of natural selection to make predictions about speciation and the fossil record (he also uses this idea in a latter chapter on the imperfection of the fossil record). I could pull quite a few other quotes from "The Origin of Species" to prove my point. I could also pull some from "The variation of Animals and Plants under Domestication" and The Descent of Man; and Selection in Relation to Sex" but I think the point is made.