Re: Natural selection and extinction

Frans K wrote:

> Hello,
>
> Every species that we know (now living or in the fossil record) has survived
> natural selection, at least for a certain period of time. We don't know of
> any species that did not survive natural selection. Many (if not most)
> species have become extinct in the course of geologic time. Extinction is a
> form of natural selection, but it is not the form that Darwin had in mind
> when he wrote his book on the origin of species by means of natural
> selection. Or did he? Species do not (cannot) originate by extinction in the
> geologic sense of the word extinction. However, Darwin does speak of
> extinction as a consequence of natural selection (pp. 109-111 of the first
> edition of the 'Origin of species'). Existing species must become extinct to
> make room for new species. A citation shows this (taken from:
> http://www.literature.org/authors/darwin-charles/the-origin-of-species/chapter-04.html )
> :
>
> "From these several considerations I think it inevitably follows, that as
> new species in the course of time are formed through natural selection,
> others will become rarer and rarer, and finally extinct. The forms which
> stand in closest competition with those undergoing modification and
> improvement, will naturally suffer most. And we have seen in the chapter on
> the Struggle for Existence that it is the most closely-allied forms,
> varieties of the same species, and species of the same genus or of related
> genera, which, from having nearly the same structure, constitution, and
> habits, generally come into the severest competition with each other.
> Consequently, each new variety or species, during the progress of its
> formation, will generally press hardest on its nearest kindred, and tend to
> exterminate them."
>
> This puzzles me. It seems to me that, in this text passage, Darwin implies
> that natural selection is due to extinction. Or that extinction is the cause
> of natural selection. Probably, I am misreading Darwin here. The dinosaurs
> did not originate by extinction. Not any species did originate by extinction
> in the course of geologic time.

I believe you have it just backwards. Darwin implies that extinction is
due to natural selection, and that natural selection is the cause of
extinction.

> Darwin was puzzled himself by by the phenomenon of extinction. On p. 318 he
> writes:
> "No one I think can have marvelled more at the extinction of species, than I
> have done."
>
> On p. 320 he writes:
> "The theory of natural selection is grounded on the belief that each new
> variety, and ultimately each new species, is produced and maintained by
> having some advantage over those with which it comes into competition; and
> the consequent extinction of less-favoured forms almost inevitably follows."
>
> On p. 322:
> "Thus, as it seems to me, the manner in which single species and whole
> groups of species become extinct, accords well with the theory of natural
> selection."
>
> It seems very clear from these two sentences, that Darwin sees the struggle
> for life as the cause of extinction of an existing species. But how came
> this existing species into being? By the same struggle for life? I don't get
> it. What is the role of extinction in the origin of species? Who can help me
> out?

You are confusing two levels of selection. To Darwin, new species came
into existence by the gradual selection of variations within
populations. That would result in individual death, but not in
extinction. Then the species formed by this process competed among
themselves, with the better adapted species causing the extinction of
the less well adapted ones. In modern terms, he was contrasting
individual selection with species selection.

.

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