Sunday, 5 June 2016

The evolution of natural selection

Modern definitions of "natural selection" include both differences in survival and differences in reproductive success. E.g. see here.

For Darwin, natural selection was all about the survival and preservation of individuals. Some quotes from Darwin should illustrate the point.

In The Origin of Species, Chapter 4, Darwin wrote:

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 of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.

In The Origin of Species, Chapter 3, Darwin wrote:

Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man's power of selection.

In The Origin of Species, Chapter 4, Darwin wrote:

Natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure.

Darwin repeatedly uses the term 'preservation' and repeatedly says he is talking about individuals (rather than traits). In The Descent of Man, Darwin wrote:

The survival or preservation of certain favoured words in the struggle for existence is natural selection.

In a letter to Charles Lyell dated 1860, Darwin regretted the use of the term "Natural Selection," preferring the term "Natural Preservation". He wrote:

Talking of “Natural Selection”, if I had to commence de novo, I would have used "natural preservation".

Of course, Darwin did also refer to differences in reproductive success. He called these "sexual selection". Darwin wrote:

this leads me to say a few words on what I call Sexual Selection. This depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring.

For Darwin, natural selection and sexual selection were separate. Most modern authors would say that natural selection includes sexual selection.

Darwin's distinction between "natural selection" and "sexual selection" corresponds broadly to my terms "natural elimination" and "natural production". Darwin's term "sexual selection" is totally unusable in this context - since both sexual and asexual reproduction can result in different numbers of offspring.

In the 1930s the term "natural selection" was hijacked by other evolutionary biologists and redefined away from what Darwin originally meant by it. Here's Satoshi Kanazawa (2003):

In the 1930s, however, biologists redefined natural selection to subsume sexual selection and began to contend that differential reproductive success was the currency of natural selection.
Here is Geoffrey Miller in The Mating Mind, 2000, p.8:

In the 1930s, biologists redefined natural selection to include sexual selection, because they did not think sexual selection was very important. Following their precedent, modern biology textbooks define natural selection to include every process that leads some genes to out-compete other genes by virtue of their survival or reproductive benefits. When one biologist says "evolution through natural selection," other biologists hear "evolution for survival or reproductive advantage." But non-biologists, including many other scientists, still hear "survival of the fittest." Many evolutionary psychologists, who should know better, even ask what possible "survival value" could explain some trait under discussion. This causes enormous confusion, and ensures that sexual selection continues to be neglected in discussions of human evolution.

I share the concern that this redefinition can be a source of confusion. Darwin is widely credited with the idea of natural selection. I wonder how many students realize that the natural selection they are taught is quite different from the natural selection that Darwin came up with.

Most of the sources I can see that mention this redefinition argue that its consequences were negative, and we should go back to Darwin's conceptual split between destruction and reproduction. That's essentially the split which I argued for in my essay natural production and natural elimination.

Satoshi Kanazawa (2003) says:

I concur with Miller (2000, pp. 8–12), Campbell (2002, pp. 34–35), and others in the current generation of evolutionary psychologists and believe that we should return to Darwin’s original definitions and treat natural and sexual selection as two distinct processes. I am fully aware that this view is still controversial and in the minority, but I firmly believe that the conceptual separation of natural and sexual selection will bring theoretical clarity to evolutionary biology and psychology.

Geoffrey Miller in The Mating Mind, 2000, p.8-9 says:

In this book I shall use the terms "natural selection" and "sexual selection" as Darwin did: natural selection arising through competition for survival, and sexual selection arising through competition for reproduction. I am perfectly aware that this is not the way professional biologists currently use these terms. But I think it is more important, especially for nonbiologist readers, to appreciate that selection for survival and selection for attracting sexual partners are distinct processes that tend to produce quite different kinds of biological traits. Terms should be the servants of theories, not the masters. By reviving Darwin's distinction between natural selection for survival and sexual selection for reproduction, we can talk more easily about their differences.

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