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You don't need to explain to me what the theory of evolution is, or how it works. This question is purely about what exact meaning the word "natural selection" is ascribed to.

There seem to be multiple meanings that are given to the term "natural selection":

  • The process by which, in a single generation of species, the most fit individuals tend to survive. The "things that are being selected", are the individual organisms, and the "moment that selection took place", is the moment that these individuals reproduced or failed to reproduce, or alternatively, the moment that they died or survived.

  • The process by which, over many generations, the genotypes that result in highest fitness tend to to be selected for. The "things that are being selected" are the genes, and the "moment that selection took place" is not a single instance in time or a single generation, but a long time-span. In fact, I sometimes hear the idea that natural selection is already selecting for genes even if those particular genes don't exist in any organism. For example, if natural selection is selecting for "height", and there is a gene X that would increase height, then it is said that X is being selected for even if X does not yet exist.

Are both of these interpretations generally accepted?

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  • $\begingroup$ Natural selection has many different nuances and interpretations different fields. A population-geneticist and an ecologist would give you vastly different answers as would the original text from Darwin. This theory has been constantly refined since its inception, in particular in response to developments in genetics and statistics. RA Fisher in particular has written about this at length. The earlier works of Dawkins also give a good summary on this topic and address common misconceptions. $\endgroup$
    – Tom Kelly ケリー・トム
    Commented Dec 20, 2018 at 13:28
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    $\begingroup$ Possible duplicate of What is the definition of “Natural Selection”? $\endgroup$
    – theforestecologist ♦
    Commented Apr 19, 2019 at 13:15

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"...in a single generation of species, the most fit individuals tend to survive.". This is not correct. "Fitness" in an evolutionary context refers to how many offspring you produce. It has nothing to do with how many pushups you can do. Yes, individual organisms are selected, and the selection process involves differential survival (to reproduce).

"The process by which, over many generations, the genotypes that result in highest fitness tend to to be selected for. "

This is largely correct.

"The "things that are being selected" are the genes..." Yes and no. It is individuals that are selected. They live or die, depending on their traits, which are encoded by genes.

Darwin noticed the extreme physical variations that humans could create in dogs, and pigeons by selective breeding. He thought that the same process could occur in nature, and termed it "Natural selection".

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    $\begingroup$ I did. It seemed from the error-filled descriptions that you could use some clarification. You set up two alternative definitions, both of which were significantly flawed, as if the true definition had to be one or the other, when the true definition contains elements of both. $\endgroup$
    – Karl Kjer
    Commented Sep 23, 2018 at 13:45
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    $\begingroup$ Biologist have a unified definition of natural selection. You presented two that no evolutionary biologist would agree with. That people outside the field are sloppy with their terminology does to mean that experts are confused. $\endgroup$
    – Karl Kjer
    Commented Sep 23, 2018 at 20:25
  • $\begingroup$ You said the most fit survive. $\endgroup$
    – Karl Kjer
    Commented Sep 23, 2018 at 20:26
  • $\begingroup$ Survival of the fittest lineage, not individual. $\endgroup$
    – Karl Kjer
    Commented Sep 24, 2018 at 10:34
  • $\begingroup$ Re ""Fitness" in an evolutionary context refers to how many offspring you produce." Not exactly. If you produce a lot of offspring, but most or all of them die before reproducing, then you are not very fit (in the evolutionary sense). Indeed, there are two different paths to success: have lots of offspring and hope that a few survive (like the codfish, which can lay 2.5 million eggs a year, and live 25 years or so), or like many mammals, invest lots of effort into caring for a few offspring. $\endgroup$
    – jamesqf
    Commented Dec 20, 2018 at 18:35

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