Wednesday, January 16, 2013

What Exactly Is Evolution? Stated Clearly Gets It Mostly Right

Stated Clearly posts videos on YouTube. It is dedicated to "spreading the love of science to the world" [Stated Clearly, YouTube] [Stated Clearly, Website].

The video below, narrated by Jon Perry, answers the question, What Exactly Is Evolution. It starts with an excellent definition of evolution that closely resembles my own preferred, minimal, defintion of evolution at What Is Evolution?. Here's the definition in the video ...
(Evolution is defined as) any change in the heritable traits within a population across generations.
The first example involves a new mutation that arises in an amoeba-like organism. Because this new mutation in inherited by a daughter cell, the video declares that "evolution has officially occurred."

What this means is that mutation becomes a mechanism of evolution. I prefer to think of evolution as "heritable changes in a population spread over many generations" in order to make it clear that change in a single generation doesn't qualify. I also prefer to think of mutation as a mechanism for generating variation and not a mechanism for causing evolution but this can be legitimately debated.

The second example is two badgers mating to produce an offspring that has a different combination of characteristics than either parent. According to the video "evolution ... has officially occurred." This is incorrect. Populations evolve, not individuals. It's quite possible for the individuals in a given generation to have different combinations of traits than either of their parents while the frequency of alleles in the population remains unchanged. Thus, evolution has NOT occurred.

The idea that it's populations that evolve and not individuals is crucial to a correct understanding of evolution and it's a shame that the video promotes a common misconception.

Bug_girl at Skepchick liked this video and so did PZ Myers at Pharyngula.



6 comments:

  1. I recall something about sexual recombination of two butterflies producing a butterfly with a novel pattern on its wings, combining the characteristics of both parents, thus increasing the complexity of the pattern.

    Well, I know Larry will say that's novel variation, not evolution.

    But anyway, does anyone recall the butterfly example I'm talking about? I've searched and can't find a reference for it in the literature.

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  2. This is how to explain the glory of biology??
    Creationists can do better!

    Evolutionism is the attempt to explain the glory of biology found by thinking mankind.
    Its largely a hunch and lines of reasoning that small changes in offspring can lead to populations being changed/prevailing and to such fantastic extents as to turn bugs into buffalos.
    hard to believe I know!

    As Darwin said however i9f theres no block to small changes then theres no block to this possibility.
    A line of reasoning.
    No evidence and the reasoning is not reasonable.

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  3. I don't think it matters greatly, but I don't see how you can avoid the conclusion that mutation - and recombination - are mechanisms causing evolution. If we could stack successive populations during a process of allele fixation, and clustered on a particular allele, it would typically describe a rough cone in this roughly cylindrical stack. At the apex of the cone lies the mutation; at its base lies the point at which its last alternative died. The points where frequency becomes 1/N, or N/N, are just as much a part of the evolutionary series as any other slice, of any thickness, one may choose to cut from the cone.

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    Replies
    1. What we're looking for in a definition is the minimal conditions that meet the criteria for evolution. In that case, the minimum requirements are a population containing variation and time. Thus, mutations isn't part of the DEFINITION of evolution.

      Now, the question is whether mutation counts as a mechanism of evolution. There's a sense in which it does and that's why mutation pressure is a real phenomenon. I don't think this is important enough to count as a real mechanism and most (all?) of the textbooks seem to agree. What we're interested in is whether a given allele will eventually become fixed or lost in a population and mutation can't accomplish this end by itself. (Well, technically it can but it's only a theoretical possibility.)

      As for recombination, that's pretty much irrelevant as far as evolution is concerned. In the long run, all species will eventually reach linkage equilibrium.

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    2. Your lowering mutation as a mechanism for evolution?
      Thats the essential ingredient!
      Without mutation there can not be important crossing of boundaries or organs and so on!
      Colour and size will not and will not persuade people that slime became sheep however long time one had.
      Variation within types is just keeping that type however varied.

      The great evidence for how there is change in nature stares us in the eye.
      its the example of the different types of people.
      Explain that and one has explained much.

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    3. What we're looking for in a definition is the minimal conditions that meet the criteria for evolution. In that case, the minimum requirements are a population containing variation and time. Thus, mutations isn't part of the DEFINITION of evolution.

      Well yes, your definition. But I don't see the merit in excluding the source of that variation, in pursuit of an abstract minimum. I'd favour a more complete definition with optional components, depending on the scale one is looking at. We don't just parachute into the middle of an evolutionary sequence (though population genetics often does). As far as a retrospective view from any given current sequence is concerned, it changed at a particular generation in a particular individual, regardless of the subsequent population shedding of alleles. Change down generations is also evolution - and more so, to the public, than the formulation beloved of population geneticists. It is important to emphasise the population aspect, but not to the exclsuion of all individual factors (individuals, after all, do all the dying and being born).

      As for recombination, that's pretty much irrelevant as far as evolution is concerned. In the long run, all species will eventually reach linkage equilibrium.

      I disagree. It rather depends how close the linkage. Recombinations within a gene, or of gene and control region, create novelty - not to the extent that mutation does, admittedly. As to whether they reach equilibrium 'in the long run', it again depends upon a number of additional factors - epistasis, extinction of alleles of one or other part of the recombinant segments, for example. I may be misunderstanding you, but I see recombination as a significant evolutionary force.

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