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.



12 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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    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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  4. Recombination, even without mutation, is an extremely important mechanism of evolutionary change. Look at the malaria resistance in humans for a clear real life example of this.

    One sickle cell gene makes you resistant, two gives you sickle cell anemia.

    When you're dealt a hand of cards, some hands are better than others, not because the individual cards are better but because the combination of cards is better. Certain allele combinations produce unique traits which can be selected for above other traits even without new mutations coming in to existence.

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    1. Recombination has nothing to do with the fact that heterozygotes for the sickle cell allele have an advantage in some environments. That would still be true even if there was no recombination in humans.

      You're talking about the possible advantage of sex in the sense of meiosis and the fusion of haploid cells from different organisms in order tp produce a new organism. While this seems to be important for organisms that enjoy sex, it can't be part if the definition since there are millions of species that have evolved quite well for billions of years without sex.

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    2. As far as malaria resistance goes, I believe you are right in saying that simple recombination isn't possible in that case. If I remember correctly they are two alleles of the exact same gene. If that is the case, you would need a gene duplication event first, then a recombination event (or a mutation of the newly duplicated gene) to make malaria resistance permanent on a single chromosome.

      In the case shown in the video however, we have a hypothetical situation with two different genes, one which codes for claw length, one which codes for paw width.

      We have two alleles for each: long claw, short claw; and wide paw, narrow paw.

      The wide paw allele could certainly come together with the long claw allele in a simple recombination event. This would create a chromosome which now codes for an entirely new trait which I'll call "shovel hands". This is a new trait which neither parent had. Sexual reproduction can therefore produce evolutionary novelty with no need of mutation. It's pretty awesome!

      If you want to exclude sexually produced novelty as a form of evolution simply because not all creatures have sex, then by that logic you would also need to exclude writing as a form of communication because not all people can read and write.

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  5. How about:
    Evolution is the change of frequencies of genes (in sensu Williams) in populations.
    I've always liked that definition (and I like Williams abstract definition precisely because it allows evolution to be defined so concisely. I'm well aware I'm in for a lot of butthurt here, but we need a decent word for that concept).

    I would drop the "across generations", because it's not neccessarily clear what constitutes a generation and leave it at change. One could add a "through time", but that's somewhat redundant.

    I would drop "heritable traits" in favor of Williams genes, because what we generally care about are histogram distributions and for other traits they are trivially at or close to uniform distributions.

    "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."

    Take Williams definition, note how wide the range of things that qualify as genes under it is and that becomes incorrect. For instance ancestor-decendent relationships are genes in this sense. In this case we know that neither parent badger is a descendent of themselves and therefore the frequency of the "is a descendent of daddy badger" allele is less than 1. The offspring badger is a descendent, so for her the frequency is 1 and the resultant weighted mean is the new frequency, which should be higher than it was previously.

    Even if you restrict yourself to genomic genes, the only way for the offspring not to affect allele frequencies is by being a homozygote for fixed alleles (do this for all of them and you are talking clones with µ=0) or a heterozygote for alleles with frequencies of precisely 0.5. That's bloody unlikely.

    "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.)"

    Are we only interested in fixation and loss? What about cases where heterozygote advantage maintains a polymorphism with high frequencies for two alleles and time ti fixation can be quite long? Ultimately if you are only interested in fixation or loss you are interested in an absorbing state at which the population would stop evolving (if there were no novel mutations). That's basically the least interesting state a population could be in...

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    1. There's no way we can change the definition of "gene" in that way. It makes no sense to refer to each allele as a gene. It also makes no sense to refer to things like origins of replication and SARs as genes. Not only that, we often talk about the evolution of junk DNA and that doesn't qualify as a "gene" by any reasonable definition of the word.

      I'm aware of the fact that there are some cases of balancing selection. It's the exception that proves the rule. We are mostly intetested in fixation.

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    2. In a lot of ways it's only reverting to Johannsens original definition. Population genetics had genes before there even was molecular genetics and to give an obvious example, amino acid sequences (also genes in sensu Williams) show the same general evolutionary dynamics as genes in the molecular sense.

      Anyway, the key thing here is that we need some term for this concept, simply because it's useful for structuring thought about evolution. Phylogenetics tries to figure out ancestry (and that's something we can treat as Williams genes). Of course that's not some physical part of organisms. So we use other genes in this sense (including classical apomorphies), which are informative about phylogenetic relationships. What makes them informative? Linkage disequilibrium between them and the ancestry genes!

      Balancing selection was not my key contention with the fixation part. The main point is that population resampling enters an absorbing state at fixation. That's when nothing of interest happens, so to look at that specifically is to ignore the part where populations actually evolve...

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