December 31, 2005 | David F. Coppedge

Do Guppies Make Good Darwinian Grandmothers?

If a report on EurekAlert is right, some evolutionary biologists used lack of evidence for natural selection as confirmation for evolution.  They predicted guppies would show no evidence of a “grandmother effect” on life history after reproduction, and “that is what they found.”
    The question under study is why evolution keeps aging individuals living if it’s only reproductive fitness that matters in keeping a species going.  Perhaps the aging are worth keeping around if they contribute to the fitness of the offspring (see 07/23/2003 entry).  After admitting that the “granny effect” is not found in many mammals, even among sociable groups, the article said:

Since guppies are livebearers that provide no postnatal maternal care, Reznick et al. predicted the populations would show no differences in postreproductive lifespan–which is what they found.
    Though overall lifespan varied among the populations, these variations stemmed from differences in time allotted only to reproduction.  Postreproductive lifespan, in contrast, showed no signs of being under selection, and appeared to be what the authors called a “random add-on at the end of the life history.”  Random or not, this is the first demonstration of a postreproductive lifespan in fish. (Emphasis added in all quotes.)

The article then stated that whether postreproductive lifespan can be under selection at all is an open question.  But then, it said that this new study helps gain an evolutionary perspective on such matters – including how they relate to humans.

What kind of reasoning says, “we predict there will be no evolutionary natural selection” on a process, then uses the confirmation of the prediction as evidence for evolution?  You can’t have it both ways.  The article stated an evolutionary principle: “For natural selection to shape the twilight years, postreproductive females should contribute to the fitness of their offspring or relatives.”   Notice that word should.  If natural selection is the be-all and end-all of existence, and if nothing makes sense except in the light of evolution, and if most biologists expected there to be a granny effect, then Reznick’s study amounts to falsification.  Grandparents everywhere should be relieved that another evolutionary principle has been falsified, because now their self-worth does not need to be tied to their tubes.
    You can’t bet at the racetrack that an aging Charlie Horse will win because it is more fit, then claim his loss also confirms your prediction.  Charlie Horse is not just a loser; he’s a pain in the arm of science.  Charlie’s hoarse cries for his theory to get to the finish line, or even past the starting gate, are increasingly falling on deaf ears among those who know how to spot winners and losers (see 09/26/2005, 08/15/2005 commentaries).

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Categories: Dumb Ideas, Marine Biology

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