Gene Expression

Selection at work?

A story in The Economist, titled the fertility bust (in the “Charlemagne” column), offers this interesting tidbit:

Germany is something of an oddity in this. In most countries with low fertility, young women have their first child late, and stop at one. In Germany, women with children often have two or three. But many have none at all.

In other words, the mean for Germany is low, but reproductive variance (or skew) is high. With a large proportion of the population not reproducing, and another proportion reproducing above replacement, this is basically very close to truncation selection if there is a phenotypic correlate (eg; only fat women reproduce within the population). If the variation in reproductive output has no genetic correlation than it is irrelevant from a microevolutionary perspective. But if the reproductive value is correlated with geography or some other parameter than it is likely that there is some genotypic bias in terms of fitness. With 80 million people it seems unlikely that Germany will suffer from a mutational meltdown, so if fitness is heritable (that is, the fecund transmit their fecundity to the next generation) a bounce back seems inevitable, all parameters held equal.


  1. #1 Dan Dare
    February 21, 2006

    Yes, I have always thought so too.

    If you introduce a new challenge like chemical contraception, initially it devastates the population. But some individuals are resistant to it. Perhaps because of religious or philosophical reasons, perhaps because they just love kids and long for a big family. Anyway, even though the contraceptives are there, they chose not to use them.

    End result: If any of that choice is heritable then these trends will be accentuated in the next generation. In time resistance of one form or another to contraception will emerge.

    There is no difference here between contraception and the effects of a new insecticide or antibiotic. In time you will get resistance.

  2. #2 Pithlord
    February 22, 2006


    Does that mean Malthus will end up being right after all?

  3. #3 Boronx
    February 22, 2006

    My firm belief is that contraception has caused the stupid, lazy, forgetful or superstitious to increase in fitness relative to everyone else

  4. #4 razib
    February 22, 2006

    boronx, probably.

  5. #5 Dan Dare
    March 1, 2006

    Sorry Pithlord, didn’t see your comment.

    Good question.
    Maybe by then we will have space colonization or something.

  6. #6 greg
    March 1, 2006

    Re Dan’s comments and contra Boronx:

    Perhaps seminal fluid that degrades latex?

    Uterine walls unaffected by estrogen?

  7. #7 Dan Dare
    March 1, 2006

    You are right Greg, that could happen. But I was thinking that anything like that could be defeated by further advances in technology.

    The reason we want contraception today is because our sex drive is stronger than our nurturing instinct. At least for many if not most people.

    What if it was the other way around. If no-one cared much about sex, they just wanted kids. Then they wouldn’t use contraceptives even if they were available. Improving the technology probably wouldn’t even be attempted.

    So a mutation that had that effect would be unstoppable even in an age of available contraception.

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