Biologists like memes (like memes)

From: Chris Taylor (
Date: Wed Jul 04 2001 - 09:59:45 BST

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    If you can't be arsed skip to the last paragraph (nothing earth
    shattering - just nice to see it presented in Nature, without too many

    Predicting the future

    Institute of Genetics, University of Nottingham, Nottingham NG7 2UH, UK.
    Nature 411, 999 (2001) © Macmillan Publishers Ltd.

    The idea of fitness is central to evolutionary biology. The British
    philosopher Herbert Spencer characterized Darwin's theory as "the
    survival of the fittest" — the survival of the individuals that best fit
    their environment. In the 1930s, J. B. S. Haldane, Sewall Wright and
    Ronald Fisher quantified 'fitness' to express the strength of natural
    selection. Thus, if a mutant genotype suffers a 10% selective
    disadvantage relative to the wild type, it has a fitness of 90%.

    This concept of fitness has prompted sterile debate along the lines
    that, as natural selection states that the fittest survive, and as 'the
    fittest' is defined as those that survive, the whole concept of natural
    selection is tautological. This misses the point: the main thing is that
    it is extremely useful to have a quantifiable, measurable description of
    a genotype.

    Fitnesses of genotypes tend, empirically, to be roughly constant.
    Calculations based on this assumption give good predictions of the time
    course of the spread of advantageous alleles in populations. Fisher
    showed that when the fitness of each genotype is constant with time, the
    mean fitness of a population increases. Fitness describes the present
    success of a genotype, not the probability of its survival in the long
    term, which might depend on its capacity to adapt to new environments.

    One fundamental misunderstanding is that any differences in survival or
    reproduction between individuals reflect differences in fitness. This is
    not what fitness means. Fitness represents an expected outcome, and what
    actually happens in small populations differs from expectation because
    each generation's genotypes represent a sample, with an attendant
    sampling error, of the gametes produced by the previous generation; this
    is the basis of the phenomenon known as 'genetic drift'. The fitness of
    a genotype is related only probabilistically to real events; weakly
    advantageous mutations are usually lost by chance. Weakly deleterious
    mutations are much less likely to spread than advantageous alleles, but
    may arise much more often. Indeed, it is probable that most evolution of
    amino-acid sequences has occurred by fixation of weakly deleterious

    Fitness is hard to measure, particularly in wild populations, because it
    summarizes expected survival and reproduction. In particular, measuring
    'lifetime reproductive success' by painstakingly tracking cohorts of
    individuals throughout their lives gives data that are difficult to
    interpret. Each individual in a sexual wild population has a genotype
    that, as an entity, is unique. An individual's genotype will have a
    fitness, which will thus be the individual's fitness. But random events
    cause the lives of individuals to differ, even those with identical
    fitnesses, and variation in the lifetime reproductive success of
    individuals does not represent variation in fitness between their
    genotypes. The only way to measure differences in fitness is to measure
    differences in mean survivorship and in mean reproductive rate between
    classes of individuals (groups of individuals that differ biologically).
    If one is interested in evolution, the only interesting classes are
    those that differ in their genotypes. A genotypic class, for example,
    might include all individuals that share the same genotype at a single
    genetic locus.

    Now, if one is looking at the mean survival and reproduction of
    genetically defined classes of individuals, there is no point in looking
    at lifetime reproductive success. It is not worth measuring the
    fertility of young adults, for example, and then monitoring the
    fertility of the same individuals year after year as they grow older.
    You can find out all you need to know by looking at the fertility of
    different age classes in the same year — the fact that the individuals
    are different has no effect on the expected mean fitness.

    Many view natural selection as an environmental force that acts on the
    phenotypes of populations, by analogy with artificial selection in
    animal or plant breeding. Although differences in genotypic fitness are
    caused by differences in phenotype, the widespread occurrence of
    pleiotropy — whereby a single genetic change has multiple phenotypic
    effects — means that it is very difficult to identify the true ways in
    which fitness differences arise. The most obvious phenotypic differences
    may not be the most important.

    Does Fisher's theorem predict whether organisms become better adapted to
    their environment with time? In principle, yes, but there are important
    caveats. First, environments change. It is futile to try to explain
    human behaviour in adaptive terms, as the environments in which the
    genes responsible evolved were so different. For example, it may be that
    bad drivers crash cars more often, and so there is natural selection
    against individuals who are poor at driving. But this has no causal
    connection with the fact that humans can drive cars. The genes for this
    skill were not created by selection against individuals who crashed. Of
    course, nobody would suggest that they were, yet there are consistent,
    misguided attempts to explain other aspects of contemporary human
    behaviour in terms of the consequences, in effects on fitness, of those
    behaviours in modern environments.

    Second, although genes that improve survival tend to increase fitness,
    so do genes that increase sexual attractiveness — such as those that
    create the peacock's tail. A population of peacocks that did not evolve
    a spectacular tail might have been more successful in terms of
    population density or the probability of survival. Equally, in a
    population in which random genetic changes reduced male fitness to make
    all males slightly less attractive to females, the females would settle
    for second-best, and the species would get along fine. There is no
    necessary agreement between mean fitness and ecological variables.

    I have used the term 'fitness' to describe differences between genotypes
    within species. What about the 'fitnesses' of different entities? Some
    species spread at the expense of others, and some ideas (memes) become
    better known by imitation. Can the spread of religion, for example, be
    explained in terms of 'memes' with high 'fitness', as some believe?
    Logically, 'fitness' could be used for these other entities, in which
    case its use would remain as circular and non-explanatory as it is for
    genotypes. But here, fitness is not constant over time, so there is no
    pragmatic justification for it as a predictive mathematical tool.

    References 1. Smith, J. M. Evolutionary Genetics 2nd edn (Oxford Univ.
    Press, Oxford, 1998).

     Chris Taylor ( »people»chris

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