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Evolution in a nutshell an alternative outline on evoution and some consequences concerning valuations by Gregor Kjellström
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The unit of selection In order for the process to be able to select something, there must exist some performance criterion or measure of fitness (the fitness of the individual) separating those who survive from those who do not; and here different definitions are available. The opinion that a particular gene or allele may have a fitness of its own and be a unit of selection is widely spread. An example is Fisher's fundamental theorem of natural selection, 1930 (see Maynard Smith), where the mean fitness is determined as a mean over the set of genes in a large population. The theorem states that “the rate of increase of mean fitness of any organism at any time is equal to its variance in fitness at that time”. Certainly, if the evolution of some organism has been driven in absurdity, then both the phenotypic variance and the increase of mean fitness may simultaneously go to zero, and the theorem is apparently valid. But a population may reach a state of selective equilibrium, in which case the increase of mean fitness is equal to zero, but not necessarily the phenotypic variance. So, this theorem can hardly be a fundamental truth (cp. theorem 6.2.2). As another example we may mention the definition of fitness given by Maynard Smith in the following way: ”Fitness is a property, not of an individual, but of a class of individuals – for example homozygous for allele A at a particular locus. Thus the phrase ’expected number of offspring’ means the average number, not the number produced by some one individual. If the first human infant with a gene for levitation were struck by lightning in its pram, this would not prove the new genotype to have low fitness, but only that the particular child was unlucky.” Even if the definition is useful in breeding programs, it can hardly be of any use as a basis of a theory of an evolution selecting individuals. It seems as if this definition denies the fitness of the individual. Nevertheless, the individual fitness is needed, because otherwise the “expected average number of offspring” from a certain class of individuals can’t be determined. In addition, if it is possible to define the fitness of an allele as an average of individual fitness values, then, it must be possible to define the mean fitness of a whole population from such values. So it can’t be forbidden to use the fitness of the individual. In this pamphlet the fitness of the individual is defined as a probability function dependent on individuals (not genes), which – in principle – makes it possible to take even the stroke of lightning into consideration. |
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