Many biologists' prime concern are traits characterizing the sort of individuals they can hold in their hand instead of traits of underlying clonally reproducing genes, of which the visible bearers are but uneasy coalitions. This section considers how the invasion fitness of genes relates to the traits of their carriers. Part of that story is contained in the developmental map from genotypes to traits and in the ecological processes translating traits into demographic parameters. Here, it is only considered how transmission genetics and demography conspire in determining invasion fitness and that only for a few ecological scenarios akin to those usually considered in population genetics: unstructured populations in discrete time or structured ones with a single birth state and nonfluctuating environmental attractors, all with randomly uniting gametes.
A second aim of this section is exemplifying the calculation of invasion fitness when there is more than one birth state.
The arguments will be couched in the well-honed formalism of population genetics, as this is far more efficient than working in terms of the population dynamics of alleles. (The reason for the earlier stress on the population dynamical viewpoint is that the latter is conceptually more encompassing.) The initial growth ratio of a population of mutant alleles is independent of whether this ratio is calculated from the approximate dynamics of low allele frequencies or from the dynamics of expected population sizes.
The trait vector produced by a genotype G will be denoted as XG with f (XG|E), also abbreviated as fG the corresponding average per capita lifetime macrogametic output, and m(XG|E), abbreviated as mG, the average per capita lifetime output of microgametes times their fertilization propensity. Thus, fG equals the average number of kids mothered by a randomly chosen G-individual and mG is proportional to the average number of kids fathered.
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