How Knowledge Grows by Chris Haufe

Author:Chris Haufe
Language: eng
Format: epub
Tags: evolutionary epistemology; kuhn; cultural evolution; Darwinian population; exemplar; disciplines; multiple discovery
Publisher: MIT Press

5.4.2 Gene Flow

The foregoing factors are sufficient to cause a full-scale branching event in sexually reproducing populations under random mating, but only if the value of each factor is on the high end. Otherwise, if grape-eaters and toast-eaters continue to mate despite their divergent lifestyles, the population of toast-eating variants will generally be perpetually pulled back from the toast-eating trajectory and toward the population mean for the ex hypothesi larger parent population of grape-eaters.

There are a few ways of preventing the swamping of toast-eaters through random mating. Most models of ecological divergence get around this obstacle by positing a nonrandom component to mating, typically in the form of mate preferences (Bolnick and Fitzpatrick 2007, 469). A preferable, more conservative approach9 would be to introduce nonrandom mating through spacial considerations, along the following lines: toast-eaters like hanging out where there are toast to eat. If toast territory and grape territory share less than 50% of their respective spaces, toast-eaters are more likely to encounter (and mate with) other toast-eaters. Either way, the toast-eating subgroup is able to preserve the progress it has made toward the toast-eating optimum, because its mean phenotype can evolve unencumbered by the effects of the orthogonal optimizing forces governing grape-eaters (Fry 2003).

The simplest and most intuitive model of ecological speciation is, in fact, the process described in section 3.1. Here, assortative mating preferences evolve in response to selection against mating with individuals who might drag one’s offspring toward intermediate phenotypic values and thus reduce the fitness of those offspring. It pays toast-eaters to mate with other toast-eaters because that is more likely to advance their offspring toward the toast-eating optimum than mating with grape-eaters. Likewise, it pays grape-eaters to mate with other grape-eaters. In general, “individuals who mate randomly risk producing intermediate offspring with lower fitness, indirectly favoring individuals that mate with their own ecotype” (Bolnick and Fitzpatrick 2007, 470). Those with sufficiently strong mate preferences will be favored by selection, because their offspring will tend to have higher fitness.

Regardless of which model one favors, each of them attempts to devise some way of interrupting gene flow between subgroups. It is generally recognized that the cessation of gene flow is a critical element in the development of a variant subgroup into a full-blown new species. Canonical models of speciation are “allopatric,” meaning that genetic divergence occurs because subgroups are confined to different areas and mating between subgroups is impossible. Because ecological models of speciation aim at describing conditions under which speciation occurs even when members of divergent subgroups could still mate with each other in principle, such models generally look for ways in which mating between subgroups is permitted but discouraged.

The reason why cessation of gene flow is thought to be so important for speciation has to do with the homogenizing effects of random mating in large populations. This principle is reflected in our discussion above as well as in section 3.1. In such populations, effectively random mating ensures that members’ genomes tend to stay relatively similar to one another.

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