Neo-Darwinism / R.J. Berry.

  • R. J. Berry
Date:
1982
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Neo-Darwinism / R.J. Berry. Source: Wellcome Collection.

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    §6.2 THE MECHANISM OF SPECIATION 49 Table 6 Reproductive isolating mechanisms EXTRINSIC: may or may not involve genetical factors 1 Geographic separation 2 Other spatial separation (e.g. certain parasites) INTRINSIC: has a genetical base 1 Asexuality, etc. 2 Premating or prezygotic (prevent the formation of hybrid zygotes) a Ecological or habitat isolation b Seasonal or temporal isolation ('mate avoidance' e.g. in flowering or hatching time) с Sexual or ethological isolation ('mate rejection') d Mechanical isolation (i.e. structure of genitalia or flower parts prevents copulation or transfer of pollen) e Isolation by different pollinators (related species of flowering plants may attract different insects as pollinators) / Gametic isolation (in organisms with external fertilization, gametes are not attracted to each other; with internal fertilization, the gametes or gametophytes of one species may be inviable in other species). 3 Postmating or postzygotic a Hybrid inviability (hybrid zygotes have reduced viability or are inviable) b Hybrid sterility (F, hybrids fail to produce functional gametes) с Hybrid breakdown (Fj or backcross hybrids have reduced viability or fertility) where a zygote is cared for by its parent, since hybrid zygotes can be discriminated against by the parent). In a few organisms, the effects of individual genes on pre- and post-zygotic barriers have been worked out. For example, there is inherited variation in Drosophila melanogaster for mating speed, the duration of copulation, and the extent of assortative mating, all of which contribute to prezygotic barriers. 6.2.2 The speed of speciation A well-worked example where parapatric (if not sympatric) speciation has taken place involves species of the land snail Bartula on the volcanic Moorea island in French Polynesia. These were originally regarded as the result of random differentiation (through genetic drift) on isolated parts of the island (see section 2.2.1) but it is now clear that an early colonizing group separated into races locally adapted to different habitats, independent of geographical isolation, and that these have diverged to form at least nine different species capable of some gene-flow but in general behaving as distinct species. The best examples of speciation occur where island populations have significantly changed from their mainland ancestors. Darwin himself was impressed by the diversity of fauna on the Galapagos archipelago. For example, there are thirteen species of ground finches (Geospizinae) which have adapted to different habitats. Presumably this adaptation occurred on different islands, and then specialized finches successively colonized other islands; there are now ten finch species on the main island of Indefatigable. In contrast, there is only one finch species on Cocos, a single high tropical island where there has been no
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