Chapter 16. Mechanisms of speciation. The three species concepts outlined in this chapter all involve first population isolation, then divergence, and later reproductive isolation.

The biological species concept (BSC): species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups. This concept is problematic for allopatric populations, fossil and asexual species, as well as bacteria and archaea where gene flow (lateral gene transfer) occurs among divergent lineages and causes new species to evolve. The BSC is overly deterministic in that reproductive isolation is somehow a goal of evolution.

The morphospecies concept (MSC): species comprise morphologically similar individuals that are distinct from individuals of other species. Morphological distance analysis often does not detect cryptic species, and species of bacteria, archaea, and fungi that may have little phenotypic variation.

The phylogenetic species concept (PSC): species are the smallest detectable monophyletic assemblages of populations. Molecular phylogenetic analysis is required, and this will be impossible to apply to fossil species. The copepod and elephant examples underscore that biodiversity will be underestimated without the PSC.

Population isolation. Geographic isolation occurs via dispersal or vicariance. Dispersal (e.g., Drosophila on the Hawaiian Islands) has a phylogenetic signature of a founder event. Vicariance (e.g., snapping shrimp along the Panamanian Isthmus) has a phylogenetic signature that does not implicate a source area. Both dispersal and vicariance reveal that allopatric speciation via ecological or geographic isolation is very common. Common genetic isolation mechanisms include mostly polyploidy (4n x 2n = sterile 3n offspring).

Divergence most commonly is the result of mutation and drift. The text contradicts this (pp. 616-617) and places most emphasis on natural selection and sympatric divergence, e.g., the Apple and Hawthorne maggot flies, the examples listed in Table 16.1, and the pea aphids and threespine sticklebacks. Sexual selection (pp. 621-623), however, must be the common cause of divergence via selection. The chapter doesn’t emphasize this, however. Sexually selected secondary sex traits are arbitrary and not prone to convergent evolution. Any discriminating mating behavior by females will reduce gene flow between populations and cause rapid divergence. The most species-rich taxonomic groups - insects, birds, flowering plants, teleost fishes, and mammals - have the common signature of divergence by sexual selection: the females among sister species have similar phenotypes whereas the males differ by secondary sex traits (in plants male traits involve petal and stamen morphologies).

Reproductive isolation includes mostly geographical isolation. Selection against the hybrids is a secondary cause, as among the benthic and limnetic forms of the threespine or between pea and alfalfa aphids. Reproductive isolation may occur late during population divergence, if ever. Indeed, most species have a close relative with which they can interbreed and produce fertile progeny when brought together artificially.

Text questions: 2 (see below for criteria and pros & cons), 3 (application of the PSC nearly always results in the detection of more species), 4 (sex in bacteria involves lateral gene transfer or gene flow that gives rise to new species; gene content shaped by ecological context defines bacterial species), 5 (dispersal implies source-founder populations, vicariance implies an original population divided into two or more descendant population and involving no dispersal), 6 (the formation of the Panamanian Isthmus facilitated dispersal for terrestrial organisms and vicariance events for marine – the opposite may be the case with respect to the presence of the Panama canal), 7 (no source area can be detected), 8 (older islands are always optimized at the older nodes and younger islands at the younger nodes of a phylogeny of Hawaiian endemics), 10 (triploids produce sterile gametes), 11 (divergent descendants can merge back into one species via hybridization, or not if selection occurs against the hybrids), 13 (discriminating mating behaviors by at least the female form a strong barrier to gene flow and cause divergence of male secondary sex traits), 14 (disruptive selection perhaps by selection against hybrids and the different forms are separate ecologically and spatially, however slight), 19 (The spiny lizards probably fit the phylogenetic species concept the best given that females are all phenotypically indistinguishable and that individuals of different species probably can produce fertile hybrid offspring if brought together - only sexual selection can give rise to related allopatric species where just the males differ in phenotype or secondary sex traits).

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