Macroevolution: Speciation and Phylogenesis

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Differentiation among populations

Local populations will inevitably differentiate (i.e. become more different in their allele frequencies) due to the five factors previously discussed.

Local differentiation in the land snail,Helix aspersa, was extensively studied.

In Bryan, Texas, snail populations have been differentiated along very short distances, such as on different sides of streets, due to very limited migration and chance events. The pie diagrams show the frequencies of different alleles.

Local phenotypic differentiation in the garden snail (Cepaea hortensis) is caused by differential predation in the various habitat types. Birds, mostly thrushes are preferentially feeding on snails that have conspicuous shell color and/or banding patterns.


Large-scale variation among populations

Allen's rule

Allen's rule: The size of extremities (limbs, tail, ear, etc.) increases from colder to warmer climates in the same species or in closely related species.

The sizes of extremities of the jackrabbit (left) of the Southwest and the arctic snowshoe hare (right) reflect adaptations to different temperature regimes.

Bergmann's rule

Bergmann's rule: The body size of organisms within species (or in closely related taxa) increases from warmer to colder climates.

Fact: The surface-to-volume ratio decreases for larger objects. Smaller objects lose heat faster, as a consequence.

Accumulation of genetic differences that manifest themselves in the phenotype may lead to the formation of clines, races and eventually new species.

Clines, races and race formation

Species formation

Different species, such as these meadowlarks, can be very similar in appearance, yet they do not interbreed.

Sometimes phenotypically different populations, such as these warblers that were earlier regarded as distinct species, can easily interbreed and have fertile offspring. Left, myrtle warbler, right, Audubon's warbler.

Reproductive isolating mechanisms

Phenotypic differences may arise or may become more pronounced due to selection for avoiding gene exchange or hybridization between populations. This phenomenon is called:

Reproductive Character Displacement

Character displacement among different species of finches in the Galapagos. Due to local competition, certain characters, in this case bill depth, changed in a way that allow these two species to coexist.

Character displacement within species

In Drummond's phlox (Phlox drummondii), change in flower color occurs when it is in sympatry with a closely related species, the pointed phlox (Phlox cuspidata). The color of the pointed phloxes' flowers is pink, whereas that of Drummond's phlox is red (area in red). When the ranges of the two species do not overlap, the flowers of Drummond's phlox are almost identical to the color of the flowers of the pointed phlox. Though hybrids do occur in sympatry, they are sterile, and therefore production of hybrid seeds decreases the reproductive success of the parental plants. This color change increases the morphological difference between the two species reducing the occurrence of mistaken interspecific pollen transfer by pollinators.

Large-scale Evolutionary Patterns

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