Macroevolution: Species Formation

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Allopatric speciation

Two models of allopatric speciation.

The Kaibab squirrel (Sciurus aberti kaibabensis, left) became geographically isolated from the common ancestor with its closest relative, the Abert squirrel (Sciurus aberti aberti, right) in the North Rim of the Grand Canyon about 10,000 years ago. Since then, several distinguishing features, such as the black belly and forelimbs have gradually evolved.

Differentiation in the butterfly, Erebia epiphron in the European mountains. Different colors designate different races or forms. Note that these races evolved in geographical isolation, that is, in allopatry.

Circumpolar ("around the pole") distribution of the gulls in the Larus argentatus group. Populations from Europe to eastward can interbreed, but in northern Europe, when the circle closes from the west, the two gulls (L. fuscus B3, B4, and L. argentatusA2) are reproductively isolated. Such species complexes are called ring species.

Allopatric speciation in North American freshwater fish, called shiners.

When two populations that were evolving in geographical isolation come in contact they may hybridize, but the gene flow can be very restricted depending on the degree of genetic differences. In Europe, the Hooded Crow (Corvus corone cornix) and the Carrion Crow (Corvus corone corone) are two subspecies of the same species, and their hybrid zone is very narrow.

Sympatric speciation

Sympatric speciation: differentiating and acquiring reproductive isolation within the same area (sym=together, patria=homeland)

In the apple maggot fly (Rhagoletis pomonella), host specificity takes the place of geographical isolation. Accidental mistakes in oviposition site selection lead to the establishment of sympatric populations of this species that did not interbreed due to their high fidelity to their host plant.

Pollinator switching in species with highly specialized pollination mechanism may also lead to sympatric speciation. The two orchids above are very similar and have completely overlapping ranges, yet are pollinated by different species of solitary bees. In this genus, pollinators are attracted by the flowers' species-specific chemical attractants that mimic the female pheromones of the targeted insects. Left, Yellow-bee Orchid (Oprys lutea), right, Ophrys sicula. The latter species was regarded as a variant of the former for long.

Sympatric speciation by polyploidy

Two mechanisms leading to polyploidy. Top, autopolyploidy: producing a tetraploid species from unreduced gametes and subsequent selfing or cross-fertilization with a conspecific plant. Bottom, allopolyploidy: formation of a tetraploid species from unreduced gametes and cross-fertilization between two different species. Note that the production of unreduced gametes are essential to polyploid speciation.

Three diploid Tragopogon species introduced from Europe some 200 years ago gave rise to new tetraploid species by hybridization.

Speciation and chromosomal rearrangements

Adaptive radiation

When one or very few species give rise to many new species within a relatively short period of time.

Six of the thirteen species of Darwin's finches.

The Hawaiian honeycreepers present another excellent example of adaptive radiation. Unfortunately, several species of this diverse group have gone extinct.

In the group of the Hawaiian silverswords (plants in the sunflower family), one founding species gave rise to several, phenotypically very different species that not only look different, but also occupy very different habitats.

Three representatives of the many cichlids in Lake Victoria, East Africa. Adaptive radiation resulted in an amazing diversity with hundreds of different species in these freshwater fishes.

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