Species concepts in biology morphological species phylogenetic species

Species concepts in biology morphological species phylogenetic species biological species

Morphological species • the familiar “taxonomic” description based on sets of distinguishing characteristics • often works but can be misleading

Morphological species of horned lizards from Futuyma (1998), p. 463

Morphological species • these morphologically similar meadowlarks sing different songs and do not interbreed: they are true species Eastern meadowlark Sternella magna photo from Shaw Creek Bird Supply Western meadowlark S. neglecta photo from USFWS

Morphological species • these allopatric warblers were previously classified as distinct species • but they freely interbreed Audubon’s warbler Myrtle warbler

Maps from USGS Breeding range map of myrtle warbler Breeding range map of Audubon’s warbler

Breeding range map of myrtle warbler Breeding range map of Audubon’s warbler In the Rocky Mountains of British Columbia and Alberta, eastern boreal forest (white and black spruce) mixes with western montane forest (Douglas fir, Engelman’s spruce, and western white spruce)

Breeding range map of myrtle warbler Intergrades between myrtle and Audubon’s warblers result from interbreeding They are now known as the Audubon’s and myrtle races of the yellow-rumped warbler Breeding range map of Audubon’s warbler

The phylogenetic species concept: species are monophyletic groups [tips of the tree are individuals, letters are species, numbers are populations]

Application of the phylogenetic concept helped uncover cryptic species in the copepod Eurytemora affinis

And in African elephants

Biological species • “. . . groups of interbreeding populations that are reproductively isolated from other such groups. . . ” – Ernst Mayr

Biological species • the most widely accepted and useful species concept • problems: – useless for asexual organisms – sometimes difficult to test (e. g. allopatric species)

Biological species • the most widely accepted and useful species concept • problems: – useless for asexual organisms – sometimes difficult to test (e. g. allopatric species)

Biological species • reproductive isolation: an objective criterion that is biologically significant • application of the concept has born a research program

Understanding speciation? study the evolution of reproductive isolation

Reproductive isolating mechanisms • prezygotic mechanisms act prior to hybrid zygote formation – behavioral (mate choice) – gamete recognition • postzygotic mechanisms act on hybrid zygotes – hybrid inviability/sterility – ecological inferiority of hybrids

Prezygotic: behavioral isolation • courtship and mating behaviors diverge dramatically, even between closely related species • Hawaiian Drosophila do not interbreed: females reject courtship of heterospecific males

Mating behaviors can evolve rapidly • Drosophila heteroneura and D. silvestris are sister species, sympatric on Hawaii from Freeman and Herron 2002 D. heteroneura has dumbbellheaded males that butt heads with other males to gain display territories on leks D. silvestris has roundyheaded males that fight other males, Greco-Roman style, for territories

Behavioral isolation • mating calls of frogs and insects are species -specific • females recognize conspecific calls

Prezygotic isolation by gamete incompatibility • marine invertebrates and other organisms that lack mating behavior: species recognition occurs between egg and sperm

Sea urchin bindin: a gamete recognition protein involved in speciation Bindin egg surface • Bindin is an adhesive protein that binds sperm to the egg surface. Without this, fertilization does not occur. • Rapid change of bindin proteins creates barriers to fertilization between species.

Postzygotic reproductive isolation • hybrids are often inviable or sterile

Postzygotic reproductive isolation • hybrids are sometimes ecologically inferior bird predation on hybrids creates abrupt boundaries between Heliconius color races