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

from Futuyma (1998), p. 463

Morphological species of horned lizards

Morphological species• these morphologically similar meadowlarks sing

different songs and do not interbreed: they are true species

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Eastern meadowlark Sternella magna Western meadowlark S. neglecta

photo from Shaw Creek Bird Supply 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

Breeding range map of Audubon’s 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

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

D.D. heteroneura heteroneura has dumbbell-has dumbbell-headed males that butt heads headed males that butt heads with other males to gain display with other males to gain display territories on leksterritories on leks

from Freeman and Herron 2002

D. silvestrisD. silvestris has roundy- has roundy-headed males that fight other headed males that fight other males, Greco-Roman style, males, Greco-Roman style, for territoriesfor 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 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.

Bindin

egg surface

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