Species concepts in biology

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Species concepts in biology

• morphological species
• phylogenetic species
• biological species

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Morphological species

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

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

• these morphologically similar meadowlarks sing
  different songs and do not interbreed they are
  true species

Eastern meadowlark Sternella magna
Western meadowlark S. neglecta
photo from Shaw Creek Bird Supply
photo from USFWS
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Morphological species

• these allopatric warblers were previously
  classified as distinct species
• but they freely interbreed

Audubons warbler
Myrtle warbler
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Maps from USGS
Breeding range map of myrtle warbler
Breeding range map of Audubons warbler
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In the Rocky Mountains of British Columbia and
Alberta, eastern boreal forest (white and black
spruce) mixes with western montane forest
(Douglas fir, Engelmans spruce, and western
white spruce)
Breeding range map of myrtle warbler
Breeding range map of Audubons warbler
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Intergrades between myrtle and Audubons warblers
result from interbreeding They are now known as
the Audubons and myrtle races of the
yellow-rumped warbler
Breeding range map of myrtle warbler
Breeding range map of Audubons warbler
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The phylogenetic species concept species are
monophyletic groups
tips of the tree are individuals, letters are
species, numbers are populations
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Application of the phylogenetic concept helped
uncover cryptic species in the copepod Eurytemora
affinis
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And in African elephants
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Biological species

• ...groups of interbreeding populations that are
  reproductively isolated from other such
  groups...
• Ernst Mayr

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Biological species

• the most widely accepted and useful species
  concept
• problems
• useless for asexual organisms

• sometimes difficult to test (e.g. allopatric
  species)

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Biological species

• the most widely accepted and useful species
  concept
• problems
• useless for asexual organisms

• sometimes difficult to test (e.g. allopatric
  species)

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Biological species

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

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Understanding speciation?

• study the evolution of reproductive isolation

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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

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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

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Mating behaviors can evolve rapidly

• Drosophila heteroneura and D. silvestris are
  sister species, sympatric on Hawaii

D. heteroneura has dumbbell-headed males that
butt heads with other males to gain display
territories on leks
D. silvestris has roundy-headed males that fight
other males, Greco-Roman style, for territories
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Behavioral isolation

• mating calls of frogs and insects are
  species-specific

• females recognize conspecific calls

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Prezygotic isolation by gamete incompatibility

• marine invertebrates and other organisms that
  lack mating behavior species recognition occurs
  between egg and sperm

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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
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Postzygotic reproductive isolation

• hybrids are often inviable or sterile

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Postzygotic reproductive isolation

• hybrids are sometimes ecologically inferior

bird predation on hybrids creates abrupt
boundaries between Heliconius color races