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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
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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