Speciation 10/22/2012
When gene flow is reduced between populations, they may diverge genetically as a result of
mutation, natural selection and genetic drift
Gene divergence may eventually lead to speciation, the creation of new species
How are species defined and identified?
Species – an evolutionarily independent population or group of populations
Gene flow eliminates genetic differences among populations, so evolutionary independence
starts with lack of gene flow
Mutation, natural selection, and genetic drift begin to act on populations independently
Allele frequencies and other characteristics diverge, over time populations become distinct
species
Biologists use three approaches to identify species
The Biological species concept
Populations are evolutionary independent if they are reproductively isolated from each other, i.e.
they do not interbreed
Therefore, no gene flow occurs between these populations
Mechanisms that stop gene flow can be either
Prezygotic – isolation occurs when individuals of different species are prevented from mating
Habitat isolation – populations live in different habitats and do not meet
Behavior isolation – little or no sexual attraction between males ad females
Temporal isolation – mating or flowering occurs at different seasons or times of day
Mechanical isolation – structural differences in genitalia or flowers prevent copulation or pollen
transfer
Gametic isolation – gametes do not form zygote because of incompatibilities
Postzygotic – prevent hybrid zygote from developing into a viable, fertile adult
Reduced hybrid viability – zygotes fail to develop or fail to reach sexual maturity
Reduced hybrid fertility – hybrids fail to produce functional gametes
Hybrid breakdown – offspring of hybrids have reduced viability or fertility
Weakness of concept – reproductive isolation cannot be evaluated in fossils or in species that
reproduced asexually, and the concept is usually only applied to populations that reproduce
geographically 2 Speciation
The Morphospecies concept
Identify evolutionarily independent lineages by differences in morphological features
Distinguishing features are most likely to arise if populations are independent and isolated from
gene flow
Weakness of concept – features used to distinguish species under this concept are rather
subjective and has led to errors such as females and males of same species being classified as
different species
The Phylogenetic concept
Based on evolutionary history of populations
On phylogenetic trees, an ancestral population plus all of its descendants is called a
monophyletic group or clade
A species is defined as the smallest monophyletic group on a tree that compares populations
Each tip is a phylogenetic species
Phylogenies are currently available for only a tiny subset of populations on the tree of life
In practice, biologists us all three species concepts, the biological, morphospecies, and
phylogenetic
Species definitions in actions
Subspecies – populations that live in discrete geographic areas and have their own identifying
traits but are not distinct enough to be considered a separate species
Hybrid zones
Sometimes hybrid offspring of related species re healthy, capable of breeding, and possess traits
that are intermediate between the two parents
A geographic ar
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