Transcript Origin of Species

The Origin of Species
Chapter 23
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Outline
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The Nature of Species
Pre and Postzygotic Isolating Mechanisms
Geography of Speciation
Hawaiian Drosophila
Darwin’s Finches
Lake Victoria’s Cichlid Fishes
New Zealand Buttercups
Diversity of Life Through Time
Pace of Evolution
Speciation and Extinction
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Biological Species Concept
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Ernst Mayr
– “…groups of actually or potentially interbreeding
natural populations which are reproductively
isolated from other such groups.”
Individuals that cannot produce fertile offspring are
termed reproductively isolated, and thus members
of different species. (sympatric species)
Application problems
difficult to apply the concept to populations that do not occur together
in nature (allopatric populations)
no meaning for asexual species
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The Nature of Species
Distinctiveness of sympatric species
– Sympatric species are distinctive entities
that are phenotypically different, utilize
different parts of the habitat, and behave
separately. Biological species concept
– If sympatric species commonly exchange
genes, their gene pools should become
homogenized.
– Two species that occur together and
appear to be nearly identical are termed
sibling species.
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The Nature of Species
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Geographic variation within species
– Populations within a species that occur in
different areas and are distinctive may be
classified as subspecies or varieties.
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Prezygotic Isolating Mechanisms
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Prezygotic isolating mechanisms prevent the
formation of zygotes.
Ecological isolation
– Even if two species occur in the same
area, they may utilize different portions of
the environment and thus do not hybridize
because they do not encounter each
other.
 lions and tigers in India
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Tiglon
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Prezygotic Isolating Mechanisms
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Behavioral isolation
– Many birds differ in courtship rituals which
tends to keep species distinct.
Temporal isolation
– varying breeding seasons
Mechanical isolation
– reproductive structural differences
Prevention of gamete fusion
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Postzygotic Isolating Mechanisms
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Postzygotic isolating mechanisms prevent
the proper functioning of zygotes after they
form.
– Hybridization often produces embryos that
die during early development.
– hybrid sterility
– abnormal sex organ development
– chromosome alterations
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Problems with the Biological Species Concept
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Extent of reproductive isolation
– high levels of hybridization
Difficult to apply to species that do not occur
together in nature
Cannot be applied to asexual reproduction
No single definition may be universally
applicable.
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Reproductive Isolation and
Evolutionary Change
Most reproductive isolating mechanisms
initially arise for some reason other than to
prevent reproduction.
– adaptations for changing environments
– selection may reinforce isolating
mechanisms
 Initially incomplete isolating
mechanisms are reinforced by natural
selection until they are completely
effective.
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Reproductive Isolation and
Evolutionary Change
Role of natural selection in speciation
– Reinforcement is driven by natural selection
favoring the perfection of reproductive
isolation.
Random changes may cause reproductive
isolation
– Given long enough periods of time, any two
isolated populations will diverge due to
genetic drift.
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Reproductive Isolation and
Evolutionary Change
Adaptation and speciation
– Adaptation and speciation are probably
related in many cases.
 As species adapt, they accumulate many
differences that may lead to reproductive
isolation.
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Geography of Speciation
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Speciation is a two-part process:
– identical populations must diverge
– reproductive isolation must evolve to maintain the
differences
Allopatric speciation
– Geographically separated populations
appear much more likely to have
evolved substantial differences
leading to speciation.
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Geography of Speciation
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Sympatric speciation
– In recent years, a number of cases have
appeared that appear difficult to interpret
in any way other than sympatric
speciation.
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Sympatric Speciation
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Instantaneous speciation through polyploidy
– individual is born that is reproductively
isolated from other members of species
 polyploidy - more than 2 sets of
chromosomes
 autopolyploidy - all chromosomes
arise from a single species
 allopolyploidy - two species hybridize
Disruptive selection
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Rapid Evolution
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Adaptive radiation - process producing a
cluster of species, occupying a series of
similar habitats, all evolving from a recent
ancestor
– requires both speciation and adaptation to
different habitats
 Adaptation may be driven by the need to
minimize competition for available
resources with other species (character
displacement).
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Darwin’s Finches
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The 14 species of finches Darwin
encountered comprise four groups, all
derived from one similar mainland species,
and radiated in the absence of competition
from other birds.
– ground finches
– tree finches
– warbler finches
– vegetarian finches
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Pace of Evolution
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Punctuated equilibrium
– proposed by Stephen Jay Gould and Niles
Eldredge
 Evolution normally proceeds in spurts,
with long periods of little movement in
between.
 contrasted to the theory of gradual
evolutionary change (gradualism)
 The proposed stasis would be expected in
large populations experiencing stabilizing
selection over long periods of time.
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Macroevolution
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Speciation and Extinction Through Time
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There have been 5 major mass extinctions interspersed
within relatively consistent extinction patterns.
– most famous - end of Cretaceous period (65 mya)dinosaurs went extinct
 Mammals quickly experienced evolutionary radiation.
 Biological diversity tends to rebound after mass
extinctions.
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The Future of Evolution
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Human impacts on the environment will
affect the evolutionary process in many
ways.
– environment alteration
 climate change
 decreased population sizes
 increased genetic drift
 increased extinction
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