Download Speciation

Working with the
Biological Species Concept
• Speciation is a two-part process
– 1. Identical populations must diverge
– 2. Reproductive isolation must evolve to maintain these
differences
• Speciation occurs much more readily in the
absence of gene flow
– This much more likely in geographically isolated
populations
• Populations can become geographically isolated
for several reasons
Consider how the amount of genetic
divergence (change) forms a
continuum:
Microevolution
small changes
Macroevolution
large changes
Microevolution = adaptation
Macroevolution = speciation
The Evolution of Species
• Significant changes in the gene pool
could lead to the evolution of a new
species over time.
• The evolution of new species, a process called
speciation occurs when members of similar
populations no longer interbreed to
produce fertile offspring within their
natural environment.
The Evolution of Species
• Geographic isolation occurs whenever a physical
barrier divides a population.
• A new species can evolve when a population has been
geographically isolated.
Reproductive isolation can result in speciation
• Reproductive isolation occurs when formerly
interbreeding organisms can no longer mate and
produce fertile offspring
• A change in chromosome numbers- polyploidy
can lead to speciation
Isolating Mechanisms
• Reproductive isolating mechanisms are the barriers
that prevent genetic exchange between species
– Prezygotic isolating mechanisms
• Prevent the formation of zygotes
– Postzygotic isolating mechanisms
• Prevent the proper functioning of zygotes after
they have formed
Prezygotic barriers:
Geographic isolation
Ammospermophilus spp
• Species occur in different areas
– physical barrier
– allopatric speciation
• “other country”
Harris’s antelope
squirrel inhabits
the canyon’s
south rim (L). Just
a few miles away
on the north rim
(R) lives the
closely related
white-tailed
antelope squirrel
Prezygotic barriers:
Ecological isolation
• Species occur in same region, but occupy
different habitats so rarely encounter each other
– reproductively isolated
2 species of garter snake, Thamnophis,
occur in same area, but one lives in water &
other is terrestrial
lions & tigers could
hybridize, but they
live in different
habitats:
 lions in grasslands
 tigers in rainforest
Prezygotic barriers:
Temporal isolation
• Species that breed during different times of day,
different seasons, or different years cannot mix
gametes
– reproductive isolation
– sympatric speciation
• “same country”
Eastern spotted skunk
(L) & western spotted
skunk (R) overlap in
range but eastern mates
in late winter & western
mates in late summer
Prezygotic barriers:
Behavioral isolation
• Unique behavioral patterns & rituals isolate species
– identifies members of species
– attract mates of same species •
• courtship rituals, mating calls
• reproductive isolation
Blue footed boobies mate
only after a courtship display
unique to their species
Prezygotic barriers:
Mechanical isolation
• Morphological differences can prevent
successful mating
– reproductive isolation
Even in closely related
species of plants, the
flowers often have distinct
appearances that attract
different pollinators.
These 2 species of monkey
flower differ greatly in
shape & color, therefore
cross-pollination does not
happen.
Plants
Mechanical isolation
Animals
• For many insects, male &
female sex organs of
closely related species do
not fit together, preventing
sperm transfer
– lack of “fit” between sexual organs:
hard to imagine for us… but a big issue for insects with
different shaped genitals!
Damsel fly penises
Prezygotic barriers:
Gametic isolation
• Sperm of one species may not be able to fertilize
eggs of another species
– mechanisms
• biochemical barrier so sperm cannot penetrate egg
– receptor recognition: lock & key between egg &
sperm
• chemical incompatibility
– sperm cannot survive in female reproductive tract
Sea urchins release sperm
& eggs into surrounding
waters where they fuse &
form zygotes. Gametes of
different species— red &
purple —are unable to fuse.
Postzygotic barriers:
• Prevent hybrid offspring from developing
into a viable, fertile adult
– reduced hybrid viability
– reduced hybrid fertility
– hybrid breakdown
zebroid
Forms of Selection
• Selection is a statistical concept
– One cannot predict the fate of any single individual
– But it is possible to predict which kind of individual will tend
to become more common in a population
• Three types of natural selection have been identified
– Stabilizing selection
• Acts to eliminate both extreme phenotypes
– Disruptive selection
• Acts to eliminate intermediate phenotypes
– Directional selection
• Acts to eliminate a single extreme phenotype
Three kinds of natural selection
Natural selection acts on variations
• Stabilizing selection is a natural selection that
favors average individuals in a population.
Selection for
average size
spiders
Normal
variation
Natural selection acts on variations
• Directional selection occurs when natural
selection favors one of the extreme
variations of a trait.
Normal
variation
Natural selection acts on variations
• In disruptive selection, individuals with either
extreme of a trait’s variation are selected for.
Selection for
light limpets
Normal variation
Selection for
dark limpets