working with the biological species concept speciation is a two-part process –1. identical...
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Working with theBiological 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
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Consider how the amount of genetic divergence (change) forms
a continuum:
Microevolution Macroevolution small changes large changes
Microevolution = adaptationMacroevolution = speciation
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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.
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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
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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
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Prezygotic barriers: Geographic isolation• 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
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
Ammospermophilus spp
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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
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
lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest
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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
Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer
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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
Blue footed boobies mate only after a courtship display unique to their species
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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.
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
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Mechanical isolation
• 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
Animals
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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.
Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.
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Postzygotic barriers:
• Prevent hybrid offspring from developing into a viable, fertile adult– reduced hybrid viability– reduced hybrid fertility– hybrid breakdown
zebroid
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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
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Three kinds of natural selection
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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
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Natural selection acts on variations
• Directional selection occurs when natural selection favors one of the extreme
variations of a trait.
Normal
variation
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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