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Speciation
• Process by which a daughter species evolves from a parent species
Genetic Divergence- when populations become reproductively isolated
Species-population of individuals who can interbreed under natural conditions producing fertile offspring
• Does evolution always result in a new species?
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2 Concepts on Speciation
• Evolutionary species concept every species has its own evolutionary history, part of which is in the fossil record, and diagnostic traits
• Biological species concept reproductive isolation rather than trait differences define a species
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Reproductive Isolating Mechanisms
• Any heritable feature of body form, functioning or behavior that prevents interbreeding between species
• Pre-zygotic and Post-zygotic
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Pre-ZygoticMechanisms
• Behavioral• Temporal• Mechanical• Habitat isolation• Gametic mortality
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Post-Zygotic Mechanisms
• Hybrid mortality lower in fitness, early death
• Hybrid sterility• Would natural selection favor pre- or
post-zygotic mechanisms?
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Types of Speciation Processes
• Allopatric Speciation-species form due to physical barriers that prevents gene flow; mountain range, river, island
BARRIER
Species A becomes distinct from species B; cannot mate with each other
A B
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Allopatric Speciation
• Salamanders in Central California
• Washington Sockeye salmon in lakes vs rivers
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Allopatric Speciation: Adaptive Radiation
• A single ancestral species gives rise to a variety of species, each adapted to a specific environment
• Hawaiian honeycreepers• Occurred after dinosaur
extinction mammals diversified in only 10 million yrs!
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Types of Speciation Processes
• Sympatric speciation-species forms within the home range of an existing species in absence of physical barrier
Species A-blue Species B-purple
Species A and B cannot mate
AB
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Sympatric Speciation
• Polypoidy in plants• Autopoidy diploid plant
produces diploid gametes• Alloploidy hybrid plant
doubles it chromosome #
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Models of Macroevolution
• Gradual Model-new species formed through gradual changes in traits over time; most consistent with fossil record
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Models of Macroevolution
• Punctual Model-new species formed in a short amount of time; occurs through bottlenecks/founder effects, mutations, directional selection (natural selection)
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Developmental Genes and Macroevolution
• Development of eyes is controlled by Pax6 gene– It did not matter if it was a
lens or compound eye
• Loss of gene results in failure of eye to develop in mice, humans
“…all animals share the same control switches for development…”
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Developmental Genes and Macroevolution
• Development of overall shape is controlled by Hox genes– Control number and
appearance of repeated structures
• Differential expression in chickens vs snakes
“…animal diversity is due to variation in the expression of ancient genes…”
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Evolution of the Modern Day Horse
• Does not fit a gradual model perfectly
• Trends observed– Increase in size– Toe reduction– Change in tooth
size/shape
• Modern horse evolved 4 mya adapted for living in open plains
“Macroevolution is not goal-oriented”
“…speciation, diversification, and extinction are common occurances…”
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Darwin’s drawing in The Origin of Species