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The flashcards below were created by user
kamrunsreno
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Speciation
Occurs between micro- & macroevolution
- • Microevolution
- – Changes in allele frequency in a population
- – Short time frame (generations)
- – Ex. Mutation, selection, drift, gene flow
- • Macroevolution
- – Broad patterns over long time spans
- – Origins of entire new groups of organisms
- – Ex. Origin of mammals, flowering plants etc.
The Formation of new species
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Biological Species Concept
Species = populations that can interbreed and produce viable and fertile offspring
- – New species form with reproductive isolation
- • Different species may appear similar
- • One species may have great diversity
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Reproductive Isolation
- Prevents two individuals from producing viable, fertile offspring
- – Maintains existing species boundaries
- – Can lead to the formation of new species
- • Reasons for isolation:
- – Prezygotic barriers
- • Before formation of the zygote
- – Postzygotic barriers
- • After formation of the zygote
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Prezygotic Barriers: habitat
- Two species rarely encounter each other
- – Occupy different habitats
- – Ex. Two species of snakes
- • One species aquatic, one species terrestrial
- • Unlikely to be in the same place
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Prezygotic Barriers: Temporal Isolation
- Species that breed at different times
- • Timeofday,differentseasons,ordifferentyears
- – Ex. Spotted skunks
- • One species mates in winter, one mates in summer
- • Unlikely to be sexually active during same period
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Prezygotic Barriers: Behavioral Isolation
Behavioral isolation
– Courtship rituals and other behaviors unique to a species are effective barriers
– Ex. Blue-footed boobies courtship display
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Prezygotic Barriers: Mechanical Isolation
Mating attempted, but morphological differences prevent completion
– Ex. Snail genital openings don’t align
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Prezygotic Barriers: Gametic Isolation
Sperm of one species cannot fertilize egg of another species
- • Sperm cannot survive in reproductive tract
- • Sperm cannot penetrate the egg
– Ex. Sea urchin egg/sperm proteins won’t fuse
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Postzygotic Barriers: Reduced Hybrid Viability
Incompatible genes from parents interact and impair development and survival
– Ex. Salamander hybrids that do not fully develop or are frail
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Postzygotic Barriers: Reduced Hybrid Fertility
Reduced Hybrid Fertility
– Vigorous hybrids produced, but are sterile
- • Often due to chromosomal differences
- – Chromosome #, chromosome structure
–Ex. Donkey+HorseMule(sterile)
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Postzygotic Barriers: Hybrid Breakdown
Some first generation hybrids are viable and virile, but next generation are weak or sterile
- • Often accumulate recessive disorder alleles
- – Ex. Many plant strains
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Mechanisms of allopatric speciation
- Geographic barriers prevent gene flow
- – Rivers, oceans, mountains, canyons etc. etc.
- Ex. 24.6 Antelope squirrels & G. Canyon
- Ex. 24.8 Shrimp species separated by isthmus
- Ex. 24.9 Salamander reproductive isolation increases with distance
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Sympatric Speciation
- Occurs in populations with range overlap
- Can arise from:
- – Polyploidy
- • Subpopulation with an extra set of chromosomes
- – Habitat differentiation
- • Subpopulation can exploit a new microhabitat
- – Sexual selection (24.12)
- • Ex. Subpopulation with different morphology
- • New subpopulations no longer interbreed
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Sympatric Speciation: Polyploidy
- More common in plants
- – Autopolyploidy
- • Errors in cell division create new subpopulation within a single original species
- – Allopolyploidy
- • Hybridization between two species creates new subpopulation with different chromosome #
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Rates of Speciation
- Can occur at different rates
- – Evidence of this in the fossil record
- • Punctuated equilibrium
- – Describes long periods of species stability followed by sudden change (rapid speciation)
- • Gradual change over time also possible
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