Speciation • A species is an interbreeding group of organisms • When a population is somehow severed from the parent population and over time its gene pool is altered. • Allopatric and sympatric
Allopatric speciation • Isolation is the result of a geographic barrier • Ex. Galapagos finches • True speciation only occurs if the remaining populations will NOT breed
Sympatric speciation • No geographic barrier • Balanced polymorphisms – insects a certain color with a certain kind of camoflage will mate and isolate themselves from the original population • Hybridization – when hybrids are more successful than parental generations
Sympatric - polyploidy • When a sub-population arises in the midst of the parent population • Many plant species have accidental polyploidy in meiosis. They produce zygotes with multiple sets of chromosomes. • They can no longer reproduce with parental generation.
Adaptive Radiation • Adaptive radiation – rapid evolution of a variety of species from a single ancestor (ex. Darwin’s finches) • Occurs after mass extinctions when 90% of organisms become extinct. Leaves room for others to colonize
What is the tempo of speciation? • Phyletic gradualism vs. punctuated • Punctuated equilibrium model – species most often diverge in spurts of rapid change instead of slow and gradually. • Over a few to ten thousand years, genetic drift and natural selection can change small, isolated species
Reproductive Barriers • Prezygotic barriers – occur before conception • Post-zygotic barriers – occurs after conception
Pre-zygotic Barriers • Temporal Isolation – mating or flowering occur at different time of the year/day Ex. Western spotted skunks breed in fall, while eastern spotted skunks breed in the late winter • Habitat Isolation – species living in the same region may occupy different habitats Ex. Some Garter snakes live in water/some on land • Behavioral Isolation-courtshiprituals are very specific Ex. Birds • Mechanical isolation-structuraldifferences in genitalia or flowers prevent copulation or pollen transfer Ex. Insects and flowers • Gametic isolation- male sperm cannot fertilize the egg
Post-zygotic Barriers • Hybrid Inviability – hybrid zygotes fail to develop or reach sexual maturity • Hybrid Sterility – Hybrid individuals fail to produce young • Hybrid breakdown – hybrids have reduced viability with each generation.
Patterns of Evolution • Divergent evolution – when two isolated population evolve independently (Ex. Brown and polar bears) • Convergent Evolution – when two organisms without a common ancestor occupy the same niche, so they have the same characteristics. (ex. Porpoise and penguin) • Parallel Evolution – two related species who have made similar evolutionary changes. Ex. Placental and marsupial wolf • Co-evolution – predator/prey relationships