Speciation & Rates of Evolution AP Biology Unit 4

Speciation A species is a population or group of populations that –Members can mate with one another to produce viable, fertile offspring –Cannot produce fertile, viable offspring with other populations

Reproductive isolation Species are usually reproductively isolated from one another (have limits to reproduction or lead to offspring of fitness 0) –Prezygotic barriers -- “before the zygote” –Postzygotic barriers – “after the zygote”

Prezygotic barriers These are things that prevent mating or fertilization even if mating occurs

Prezygotic Barriers Habitat Isolation –Species live in different habitats and don’t meet much (if at all) Temporal Isolation –Mating occurs at different times Behavioral Isolation –Mating behavior differs between species Mechanical Isolation –Body parts aren’t able to match up (even in plants) Gametic Isolation –Sperm from one species is not able to fertilize the egg from another species

One example: Behavioral Isolation Blue footed boobies engage in a courtship display in which the male high steps to draw attention “show off” his feet to the female They also fly over the female’s heads to flash their feet Image taken without permission from

Postzygotic barriers These are things that prevent the offspring from being viable or fertile

Postzygotic barriers Reduced Hybrid viability –Genes from different species interact and prevent or impair development Reduced Hybrid fertility –Meiosis in hybrids can’t produce gametes since chromosomes are different Hybrid breakdown –When hybrids mate their offspring are sterile or frail

Allopatric vs. Sympatric Speciation Allopatric speciation –Speciation occurs as a result of goegraphic isolation Sympatric speciation –Speciation occurs without geographic isolation

Polyploidy Polyploidy = having too many chromosomes In plants, errors in cell division may result in polyploid individuals who are reproductively isolated from the parent species

Polyploidy In other cases, sometimes a fertile hybrid can also result (allopolyploid)

Booming Diversity Adaptive radiation can occur when there is sudden opportunity in the environment extinction of a competitor, parasite, or predator –Mammal adaptive radiation after extinction of dinosaurs genetic drift (specifically founder effect) –Plant adaptive radiation when they arrived on Hawaiian Islands

Rates of Evolution Biologists also are interested in the rate of evolution –Is it happening at a slow steady speed? –Or is it happening in bursts? There are 2 proposed models

Gradualism Model that says evolution is occurring at a constant, steady rate

Punctuated Equilibrium Periods of a lots of evolution (speciation) followed by periods with no noticeable evolution “Spurts” of evolution Which is correct? Probably a mixture of the 2.