Transcript Slide 1
Speciation How Species Form Species • How are new species defined? • Used to be on basis of structure • These are different species: – Top: Grevy’s zebra (endangered) – Bottom: Plains zebra (widespread in Africa) Speciation • When some members of a sexually reproducing population change so much that they are no longer able to produce viable, fertile offspring with members of the original population Reproductive Isolation • Can produce new species if there is no gene flow between two populations • Many isolating mechanisms; some which occur before fertilization and some after Pre-zygotic Isolating Mechanisms • Behavioural – Any special signals (bird song, pheromones, mating rituals, etc.) prevent interbreeding – Ex. Eastern and Western meadowlark look the same, have overlapping ranges but have very different songs Pre-zygotic Isolating Mechanisms • Habitat – Two species may live in the same general area but have different habitats – Eg. The common garter snake is commonly found near water but the Northwest garter snake prefers open meadows Pre-zygotic Isolating Mechanisms • Mechanical – Many species are separated by temporal (timing barriers) – Diurnal vs. nocturnal; mate or flower at different times – Eg. Tropical orchids each flower in response to weather stimuli Pre-zygotic Isolating Mechanisms • Mechanical – Some species fail to mate because they are anatomically incompatible – Lock and key genitalia (dogs, insects, etc.) – Plant structure may impede pollination – Some bees carry pollen on legs, some on backs, some on wings Pre-zygotic Isolating Mechanisms • Gametic – Ensures that sperm and egg will rarely form a zygote – Sperm may not be able to survive in the internal female environment – Pollen grains of one species fail to germinate on stigma of another species Post-zygotic Isolating Mechanisms • Hybrid Inviability – Genetic incompatibility between two species – Stops development of hybrid zygote – Eg. Sheep/goat crosses usually die in early development (otherwise we’d have shoats…or geep!) – Doesn’t always fail Post-zygotic Isolating Mechanisms • Hybrid Sterility – Can mate and produce hybrid offspring which are sterile – Failure of meiosis due to chromosome number – Eg. Horse and donkey make a mule Post-zygotic Isolating Mechanisms • Hybrid Breakdown – F1 generation of hybrid offspring is viable – If two F1 hybrids mate then F2 offspring are sterile or weak – Eg. Some cotton plants produce fertile hybrids, but their offspring die as seeds Modes of Speciation 1. Allopatric • 2 populations geographically separated from each other (physical barriers) 2. Sympatric • A population may split into separate gene pools, even within same geographic area