8.3 Flashcards
Species:
a group of similar organisms that can interbreed to produce fertile offspring
Speciation:
the evolution of a new species
New species may arise in 2 different ways:
Allopatric speciation
Sympatric speciation
Individuals from 2 different species cross-fertilise and produce a…
fertile hybrid
Groups within the population become isolated from one another and cannot interbreed, this is called…
reproductive isolation
Reproductive isolation, gene flow amongst the separate groups is prevented and the population’s single gene pool is now…
split into several separate gene pools
Reproductive isolation, allele frequencies within each gene pool may change because….
selection pressures acting on the phenotypes of the organisms may be different
Reproductive isolation, the gene pools may become so different from each other that…
successful interbreeding is no longer possible and each group is now a different species within its own gene pool
Allopatric speciation occurs when:
populations are prevented from interbreeding because they become geographically isolated
Allopatric speciation, the selection pressures in the 2 areas result in the gene pools of the 2 groups becoming very different there is no…
gene flow between the 2 groups
Sympatric speciation occurs when…
populations living together become reproductively isolated
Sympatric speciation may take place :
before mating occurs - preventing the exchange of gametes (prezygotic mechanisms)
after mating - preventing the development of the zygotes into offspring (postzygotic mechanisms)
5 prezygotic variations:
geographical ecological temporal behavioural mechanical
1 post-mating but prezygotic variation:
gametic
3 postzygotic variations:
hybrid sterility
hybrid inviability
hybrid breakdown
what is geographical variation:
populations are isolated by physical barriers (oceans, mountains, rivers)
what is ecological variation:
populations inhabit different habitats within the same area and so individuals rarely meet
what is temporal variation:
the breeding seasons of each population do not coincide and so they do not interbreed
what is behavioural variation:
mating is often preceded by courtship, which is stimulated by the colour or markings of the opposite sex, the call or particular actions of a mate. Any variations in these patterns may prevent mating.
What is mechanical variation?
Anatomical differences may prevent mating occurring
What is gametic variation?
The gametes may be prevented from meeting due to genetic or biochemical incompatibility
What is hybrid sterility?
Hybrids formed from the fusion of gametes from different species are often sterile because they cannot produce gametes
What is hybrid inviability?
despite fertilisation taking place, further development does not occur or fatal abnormalities arise in early growth
As the offspring do not reach sexual maturity, breeding does not occur
What is hybrid breakdown?
The first generation of hybrids is fertile but the second generation fails to develop or, if it does, it is sterile
