First 287 words of the document:
Biology Unit 4
Topic 5: On the wild side
22. Explain how reproductive isolation can lead to
In order for a new species to form, part of an existing population must become reproductively
isolated from another part. This usually happens when a barrier comes between two or more parts
of an existing population. Over time, natural selection may cause the different parts of the
population to change to such an extent that they can no longer interbreed to produce fertile
offspring and this makes them two or more different species.
Prezygotic reproductive barriers Explanation
Habitat isolation Populations occupy different habitats in the same area so
don't meet to breed.
Temporal isolation Species exist in the same area but are active for reproduction
at different times.
Mechanical isolation The reproductive organs no longer fit together.
Behavioural isolation Populations do not respond to each other's reproductive
Gametic isolation Male and female gametes from two populations are simply
incompatible with each other.
Postzygotic reproductive Explanation
Low hybrid zygote vigour Zygotes fail to develop properly and die during embryonic
development or result in offspring with severe abnormalities
so they can't reproduce successfully.
Low hybrid adult viability Offspring fail to thrive and grow properly.
Hybrid infertility Offspring may appear healthy but are infertile, such as the
mule, which is the healthy, useful but infertile offspring of a
horse and a donkey.
Text Book: p. 56-57