Lets say that a group of birds only breed in cool climates and the climate warmed and the group of birds split up to find cooler places.
In this case their food source may change and lead to a different beak shape or different length claws to get food. Over a prolonged period of time this will cause changes so great that the two groups of bird is forced back together by further enviromental change would no longer be able to breed and have fertile young. If this was the case they are now two different species of bird. The has arisen from geographical isolation.
Fertile young may be born but be sterile because they have an odd number of chromosomes ( this is the case of a mule). Some embryos may not even implant .The Gentailia of the birds or the courtship rituals may have changed so much that they may choose not to breed with one another.Gametes may also not survive in the body of another species. This type of isolation is reproductive isolation.
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