1.a.5 – How a new species appears?
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How a new species appears?
When in evolution appears a mutation that prevents the possibility of crossing with the original species, it is passed a point of no return, a point after which there can be no longer the genetic mixing. This reproductive isolation, leads to the formation of a new species: it is not due to physical or geographical barriers but it is physiological, is part for the intrinsic nature of the species.
This can happen in many ways, for example, two people of the same species, separated in different regions, can develop mutations that lead to the creation of two separate races. Then if the race of the first region develops a ritual of courtship incompatible with the other members of the second region, these latter will always be discarded in case of encounter. Another case occurs when the two races develop a physical shape too different; think of the difference in size between a St. Bernard dog and a Chihuahua dog that makes mating mechanically impossible.
The most important case is however the one due to genetic causes, that is when the accumulation of several mutations makes the fusion of DNA of two individuals increasingly difficult, until the total incompatibility is reached, causing sterility of the offspring or its failure to develop.
ROYAL BOX
ERNST MAYR 
