My favourite example of the complicated link between genes and growth is tortoiseshell cats. (The kind with random blotches of colour.) When they've cloned this breed, they get kittens that all look different. Apparently the tortoiseshell genes say "make blotches," but where, and what colour, is determined by something else - maybe an internal randomiser, maybe microscopic chance influences in the womb.
I'm guessing a lot of other things work the same - genes set the parameters, chance or early influence set the exact values. Variety is good for a species, particularly a species like ours that specialises in everything. Contrary to the alpha-male theory of evolution, there is not one "fittest" type to which the whole species is trying to evolve: what type is fittest changes all the time.






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