The adaptation is subject to environmental context?

For adaptation we mean the sum of changes that a species undergoes because of natural selection in a given environment.  Natural selection favors the mutations that promote the survival of future generations, who consequently will be better fit to live in that environment. A classic example of adaptation is the development of a thick, warm fur in a region having very cold climate, or the development of long fast legs to escape predators.
Adaptation is always related to a certain environment because this determines the chances of survival and therefore the natural selection. However, a positive change in a certain context may also prove to be useful in other circumstances: for example hands, that the first monkeys used to cling to branches and to collect fruits, were then used by men to build instruments of all kinds, to write and to drive a car; another example is the stratum of fat, developed to deal with periods of famine and then became a protection against the cold for animals of polar regions. In these cases, we talk about pre-adaptation, as if they were an early adaptation, but in fact these are fortunate combinations which are anyway of great importance for the evolution, because increase the chances of adaptation to new environments, as well as to meet new needs and exploit new resources. In general, the evolution progresses by small steps but cannot create from scratch the new organs: it can only modify the existing ones, which therefore constitute a pre-adaptation, that is a basis for future developments.
In any stable environment, which does not change over time, positive mutations would sum up to exhaust all possibilities; over time, the appearance of new positive changes would increasingly become less likely and the different species would  find themselves in a sort of state of equilibrium given by the maximum adaptation to their environment.
The evolution is slow, gradual, but not uniform; often presents a variable speed precisely because of stability or instability of the environment. A situation of perfect stability is also very unlikely, in fact every mutation of any species is actually a small or large environmental change and thus the evolution will not stop ever, even without considering geological and climatic changes.

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NILES ELDREDGE  

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