If the offspring of this novel hybrid organism have survival capabilities equal to or superior to either of its parent organisms, the new hybrid will persist over time.
Further complicating evolution in multi-cellular organisms like humans is the need to produce a large variety of cells, each with vital but different functions. Thus, beginning with all the same genes, our embryonic cells differentiate into brain, kidney, liver, and skin cells among others, in order to produce a whole human being.
This process of differentiating is accomplished by accelerating the activity of some genes and suppressing the action of many others. This complex regulation of gene activity is accomplished by a large group of genes which don’t code for proteins (and therefore don’t contribute directly to such obvious characteristics as eye color) but code instead for regulator molecules which have the ability to regulate the activity of other genes.
It is now thought that many of dramatic changes seen during evolution (say from small primate to human) are less the result of random mutations in protein producing genes and more the result of heritable changes in the regulation of the activity of these genes. This could account for the fact that, although our genetic constitution (genotype) is only slightly different from that of a chimpanzee, there are much more marked differences in our appearance and behavioral characteristics (phenotype).
The attributes of the environment in which these processes take place provides much of the selection pressure which determines which organisms perish and which persist. In these multiple ways evolution has proceeded over millions of years to give us the huge variety of organisms we share the Earth with today.
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