This is a form of reproductive isolation . Over time, changes in the gene pools of each sub-group will occur, as described earlier. This can eventually lead to the evolution of new species. The correct term is 'adaptation' , not 'adaption' please try to remember this. Adaptations are acquired by an individual as a set of genetic information - the chance combination of alleles obtained at fertilization.
Individuals can only express in their phenotype the characteristics for which their genotype codes .
This can include many variants of phenotype, within the one individual. What the weight lifter has done is manipulate his own environment to realise all his genetic potential in his phenotype. They are not meant to mean that only the strongest, physiologically fittest individuals will survive and reproduce. Relative fitness is the contribution of a genotype to the next generation compared to the contribution of alternative genotypes at the same locus. From this it can be seen that the phenotype corresponding to a high frequency genotype is probably advantageous in the environment of the species at the time - or at least it is not disadvantageous.