Evolution generally occurs by “divergence“; in other words, gene duplication or chromosomal rearrangement events, or DNA slippage during transcription resulting in mRNA that no longer corresponds to the DNA from which it was derived, in the original species … result in a different expression of activity (a new trait). And –– if this activity (this new trait) improves the reproductive and survival rates of the newly formed organism –– this genetic or chromosomal change becomes “fixed” in future generations, leading to new speciation. More efficient finding or processing food, avoiding predators and other dangers in the environment are the principal means by which an organism achieves improved reproductive and survival rates.
Evolution can also occur by “convergence“; in other words, already-existing genes, gene segments, or other DNA sequences (including epigenetic events) –– randomly mixed/combined as a function of evolutionary time –– are used in creating new genes that are more useful in achieving improved reproduction and survival rates for the new organism. It is highly likely that our complex genomes are capable of producing endless possibilities for adapting to any ecological challenge.
In the attached article, authors show that evolution of a new protein function –– in response to low-oxygen, high-altitude conditions –– can occur through different genetic mechanisms, across a wide diversity of avian species living on land regions at high vs low altitudes. The molecular basis of convergence in hemoglobin function was examined by comparing 56 avian taxa that have contrasting altitudinal range limits. Convergent increases in hemoglobin-oxygen affinity were persistent among high-altitude taxa, but few such changes were attributable to parallel amino acid substitutions at key residues of the hemoglobin protein.
Hence, predictable changes in biochemical phenotype do not appear to have a predictable molecular basis. Experiments involving resurrected ancestral proteins revealed that historical amino-acid substitutions have context-dependent effects, indicating that possible adaptive solutions are contingent on prior history. Mutations that produce an adaptive change in one species may represent precluded possibilities in other species because of differences in genetic background.
Science 21 Oct 2o16; 354: 336–339 [article] and News-N-Views p 289 [editorial]