Obviously, if there were genes that coded for chicken brain speed. Those chicken brains that produced more of whatever neurotransmitter, or neuron growth stimulant, or whatever else might be involved, would have been the outcome of information, in the form of a genome, that was passed on from speedy parents to their offspring.
That genome would in pretty much all cases, pre-exist. No evolution here in terms of a greater complexity, that might ultimately explain how plants and animals would have come about simply as a result of random chemical changes, from one-cell ancestors.
As the Nature article quoted by Buffalo above demonstrated, gene deletion looks to be a major driver of speciation. Biological entropy, devolution rather than evolution may be a better explanation for diversity in species.
Except in childhood and adolescence, physically we wind down. Life teaches us that physical mutations result in bad things, not something amazingly superior. Stay out in the sun too long smoking cigarettes is not going to raise one’s IQ or quicken one’s stride. It’s the other way around.
People or their families chose who they will marry, natural selection so 200,000 BCE.