Evolution and natural selection is not only connected to the DNA, but is also a function of environmental potentials. Say we have a herd of similar animals living in a warm climate. Some of the offspring develop extra thick fur due to a mutation. This change may not be selected, since this may cause the animal to be too warm in extended activity like hunting or the male mating Olympics. Although this may not work in the warm, this same genetic variation, if it occurred in a cooler climate, might now mean selection. Genes throw up a variation but it is up to the environment to give it a thumbs up or a thumbs down in terms of persistence due to practicality.
The earth is not a static place, as we see with climate change. The earth will change locally and globally with time, such as the last ice age and the eventual warming again to the present age. These changing potentials have an impact on previous selections, as well as on new selections. The ice age fur may not be useful after the warming started to occur. We don't see many wooly elephants which was predictable.
Darwin developed his theory on a controlled island environment where change is slower than many places on earth. It would have been hard to make his same inferences if he had stayed in England to develop this theory, which has environmental pressures from humans populations. Or if he had done his research in a place like the YellowStone national park, subject to large forest fires, where nature burns and recovers in a decade with new flora and fauna dominating in such a short time.
Humans bring to light another factor, connected to choice, which implies the brain. For example, if a male bird is colorful to help attract females. The bottom line is the female's brain is making a choice for natural selection. If you altered her brain new selection could occur. She is using sensory input to trigger the brain, with her brain making a gut choice.
The brain brings up another feature of evolution called migration. If an animal migrates, with a given set of genetic qualities, it becomes possible to find an environment that suits its DNA, giving itself the possibility of natural selection. The brain will help drive its genetic selection process. For example, if the hairy critter in the first example, had the urge to migrate from the warmer weather (driven out by the herd), to where it was cooler, now it can become selected. When people write in these forums each has certain areas of expertise where we feel more comfortable; gives each person better odds for selection. If you are a troll you will be driven out to another environment. This is all brain not genetics.
Migration not only helps the critter that migrates, but it will also impact the new environment. In the case of human migration, during the settling of the America's, the settlers brought germs and disease to the indigenous people, thereby changing the parameters of local selection process. Immunity to these particular diseases was not a main factor for local selection before the migration. Immunity, which was always there for some, which allowed many to be resistant, had no real value, until after the environment changes. Now the change bring it out as important.
With humans, fads change the image in the mind of the ideal women, from thin to full figure and then back to thin or the new normal which is heavier. The distribution of ladies may be the same, but environmental pressures, and the brain, change breeding selection back and forth. As we control the potentials of the environment, like in Galapagos, some of these effects become factored out allowing a simpler theory that create intuitive yellow flags for many like myself.
