>>11102851The key difference between Darwinian or natural selection and genetic drift is the impact the respective traits being passed on to the next generation has on survival fitness. Both occur all the time in parallel.
Genetic drift refers to how, when "neutral traits" that have little or no impact on survival, selection pressures do not select based on those traits. This does not mean all traits are neutral though, and natural selection still operates on beneficial or detrimental traits. This also doesn't mean that neutral or non-neutral traits are passed down through different mechanisms to the next generation, just that some traits don't factor in to selection but are still passed on and mutates as usual.
For a contrived example, imagine a population of predators with two alleles for eye color, green and yellow, and two for claw shape: thin and long but sharp or thick and stubby but strong. Whether the eyes are green or yellow is a neutral trait is not going to impact selection for the predators. However, when it comes to claws, those two variations will have a huge impact on surviving to produce offspring, depending on the prey that is available.
So, you can therefore conclude that in that same population, if the only prey left are ones that burrow due to a change in the environment, those predators with short stubby claws will be able to dig them out without breaking their claws, while those with the thinner ones(which might be better for catching prey above the ground) are at a significant disadvantage. Their eye color has no impact on this selection process, and if for example by chance most of those with stubby claws also mostly had green eyes, then the next few generations will see yellow eyes fade from the population because those alleles were just coincidentally in the wrong subset of the species. If the alleles are evenly distributed, it should still drift due to other chaotic factors.
The unimportant genes drifted, the important ones selected.
