What is the difference between natural selection and genetic drift?

Genetic drift and natural selection- can they happen simultaneously?

• I understand the difference between genetic drift and natural selection, but I am unsure of whether these two phenomena can happen at the same time in a given population, and if so, ...show more

• Answer:

  Yes, and even on the same gene locus. Have a nice day. Imagine a field of grasses. One species has a gene that controls flower colors. The flowers can be yellow or red. Let's put a big herbivore in there. This big herbivore eats and stomps plants indiscriminately. But this year, the herd of big herbivore ate and stomped more yellow-flowered plants of this species than red-flowered, proportionate to their representation in the field. That's genetic drift. Now let's put a small herbivore in there too. The small herbivore preferentially eats the yellow-flowered plants because they're easier to see at dusk (when the big herbivores are stomping around less than in full daylight). That's directional selection, a subset of natural selection. > 2. If they can happen simultaneously, how would population size effect the changes seen? Smaller effective population size is going to increase the rate of change. Say you have 10000 plants, equally red-flowered and yellow-flowered. You randomly stomp 5000 plants. You are probably going to be left with 5000 plants, equally red-flowered and yellow-flowered. Now say you have two plants. You randomly stomp one. Get the picture?