What is the ultimate source of genetic variation needed for natural selection to act upon?

Answer 1

Primarily the natural variety in the sperm and the egg when they meet. If you remember your biology class when you probably drew out those little 4 boxes to determine the possible combinations from a parent with gene TT and another with Tt.

That sort of random parental genetic transferral is much of the acting force behind natural selection. Each of your offspring could potentially be very different. One could be very tall/short, another could be inclined towards math/literature, one could have good vision while another has poor vision, and so on. It is generally the random assortment of physical traits in the offspring that go through the filter of natural selection.

Example: In a world where predators are very fast, whichever of your children is the fastest is the most likely to survive. He/she is likely to meet a mate who is similarly fast (the slightly slower people being eaten more often) and have kids that range in speed but are generally more inclined to be fast and so on.

I should probably note that things like disease and changed environment serve as a "filter" for natural selection, but they're not the source of variation. Genes don't respond to environment directly or consciously. A change in environment is a change that may favor different traits: if for some reason all the trees suddenly die off then the taller people will be easier for predators to spot. So while before there may have been a lot more tall people, after an environmental change the shorter people could be more suited to the natural environment (this is also seen in the famous peppered moths instance).

Genetic variation comes from sex combinations and minor mutations (something minor like if you're taller than anyone else in your family - a major mutation is typically deadly to the organism or doesn't readily spread) and it's the changes in nature that "select" what traits survive.

Answer 2

The mechanism that makes it work is the random combination of DNA from two parents to make one child.
This, however does not introduce any new variation where it did not existed.
The ultimate source of the variation is DNA-errors made during DNA copying or by environmental influences (like UV radiation).

Answer 3

Usually genetic variation is a response to disease or a change in environment.

Basically any long term change in a small population or short term change in many populations creates the need to adapt genetically.

Answer 4

Both mutation and selection of genes from parents. For example if parents have some good and some "bad" genes. The children who get a higher percentage of "good" genes will spread their genes faster thereby flooding out the bad genes.

Answer 5

Mutations