We now want to think a little bit about the sources of genetic variation, and that's just because remember, evolution requires genetic variation to be in a population. So here, you want to just think a little bit more specifically, what do we mean by genetic variation, and also where does that variation come from? Let's start just by being clear. There is no single definition for genetic variation. We're going to say here there're really sort of two ways to talk about genetic variation in a population.
We can talk about new or different alleles, or we can talk about new or different combinations of alleles. Alright. So, new alleles. Where do new alleles come from? Well, they come from mutations.
Mutations to the DNA sequence. So we have here, sort of, in the classic Mendelian sense, we could have a big A and a little a. Those are different alleles of the same gene. But, of course, these are actually genes on a chromosome. So here we see the chromosomes and there are other genes there.
We have the A, B, C, and D genes, but let's just look at the As right now. So for this to be a big A and this to be a little a, there has to be some difference between these alleles. And if we zoom in on the sequence, we see it right here. There is a difference in those DNA sequences that had to, at some time in the past, come from a mutation. Alright.
But we could also have new combinations of alleles, and combinations of alleles you can just think of as genotypes. And these different genotypes get put together from sexual reproduction and from recombination, or you can think of crossing over. Alright. So, you know, you could have a big A allele or a little a allele, but, of course, in diploid organisms, right, we have two alleles, one from each parent. That's the sexual reproduction part.
Right? We could have a big A and a little a. We could have two big As. We could have two little as. But, again, all of these are genes on a chromosome, so we can think about all the genes on the chromosome.
Right? You can see here, well, all the big alleles are together, and all the little alleles are together. But if we look here, there's been a crossing over event. Right here, we have a little d paired with our other big alleles. And over here, well, here we have a big D paired with our other little alleles.
Right? So that's a new genetic variant. Right? So both new alleles and new combinations of alleles produce new variants, which is genetic variation.