Speciation is how a species splits into two or more species.
What to say
1) Two populations of the same species are isolated geographically.
2) That means they can’t interbreed.
3) The two areas have different environments and different selection pressures.
4) Genetic differences develop between the two populations.
5) That could be due to selection or just random mutations.
6) Differences in allele frequency accumulate between populations.
7) Over time…
8) …the two populations are so different that they cannot mate and have fertile offspring.
9) That makes them different species.
What not to say
Don’t say genes, say alleles. Evolving new genes takes a long, long time-evolving different alleles, variant forms of genes you already have, happens much faster.
You should still emphasise that this process takes quite a long time, though.
An old problem
For many years after Darwin explained evolution, scientists had a problem: they couldn’t see how it could happen with sexual reproduction. If children are (basically) some sort of average of their parents, why don’t successful animals have their successful traits diluted down in their offspring until they disappear? Why don’t tiny subspecies with unusual, successful characteristics see those advantages wash away like a drop of coffee in a glass of water when they mate with other animals? Fleeming Jenkins, a Scottish engineer and creationist, explained this vividly just seven years after Darwin published Origin of Species-you can read his argument here, in the passages beginning “An illustration will bring this conception home.” and “a very highly-favoured”. (Trigger warning: his choice of example is quite spectacularly racist.)
It wasn’t a question Darwin could give an answer to, and an answer didn’t emerge until the arrival of genetics in the early twentieth century explained what was going on. Dominant and recessive alleles explained the answer: people don’t have to be an average of their parents. Instead, they can completely take after one parent in some ways, and completely take after the other in other ones.