Take the time to watch the Crash Course video, Population Genetics: When Darwin Met Mendel. The key concepts you need to pay attention to in this video are: gene flow, genetic drift, bottleneck effect, sexual selection and founder effect.
To sum up some of the key points:
- Gene flow - occurs when individuals join new populations and reproduce, this is what keeps neighboring populations similar. Low gene flow increases the chance that two populations will eventually evolve into different species
- Genetic drift - is a change in allele frequency due to chance, this causes a loss of genetic diversity. This is common in small populations, and it can be caused by a population bottleneck. Genetic drift can have negative effects on a population
- Founder effect - this occurs after the start of a new population
- Sexual selection occurs when certain traits increase mating success
For a short review, visit these Berkeley Evolution 101 pages: