The field of evolutionary psychology is a relatively new one. It's also controversial, as many mainstream psychologists don't agree with the theories and conclusions that evolutionary psychologists have drawn from their research.
The main disagreement between these two groups has to do with how they interpret human behaviour -- specifically, why humans behave the way they do. Mainstream psychology looks at human behaviour through a social lens; it assumes that people are born without any innate tendencies or instincts and then learn them over time based on what society teaches them.
Evolutionary psychology takes an entirely different approach by looking at humans' genetic makeup in order to understand why we act the way we do today.
Evolutionary psychologist David Buss explains this difference in his book "The Dangerous Passion" by using an analogy about dogs versus wolves. He says that if you were to look at dog breeds like poodles or beagles, you'd see all sorts of variations among those breeds -- some are small while others are large; some have long hair while others have short hair; etcetera (pardon our pun).
But when you look across all dog species, there's no variation whatsoever because dogs share common ancestors who had specific traits passed down through generations until those traits became ingrained into each breed's DNA code.
In other words, evolutionarily speaking (and pardon another pun), every single dog shares certain characteristics because their ancestors did too -- even though individual dogs may vary greatly from one another within each breed group. "Humans show much more variability than can be accounted for genetically," writes Buss on his website "Why? Because culture plays such a powerful role in shaping us".