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Genetic changes happen spontaneously all the time. They are called random mutations. Most of them are lethal, because most species are already well adapted to their current environment, and any significant change is therefore likely to make them less well adapted. But sometimes changes occur that do not seriously compromise an organism's current relationship to its environment, and those changes become part of that individual's genetic code, even though they are no great advantage at that time. Environments do change over time however, and in some cases a mutation that had no benefit initially may become strongly advantageous over time. For example, a mutation that causes thicker fur may not be a great advantage in a temperate climate. But if the average temperature in that locality becomes significantly colder over time, the thicker fur may become a distinct advantage to survival, so much so that the individuals with less fur, which did just fine in a temperate climate, may no longer be able to survive. At that point, thick fur, which was originally a deviation from the norm for that species in that locality, may become the norm. The species has evolved a new characteristic. Once a species accumulates a sufficient number of such changes, it is distinctly different from the ancestral stock it came from, and at that point it no longer makes sense to consider it the same species.

All organisms have different traits. Because of this, some animals can survive in certain situations better. The ones that survive better end up having more children. Because children are usually similar to their parents, the traits that help animals survive usually become more prevalent in a population.