Reproductive Success

Natural selection does not simply reward survival. What matters in evolution is reproductive success: the number of offspring an individual produces that survive to reproduce themselves. An organism that lives a long time but never breeds contributes no alleles to the next generation.This is why biologists describe fitness as an organism's ability to survive and reproduce in its environment, passing its alleles on. An allele increases in frequency in a population only if individuals carrying it leave, on average, more surviving offspring than individuals without it.Reproductive success depends on several things:1. Surviving to reproductive age, for example by avoiding predators, disease and starvation.2. Finding and winning a mate. Where individuals compete for mates, or where one sex chooses mates, features that attract mates are selected even if they reduce survival. This is sexual selection. The peacock's large tail makes the male more visible to predators, but peahens prefer males with bigger, brighter tails, so the alleles for these tails are passed on.3. Producing viable offspring and, in many species, keeping them alive through parental care.The key idea is that selection is measured in the next generation. A trait that shortens life but greatly increases the number of offspring can still become more common, because the alleles for it are passed on to more individuals. When answering exam questions, always link an advantage to both survival and reproduction, and finish by stating that the allele is passed on and its frequency increases.