How does a species evolve? Occasionally, changes (DNA mutations) will occur in a gene as it passes from one generation to the next. Some of them affect an animal's appearance, behaviour or the way its body works. Sometimes, an animal with a new version of a gene survives better than an animal with the old version - 'the survival of the fittest': then that animal is likely to have more offspring. They will inherit the new version of the gene, and in turn pass it on to their young.