Assuming that variability of populations were non-genetic, that is, not controlled by genetic material, once again chance events alone would determine which of the organisms would survive. But phenotypic traits of a1 organisms'have a genetic component and heritable to some degree. If an individual has some form of a trait that gives him a better chance of survival in a given environment, evidently his offspring inherit the trait. This is known as differential survival. Differential survival *auld refer to the survival of individuals in an environment to which they are well adapted. Individuals who do not possess such adaptations for the given environment may not survive to reproduce. The adaptations are genetically controlled and heritable. Differential survival leads to differential reproduction of alleles responsible for the survival of the next generation. In fact it is customary to say that natural selection is synonymous with differential reproduction. The consequence of differential reproduction is not so much that individuals survive, but that such individuals are able to perpetuate their genes in subsequent generations. Thus what really survives is the set of genes.