In the genetic lottery drawn at the individual's conception, the best insurance for a long and biologically successful life is to have the right parents. Challenges which the environment may impose include variations in climate, food availability, diseases, parasites, living space, predation and competition from other species. Genes, on the other hand, allow for individuals to exhibit variations in all sorts of personal abilities, some obvious and others latent until a specific need arises. Some of these possible variabilities include such things as strength, burst speed, endurance, environmental irritability, heat or cold resistance, metabolic rate, immune response, behavioural flexibility and diversity of diet preferences. Those individuals which are best equipped to cope with the prevailing conditions thrive, whilst the remainder are, to some extent, disadvantaged.
Genes make an individual unique.
Organisms which form a single species can, by definition, breed and leave viable offspring. Those from similar, yet different species, may breed and may even produce offspring, but they are, with few exceptions, sterile. Examples include mules, tiger x lions, donkey x zebras. Such crosses rarely occur in nature as the species involved live in different environments. Reproductive effort resulting in sterile hybrids represents a wastage of biological resources.