Darwin and Wallace recognized the implications and significance of these key facts: the hereditable nature of variation between organisms, the ability of organisms to reproduce many more offspring than are needed to replace themselves, and the constraints on population size due to limited environmental resources. Based on these facts, they drew a logical implication, namely that individuals would differ in their reproductive success – that is, different individuals would leave behind different numbers of descendants. Over time, we would expect that the phenotypic variations associated with greater reproductive success (and the genotypes underlying these phenotypic differences) will increase in frequency within the population; over time they would replace those organisms with less reproductively successful phenotypes. Darwin termed this process natural selection, in analogy to process of artificial selection practiced by plant and animal breeders. As we will see, natural selection is one of the major drivers of biological evolution.
Just to be clear, however, reproductive success is more subtle than survival of the fittest. First and foremost, from the perspective of future generations, surviving alone does not matter much if the organism fails to produce offspring. An organism’s impact on future generations will depend not on how long it lives but on how many fertile offspring it generates. An organism that can produce many reproductively successful offspring at an early age will have more of an impact on subsequent generations than an organism that lives an extremely long time but has few offspring. Again, there is a subtle point here. It is not simply the number of offspring that matter but the relative number of reproductively successful offspring produced.
If we think about the factors that influence reproductive success, we can classify them into a number of distinct types. For example, organisms that reproduce sexually need access to mates, and must be able to deal successfully with the stresses associated with normal existence and reproduction. This includes the ability to obtain adequate nutrition and to avoid death from predators and pathogens. These are all parts of the organism’s phenotype, which is what natural selection acts on. It is worth remembering, however, that not all traits are independent of one another. Often the mechanism (and genotype) involved in producing one trait influences other traits – they are interdependent, after all they are aspects of a single organism. There are also non-genetic sources of variation. For example, there are molecular level fluctuations that occur at the cellular level; these can lead genotypically identical cells to display different behaviors, that is, different phenotypes. Environmental factors and stresses also influence the growth, health, and behavior of organisms. These are generally termed physiological adaptations. An organism’s genotype influences how it responds phenotypically to environmental factors, so the relationship between phenotype, genotype, and the organism’s environment is complex.