An understanding of natural selection is important to dispel the myth of Social Darwinism. This unfortunately named myth, which flourished in the late 19 and early 20th Centuries, was applied to human society. It held that the strong prosper whilst the weak founder.
Natural selection may occur at several biological levels: the level of the individual organism; the level of the kin group, i.e., a family of organisms related through reproduction; the level of the social group; at species level, or even at ecosystem level. These biological levels form a hierarchy with individual organisms at the bottom and ecosystems at the top.
Selection at each of these levels can be understood as competition between organisms, kin groups, social groups, species, or ecosystems for the resources in a particular environment. The one which best fits that environment is the one which will survive, propagate and, ultimately, predominate.
There are two main theories of natural selection. Firstly, that selection only occurs at the individual and kin levels. Secondly, that selection occurs at multiple levels. All theories accept that natural selection occurs primarily at the level of the individual organism, but opinions differ over whether it can also occur at higher biological levels and where the cut-off point is as we rise up through those levels.
Because the subject is complex, it will be split over five posts, one for each biological level beginning with individual level selection.
Darwin believed that natural selection occurred primarily at the level of the individual organism, i.e., that a trait in an individual organism which made it fitter in the context of its environment would enable it to survive and reproduce better than others without that trait.
An organism’s environment comprises not only the physical world but also other members of its own species and members of other species. This can lead to more complex selection processes such as sexual selection and co-evolution. These processes take place at the level of the individual organism, nevertheless.
Sexual selection can occur in organisms which reproduce sexually. Generally, partners in procreation are chosen based on their appearance of health and success. This appearance suggests that they do not carry adverse genes which may prejudice the survival of any joint offspring. In many species this has led to the evolution of traits which overtly demonstrate health and success, for example the plumage of birds. Clearly, successful partner selection will propagate the genes on which an organism relies for its survival and will eventually become a species trait, therefore.
There are, of course, many other traits and ways of displaying them which improve an organism’s likelihood of mating, an example is the support that one parent provides for the other while offspring are being reared.
The environment of any species includes other species with which it interacts. Thus, new traits in one species can evolve in response to new traits in another and vice versa. This effect is known as co-evolution, a concept first proposed by ecologists P.R. Erlich and P.H.Raven in 1964. One example is the evolutionary arms race between a predator, in the form of improving predatory skills, and its prey, in the form of increasing ability to avoid predation. Similarly, a plant and its pollinator can co-evolve traits to the point that there is a clear interdependence between the two species. Examples of co-evolution are widespread in all natural ecosystems.