Unit of selection

A unit of selection is a biological entity within the hierarchy of biological organisation (e.g. genes, cells, individuals, groups, species) that is directly subject to natural selection. There has been intense debate among evolutionary biologists about the extent to which evolution has been shaped by selection pressures acting at the different levels of biological organisation.

The debate

Evolutionary biologists have debated for several decades about the levels and units of selection that are most likely to result in adaptive evolution, e.g., about the relative importance of group and individual selection in driving the evolution of biological altruism in several taxonomic groups (altruism is a behavior which, unlike many other traits, reduces the fitness of individuals that engage in it, and thus it cannot have evolved because of selection acting on single individuals in isolation; see Kin selection).

The topic has also enjoyed high resonance with the general public because many mistakenly believe that deep meaning about evolution and life in general and about human nature in particular, may flow from the fact that evolution may have mainly been shaped by competition between units at this or that level of biological organisation.

Richard Dawkins, for instance, has written several books championing the view that the unit of selection is the individual gene. He argues that genes are indeed selfish and that most if not all of the history, structure, and dynamics of the living world is the result of an ongoing race among selfish genes to produce more copies of themselves than competing genes by manipulating the characteristics of individuals, (kin) groups, etc.

Almost without exception, however, modern evolutionary biologists view evolution as a natural process of cause and effect that is deprived of ultimate teachings about the meaning of life.

The philosopher of biology Elliot Sober has indeed stressed that causal explanations of the process of selection should be as central to narratives describing natural selection as they are to scientific narratives describing any other natural process. He contends therefore that such narratives should be descriptions of how differences in fitness are generated in each specific case. He points out, indeed, that natural selection can entail the differential reproduction of many things ("selection of") but that it is the cause of the differences in reproductive output what points to the target of selection and thus defines what the level and the unit of selection are.

Sober's focus on how fitness differences are generated is simple enough, but it comes after decades of confusion about which criterion should be adopted to determine the unit of selection in specific cases. G.C. Williams and Dawkins, for instance, treat the problem of the unit of selection together with that of the unit of heredity and of the unit of evolution, which did not help them focus view selection as a process that generates fitness differences.

Below, undisputed cases of selection at the genic, cellular, individual, and group level are presented and discussed, starting with individual selection which is the most recurrent type.

Selection at the level of individual organism

Selection at the level of the organism can be described as Darwinism, and is well understood and considered common. When a gazelle, for instance, has a trait that allows it to run faster than others and therefore to avoid predators more effectively so that ultimately it manages to stay alive longer and reproduce over more breeding seasons, the causation of the higher fitness of this gazelle can be accounted for fully only if one looks at how individual gazelles fare under predation so one can come to the conclusion that the faster gazelle's speed allows it to avoid predation better.

The speed of the faster gazelle could be caused by a single gene, be polygenic, or be fully environmentally determined, but the unit of selection in this case is the individual since the speed of the gazelles that selection is evaluating, is a functional property of each individual gazelle, i.e., individual speed is the property being selected-for.

The description of the causation chain in this case of selection can be stopped at the individual level because the generation of fitness differences is supervenient to the (various possible) causes below the individual level (i.e. differences in speed were necessary and sufficient in this case of selection).

Note, however, that in this case of selection it is the presence of predators what creates an opportunity for faster individual gazelles to be selected and for their speed to be selected-for, so that this presence is, in the deepest ultimate sense, the cause of the selection regime that is active

Selection at the level of the group

Specific syndromes of selective factors can create situations in which groups are selected because they display group properties which are selected-for. Some mosquito-transmitted rabbit viruses, for instance, are only transmitted to uninfected rabbits from infected rabbits that are still alive. This creates a selective pressure on every group of viruses already infecting a rabbit not to become too virulent and kill their host rabbit before enough mosquitoes have bitten it, since otherwise all the viruses inside the dead rabbit would rot with it. And indeed in natural systems such viruses display much lower virulence levels than do mutants of the same viruses which in laboratory culture readily outcompete non-virulent variants (or than do tick-transmitted viruses since ticks do bite dead rabbits).

Selection at the level of the gene

Evolutionary biologists have described clear-cut examples of selection at the level of the gene, such as meiotic drive and retrotransposons. In both of these cases, gene sequences increase their relative frequency in a population without necessarily providing benefits at other levels of organization. Meiotic-drive mutations (see segregation distortion) manipulates the machinery of chromosomal segregation so that chromosomes carrying the mutation are later found in more than half of the gametes produced by individuals heterozygous for the mutation, and for this reason the frequency of the mutation increases in the population. Retrotransposons are DNA sequences that generate copies of themselves that later insert more or less randomly in the genome. Such insertions can be very mutagenic and thus reduce drastically individual fitness, so that there is strong selection against elements that are very active. Meiotic-drive alleles have also been shown to be reduce strongly individual fitness.

Selection at the level of the cell

Leo Buss in his book The Evolution of Individuality proposes that much of the evolution of development in metazoans has been constrained and shaped by the conflict between selection at the level of the cell and that at the level of the multicellular individual. With this theory it is possible to address phenomena as diverse as cancer, gastrulation, and germ line sequestration.

Species selection and selection at higher taxonomic levels

That selection can operate at and above the level of species remains controversial among biologists. One particular defender of the idea of species selection is S.J. Gould who has proposed the view that there exist macroevolutionary processes which shape evolution beyond the level of species and are not driven by the microevolutionary mechanisms already characterized by the Modern Synthesis. If one views species as individuals that replicate (speciate) and die (go extinct), then species could be subject to selection and thus could change their occurrence over geological time, much as heritable selected-for traits change theirs over the generations.

For species selection to occur however, the patterns of differential persistence of species over geological time must be the result of selection for species-intrinsic properties rather than for properties of genes, cells, individuals, or populations of the species involved. In other words, species must be shown to have been units of selection that have their own properties directly evaluated by selection. While the fossil record clearly shows differential persistence of species, examples of species-intrinsic properties subject to natural selection have been much harder to document.

Elliot Sober

Elliot Sober stresses that causal explanations of the process of selection should be as central to narratives describing natural selection as they are to scientific narratives describing any other natural process. For this reason he contends that such narratives should be causal descriptions of how differences in fitness are generated in each specific case. He argues, indeed, that natural selection can entail the differential reproduction of many things ("selection of") but that it is the cause of the differences in reproductive output what points to the target of selection and thus defines what the level and the unit of selection are.

References

• Buss, Leo W. (1988) The Evolution of Individuality (ISBN 0691084688)

• Dawkins, Richard (1976; second edition 1989) The selfish gene. ISBN 0192860925.

• Gould, Stephen Jay (2002) The Structure of Evolutionary Theory. Harvard University Press.

• Sober, Elliott (1984; 1993) The Nature of Selection: Evolutionary Theory in Philosophical Focus. The University of Chicago Press.