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 altruism in several taxonomic groups (altruism reduces the fitness of individuals which engage in it, so that it cannot have evolved because of selection acting on single individuals in isolation; see Kin selection).

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

Richard Dawkins, for instance, has written several books championing the selfish gene view that the unit of selection is the individual gene. He argues that genes are "selfish" because they manipulate the characteristics of individuals, (kin) groups, etc., in order to produce more copies of themselves than competing genes in successive generations. In Dawkins's view, most if not all of the history, structure, and dynamics of the living world is the result of past and ongoing races among selfish genes.

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.

And this focus on causation has indeed led the philosopher of biology Elliot Sober, to stress that causal explanations of the process of selection should be as central to the narratives describing natural selection as they are to any scientific narrative. He contends therefore that the unit of selection can be found by dissecting how differences in fitness are generated in each specific case. He points out, that natural selection entails the differential reproduction of many things ("selection of") but that it is the causation of differences in reproductive output what points to the functional property directly "selected-for", and thus to the level and the unit of selection.

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 in determining what the unit of selection is in each specific case. G.C. Williams and Dawkins, for instance, have treated the problem of the unit of selection together with that of the unit of heredity and of the unit of evolution, which does not help one in dissecting selection as a causal 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 familiar 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

 * Main article: Group selection

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 which 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 that 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) manipulate 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 themselves in the genome more or less randomly. 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 strongly reduce 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 reflects the conflict between selective pressures acting at the level of the cell and those acting at the level of the multicellular individual. This perspective allows one to make sense straightforwardly of phenomena as diverse as cancer, gastrulation, and germ line sequestration. Cancer, e.g., occurs when individual cells in the body mutate and develop the ability of proliferating without the restrains acting on normal cells which this way are forced to serve the needs of the individual organism. However, one must be careful not to abuse such verbalizations to avoid that they be trivialized. The proliferation of specific cells of the vertebrate immune system to fight off infecting pathogens, e.g., could be described as a case of cellular selection, but it is better described as a case of programmed and exquisitely contained cellular proliferation.

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 at and above the level of species and are not driven by the microevolutionary mechanisms that are stressed 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 evolution to be driven by species selection, 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 whose properties were 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.