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Since the 1960s, controversial debates have arisen regarding the level of the biological hierarchy at which natural selection takes place. In particular, views in opposition with one another have arisen between those that believe that the level of selection acts at the gene (e.g. Dawkins), the individual is the only biological level at which selection acts (e.g. Williams), and those who believe that natural selection may act on groups of organisms (e.g. Wynne-Edwards). According to Darwin, natural selection acts on the level of the organism, and it is the differences in reproduction and survival of individual organisms that drive the evolutionary process (Darwin, 1871). On the other hand, Dawkins amongst others would argue that the exact unit of selection is the gene, for genes alone are the 'ultimate beneficiaries' of the selection process (Dawkins, 1982). This essay will explore the key theories that surround the 'unit of selection' debate, in an attempt to discuss whether natural selection acts solely on genes.
The term 'natural selection' was first introduced by Charles Darwin in his innovative and groundbreaking book On the Origin of Species in 1859. Natural selection is the key mechanism of evolution, and can be defined as the process by which heritable traits that facilitate increased survival and reproductive success become more common in a population over successive generations (Darwin, 1859). Natural selection is considered to be the primary factor for species and genomic diversity, as genetic variation within a population of organisms indicates that some individuals will survive and reproduce more successfully than others. Natural selection operates on the phenotype, which are the observable traits or characteristics of an organism, with the genetic and heritable basis of any phenotype which gives a reproductive advantage becoming more common within a given population. Over evolutionary time, this process may result in certain adaptations that specialise organisms for specific ecological niches, and may ultimately result in the surfacing of a new species. Those individuals that are selected for will be best able to find food and mates in environments where there is competition, and avoid predators for example (Darwin, 1859).
At the genetic level, natural selection causes changes in gene frequency, for genes which code for protein synthesis determine aspects of an individual's behaviour. Variation within a population occurs when a genes alleles code for different forms of the same protein. There is competition between alleles for a specific site on the chromosomes, and thus natural selection is the differential survival of alternative alleles. Because selection of genes is mediated though phenotypes, the most successful genes will belong to those which promote an individual's survival and reproductive success. Consequently, we would expect individuals to behave so as to promote gene survival (Krebs & Davies, 1981). The question rests as to what unit is the 'true' fundamental unit of selection; whether genes or individuals are best seen as the true units of selection, or whether groups of individuals can be units of selection.
Darwin's theory of evolution explained above has been interpreted as implying that the unit of selection is the individual, yet he also believed that selection could act at different levels of biological organisation, for example genes, organisms, and kin groups. However, a fundamental difference exists between natural selection at the level of the gene, and all other species levels. This difference is explicated by Dawkins (1982), as he suggests that the gene is the unit of selection due to the fact it is the replicator, and all other proposed units of selection such as the organism itself, are levels of vehicles (Bourke & Franks, 1995). Therefore, it is the gene, and not the individual or the population, upon which natural selection acts (Dawkins, 1976).
In The extended phenotype: the long reach of the gene (1982), Dawkins sets out to clarify the differentiation between replicators and vehicles. He states that genes are replicators, whereas organisms and groups of organisms are vehicles in which replicators travel about. "Replicator selection is the process by which some replicators survive at the expense of other replicators. Vehicle selection is the process by which some vehicles are more successful than other vehicles in ensuring the survival of their replicators" (Dawkins, 1982:82). Therefore, the question as to what the unit of selection may be depends on whether we should include the vehicle level as well as the replicator level. DNA is an active replicator in that DNA has some influence over its probability of being copied through the phenotype, and genes are germ-line replicators, for they are potentially the ancestors of a long line of descendent replicators. The phenotypic effects of germ-line replicators ensure a gene's successful replication, and it is these effects that we see as adaptations to survival. Dawkins states that rather than the group or the organism's survival, these adaptations are to ensure the relevant replicators survival themselves. This reiterates his argument that genes alone are the 'ultimate beneficiaries' of the selection process (Okasha, 2006).
The replicator concept looks for pieces of chromosome of indeterminate length which become more or less numerous than alternatives of the same length. Dawkins ultimately states that "An active replicator is a piece of genome that, when compared to its alleles, exerts phenotypic power over its world, such that its frequency increases or decreases relative to that of its alleles" (Dawkins, 1982:91). However, Dawkins has received criticism for his vague explanation of the term 'replicator' from Bateson (1981) amongst others. Bateson (in Dawkins, 1982:92) expresses Dawkins' replicator as "that bit of genetic material making the difference between the winning and losing characters". However, Dawkins has retorted that a genetic replicator is defined by reference to its alleles, and that this is not a weakness of the concept. He uses the example of the peppered moth Biston betularia to demonstrate the gene as the unit of selection, only defined by comparison with its alleles. The peppered moth's dark coloration has increased in frequency in industrial areas because of the production of phenotypes that favour its ecological niche, and there is widespread acceptance that this coloration can be determined by a particular gene. However, Dawkins disputes that this gene is only one of thousands vital for the dark coloration to show itself (Dawkins, 1982).
To summarise Dawkins' theory that the gene is the unit of selection, the genes that exist today are the ones that are good at surviving in company with the ensemble of companion genes. These fragments of DNA qualify as active germ-line replicators and thus facilitate natural selection. Adaptations are the replicator's effects on the world, and therefore the organism itself is not a replicator, but rather a vehicle ensuring the survival of its replicators (Dawkins, 1982). Similarly, groups of organisms are not replicators, but rather vehicles. Organisms themselves are not replicated in the process of reproduction, for they die and only their genes are passed on (Reeve &Keller, 1999).
Dawkins' theory has been criticised as too reductionist by those who argue that genes are not directly visible to natural selection. Similarly, Gould (1984) suggests that natural selection cannot pick among genes directly, but must select between different packages created by and containing these genes for example organisms. However, Dawkins does acknowledge the importance of the units directly confronting natural selection, and terms them vehicles. Similarly, Dawkins has been criticised for denouncing multi-level selection theories, as they do not overtly recognise the replicator-vehicle dichotomy, and some of these theories deny any special quality of the gene level (Sober 1984).
However, Dawkins does receive much support as a merciless opponent of group selection, insisting on the incapacity of selection for the benefit of the group as an evolutionary mechanism, compared with individual selection (Okasha, 2006). Group selection advocates the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, this lies regardless of the alleles direct effect on the fitness of individuals within the group. This theory dismisses the very obvious reality; many traits evolve that are advantageous to the individual even though they are disadvantageous to others in the population. Infanticide is a clear example of a practice common in the animal kingdom that has evolved because of its only advantage to the male that practices it. Infanticide is caused by sexual conflict, with the killer (male) becoming the new sexual partner of the victim's mother which would otherwise be unavailable to him. This corresponds to a gain in reproductive fitness by the killer, and a loss of reproductive fitness by parents of the offspring killed (Krebs & Davies, 1981). Nevertheless, some evolutionists are proponents of group selection theory, with Wynne-Edwards (1962) leading the way.
Wynne-Edwards (1962) proposed the idea that animals behave for the good of the group. He suggested that if a population over-exploited its food resources it would become extinct, so therefore adaptations have evolved to ensure that each group of organisms controls its rate of consumption. Groups consisting of selfish individuals die out because they over-exploit their food resources. Therefore, if a group of organisms, owing to their interactions or division of labour, provides superior fitness compared to other groups, where the fitness of the group is higher or lower than the mean fitness of the constituent individuals, group selection can be declared to occur (Reeve & Keller, 1999). However, it has been proven that most examples of group traits can be reducible to individual traits. Williams (1966) articulated a very important criticism of natural selection to act on the level of the group. Group selection is theoretically possible, however this form of selection will be weak because groups do not go extinct as fast as individuals, and therefore selection at the level of the individual will be more powerful. As a result, group selection is very unlikely to be an important force in evolution, yet the group still remains a feature of selection.
In more recent times, the limitations of earlier theories have been addressed and newer theories have been implemented that combine earlier viewpoints that selection may occur at different levels. Ultimately, the organization of the living world is hierarchical, with lower level units such as cells grouping together and cooperating to form higher level units of organization such as organisms and societies (Michod & Roze, 2000). The levels-of-selection theory defines all levels of the biological hierarchy as potential units of selection, including the level of the gene, organism and groups of organisms, and most evolutionary biologists would acquiesce with this concept.
Levels-of-selection theory takes the notion that natural selection can function concurrently at different levels of the biological hierarchy, above that and including the level of the gene. Evolution of a given trait may be affected by selection at more than one level (Okasa, 2006). For example, selection at the level of the group has been most often seen in humans and notably social insects that make cooperation a basis of their adaptations overtime (Boyd & Richerson, 1990). Social insects attract special interest because of the intricate co-operation within their societies. Social insect colonies have been described as super-organisms because of the extent to which individuals appear to function as a unit that is dedicated to the continuation and reproduction of the colony as a whole (Wheeler, 1911). Natural selection must favour this kind of co-operation, and Hamilton's (1964) kin selection theory provides the framework for comprehending this form of social behaviour.
Therefore there need be no single answer to the question of what level of biological hierarchy natural selection acts upon. Rather, a gene-centred perspective should be maintained, with the levels-of-selection theory adopted if it is convenient. Rinkevich (2000) suggests that a "unit of biological organization upon which selection might act should be both an autonomous functional entity and physically and structurally coherent, even if it is in the form of a gene" (2000:232). This opens up the scope as to what the unit of selection may be. Similarly, Kitcher et. al. (1990) have pointed to a major drawback of biologists in deciphering the unit of selection, in that they assume that for every natural selection episode, there is an exclusive account that will identify the level of selection.
In an attempt to reflect upon the question in hand, it appears that different selective forces operate on different levels of biological organizations, and that this may account for several units that natural selection may act upon. Natural selection at the level of the gene theory and levels-of-selection theory "represent not opposing theories of natural selection, but alternative ways of talking about it, respectively replicator-centred and vehicle-centred" (Bourke & Franks, 1995:56). As with organisms, groups of organisms can fit into Dawkins' model of replicators and vehicles, with them simply being viewed as vehicles above the level of the organism. Natural selection favours individuals who adopt life history strategies which maximize their gene contribution to future generations, and therefore the gene is the replicator as it is the only entity being passed on and as a consequence is perhaps the most important level of selection. However, to be in complete agreement with the statement that natural selection acts only on genes, denies the interactions that occur between biological planes, and all hierarchical levels of complexity and organization in all of biology.