Evolution is a relatively old concept but is still significant today, underlying most biological theory. It can be broadly defined as a change over time, or in biological terms as a descent with modification from shared ancestors. There are four main driving forces of evolution – natural selection, gene flow, genetic drift and mutation. Natural selection is the process in which beneficial alleles increase in frequency over time in a population due to increased fitness and therefore greater reproductive success. Gene flow or migration refers to the transfer of genes from the gene pool of one population to another. Genetic drift is the random fluctuation in allele frequencies over time due to sampling effects in finite populations. Lastly, mutations are heritable changes to DNA.
Charles Darwin in the ‘Origin of Species’ focused on natural selection as the driving force of evolution. He used his understanding of Finch adaptation in the Galapagos Islands to demonstrate the power of natural selection in differential reproductive success and the endurance of favourable traits in future generations (Stix, 2009). This theory has become a foundation of modern biology. Natural selection is considered a two-step process, the first is the variation apparent within the population – which can be considered random, followed by natural selection, a non-random process which determines the evolutionary direction (Mayr, 1978). Darwin did not understand the source of this variation due to a lack of understanding of genetics at the time of his research. However, in the time since Darwin’s death, an appreciation for the importance of random processes both in causing this variation and in driving evolution has been established. The original ‘Selectionist’ theory suggests that only alleles that confer increased fitness will persist in the populations, yet random processes can assist in explaining why many neutral alleles still persist and may even fixate. This essay will assess the importance of random processes as a driving force of evolution.
Mutations are random, spontaneous and heritable (if in the germ-line) changes in DNA. If mutations were not random then one would assume they would be largely beneficial to result in increased fitness of the individual. This random process generates variation and leads to heterozygosity. Upon this, natural selection can act to fix beneficial mutations or eliminate deleterious mutations and without these, evolution would quickly cease (Whitlock, 2000). The selection acting upon these mutations increases the frequency of these beneficial alleles in the population, increasing fitness, promoting specialisation and therefore resulting in adaptive radiation (Stix, 2009). For example the mutation that allowed aggregation of cells gave rise to multicellular organisms and the subsequent radiation (Mayr, 1978).
The discovery in the 1960’s that polymorphism (2 or 3 forms of an allele at the same locus) was fairly common, despite selection predicting it to be low led to the rise of the neutral theory. This theory suggested that many mutant alleles are neutral and have no selective advantage or disadvantage (Kimura, 1984). For any neutral mutation in the population, it is equally likely to be lost or to reach fixation and the probability of fixation is higher if the initial allele frequency is higher. It predicts a constant rate of evolution if all species have the same generation time, dependent on mutations. The fate of these neutral mutations is largely determined by another random process known as genetic drift, which causes the frequency of these alleles to fluctuate over time. Genetic drift is essentially random sampling of the gene pool at each generation, meaning that no new alleles can be gained, but some may be lost as they are not passed on to the next atherefore reduces heterozygosity and genetic variation. If neutral mutations are occurring frequently and random genetic drift is constant over a long period of time then the genetic composition of a population would show significant changes over this time period (Kimura, 1984), suggesting that drift on its own is a strong evolutionary force. This model was later updated to the ‘nearly-neutral theory’ in which there is a slight selective difference between alleles, which makes population size more important.
In cases where the effective population size is smaller, random genetic drift is more important than selection at determining the fate of new alleles (Whitlock, 2000) as larger populations can harbour mutations for longer. In smaller populations, beneficial and deleterious alleles are more likely to act as though they are neutral mutants. This means many beneficial alleles, unless being strongly acted upon by selection, will be lost. One example of a reduced effective population size is the bottleneck effect. This is when there is a sudden and sharp decrease in population size as a result of an event such as a natural disaster e.g. forest fire. When the population size is rapidly reduced in this way, the average heterozygosity is likely to decline as low frequency alleles are eliminated (Nei et al., 1975). The combination of mutations and genetic drift results in a form of evolution that is non adaptive. It does not follow the same direction of natural selection to increase fitness, but instead acts in a ‘random walk’ fixing alleles and changing genetic composition. This provides evidence that evolution is not always selective as initially proposed by Darwin.
However mutations are a relatively rare event and it has been suggested that much of the variation in a population actually arises from the reshuffling of previous mutations (Ayala, 1978). This is particular true in larger populations that have a greater standing variation where the random process of recombination during sexual reproduction can reveal new combinations of alleles that may present a selective advantage. The recombination during meiosis does not change allele frequencies, just exposes new combinations. In these larger populations, selection is more deterministic and any new combination of alleles that presents a slight phenotypic selective advantage will be selected upon. This also provides additional evidence that neutral mutations are harboured within a population as they may one day confer a selective advantage as the biotic and abiotic conditions change. It can only aid selection to increase fitness if there are negative associations between favourable alleles, meaning there is linkage disequilibrium that could have been caused by random genetic drift (Barton, 2010).
Arguably, random processes and events underlie much of evolution and throughout evolutionary history. Mass extinction events tend to be random but result in empty niches which lead to huge adaptive radiations. Equally, mutations leading to new innovations such as being able to colonise land leads to big radiations too. The effect of random processes also depends on the effective population size, as previously mentioned, in smaller populations, random genetic drift is more deterministic than selection and vice versa in large populations. Even when selection appears to be stronger than the random process of genetic drift, it is acting upon variation provided by random mutations and novel combinations of alleles presented due to recombination.
The random and non-random process of evolution are all interdependent and none of them act in isolation (Andrews, 2010). Mutations are the ultimate source of evolution, upon which genetic drift randomly fluctuates their frequency and natural selection acts to fix or eliminate them from the population. Migration or gene flow acts to move allele frequencies towards the average for the metapopulation and as a result can reduce the likelihood of speciation events and gene pools become less isolated. The importance of natural selection cannot be denied, and it is the only mechanism that works towards adaptive evolution, however it could not work without the underlying random processes that generate variation. In conclusion, no single one of these mechanisms is more important than the other, but work together to generate genetic diversity and evolve populations.