Variation Within Populations

The Raw Material of Evolution

Imagine a population of beetles living in a forest. If every single beetle were identical in colour, size, and metabolic rate, the population would be static. If the environment changed, for instance, if the forest floor became covered in dark moss, there would be no beetles with a dark colouration to hide from predators. The entire population would likely be eaten, leading to extinction. However, in reality, beetles vary. Some are slightly darker, some are slightly larger, and some have slightly different wing shapes. This variation is not random noise; it is a structured diversity generated by genetic processes. The key distinction to grasp here is between phenotypic variation and genotypic variation. Phenotypic variation refers to the observable differences in traits, such as the actual colour of the beetle's shell. Genotypic variation refers to the differences in the DNA sequences that code for those traits. While all phenotypic variation is influenced by the environment to some degree, only the component of variation that is genetically based is heritable. For example, a beetle that is darker because it has been exposed to more sunlight is not necessarily darker because of its genes. If that beetle reproduces, its offspring will not inherit the "sun-tanned" colour; they will inherit the genetic potential for colour. Therefore, evolution by natural selection requires that the variation in fitness-relevant traits has a genetic basis. This genetic variation arises primarily through mutation, which creates new alleles, and through sexual reproduction, which shuffles existing alleles into new combinations. This shuffling ensures that even in a stable environment, new phenotypes appear in every generation, providing the continuous supply of raw material that selection can act upon.

Sources of Genetic Variation

Genetic variation within a population is generated and maintained by several distinct mechanisms. The ultimate source of all new genetic variation is mutation. Mutations are changes in the DNA sequence. They can occur as point mutations, where a single nucleotide is changed, or as larger structural changes. While most mutations are neutral or deleterious, some are beneficial in specific environments. However, mutation rates are low: roughly \(10^{-9}\) to \(10^{-8}\) per nucleotide per generation in many eukaryotes (about \(1.2 \times 10^{-8}\) in humans), or about \(10^{-6}\) to \(10^{-5}\) per gene. Each generation therefore adds only a little new variation; most of the variation present at any moment is old variation carried forward. The primary driver of variation in sexually reproducing populations is recombination. During meiosis, homologous chromosomes exchange segments in a process called crossing over. Furthermore, the independent assortment of chromosomes during meiosis ensures that each gamete receives a unique combination of alleles. When two gametes fuse during fertilisation, the resulting zygote has a novel genetic makeup. This process, known as genetic shuffling, creates vast amounts of phenotypic variation without requiring new mutations. For instance, in humans, the number of possible chromosome sets from independent assortment alone (ignoring crossing over) is approximately \(2^{23}\) for each parent, leading to a theoretical maximum of \(4^{23}\) combinations in the offspring. This mechanism ensures that siblings are genetically distinct, providing the diversity necessary for selection to act upon.

where \(n\) is the haploid chromosome number. This exponential increase in diversity highlights why sexual reproduction is so effective at maintaining variation compared to asexual reproduction, which relies solely on mutation for new genetic combinations.

Quantifying Variation: The Variance Components

To understand how much of the observed variation is heritable, biologists use the concept of variance. The total phenotypic variance (\(V_P\)) in a population for a quantitative trait is the sum of the variance due to genetic differences (\(V_G\)) and the variance due to environmental differences (\(V_E\)). Mathematically, this is expressed as \(V_P = V_G + V_E\). This equation assumes that genetic and environmental effects are independent and additive. The genetic variance (\(V_G\)) can be further decomposed into additive variance (\(V_A\)), dominance variance (\(V_D\)), and interaction variance (\(V_I\)). Additive variance is the component that is most important for evolution by natural selection because it is the portion of genetic variance that is passed predictably from parents to offspring. Dominance variance arises from interactions between alleles at the same locus, while interaction variance arises from interactions between alleles at different loci (epistasis). The proportion of phenotypic variance that is due to additive genetic variance is known as narrow-sense heritability (\(h^2\)), defined as \(h^2 = V_A / V_P\). A high \(h^2\) indicates that the trait is strongly influenced by additive genetics and is therefore more likely to respond to selection. Conversely, a low \(h^2\) suggests that environmental factors play a larger role in determining the trait's expression.

It is crucial to note that heritability is a population-specific statistic. It does not describe the proportion of a trait that is "genetic" in an absolute sense, but rather the proportion of variation in that trait within a specific population at a specific time that is attributable to genetic differences. If the environmental variance decreases, heritability may increase, even if the genetic variance remains constant.

Maintaining Variation in the Face of Selection

A common misconception is that natural selection should eventually eliminate all variation, leaving a population of optimal, identical individuals. In reality, variation persists due to several mechanisms. First, balancing selection can maintain multiple alleles in a population. For example, heterozygote advantage occurs when individuals with two different alleles at a locus have higher fitness than homozygotes. A classic example is sickle-cell anaemia in humans, where heterozygotes have resistance to malaria, while homozygotes for the normal allele are susceptible to the disease, and homozygotes for the sickle-cell allele suffer from severe anaemia. This selective pressure maintains both alleles in the population. Second, mutation-selection balance ensures that new deleterious mutations are constantly introduced into the population, counteracting the removal of these alleles by selection. Third, spatial and temporal heterogeneity in the environment can favour different alleles in different places or times. If the environment fluctuates, selection may favour different phenotypes in different years, preventing any single allele from fixing. These mechanisms ensure that populations retain the genetic diversity necessary to adapt to future environmental changes, providing a buffer against extinction.