Question

What role does mutation play in population change? how does the mechanism of evolution change between differing study systems?

Answer 1

Evolution by mutation occurs whenever a mistake in the DNA occurs in the heritable cells of an organism. In the single-celled asexual organisms, such as bacterial, the whole cell and its DNA is passed on to the next generation because these organisms reproduce via binary fission. For sexual organisms, mutations are passed to the next generation if they occur in the egg or sperm cells used to create offspring. Mutations occur at random in the genome, but mutations of large effect are often so bad for the organism that the organism dies as it develops, so mutations of smaller effect or even neutral mutations are theoretically more common in a population. The variation that is created in a population through the random process of mutation is called standing genetic variation, and it must be present for evolution to occur. Mutation is the raw stuff of evolution because it creates new heritable phenotypes, irrespective of fitness or adaptation. Mutation rates are actually pretty low for most genes, ranging from 10^-6 for the average human gene to 10^-10 (per base pair) for the average bacterial gene.

Because mutation rates are low relative to population growth in most species, mutation alone doesn’t have much of an effect on evolution. But mutation combined with one of the other mechanisms of evolution (genetic drift, natural selection, non-random mating, and gene flow) can result in meaningful changes in allele frequencies in a population.

Evolution by genetic drift causes changes in populations by chance alone

Evolution by genetic drift occurs when the alleles that make it into the next generation in a population are a random sample of the alleles in a population in the current generation. By random chance, not every allele will make it through, and some will be overrepresented while other decline in frequency regardless of how well those alleles encode for phenotypic suitability to the environment, so sometimes drift reduces the average fitness of a population for its environment. Populations are constantly under the influence of genetic drift. The random drifting of allele frequencies always happens, but the effect is subtle in larger populations. In these cases, the signal of genetic drift is easily swamped out by the stronger effects of selection or gene flow, so we often ignore drift except in small or endangered populations, where a random draw of alleles can dramatically change the population’s chance of survival in the next generation.

Evolution by gene flow (migration) makes two different populations more similar to each other

Two different populations are often subject to different selective pressures and genetic drift, so they would be expected to have different allele frequencies. When individuals from one population migrate into a different population, they bring those different allele frequencies with them. If enough migration and mating occurs between two populations, then the two populations will experience changes in allele frequencies and such that their allele frequencies become similar to each other.

Non-random mating results from mate choice

Selecting a mate at random is a pretty risky idea because half of your offspring’s genes come from your mate. Non-random mating is a more common approach in real populations: think about male birds being selected as mates by females who choose males for their vivid colouration or beautiful and complex birdsong. There is evidence that fish, birds, mice, and primates (including humans) select mates with different HLA genotypesthan themselves. We humans also tend to mate more often with individuals who resemble us phenotypically (positive phenotypic assortment). Non-random mating with “like” individuals will shift the genotype frequencies in favour of homozygotes, while non-random mating with “unlike” individuals (negative phenotypic assortment) creates an over-representation of heterozygotes. These shifts can occur without changing the proportion of each allele in the population, also called the allele frequency.

Natural selection can cause microevolution, or a change in allele frequencies over time, with fitness-increasing alleles becoming more common in the population over generations. Fitness is a measure of relative reproductive success. It refers to how many offspring organisms of a particular genotype or phenotype leave in the next generation, relative to others in the group.

Natural selection can act on traits determined by different alleles of a single gene, or on polygenic traits (traits determined by many genes). Polygenic traits in a population often form a bell curve distribution. Natural selection on polygenic traits can take the form of:

•             Stabilizing selection: Intermediate phenotypes have the highest fitness, and the bell curve tends to narrow.

•             Directional selection: One of the extreme phenotypes has the highest fitness. The bell curve shifts towards the more fit phenotype.

•             Disruptive selection: Both extreme phenotypes have a higher fitness than intermediate phenotypes. The bell curve develops two peaks.

For speciation to occur, two new populations must be formed from one original population, and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists often divide the ways that speciation can occur into two broad categories:

•             Allopatric speciation—allo meaning other and patric meaning homeland—involves geographic separation of populations from a parent species and subsequent evolution.

•             Sympatric speciation—sym meaning same and patric meaning homeland—involves speciation occurring within a parent species remaining in one location.

Mutation

Mutation, a driving force of evolution, is a random change in an organism’s genetic makeup, which influences the population’s gene pool. It is a change in the nature of the DNA in one or more chromosomes. Mutations give rise to new alleles; therefore, they are a source of genetic variation in a population.

Mutations may be harmful or benign, but they may also be beneficial. For example, a mutation may permit organisms in a population to produce enzymes that will allow them to use certain food materials. Over time, these types of individuals survive, while those that don’t have the mutations are more likely to perish. Therefore, natural selection tends to remove the less-fit individuals, allowing more-fit individuals to survive and form a population.

Gene flow

Another mechanism of evolution may occur during the migration of individuals from one group or location to another. When the migrating individuals interbreed with the new population, they contribute their genes to the gene pool of the local population. This establishes gene flow in the population.

Gene flow occurs, for example, when wind carries seeds far beyond the bounds of the parent plant population. As another example, animals may be driven off from a herd. This forces them to migrate to a new population, thereby bringing new genes to a gene pool. Gene flow tends to increase the similarity between remaining populations of the same species because it makes gene pools more similar to one another.

Darwin’s theory included the observation that evolutionary changes take place slowly. In many cases, the fossil record shows that a species changed gradually over time. The theory that evolution occurs gradually is known as gradualism.

In contrast to gradualism is the theory of punctuated equilibrium,which is a point of discussion among scientists. According to the theory of punctuated equilibrium, some species have long, stable periods of existence interrupted by relatively brief periods of rapid change.

Both groups of scientists agree that natural selection is the single most important factor in evolutionary changes in species. Whether the change is slow and gradual or punctuated and rapid, one thing is certain: Organisms have evolved over time.