Mechanisms of Evolution

Learning Objectives

1. Identify, explain, and recognize the consequences of the mechanisms of evolution in terms of fitness, adaptation, average phenotype, and genetic diversity
2. Know and recognize the five assumptions of the Hardy-Weinberg principle
3. Use the gene pool concept and the Hardy-Weinberg principle to determine whether a population is evolving at a locus of interest

Biologists organize their thinking about biological processes using evolution as the framework. There are four key mechanisms that allow a population, a group of interacting organisms of a single species, to exhibit a change in allele frequency from one generation to the next. These are evolution by: mutation, genetic drift, natural selection, and gene flow. Each type of evolution can be characterized by how it affects fitness, adaptation, the average phenotype of a trait in a population, and the genetic diversity of the population.

Mutation generates variation

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 for the average bacterial gene (from https://bionumbers.hms.harvard.edu/).
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 natural selection results in individuals that are a better fit to their environment

Evolution by natural selection occurs when the environment exerts a pressure on a population so that only some phenotypes survive and reproduce successfully. The stronger the selective pressure or the selection event the fewer individuals make it through the sieve of natural selection. Those phenotypes that survive a strong selection event, such as a drought, are a better fit for an environment that suffers drought. Another way to say this is that they have higher Darwinian fitness.
The finches on the Galápagos islands have provided a robust study system for observing natural selection in action over the past decades (see the work of Peter and Rosemary Grant and their collaborators). The small finches on the island of Daphna Major have strong beaks to feed on seeds. Smaller beaked birds can only crack open the smallest seeds, while birds with larger beaks prefer larger seeds. In 1977, drought reduced the number of small seeds, so many small-beaked finches starved to death.

In the finch example above, the average phenotype has shifted so most individuals have larger beaks, which is a genetically controlled-trait in the finches. The larger beak size is an adaptation to the seed sizes available during drought conditions. A result of this shift is that small beak phenotypes have become rare or disappeared, so there is reduced phenotypic and therefore reduced genetic diversity in the finch population after selection.
When a population displays a normal distribution for a particular trait, natural selection can drive change in populations in different directions depending on the type of selection. Stabilizing selection results in a narrowing of the normal distribution, because individuals who had the ‘average’ phenotype, or the phenotype closest to the mean, tend to leave more offspring than those with phenotypes at either extreme. Directional selection results in a shift toward one end of the normal distribution, because individuals who had one extreme of the phenotype tend to leave more offspring than those with the other extreme. Disruptive or diversifying selection results in separation of the normal distribution into two distributions with elimination of the middle of the peak, because individuals with either extreme phenotype tend to have more offspring than those with the intermediate phenotype.
The image below illustrates these three types of selection:

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 genotypes than 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.
Watch this Ted Ed video to review these concepts with an easy way to remember them:

 

Measuring Evolutionary Change: the Hardy-Weinberg Equilibrium Principle

How would a researcher know if selection or drift or even mutation were altering the allele frequencies for population? In other words, can we use the mechanisms above to detect evolution happening in real populations? To do that we’d need a null expectation or a baseline against which to measure change. We call that baseline the Hardy-Weinberg equilibrium (HWE). To calculate what the alleles frequencies (p and q in the example below) should be in the absence of any evolution, we need to assume that the population is undergoing no selection, no mutation, no drift, no gene flow, and that individuals are selecting mates at random.
Also recall that each individual is a diploid, carrying two copies (alleles) of each gene. Assume that the entire population only has two variants, or alleles, for a gene for pea color. Individuals that carry at least one Y allele have yellow coloration, while those who carry two copies of the y allele are green. If the frequency of the y allele is 0.1 = q, then the frequency of the normal allele is p = 1 – q = 0.9. Hardy-Weinberg equilibrium assumes those frequencies will not change from one generation to the next. To show that mathematically, we need to count the alleles in each generation. For instance, we can start by saying that if q = 0.1, then the green pea plants, who have two copies of y, have a genotype frequency of q^2 = 0.01. Likewise the yellow homozygotes have a frequency of p^2 = 0.81. But there’s a third type, the heterozygote, that has one copy of each. We can simply subtract the frequencies to see what the proportion of heterozygotes is: 1 – 0.81 – 0.01 = 0.18. Hardy-Weinberg gives us the simple equation to figure out where the alleles are, assuming no evolution: p^2 + 2pq + q^2 = 1. So, 0.18 = 2pq. Why is that? A heterozygote can inherit it’s green color y allele from either parent, mom or dad. Because there are two ways to arrive at the probability of a heterozygous state (pq), we need to add those two options together, and pq + pq = 2pq.

 
Below is a Crash Course Biology video on Population Genetics that explains Hardy-Weinberg equilibrium dynamically…using ear wax phenotype in humans.

 
Recommended Readings
Grant and Grant. 2002. Unpredictable Evolution in a 30-Year Study of Darwin’s Finches. Science 296: 707-711.