Introduction

Evolution occurs at the population level (a population is defined as a group of individuals of the same species in a given area). Because the individuals of a population can interbreed, they share a common group of genes known as the gene pool (all the alleles for all the traits for the population). For evolution to occur, some of the allele frequencies must change with time.

Two investigators, English mathematician Godfrey Hardy and German physician Wilhelm Weinberg theorized that the frequencies of a population’s genotypes and alleles are always stable or in equilibrium unless some type of force acts on that population. Population geneticists have predicted five causes that can disturb this equilibrium or cause Evolution. These five are mutation, gene flow, genetic drift, non-random mating, and natural selection.

Learning

Let us look at the conditions that cause Evolutionary change:

Mutation

Mutations are merely changes to a DNA sequence. The times where mutations can occur:

• Cell Division – in Eukaryotes, during the S-Phase, DNA is replicated; errors in nucleotide base pairing can occur during this process.
• Exposure to radiation – can cause ‘breaks’ in the DNA chain; when these ‘breaks’ are repaired by the cell, sometimes incorrect nucleotide bases can be inserted into the DNA molecule, resulting in a mutation.
• Exposure to certain chemicals – can reduce the accuracy of the enzymes responsible for DNA replication and/or repair, which can lead to an increase in mistakes/mutations.

To have an effect on evolution, we must also keep in mind that a mutation has to be ‘heritable’ (passed on to offspring), so it must occur within the gametes (sperm or egg).  The only way for an offspring to ‘inherit’ the mutation, it must be obtained from either the mother’s egg cell or the father’s sperm cell. 

On average, only about 1 in every 100,000 gametes will contain a mutant gene, making mutation a rare, but important force in shaping evolution.

The most important concept to understand about mutation is the fact that mutations occur randomly – that is, mutations never arise because of a need for the organism.  In some cases, the mutation may benefit the organism; in some cases, the mutation may harm the organism; in most cases, there will be a neutral effect on the organism. With that in mind, it is important to recognize that mutations merely provide the potential for evolutionary change to occur, but mutation itself is never the driving force behind evolutionary change. The flower color allele on the roses pictured has mutated, so now there are two colors.

Natural Selection

New alleles generated by mutation are rare and are often ‘neutral’ in their ability to provide any benefit to the individual possessing the allele.  However, in some instances, alleles do confer an advantage to their possessor.  When an allele (or trait) helps an organism to survive or to reproduce, that is what we refer to as ‘natural selection’.

The environment ultimately “selects” individuals with the best phenotypes to survive to reproduce.  Those individuals who have more surviving offspring pass on more of their genes to the next generation resulting in a shift in allele frequencies favoring their alleles.

Natural selection constantly acts as a ‘selector’ of winner and loser alleles. If the right combination does not exist within a population to survive the environment, that population may go extinct!

Non-Random Mating

In almost all cases in the animal kingdom, mating occurs in a decidedly non-random fashion. Because animals display biases that influence the selection of mates, this can have a profound impact on the composition of the gene pool.

In human populations, people select mates non-randomly for traits that are easily observable. Cultural values and social rules primarily guide mate selection. Most commonly, mating is with similar people in respect to traits such as skin color, stature, and personality.

Animal breeders do essentially the same thing when they intentionally try to improve breeds or create new ones by carefully selecting mating pairs.  When they select mates based on desired traits, farmers hope to increase the frequency of those traits in future generations of farm animals. 

Even without the intervention of farmers, most animals select mates carefully.  Charles Darwin noted this fact in his 1871 book “Descent of Man and Selection in Relation to Sex”.  He suggested that mate selection is a powerful force of evolution.  This idea was widely rejected in Darwin’s time, but later research showed that he was correct.

Genetic Drift

Genetic Drift is defined as the “random chance events which can result in changes in allele frequencies within a population”. When random chance events interfere with an individual’s survival or ability to mate, that individual’s alleles are essentially ‘removed’ from the gene pool. There are two types of Genetic Drift which can shape the allele frequencies of a population:

• Population Bottleneck
• Founder Effect

Population Bottleneck

Catastrophic events, such as natural disasters, can have a huge impact on the survival of individuals within a population. When chance catastrophic events occur, there is the potential to remove a significant number of alleles from the gene pool and affect the overall allele frequencies for that population.

Now, assume we have a population of beads and a catastrophic event occurs where the number of beads is drastically reduced.  In this example, we will visually ‘restrict’ the number of ‘surviving’ beads through the neck of the bottle.

By chance, the genetic structure of the surviving beads becomes the entire population’s surviving gene pool which may be very different from the pre-disaster beads/population.

If allowed to grow, subsequent generations of this population would now reflect a higher proportion of surviving beads and differ from the makeup of the original population.

Founder Effect

The founder effect occurs when some portion of the population leaves to start a new population in a new location or a physical barrier arises dividing a population. The genetic structure of the original population changes to match that of the new population’s founding members.

Gene Flow

Gene Flow is defined as the “movement of alleles between populations” and results in the change in allele frequencies within populations.

This variable flow of individuals in and out of the group not only changes the population’s gene structure, but it can also introduce new genetic variation to populations in different geological locations and habitats.

Summary

• Mutation – New alleles generated by mutation are rare and are often ‘neutral’ in effect.
• Natural Selection – Individuals who have more surviving offspring pass on more of their genes to the next generation resulting in a shift in allele frequencies.
• Non-random mate selection – a powerful force of evolution.
• Genetic Drift –
• Bottleneck – when a catastrophic event drastically reduces the population.
• Founder Effect – a population leaves to a new location.
• Gene Flow – the movement of alleles between populations, resulting in an overall shift in allele frequencies.

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Sources:

“Mechanisms of Evolutionary Change.” https://www.khanacademy.org/science/ap-biology/natural-selection/hardy-weinberg-equilibrium/a/hardy-weinberg-mechanisms-of-evolution Licensed CC BY-NC-SA 4.0

Gene Flow and Mutation. (2020, August 15). Retrieved May 24, 2021, from https://bio.libretexts.org/@go/page/13484

Population Genetics.” By OpenStax Biology 2e. Retrieved from: https://openstax.org/books/biology-2e/pages/19-2-population-genetics Licensed under: CC-BY: Attribution