3.1 The four forces of evolution

3.1 The four forces of evolution#

We will focus here on four major forces driving patterns of genetic variation in populations. More details about each of these forces is described in future sections.

Force 1: Mutations#

Mutations are the fuel for evolution. They are also what makes studying our genomes interesting!

Any time a cell divides, the DNA must be replicated, and errors, or mutations, may be introduced. If these errors occur in germ cells such as sperm or eggs, the error will be passed on to offspring. That offspring can then spread the mutation to their offspring, who can then spread it to their offspring, and so on. And so a genetic polymorphism in the population is born. There are many types of genetic variation. However for most topics we will focus on single nucleotide polymorphisms (SNPs) which result in the change of a single base pair from one nucleotide to another (e.g. G to A).

Because mutation rates are relatively slow, we often assume most bases in our genome will have only been mutated at most once. (This isn’t really true. Most bases actually have been mutated at some point, but most never become common enough to be observed as SNPs). This forms the bases of the infinite sites model, which assumes that every new mutation occurs at a new site in the genome that was not previously mutated.

Two nice properties follow from this:

All copies of a particular variant in the present-day population likely descended from a single common ancestral chromosome. For example, consider a position in the genome for which the ancestral base is “G”. But some present-day individuals have an “A” at that site. We can usually assume all the copies of “A” variant descend from a single common ancestor.
The vast majority of genetic variants will be bi-allelic, meaning there are only two possible variants (called alleles). Note, SNPs with more than 2 alleles do exist, but are rare.

Force 2: Genetic drift#

As mutations arise, they are then spread to offspring, and some may eventually spread throughout the population. This is a stochastic process: the process happens randomly, within a finite population. As a result, there will be random fluctuations of allele frequencies over time. This phenomenon is known as genetic drift, which is described in more detail in section 3.3.

Force 3: Selection#

In some cases certain alleles can be advantageous, whereas others may be disadvantageous. These in turn increase or decrease your fitness, defined in evolutionary terms as the number of offspring you can produce. For example, an allele that causes a severe condition in which an individual does not to survive until adulthood has a severe negative fitness effect and is unlikely to be passed on to future generations. On the other hand, an allele that causes an advantage, such as the ability to digest lactose, might result in an individual being more likely to survive longer and produce more offspring, and so has a positive fitness effect. See section 3.4 for more details.

Force 4: Gene flow#

Finally, new genetic material may be introduced by admixture between population groups. This can take multiple forms. For example, we now know that our ancestors interbred with Neanderthal and other ancient human species. These ancient groups contributed to genetic material that is still in present-day populations. Gene flow can also occur when there is admixture between two or more groups that had been previously separated for some time. We will talk much more about admixture in coming weeks as we look at global vs. local ancestry and human history.