Where Mendel’s experiments with pea plants and Darwin’s observations of finches and their beak size merge, the field of population genetics begins. Mendel described how genes determine the traits of individuals and their offspring; Darwin observed that the traits of individuals of a population change over time as certain traits are more beneficial to the survival of individuals living in a certain environments. Ironically, at the time of their discoveries, most people believed that either Mendel or Darwin was correct in their thinking, but not both. We now understand how both Mendel’s and Darwin’s ideas fit together to create a complete picture of how one gene affects one organism; how organisms come together with specific genes to create populations; and how the environment imposes forces on the genes of a population to ensure its survival over time.
A species is a group of organisms that can reproduce to produce offspring that are capable of reproducing. A population is a subdivision of a species that shares a common environment. Members of a population must live in close proximity to other members without physical barriers, such as mountains, walls, or bodies of water. A population’s gene pool is all the genes of all the members of that population. The frequency of genes and alleles within a gene pool and how the gene pool changes over time is the primary focus of population genetics.
To begin to understand how genes behave within a population, it is important to describe the conditions that prevent a population’s gene frequencies from changing, and thus prevent the population from evolving. These factors were discovered independently by Godfrey Hardy and Wilhelm Weinberg in 1908. The Hardy-Weinberg principle states that the frequency of alleles in a population will remain constant provided the following conditions exist:
- The population is large.
- Members of the population mate randomly.
- Mutations do not occur.
- Selection does not occur.
- Members of the population do not leave and new members are not added to the population.
However, as Darwin first observed, the traits in a population do change or evolve over time. These changes are the result of factors that cause genetic changes within the gene pool of a population. These factors include: natural selection, mutation, migration, and genetic drift.
Crucial to the evolution of a species, and thus its survival is the mechanism of natural selection. Natural selection is commonly known as “survival of the fittest.” It is a process that promotes the traits within a population that are beneficial to the survival of members of that population. Natural selection increases the frequency of the alleles for this beneficial trait. The presence of traits that increase an organism’s rate of survival will also increase its rate of reproduction. This cycle will increase the number of “strong” members of a population by eliminating “weak” alleles. The repetition of this process is the evolution of a population.
The change in the frequency of alleles within a population can also result from “error” or mutation. A mutation is a change in the DNA sequence of a cell. Mutation is a very important factor in causing change in a gene pool because it can be a source of new genetic material in a population, which can be spread to other populations. Mutations can be caused by one of many environmental exposures, UV radiation from the sun, chemicals, or viral infections. They can also be caused by errors during DNA replication or meiosis. Mutations can harm, benefit, or have no effect on an organism, but within a population, it is a source of new alleles and new potential for change.
Migration or gene flow is another factor that can alter the gene pool of a population. When a member of one population moves to another population the gene pool of that population is altered. The new member can introduce new alleles that were created by mutations or bring back alleles that were lost when members leave a population. It has the effect of making populations within a species more similar to one another and more capable of adapting.
Genetic drift is a change in the frequency with which an allele occurs in a population due to random events. When alleles are separated during meiosis, the allele that will be selected during fertilization to produce offspring is a random event. Other random events might include an event that randomly causes the demise of an organism, thereby eliminating certain alleles. Over time, this random sampling of genes eliminates some alleles from the population, thereby altering the gene pool. Genetic drift is most visible in very small populations. The importance of this effect is debated due to its random nature that is absent of the goal of promoting stronger members of a population.
Within a population, the combined effects of natural selection, mutation, and genetic drift exhibit a controlling force on populations with an overall intent of increasing the survivability of a species. A species can change over time in big ways through a series of small changes within populations exerted by environment, error, and chance. These changes can be followed from organism to population to species to evolution of the species.