Darwin meets Mendel—not literally
When Darwin came up with his theories of evolution and natural selection, he knew that the processes he was describing depended on heritable variation in populations. That is, they relied on differences in the features of the organisms in a population and on the ability of these different features to be passed on to offspring.
Darwin described evolution as "descent with modification," the idea that species change and give rise to new species over extended periods of time and that all species can trace their descent to a common ancestor. Today, evolution is typically defined as a change in the genetic makeup of a population over generations—a definition that encompasses both the large-scale evolution Darwin envisioned and the smaller-scale processes we'll discuss in this article.
Natural selection is the mechanism that Darwin proposed to explain how evolution takes place and why organisms are typically adapted, well-suited, to their environments and roles.
The basic idea of natural selection is that organisms with heritable traits that help them survive and reproduce—in a certain environment—will leave more offspring than organisms without those traits. Because the traits are heritable, they will be passed on to the offspring, who will also have a survival and reproduction advantage. Over generations, differential survival and reproduction will lead to a progressive increase in the frequency of the helpful traits in the population, making the population as a group better-suited to its environment.
Natural selection is not the only mechanism of evolution. Populations can also change in their genetic composition due to random events, migration, and other factors. However, natural selection is the one mechanism of evolution that consistently produces adaptation, a close fit between a group of organisms and its environment.
Darwin did not, however, know how traits were inherited. Like other scientists of his time, he thought that traits were passed on via blending inheritance. In this model, parents' traits are supposed to permanently blend in their offspring. The blending model was disproven by Austrian monk Gregor Mendel, who found that traits are specified by non-blending heritable units called genes.
Although Mendel published his work on genetics just a few years after Darwin published his ideas on evolution, Darwin probably never read Mendel’s work. Today, we can combine Darwin’s and Mendel’s ideas to arrive at a clearer understanding of what evolution is and how it takes place.
Microevolution is sometimes contrasted with macroevolution, evolution that involves large changes, such as formation of new groups or species, and happens over long time periods. However, most biologists view microevolution and macroevolution as the same process happening on different timescales. Microevolution adds up gradually, over long periods of time to produce macroevolutionary changes.
Let's look at three concepts that are core to the definition of microevolution: populations, alleles, and allele frequency.
A population is a group of organisms of the same species that are found in the same area and can interbreed. A population is the smallest unit that can evolve—in other words, an individual can’t evolve.
An allele is a version of a gene, a heritable unit that controls a particular feature of an organism.
For instance, Mendel studied a gene that controls flower color in pea plants. This gene comes in a white allele, w, and a purple allele, W. Each pea plant has two gene copies, which may be the same or different alleles. When the alleles are different, one—the dominant allele, W—may hide the other—the recessive allele, w. A plant's set of alleles, called its genotype, determines its phenotype, or observable features, in this case flower color.
Genotype—pair of alleles
Allele frequency refers to how frequently a particular allele appears in a population. For instance, if all the alleles in a population of pea plants were purple alleles, W, the allele frequency of W would be 100%, or 1.0. However, if half the alleles were W and half were w, each allele would have an allele frequency of 50%, or 0.5.
In general, we can define allele frequency as
Total number of A/a gene copies in population
Number of copies of allele Ain population
start subscript, i, end subscript_ alleles of a gene). In that case, you would want to add up all of the different alleles to get your denominator.
Let’s look at an example. Consider the very small population of nine pea plants shown below. Each pea plant has two copies of the flower color gene.
The frequencies of all the alleles of a gene must add up to one, or 100%.
Phenotype frequency: How often we see white vs. purple
Allele frequency: how often we see each allele