Evolution, Natural Selection, and the Theory of Evolution by Natural Selection

It is very common for people to use the terms “evolution,” “natural selection,” and “theory of evolution” interchangeably, but, in reality, all three of these terms refer to different things and it is important to keep them straight. In this post, I will explain the differences between and give a brief primer on what evolution actually is and how it actually works.


In technical terms, evolution is the change in a population’s allele frequencies over time. In other words, evolution is simply a change in the genetic makeup of a population. It is very important to point out that populations evolve, not individuals. Because evolution deals with changes in the gene frequencies, it is impossible for an individual to evolve. There are, however, a great many ways for populations to evolve.

For example, suppose that a population of 1,000 frogs is living at the base of a volcano. Nine hundred of the frogs (90%) have genes for being green, and 100 (10%) have the genes for being brown (to keep the math simple, let’s assume complete dominance and that all individuals are homozygous). Now, imagine that the volcano erupts and kills off 500 frogs, but, just by chance the eruption missed all of the brown frogs and only killed green frogs. Now, the remaining population has 400 green frogs (80%) and 100 brown frogs (20%). Evolution has occurred because the genetic makeup of the population has changed (brown genes went from 10% to 20%). This is an example of an evolutionary mechanism known as genetic drift.

To keep running with the frog example, let us now suppose that 100 additional individuals immigrate into the population from a neighboring population, and let’s suppose that they are all brown. Now, we have 400 green frogs (66.667%) and 200 brown frogs (33.333%). Our fictional population has once again evolved because the frequency of genes has again changed (i.e., the brown gene became more frequent or abundant). This example illustrates an evolutionary mechanism known as gene flow. There are several other mechanism that can cause populations to evolve, but I am only going to talk about one of them: natural selection.

Natural selection

Hopefully at this point it is obvious that natural selection and evolution do not mean the same thing. Rather, natural selection is one of the mechanisms that causes evolution. Natural selection is a fairly simple concept that requires only three things:

  1. Variability for a trait within a population (i.e., not all individuals are the same)
  2. Heritability for that trait (i.e., parents can pass the trait to their offspring)
  3. Selection for that trait

We know from countless studies that all three of these criteria are almost always met. Also, anytime that those three criteria are met, natural selection will occur and will cause evolution. This is a mathematical certainty. In fact, we can actually calculate the amount of evolutionary change that will take place, but I won’t go into the details of the math here.

Let me give an example to illustrate how this works. Suppose we have a population of lizards. Obviously, different individuals are going to have different body sizes (variation), and body size is heritable. So large individuals should produce offspring that are, on average, larger than the average size of the population, and small individuals should produce offspring that are, on average, smaller than the average body size of the population. Now, let’s propose the very realistic scenario that large individuals are able to escape predators more frequently than smaller individuals. This results in larger individuals living longer than smaller individuals which means that they produce more offspring than smaller individuals, which results in them passing on more genes to the next generation. In other words, the genes for being large will be slightly more common in the second generation than they were in the first, and the second generation will be, on average, slightly larger than the first. The second generation will then go through the same process of selection: larger individuals will live longer and produce more offspring, therefore they will pass on more genes. So, large genes will be slightly more common in the third generation and, on average, individuals will be slightly larger than they were in the second generation. This process will keep occurring until either the population reaches an optimal body size (this is an equilibrium state at which being any larger is no longer advantageous) or the environment changes such that being larger is no longer beneficial. That, in a nutshell is natural selection. It is an amazing process that can result in almost anything you can imagine.


Mutations in some ways are actually the opposite of natural selection. Natural selection, if left to itself, actually decreases genetic diversity. In other words, if allowed to run without interference, natural selection would eventually reach a stopping point at which there would be no variation for a trait. Mutations counteract this by generating new genetic information (note: it is a common myth that scientists have never documented a mechanism for creating new genetic material, in reality, we know of a great many mutations that can add genetic information). So natural selection and mutations are, in many ways, opposing forces. Natural selection reduces variability, and mutations increase it. Ultimately, this is a good thing, because natural selection can act on the new genetic information provided by mutations, resulting in the evolution of new traits that were not possible prior to those mutations. So mutations are necessary for natural selection to continue, because without them, populations would eventually consist entirely of “clones,” and natural selection would grind to a halt (fun fact, lab mice are actually genetically identical “clones” that were created by carefully inbreeding them until there was no variation left).

It is common to hear people argue that evolution can’t be true because mutations are usually harmful, but this is a misnomer. In fact, most mutations are neutral, a few are harmful, and a few are beneficial. The neutral ones are irrelevant, but the harmful ones are selected against, and the beneficial ones are selected for (in other words, harmful mutations becomes less common in a population and beneficial mutations become more common). So there is no reason why the nature of mutations would prevent natural selection from working.

The theory of evolution by natural selection

Before I explain the theory, I want to point out that even ardent young earth creationist organizations like Answers in Genesis agree with everything that I have said thus far. It is obviously undeniable that both evolution and natural selection occur. Creationists simply take issue with the scope of evolution. In other words, they place arbitrary limits on it and say, for example, that it is possible that all species of finch evolved from a common ancestor, but it is not possible that all birds evolved from a common ancestor. This division has no scientific support and is completely arbitrary and logically invalid, but I will deal with it in detail in a later post. For now, I simply want to point out that the concept that all life on this planet evolved from a common ancestor is considered a scientific fact (despite creationists vain arguments). So, the theory of evolution by natural selection simply states that natural selection has been the primary driver of the evolution of life on this planet. Recall that there are multiple mechanisms that can cause evolution. So this theory proposes that natural selection has been the most important of those mechanisms.

In a previous post, I stated that theories explain facts. That is exactly what is happening here. The theory of evolution explains the fact that life on this planet has evolved for billions of years. Also, please note that the theory of evolution has absolutely nothing to do with the origins of life (that’s the theory of abiogenesis) or the origins of the universe (that’s the big bang theory). It only deals with what happened after life formed. So even if you could disprove the big bang or abiogenesis, you would have done nothing to the theory of evolution.

So to briefly summarize, evolution is simply a change in the genetic makeup of a population over time, natural selection is one among several mechanisms that cause evolution, and the theory of evolution by natural selection states that natural selection has been the primary driver of evolution on this planet.

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