Definitions of Evolution

If you were to wander the streets of Morris asking, "How do you define the word 'evolution'?", you'd find that most people's definition boils down to the most basic elements of the concept: "change through time." This is the general, non-technical definition. in our everyday language, we use the word evolution just to mean change, and generally with a positive connotation. This is what we mean when we say a politician's opinions on a topic have "evolved", or that the university has "evolved" to better meet student needs.

The problem with this definition is that it's different from the definition that we use in anthropology, biology, and other sciences. If we don't recognize those differences, it causes a lot of confusion and mis-communication.

The technical, biological definition of evolution is a change in allele frequencies within a population over time. In other words, gene variations within a population become more or less common from one generation to the next. That's evolution.

Note that by this biological definition, evolution cannot happen to one individual. One individual doesn't have gene frequencies (each person has two sets of genes, one from each parents), and one individual's genes cannot change over time (only from generation to generation). Evolution, therefore, only happens to populations, to large groups of individuals.

The Four Forces of Evolution

Evolution (the change in allele frequencies over time) can occur in a variety of different ways. The most important reasons for allele frequencies to change are called the four forces of evolution: mutation, gene flow, genetic drift, and natural selection.

Evolution is not the same as natural selection, gene flow, genetic drift, or mutation. Evolution is the event itself, while the forces of evolution are the causes of that event. Consider an analogy: death is an event or process. A parrot dies, just like a population evolves.* What causes the parrot to die, though, is different from the death itself. The parrot could be a victim of homicide, disease, or an accident. Similarly, what causes evolution to occur is different from evolution itself. A population could evolve because of natural selection, gene flow, genetic drift, or mutation. The four forces of evolution may be better understood as the four causes of evolution.

Mutation

In this class, we won't go through the technicalities of mutation. We can simply consider a mutation to occur when an individual has a gene/allele combination or state that neither of its parents had. In other words, through a mistake in copying the DNA, a failure to pass on a chromosome, or through some other mechanism, an individual's genotype could not be predicted by looking at the genotype of either of their parents. When this happens, allele frequencies change over time in a population (evolution occurs).

Mutation is the only way that truly new genetic material can be added to a species. All other types of evolution change the frequencies of already existing genes, but mutation adds something new and different. Mutations can be good, bad, or neutral. Most are negative, but positive ones can catch on quickly if they offer an advantage to the bearer.

Gene Flow

Gene flow is the movement of genes from one population to another through interbreeding. This can change the frequency of genes in both populations. If gene frequencies change, then evolution has occurred, by definition.

Genetic Drift

Genetic drift is the role of chance in evolution. Gene frequencies change due to genetic drift, if the reason that they increase or decrease is related to luck, rather than their importance to survival. For example, if the gene for a certain eye color was lost from a population because the only carrier fell off a cliff, that loss is not natural selection (his eye color didn't make him fall off the cliff), but gene frequencies have still changed in the population. This is much more common in small populations than large ones, since large populations usually have a lot of carriers for genes, and losing a few by chance doesn't change their frequency significantly.

Founder's Effect is a type of genetic drift (but it is NOT the same, it's just one way that genetic drift can occur). Founder's effect is when a small population is isolated, and as a result, the gene frequencies in that small population are very different from the frequencies in the larger population.

Natural Selection

Natural selection is what most people think of when they hear the word "evolution". But natural selection isn't the same as evolution; it's only one way that allele frequencies can change through time. Natural selection is part of evolution, one process that can make it happen, just the way the cold virus is one thing that can make you sick, but it's not the same as "being sick", since there are lots of other ways that you can become ill.

Natural selection is "survival of the fittest". Some individuals are better at surviving and reproducing than others. A gazelle who runs fast is more likely to survive and pass his genes on to his offspring. If his genes are responsible for his ability to run fast, then his offspring may inherit that ability. Then they, too, will survive and pass their genes on, until those genes become more and more common in the gazelle population.

Being "fittest" doesn't necessarily mean that you're the strongest or in best shape. It may mean being the smartest, or resistant to a common disease, or the least tasty to predators, or more fertile under drought conditions. When we use the term "fittest", we're referring to the individuals who are most successful at reproducing. This is what we call reproductive fitness: an individual's ability to pass their genes down to the next generation.

In order to be subject to natural selection, a trait must be heritable (it must be able to be passed down through the genes); it must be variable in the population (some individuals must be different from others); and there must be competition between individuals.

Why does it have to be heritable? Because if it is not heritable, then it is not genetic. Since natural selection is a type of evolution, it only works if it is changing allele frequencies. If a gazelle is very fast because it practices running more than other gazelles, but it doesn't have any genetically-based tendency to be faster, then it might reproduce more, but it's offspring don't inherit the ability to run faster, so it's genetic contribution to the population doesn't increase in the subsequent generation.

Why does it have to be variable? Because natural selection can only work on genes that already exist. Natural selection can't create faster gazelles if all gazelles have the same set of genes that relate to running. If one gazelle has a mutant gene that helps it run faster, however, it will be more likely to survive, and if that gene is passed down to it's offspring, then they will be more likely to survive. This leads to the greater and greater representation of the gene in the population from generation to generation.

Why is competition necessary? There must be a real advantage or disadvantage (in terms of reproduction) to a particular trait, or there is no difference in survival. For example, if a gazelle has a mutant gene that allows it to run faster, however, there are no predators in the environment, so running fast has no effect on survival, then that gene won't be passed down at a greater or lesser rate than any other set of genes.

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*"'E's passed on! This parrot is no more! He has ceased to be! 'E's expired and gone to meet 'is maker! 'E's a stiff! Bereft of life, 'e rests in peace! If you hadn't nailed 'im to the perch 'e'd be pushing up the daisies! 'Is metabolic processes are now 'istory! 'E's off the twig! 'E's kicked the bucket, 'e's shuffled off 'is mortal coil, run down the curtain and joined the bleedin' choir invisible!! THIS IS AN EX-PARROT!!"