12.1: Background

We have been studying genetics and the biological explanations for variation of traits in living things. Mendel described “particles” that were passed from parent to offspring. We now know those particles were genes, and that variation in the traits he studied was the result of inheriting different variants, alleles of each gene. It is important to have a good foundation in genetics to understand evolution.

Evolution is formally defined as a change in allele frequency over time. In other words, if you analyze a population of organisms today, and then sometime in the future, you may note a shift in the percentages of specific alleles. Since evolution is merely a change in allele frequency over time, then all life is constantly evolving. What fuels these changes?

The principal process involved in evolutionary change is natural selection, which is defined as “differential survival and/or reproduction.”

There are three requirements that must be met for natural selection to actually take place:

1. There must be variation among the members in the population for the trait in question

2. The variants of the trait must result in differences in survival and/or reproduction.

3. The trait in question must be controlled to some degree by genes (e.g., when you dye your hair, this new variant is not controlled by genes).

There is one well-known classic case of natural selection: melanism mutation in peppered moths. Prior to the Industrial Revolution in England, nearly every peppered moth had light-colored speckled wings, only some months had the mutated black peppered phenotype. This coloration enabled the moths to blend in on the lichen-covered tree trunks. The much rarer black peppered moths were so obvious that birds (the primary killer [i.e., the important "selector"] of peppered moths) easily caught and killed them. The result is that these forms differed in survival probability and thus the light-colored moths survived and reproduced. Black moths were only maintained because of the rare mutation.

The Industrial Revolution was responsible for the production of huge amounts of soot (from factory smokestacks). The soot was produced so rapidly that it covered the once-light-colored tree trunks. The now-dark tree trunks exposed the light-colored moths and the dark moths now were blending in. The birds now caught mostly light-colored moths, thus dark moths had higher survival and reproduction. Because they were producing more offspring (which had better survival probability), the dark moths now became abundant. The population now contained a larger fraction of dark moths, and this follows the definition of evolution (i.e., change in allele frequencies).

Note that if no variation exists in a population, then natural selection cannot work. Other modes of evolution exist as well (mutation, genetic drift, migration). These can work alone or in conjunction with one another:

Mutations

in a broad sense, is the only source of evolution novelties. However, most of mutations are results of somehow broken DNA and therefore probability of useful mutation is similar to probability to upgrade your smartphone with a hammer.

Migration

brings new genotypes to the population and therefore able to change the local course of evolution.

Generic drift

is usually a result of catastrophic factors applied to small populations. The smaller is a population, the bigger is probability that it will simply die out. Another name of this thing is an evolution bottleneck: few will survive, and these few will be chosen not because they are better but simply by random.