There is more to say about inheritance, and I will get back to it in the future, but for the moment I want to move on and give an overview of the forces of evolution. As before, it is useful to return to the Hardy Weinberg Castle (HWC) equilibrium I discussed a couple of weeks ago. To remind you, the one-locus two-allele HWC equilibrium is a situation in which the genotype frequencies:
remain constant through time. That is, the gene frequencies, genotype frequencies and phenotype frequencies don’t change. As mentioned before, the beauty of the HWC equilibrium is that if we use a genetic definition evolution (change in gene frequencies), then when the HWC equilibrium is holding, no evolution is occurring.
The reason the HWC equilibrium is a convenient starting point is that we know exactly the conditions under which it holds. That is, the HWC equilibrium will hold if five conditions are met. These are: no mutation; no migration; No selection; very large population size (no genetic drift), and random mating.
Thus, we can quickly see that the forces of evolution are mutation, migration selection and genetic drift. It turns out that non-random mating (the fifth condition) does not change gene frequencies (it changes genotype frequencies), and thus does not qualify as a force of selection under this narrow definition.
To make it stand out, the forces of evolution under this narrow definition are:
The problem, of course, is that “change in gene frequencies” is not an adequate definition. For the phenotypic based definition I am using: Change in the distribution of phenotypes in a population due to the gain, loss, or replacement of individuals. It will be necessary to alter the definitions of the forces of evolution.
These four forces continue to be valid when moving to a phenotypic view of evolution. In particular they are actually the only four logical ways that we can have the addition or loss of individuals from a population in a constant environment. What changes is the tendency to tie the forces explicitly to genes. So, without further adieu:
Selection: Selection is in many ways the easiest of the four forces. Selection is a main subject of quantitative genetics. In the standard quantitative genetics view evolution by selection is broken up into selection and the response to selection. Selection is an ecological process in which some individuals have a greater chance of reproducing than do other individuals. Note that it says nothing about genetics or inheritance. Thus, selection is qualitatively no different than what you do when you go to the grocery store and choose chicken noodle soup rather than cream of mushroom soup. The “genetics” comes in in the form of heritability, which in standard quantitative genetics is simply a constant or proportionality that converts within generation change in the mean of population due to selection into between generation changes in the appearance of offspring.
Selection changes the distribution of sources of information (parents) that can contribute to that transition equation, but does not change the actual transition equation. Any time the probability that a source will be included in the transition equation is correlated with the sources phenotype (i.e., not random with respect to phenotype) it can be considered selection.
Mutation: Campbell and Reece (Biology 8th edition) defines mutation as “A change in the nucleotide sequence of an organisms DNA, . . .”. An appropriate definition of genetic mutations, but actually not quite appropriate for an evolutionary perspective.
If an individual has a mutation occur during their lifetime this is not evolution. It is actually (and rather counter intuitively) “development”. That is, it is a change in an individual, not the gain or loss of an individual. On the other hand, if an individual is formed from a pair of gametes, one of which is mutant, then this is an evolutionary change. Thus, if an individual starts with one genotype, and due to a mutation ends up with another, that would technically be development, however, it would be evolutionary change if those mutational changes were passed on to their offspring.
From a phenotypic perspective mutation is a random change in the information contributing to the transition equation. Thus, for example if the offspring of two AA individuals has an Aa phenotype, this indicates that there was a random change in the information contributing “genotype” to the patterning node, and results in the gain (or replacement) of an individual with a randomly different phenotype. Note that this works perfectly well for any random change in the phenotype of an individual, whether it is genetic, cultural, or due to some other cause. Thus mutation can be defined as a random change in the information contributing to the patterning node of an individual.
Migration: This one is easy. Migration is simply the gain or loss of individuals (and their phenotypes) because they move into or out of a population.
Drift: Drift is traditionally seen as a random change in gene frequency due to sampling effects in a small population. A phenotypic view of drift would be similar. Drift is the random change in distribution of phenotypes that occurs when population sizes are less than infinite, and as a result this randomly changes the distribution of phenotypes that can contribute to the next generation.
I believe these four forces logically cover all of the possible forces of evolution in a constant environment. Selection is deterministic changes due to fitness differences, mutation is random changes due to unpredictable changes in the patterning node of individuals, migration is adding or removing individuals from the population, and drift is random changes due to sampling in a finite population.
Now the thing that has disturbed me: I have always accepted that all evolutionary change can be attributed to one of these four forces. This may be true for a gene centered view of evolution, but it will not be true for the phenotypic centered view. In one of the earlier postings I pointed out that a secular change in the environment, such as global warming, could cause phenotypic changes in a population that would have to be considered evolution. This form of evolutionary change does not fall into any of the other previous categories, and thus we must include a new force: environmental change. For those readers who are mathematically inclined it should not come as a surprise that this force exists. It is equivalent to the “transmission bias” term that is a part of the Price equation discussed by Okasha (2006, Evolution and the levels of selection). I will present a full discussion of why we need to include this force in a future post.