Evolutionary Genetics

On this webpage I address a subject dear to my heart, evolutionary genetics, the focus of my first adult incarnation. My doctorate is in evolutionary genetics, and I did research and taught at the university level for some 23 years before burnout and a need for change overcame me. In my career as a graduate student, I had the privilege of working with three outstanding evolutionary biologists: world renowned ichthyologist, Professor Clark Hubbs, at the University of Texas at Austin for my master's working on fish population genetics; Professor Wyatt W. Anderson, for my doctorate at the University of Georgia in Athens; and during that same period, one of the fathers of evolutionary genetics, Professor Theodosius Dobzhansky. Below are some fundamental concepts of evolutionary genetics which will be helpful in understanding related concepts on other pages, e.g. Adaptive Landscapes.

A closely related area is population genetics. Population genetics studies and describes, usually in mathematic terms, gene frequency changes in populations and what causes those changes. Evolutionary genetics incorporates population genetics, but is more global, studying changes in morphology, complex genetic traits, and a broader evolutionary topics.

The "fathers" or founders of population genetics were J. B. S. Haldane, Sewell Wright, and Sir Ronald Fisher. Fisher was also the father of statistics. I had the honor of hearing Sewell Wright speak at a national conference. At 90 years old, he was still lecturing on complex mathematical population genetic theory that was way over my head. His mind was still sharp. I also had the privilege of taking a course under James Crow, one of Wright's most outstanding students and a legend himself in population genetics.

Evolutionary Process vs Evolutionary History

Evolution is, at its basis, the change in gene frequencies in a population or species over time. This is the heart of the evolutionary process. The evolutionary process is about how populations and species evolve, the underlying genetic changes. At the biological level, there can be no evolution that does not ultimately go back to underlying genetic changes in gene frequencies.

Evolutionary history describes or studies the actual history that organisms evolved. The evolution of the human species is an example. the substrate studied in this case is usually the fossil record. It is usually a longer time perspective than evolutionary genetics.

Populations Evolve, Individuals Are Selected

Populations or species evolve. Gene frequency changes are a population/species phenomenon. Populations are the units of evolution. Individuals do not evolve in terms of biological evolution.

Individuals are the units of selection. Selection acts for or against individuals. More technically selection acts for or against a genotype, i.e. the specific combination of genes an individual has.

Individual Selection versus Group Selection

One phrase that drives me as an evolutionary geneticist up a wall is when people say something "is for the good of the species". For example, someone might say we have children for the "good of the species", or to "perpetuate the species". This is a type of group selection statement. Group selection is when selection is acting at the level of the group. Research and theory have both shown over and over that selection acts at the the level of the individual. Only secondarily, and at a much weaker level, can it act at the group level. Two classic books on this topic are Richard Dawkins, The Selfish Gene, and G. C. Williams, Natural Selection and Adaptation.

Altruism

This point becomes especially important when we consider such traits as altruism. Altruism is the sacrifice of oneself for someone else. In more Christian terms, it is the unselfish regard for others. The problem evolutionary biology has with altruism is that altruism lowers the fitness of the one doing it (the donor or altruist) and raises the fitness of the one receiving it. (Fitness is defined more technically below.) In the evolutionary game, this is a no-no, i.e. to do something that lowers your fitness. So how could altruistic genes increase in frequency in a population if they are being selected against? The answer: they can't.

Aside:

This is the case for "priest" genes if such genes exist, where priest practice celibacy and do not leave any offspring. The only way to keep such altruistic genes in the population is through mutation. Each generation the priest genes would be eliminated because they do not reproduce. And we wonder why the number of priest remains so low...

Love Thy Neighbor as Thyself

This is why evolutionary biologist have trouble with the Golden Rule. From an evolutionary biology perspective, are you crazy!? Such behavior would most likely lower your fitness. It makes no sense to be altruistic. Because, for one thing--

Cheaters Win

Let us say we have a population and they are going around loving their neighbor as themselves. There is one sneaky guy in the group that really only looks out for himself and his family. He's a cheater. He doesn't do any self-sacrificing; no altruism with this guy. Consequently, he will most likely have higher fitness, that is leave more offspring, and more of his genes will make it to the next generation. He is trying to avoid doing altruist stuff that might lower his fitness. 

Kin Selection

One explanation for altruism is kin selection. If individuals share genes, i.e. have genes in common, then altruism can be explained in terms of kin selection. Kin selection is where you still benefit from being altruistic because the individual(s) you are sacrificing yourself for, is genetically related to you. That individual is carrying some of your genes. Examples of this would be saving your children. This is the biggest payoff. What is you save your brother or sister? Still a high payoff. What about a distant cousin? Well now, this gets into a gray, iffy area. Probably not, but maybe. The more genes you have in common, the greater the fitness payoff.

Kin selection, while not affecting the donor's individual fitness, affects his or her inclusive fitness. Inclusive fitness includes an individual's individual fitness plus his contribution through his relatives (kin). In human populations, this is one explanation why elderly relatives have evolved to stay around: they help in the care and survival of their grandchildren, thus increasing their own inclusive fitness.

Altruism is seen in many other species other than humans. Social insects, other mammals, higher primates, fish, and so forth. So it is a general phenomenon.

The Hardy-Weinberg Equilibrium Equation

This is the starting point for understanding the mathematics of gene frequency changes in populations. In a  large population, let p be the relative frequency of the A allele and q, the frequency of an alternative allele, a, in the population. (Alleles are alternatives or variants of a specific gene. See discussion on Basic Genetics' page.)

The frequency of the three genotypes, AA, Aa, and aa, will then be

(p+q)2=p2AA+2pqAa+q2aa
which is the Hardy-Weinberg equation. It is also a binomial expansion of the alleles' two frequencies. The Hardy-Weinberg equation gives the relationship between gene (or allele) frequencies and genotypic frequencies in a population.

Graphically this can be represented by a classical Punnet's square:

pAqa
pApxpAApxqAa
qapxqAapxpaa

Which again gives you: p2AA+2pqAa+q2aa

After one generation of random mating and in the absence of selection and other forces of evolution (below), a population will be in Hardy-Weinberg frequencies and remain so. Consequently, it is referred also as the Hardy-Weinberg equilibrium equation.

Forces of Evolution

These are the forces that cause gene frequencies to change in a population or species:

  • natural selection
  • mutations
  • migration
  • random genetic drift

Natural Selection

Natural selection is the differential survival and reproduction of genotypes. More on this below. It is natural selection that adapts and attunes organisms to their environment.

Mutations

Mutations are changes in the DNA. These can be point mutations of individual DNA bases (A,T,G,C), segments of DNA involving many bases, chromosomal rearrangements, gene duplication (much like the story of alpha, beta, gamma, and fetal hemoglobin genes), or even gene loss. All genetic variants ultimately arise by mutations.

Migration

Migration is the movement of individuals and their genes between populations. These individuals may have different genes and can introduce these new genes to a new populations. Alternatively, if the migrants have alleles in different frequencies than the population they are going to or leaving, this can cause a change in gene frequencies. An excellent example of the this is the invasion of Genghis Khan and his army into Europe in the 13 century. The Mongolian race has a higher frequency of the B allele in the ABO blood gene than the European, Caucasian populations. Even today in the places his army invaded as they swept down through Europe, there is a higher frequency of their B allele in those populations compared to the rest of Europe's populations. Apparently his solders left some of their DNA behind.

Random Genetic Drift

Random genetic drift causes changes in gene frequencies through sampling error in small populations. It is a random process. In small populations there will be sampling error from generation to generation resulting in the loss of genetic variation and variants. The smaller the population, the greater the drift effect, i.e. the more rapid the loss of variation. And, populations want genetic variation.

Genetic  Variation

Genetic variation is good. In general that is. This is the reason we see hybrid vigor--in hybrids between different species (e.g. the mule) or between different strains or, in agriculture, cultivars. Experiments with Drosophila (fruit flies), decades ago took strains of fruit flies that had been made completely homozygous for every gene--that is they had no genetic variation. Geneticists irradiated the treatment group, which caused random mutations. The irradiated treatment group had higher fitness and were healthier than the non-irradiated control group. Similar experiments have been done with a variety of organisms from bacteria to mammals and showed the same affect. Even random genetic variation was better than no genetic variation.

Fitness

Finally, we come to a key concept in evolutionary genetics: fitness. The fitness of a genotype is a measure of how well it survives and reproduces. Let us designate wij as the fitness of the ij genotype where i and j are alleles in a population, then

wij = vij x fij

where  vij is the viability of the ij genotype and  fij is the fertility of that genotype. Viability means how well that genotype survives and fertility is the number of offspring left by that genotype.

Note, fitness is the product of viability and fertility. As either component, v or f, approaches zero, fitness approaches zero. A way to think about this is that if you have some real tough hombre, the meanest and baddest guy on the block, he's top dog, numero uno, the big cheese, but he is sterile. from an evolutionary perspective it does him no good: his fitness (w) is 1 x 0= 0. You have to have both viability and fertility to stay in the evolutionary game.

Fitness is related to selection, w= 1-s, where s is the selection coefficient, a measure of the intensity of selection. Selection acts on individuals or, more accurately, individual genotypes.

Average Fitness of a Population

A related concept is average fitness of a population, designated, W-bar, that is a W with a bar across the top (too hard to do with html), and is is equal to the sum of the various genotypes weighted by their frequencies in the population. This concept will be important when I discusss Adaptive Landscapes as  a prerequisite for talking about psychosocial evolution in relation to spiritual and personal growth.

Fisher's Fundamental Theorem of Natural Selection (1930)

Sir R. A. Fisher, who would lecture and give seminars drinking beer, showed that the rate of change of the average fitness in a population is equal to the variance in fitness between the genotypes. What this means is that if there is a genotype with high fitness, selection will cause rapid changes in the population to increase the population's average fitness.

Types of Selection

I want to mention four types of selection. Most people when they think of natural selection, if they think of it at all, is in terms of what is called directional selection.

Directional Selection

This is selection for or against a specific gene or genotype. In the case of two alleles in a population, say A and B, its end result is fixation (p=10) of one allele (A) and elimination of the other (q=0). A deleterious mutation would be an example here. This type of selection eliminates genetic variation from the population. In contrast to directional selection, we have balancing selection, which is quite common.

Balancing Selection

In balancing selection, selection acts to maintain multiple alleles in a population and results in maintaining or increasing genetic variation. Two mechanisms of balancing selection are overdominance and frequency dependent selection:

Overdominance

In overdominance the heterozygous genotype (A/B) has the highest fitness over both homozygous genotypes, AA or BB. The sickle cell hemoglobin gene in human populations is an example. In a malarial infested environment, the heterozygote that carries one wild-type hemoglobin alpha chain allele and  one sickled allele is resistant to malaria. Unfortunately, the homozygote for the sickle allele is lethal. In a non-malarial environment, the wild-type alpha homozygote has highest fitness.

Frequency Dependent

This is where the frequency of a genotype depends on its frequency in the population. Lets take "blond" genes as a humorous example. Lets assume a population in which most of the individuals have dark hair. Blonds when fairly rare, have a mating advantage because they attract attention. They stand out. However, as their numbers increase in the population and they become common, they loose their mating advantage.

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