Individuals live and die with the genetic endowment received at conception. The number of alleles, the possible genes for a given locus are one or two, never more in any one individual.
Populations are a group of interbreeding individuals and all of the alleles found in that population are referred to as the gene pool. While all members of Homo sapiens are capable of interbreeding, mate choice is in our lives is really quite limited. Factors that determine with whom we mate are geographical, ecological, and social.
Within a population, geneticists are concerned with gene frequencies for specific traits. If mating is random, without gene flow, without mutation, no natural selection, and the population are infinitely large, then there will be no change in gene frequencies from generation to generation. These conditions are rarely achieved.
The equilibrium that occurs with no evolution taking place is illustrated by the Hardy-Weinberg Equilibrium. It predicts that IF all conditions for stability are satisfied, THEN allele frequencies will not change from one generation to the next. Hardy was a mathematician, and Weinberg was a physician. Both arrived at the conclusion independently.
Mathematically it is p2 + 2pq + q2 = 1
It is a binomial expression. The two men are said to have discussed it over lunch; Hardy the mathematician, knew about it, but had previously considered it too trivial to publish. In it, p and q are allele frequencies.
Population genetics is about the factors that produce and redistribute variation. The theoretical basis for population genetics is the Hardy-Weinberg theorem. A very important point to remember is this: population genetics is the concept of Mendelian genetics applied to populations. It applies to alleles at the genetic level, not phenotypic expression of traits.
From an evolutionary perspective, mutation is the creative force in evolution. It is the only way to produce a new variation. For changes to have evolutionary significance, they must occur in sex cells. Mutation can be at the chromosomal level or a single base substitution. In this discussion, we focus on point mutations, the change in a single DNA base. Keep in mind that this is mutation from a Mendelian perspective.
Let us clarify a point here. Somatic mutations are not passed on to offspring. Cancer, for example, is a somatic mutation. Mutations that occur in germ cells are passed on to offspring. Sometimes a mutation will on its own return to the normal state spontaneously.
It is constructive to think of allele frequency changes in a population as the product of evolution. How frequently do mutations occur? The mutation rate for any given locus is estimated at less than one per 10,000 gametes per generation. Because we have many loci (estimated at about 100,000, we all possess numerous mutations that have accumulated, but most remain 'hidden' as recessive alleles. That collective burden of hidden mutated genes is called the genetic load.
Sickle cell anemia, cystic fibrosis, and albinism are point mutations whose expressions behave as recessive traits.
They become medically significant because there are enough carriers in the population so that from time to time some individuals inherit a copy of the gene from both parents and are afflicted. You'll recall that the heterozygote for sickle cell and cystic fibrosis enjoy an advantage in malarial and cholera environments, respectively. I am unaware of a heterozyogte advantage for albinism.
A rare mutation that is recessive will not be a problem if it never 'meets its match' as a homozygote. It should in time disappear. This is the underlying basis of incest taboos: they maintain heterozygosity and prevent the expression of deleterious recessive alleles.
There are some well-known Mendelian dominant disorders. Unlike recessives, if a dominant gene is there in the heterozygote, it gets expressed. A good example of a Mendelian disorder is achondroplasia, a form of dwarfism. What happens if an individual receives copies of the gene from both parents and is a homozygote for achondroplasia? It is fatal, usually before birth.
II. GENE FLOW
You can understand this process best by the old aphorism; where people go, genes flow. The offspring of U.S. soldiers and women in Korea and Viet Nam are an example. The appearance of shovel shaped incisors (a polygenic trait) in increasing frequency amongst certain Eastern European populations may be a heritage of the Mongolian expansion eastward under Genghis Kahn. Basque genes appear in Easter Islanders thousands of miles away from the Iberian Peninsula; their intrusion has been traced back centuries ago to a shipwreck with a crew of Basques sailors.
The most remarkable example of gene flow is from animal husbandry: frozen sperm is routinely transported long distances for commercial artificial breeding of dairy cattle.
Population movements are revealed by an interesting statistic: marital distance (distance between birthplaces). It has increased steadily since the building of the railroads in the last century. Physical barriers can limit gene flow; there are countless examples, but the many isolated South Pacific islands come to mind. Cultural barriers can produce breeding isolates: the Old Order Amish are an example. At the other extreme, some California Indian groups had a deliberate cultural preference for 'marrying out' which led to a virtual disappearance of them as a distinct group.
III. GENETIC DRIFT
The random factor in evolution is called genetic drift. Now, listen up on this one: genetic drift is due primarily to chance.
Once aspect of random genetic drift is how a sample is obtained.
Consider this fanciful notion of sampling: a flying saucer comes from a distant galaxy to obtain typical humans from planet Earth to establish breeding colonies back on the home planet. They snatch ten people and head on home across the galaxies. What if they snatched ten from a Navajo Indian reservation? Or ten from the women's prison at Dwight? Or ten sumo wrestlers from Japan? In the chance game of life, circumstances 'select' some who pass on many offspring and others who pass on not at all.
When small populations become isolated on an island and are the founder stock for a population, this is called the founder effect. It is a form of sampling. If the founders lack one or more alleles found in the larger home country population, those alleles will be missing in the new gene pool. The remote islands of Tristan da Cunha and Pitcairn are examples of populations with a small number of founding persons. The Old Order Amish represents a similar process, but biological isolation is enforced by cultural barriers.
Inbreeding is a problem in small populations. In successive generations, members of the population will become increasingly related to one another.
Every culture known has incest taboos that restrict marriages that are too close. The people may not understand heterozygosity, but they certainly understand its beneficial effects. Mating too close usually reduces vigor and increases the chance for deleterious alleles to appear in homozygotes. Inbreeding becomes a problem in small populations because in time everyone becomes a relative.
Close inbreeding can produce unfit offspring, but such is not always the case. The most remarkable case of human inbreeding comes not from an island, but from the Australian Outback. The story (documented by J. B. Birdsell) is as follows: An Afghan camel drover, one Jack Abdullah, arrived there and took to bed a full blooded aboriginal woman who produced a number of children. In time, he took one of their daughters and produced a second family, now three-quarters Afghan. Eventually he fathered yet a third family of no less than eight children by his granddaughter Annie--a family now seven-eighths Afghan. Six of the eight children survived into adulthood--a lower infant mortality than most living in central Australia.
If a population goes through a period of reduced number (called a genetic bottleneck) only the genes of the survivors will persist. Epidemic diseases and other disasters have produced these events many times. The chance migration of a small group of hunter-gatherers to a new land can also produce this effect. It can be inferred from the genes that are present and those that are missing. In some literature, the genetic bottleneck is called the survivorship effect.
For instance, the blood group O is extremely common among American Indians, and it reaches a frequency of 95% among the Indian populations of South America. The implication is this: the first Native Americans are believed to have come across from Siberia 12,000 years. If the allele for type O predominated amongst those first founding pioneers, then that could explain why it predominates amongst their descendants in the Americas today.
Random genetic drift occurs in small populations in a way that may seem counterintuitive to you.
You'll recall that in a large population, allele frequencies from generation to generation should remain constant if everything else affecting the system is neutral.
In a small population that is isolated, chance (over many generations) will lead to one allele predominating over the other if two alleles are initially present.. This can be demonstrated by computer program simulation. It has also been demonstrated with small populations of fruit flies. Chance becomes important in small populations. Hominids seem to have lived in small populations during much of their history, so random genetic drift may have been a significant factor in human evolution.
Something else happens in small populations. Weak environmental pressures can produce rapid change in small populations. They are very susceptible to the selective forces in their environment and evolution may be rapid. Thus, evolutionary changes can occur very rapidly in small, isolated populations because mutations have to occur only once or a comparatively few times to be potentially able to spread throughout the entire group as a result of genetic drift.
IV. NATURAL SELECTION
The three factors we have discussed--mutation, gene flow, genetic drift--all interact to produce variation and distribute genes within and between populations. In an earlier unit, we have discussed the re assortment of alleles at meiosis and the reshuffling of genes within homologous chromosomes. All of these events are random. They do not account for directional change. The mechanism of directional change is natural selection.
Natural selection is the central determining factor influencing the long term direction of evolutionary change. Natural selection can best be described as differential reproductive success. In other words, individuals that produce the most viable offspring are the most successful.
With isolation and a sufficient change in gene frequencies, a new species can emerge. Malaria and sickle cell anemia provide a clear Mendelian model for natural selection. When early African farmers invaded swampy lands for food production, they were exposed to malaria spread by Anopheles mosquitoes. By chance a mutation producing the sickle cell allele appeared. It provided a selective advantage in the heterozygote and became established amongst populations in malarial environments.
The tendency of humans and their hominid ancestors to live in small bands may have unwittingly contributed to human evolution. Allele frequencies and the (presumably advantageous) traits they represent may have changed rapidly--a change not possible with a large breeding populations.
The causes of long term trends in human evolution are less clear that those in simple Mendelian patterns such as sickle cell anemia. How did upright walking get started? How did symbolic thought get established? Why have human teeth declined in size over the last 35,000 years?
We can summarize these units on genetics as follows: Phenotypic traits arise from hereditary units of DNA located in chromosomes. Offspring are similar to their parents because of inherited genes, but genetic reshuffling during gamete production provides variation.
Working along with other factors such as mutation, gene flow, founder effect and genetic drift, natural selection can in time lead to an entirely new species.
..... CJ '99
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