History of Population Genetics
Population genetics is a field of biology that studies the genetic composition of biological populations and the observed changes in the genetic composition that result from the operation of various factors, including natural selection, genetic drift, mutation and gene flow. The above four are main evolutionary processes and natural selection is one of the most important factors that can affect a population’s genetic composition. Natural selection occurs when some variants in a population out reproduce other variants, as a result of being better adapted to the environment, or ‘fitter’. Presuming the fitness differences are at least partly due to genetic differences, this will cause the population’s genetic makeup to be altered over time. Population genetics is concerned with gene and genotype frequencies, the factors that tend to keep them constant, and the factors that tend to change them in populations. Population genetics is the study of the frequency and interaction of alleles and genes in populations. Ordinary genetics in comparism with population genetics, looks at how one selects breeding stock to produce the best possible offspring. Population genetics looks at the statistical distribution of genes in a particular breeding population, such as a breed of dog, and how different kinds of selection can affect that gene distribution. Ordinary genetics is seen as predicting the phenotypic makeup of the next generation, while population genetics predicts the genetic makeup of the breed as a whole, often several generations away. Population geneticists usually define ‘evolution’ as any change in a population’s genetic composition over time.
Primary founders of population genetics were Sewall Wright, Haldane J. B. S. and R. A. Fisher, who also laid the foundations for the related discipline of quantitative genetics.
In the 1920s and 1930s the field of population genetics came into light due to the major work of British biologist and statistician R.A. Fisher. In a series of papers starting in 1918 which he culminated in 1930 and publish a book tittle The Genetical Theory of Natural Selection, Fisher showed that the continuous variation measured by the biometricians could be produced by the combined action of many discrete genes, and that natural selection could change allele frequencies in a population, resulting in evolution.
J.B.S. Haldane in 1924 worked out the mathematics of allele frequency change at a single gene locus under a broad range of conditions. Haldane also applied statistical analysis to real world examples of natural selection, such as the evolution of industrial melanism in peppered moths, and showed that selection coefficients could be larger than Fisher assumed, leading to more rapid adaptive evolution.
In 1932 Sewall Wright an American biologist, who had a background in animal breeding experiments, centered his research on combinations of interacting genes and the effects of inbreeding on small, relatively isolated populations that exhibited genetic drift.,
After his experiment he argued that genetic drift and inbreeding could drive a small, isolated sub-population away from an adaptive peak, allowing natural selection to drive it towards different adaptive peaks.
Scientist who also contributed to the field of population genetics were John Maynard Smith was Haldane’s pupil, Hamilton W.D who was heavily influenced by the writings of Fisher, and American George R. Price who worked with both Hamilton and Maynard Smith. Others who were heavily influenced by Wright were American Richard Lewontin and Japanese Motoo Kimura.
Theodosius Dobzhansky was influenced by the work on genetic diversity by Russian geneticists, Sergei Chetverikov in his book in 1937 titled Genetics and the Origin of Species helped bridged the gap between the foundations of microevolution developed by the population geneticists and the patterns of macroevolution observed by field biologists. Dobzhansky examined the genetic diversity of wild populations and showed that, contrary to the assumptions of the population geneticists, these populations had large amounts of genetic diversity.
The achievement of the founders of population genetics as field of study was integration of the principles of Mendelian genetics, which had been rediscovered at the turn of century, with Darwinian natural selection. Many of the early Mendelians did not accept Darwin’s account of evolution, believing instead that novel adaptations must arise in a single mutational step; conversely, many of the early Darwinians did not believe in Mendelian inheritance, often because of the erroneous belief that it was incompatible with the process of evolutionary modification as described by Darwin. By working out mathematically the consequences of selection acting on a population obeying the Mendelian rules of inheritance, Fisher, Haldane and Wright showed that Darwinism and Mendelism were not just compatible but excellent bed fellows; this played a key part in the formation of the ‘neo-Darwinian synthesis’, and explains why population genetics came to occupy so pivotal a role in evolutionary theory.
Processes of Population Genetics
Population genetics is governed by four fundamental processes which are natural selection, genetic drift, mutation and gene flow and transfer.
1) Natural Selection
This is a process by which random evolutionary changes are selected by nature in a consistent, orderly, and non-random way. Selection refers to changes in allele frequencies due to the effects of the gene on its host. Population genetics describes natural selection by defining fitness as a propensity or probability of survival and reproduction in a particular environment. To better understand natural selection, it is important to understand Descent with Modification and Common Descent. All offsprings descent from their parents with modifications. When parents have children, the children often look and behave slightly different than the parents and each other. The differences found in offspring are partially due to random genetic mutation which results to random variation. Common descent on the other hand is the idea that all life on earth are related and so descended from a common ancestor. So through gradual process of descent with modification over many generations, a single original species is thought to have given rise to all the life we see today.
Through the process of descent with modification, new traits are randomly produced. Nature then carefully decides which of those new traits to keep. Positive changes add up over multiple generations, negative traits are quickly discarded. For example an organism with a particular gene may survive certain natural occurrences like diseases, cold or heat, predator due to colour adaptation while other organisms of the same species may not stand this condition because they lack the genes. Because organisms cannot simply survive certain natural occurrence, nature decides which of those variations get to live and reproduce and which do not. In multiple generations, species of organisms become fit for survival and reproduction within their specific environment. Natural selection acts on phenotypes, or the observable characteristics of organisms, but the genetically heritable basis of any phenotype which gives a reproductive advantage will become more common in a population. In this way, natural selection converts differences in fitness into changes in allele frequency in a population over successive generations. Prior to the advent of population genetics, many biologists doubted that small difference in fitness was sufficient to make a large difference to evolution. For natural selection to occur three condition must be met, viz:
- Over production of offspring. For natural selection that will lead to evolution to occur, there is need for the parent organisms to reproduce more offspring. Over population does not necessarily have to occur before in order for natural selection to happen within a population, but it must be a possibility in order for the environment to put selective pressure on the population and some adaptions to become desirable over others. than it can normally sustain.
- variation, natural selection can only occur if there is variation within the specified trait. If all individuals are clone then there will be no variations and therefore no natural selection at work in such population. Variations may be inform of differences in colour, size etc. increase in variation of traits in a population actually increase the likelihood of survival of a species as a whole. Even if part of a population is wiped out due to various environmental factors such as diseases, natural disasters, climate change etc, it is more likely that some individuals will posses traits that will help them survive and repopulate the species
- heredity, natural selection occurs when a characteristic or feature is passed from generation to generation where the successive generation inherit the fittest phenotype so that evolution proceeds in this way. The undesirable traits are less selected and hence appear less.
While natural selection favours desirable heritable traits that increase species survival and reproduction naturally, artificial selection though favours desirable heritable traits , it is selected by humans.
2) Genetic Drift
Genetic drift is referred to as a change in allele frequencies due to random sampling. Genetic drift may be as a result of chance effects of mating and survival in a small population. Small population is vulnerable to genetic drift where its impact is significant. Species of organisms that show genetic drift are not necessarily fit but win the game of chance. Genetic drift may cause gene variants to disappear completely, and thereby reduce genetic variability. In contrast to natural selection, which makes gene variants more common or less common depending on their reproductive success, the changes due to genetic drift are not driven by environmental or adaptive pressures, and may be beneficial, neutral, or detrimental to reproductive success. Genetic drift can occur as a result of bottleneck effect. for instance if natural disaster such as forest fire occur, of course so many organisms in the population will be destroyed, the surviving organisms are not better adapted but are only lucky to be found in an area where they are not directly affected. The survivors do not represent the original population as a result, there is definitely going to be a new allele frequencies among the surviving population. Founder Effect can also lead to genetic drift. When an organism is founded in an island or new area, the few organisms that arrive to start a new population do not necessarily represent the original population that they come from. For example, a seed that is dispersed by wind end up in new area that happen to be perfect for their growth, of course the new population will grow well. Those seeds may not necessarily represents the original pop of the plant from which they came from.
3) Mutation
Mutation is a change of genetic material especially within nucleic acid (DNA & RNA) and it’s random. Mutation is the ultimate source of genetic variation in the form of new alleles. Mutation also functions as a mechanism of evolution. Mutation can result in several different types of change in DNA sequences which can either have no effect, alter the product of a gene or prevent the gene from functioning. An organism cannot “will” itself to get a certain mutation. All organisms with DNA and RNA, plants, animals, bacteria and viruses, can have mutation. Mutation can have harmful, neutral or helpful effects. Bacteria possess a helpful mutation that allows it to survive an antibiotic but the bacteria did not willingly mutate to get this trait. Vulnerable time when mutation occurs is during DNA replication and meiosis (stage when sperm or egg cell divides to half the number parent chromosomes). Sometimes the chromosomes do not separate completely which can result in an egg or sperm having too many or few chromosomes. External factors like certain types of chemicals or excessive radiation can make mutation more likely to occur. According to the phenotype-first theory of evolution, mutations can eventually cause the genetic assimilation of traits that were previously induced by the environment. If natural selection favor either one out of two mutations, but there is no extra advantage to having both, then the mutation that occurs the most frequently is the one that is most likely to become fixed in a population. Mutations leading to the loss of function of a gene are much more common than mutations that produce a new, fully functional gene.
Types of mutation.
Novel genes are produced by several methods, commonly through the duplication and mutation of an ancestral gene, or by recombining parts of different genes to form new combinations with new functions. It is based on this that mutation is subdivided into gene and chromosomes
(a) Gene/DNA Mutation.
This occurs in the DNA which make up genes that can code for protein that influence different traits. Novel genes are produced by several methods, commonly through the duplication and mutation of an ancestral gene, or by recombining parts of different genes to form new combinations with new functions. These duplications are a major source of raw material for evolving new genes, with tens to hundreds of genes duplicated in animal genomes every million years. In protein synthesis, bases are read in threes. So if a base is added or removed it changes the total number of bases leading to frameshift mutation. DNA mutation include
i Substitution. This happen when a wrong protein base in the DNA is matched.
ii Insertion. This happen when an extra protein base or bases are added in a DNA
iii Deletion. This is when a protein base is removed.
(b) Chromosome mutation. Chromosomes are made up of DNA and protein and are highly organized with a lot of genes. Examples of chromosomal mutation include
i Duplication: this occur when extra copies of genes are generated
ii Deletion: this occur when some genetic materials break off
iii inversion. This occurs when broken segment is inversed (reversed and put back on the chromosomes)
iv translocation when fragment from one chromosome breaks off and attaches to another.
4) Gene Flow and Transfer
Gene flow is the exchange of genes between populations, which are usually of the same species. Examples of gene flow within a species include the migration and then breeding of organisms, or the exchange of pollen. Gene transfer between species includes the formation of hybrid organisms and horizontal gene transfer. Migration into or out of a population can change allele frequencies, as well as introduce genetic variation into a population. Immigration may add new genetic material to the established gene pool of a population. Conversely, emigration may remove genetic material.
Reproductive isolation
For a population to become new species, there must be barriers to reproduction between two diverging populations, but gene flow may slow this process by spreading genetic differences between the populations. Gene flow is hindered by mountain ranges, oceans and deserts or even man-made structures such as the Great Wall of China, which has hindered the flow of plant genes. Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules. Such hybrids are generally infertile, due to the two different sets of chromosomes being unable to pair up during meiosis. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype.
Genetic structure
Because of physical barriers to migration, along with limited tendency for individuals to move or spread, and tendency to remain or come back to natal place, natural populations rarely all interbreed as convenient in theoretical random models. There is usually a geographic range within which individuals are more closely related to one another than those randomly selected from the general population. This is described as the extent to which a population is genetically structured. Genetic structuring can be caused by migration due to historical climate change, species range expansion or current availability of habitat.
Hardy-Weinberg Principle
- Introduction
Evolution is not only the development of new species from older ones, as most people assume. It is also the minor changes within a species from generation to generation over long periods of time that can result in the gradual transition to new species. Evolution has been defined as the sum total of the genetically inherited changes in the individuals who are the members of a population’s gene pool. It is clear that the effects of evolution are felt by individuals, but it is the population as a whole that actually evolves.
Modern Theories of Evolution
Evolution is simply a change in frequencies of alleles in the gene pool of a population. For instance, let us assume that there is a trait that is determined by the inheritance of a gene with two alleles–B and b. If the parent generation has 92% B and 8% b and their offspring collectively have 90% B and 10% b, evolution has occurred between the generations. The
entire population’s gene pool has evolved in the direction of a higher frequency of the b allele–it was not just those individuals who inherited the b allele who evolved. This definition of evolution was developed largely as a result of independent work in the early 20th century by Godfrey Hardy, an English mathematician, and Wilhelm Weinberg, a German physician. Through mathematical modelling based on probability, they concluded in 1908 that gene pool frequencies are inherently stable but that evolution should be expected in all populations virtually all of the time. They resolved this apparent paradox by analyzing the net effects of potential evolutionary mechanisms.
Hardy Weinberg equilibrium or principle state that a population’s allele and genotype frequencies are constant unless there are some type of evolutionary forces acting on them.
Hardy, Weinberg, and the population geneticists who followed them came to understand that evolution will not occur in a population if five conditions are met:
- No mutation
- No natural selection
- the large population
- Random mating
- no migration
If no mechanisms of evolution are acting on a population, evolution will not occur–the gene pool frequencies will remain unchanged. However, since it is highly unlikely that any of these five conditions, let alone all of them, will happen in the real world, evolution is the inevitable result. Hardy Weinberg equilibrium is unrealistic in nature but it does not matter because it gives a baseline to compare an evolving population to that which remain constant without evolutionary forces acting upon it. Godfrey Hardy and Wilhelm Weinberg went on to develop a simple equation that can be used to discover the probable genotype frequencies in a population and to track their changes from one generation to another.
