In this essay we will discuss about the factors influencing allele frequency.
Essay on Allele Frequency
- Essay on Selection
- Essay on Mutation
- Essay on Meiotic Drive and Migration Pressure
- Essay on Random Genetic Drift
- Essay on Founder Principle
Essay # 1. Essay on Selection:
Evolution has been described by Lewontin (1967) as a process that converts variation within a population into variation between populations, both in space (race formation and speciation) and in time (the evolution of phyla).
The most prominent theory regarding the driving force behind evolution is of course the Darwin-Wallace theory of natural selection proposed in 1958 by C. Darwin and A. Wallace. The natural selection theory holds that as genetic variants arise within a population the fittest will be at a selective advantage and will be more likely to produce offspring than the rest. As the fit continue to enjoy greater survival, reproductively new species will eventually evolve.
II. Natural Selection:
Natural selection is generally believed to be the prominent agent for determining the relative frequency of alleles in a population. Natural selection differentiates between phenotypes in a population with respect to their ability to produce offspring.
One phenotype may better survive endemic onslaughts of parasites or predators than another, one may penetrate new habitats more effectively than another; one may mate more efficiently than other; one may even prey on the other.
The important point is that some natural situation or environmental feature selectively allows one organism to develop and to propagate more efficiently, and one genotype is thereby afforded greater representation in the population’s gene pool. If this selective process continues over many generations, allelic frequencies will change significantly and the potential favorable mutations more will arise for evolutionary change.
Further, the natural selection process, while acting on the total phenotype, will, in fact, influence only the heritable portion of the phenotype. If a trait has a high heritability, selection can rapidly affect its frequency within a population, whereas selection will take a great deal longer to have any effect on a trait with a low heritability.
Natural selection includes the following three parameters—survival rate, relative fitness and selection coefficient. To calculate values for these parameters and, best understand their meaning, we can consider the data from a particular population.
For simplicity, we shall consider only a single gene locus in the population defined by the A and a alleles and we shall assume incomplete dominance, so that A/a heterozygote can be distinguished phenotypically from A/A, and a/a homozygotes.
We then count the number of A/A, A/a, and da individuals in a given generation immediately before and immediately after some selective event—the introduction of a parasite, a change in temperature—has occurred. The representative numbers have been given in Table 47.4. From these we can calculate the survival rate, relative fitness and selection coefficient, as shown in Table.
The genotype with the largest survival rate is defined as the fittest, and it is used as the standard for the relative fitness (W) of all other genotypes. Specifically, in our example, A/A has a survival rate of 0.95 and is, thus, fitter than A/a or a/a (Table 47.4).
In determining values for W, therefore, all survival rates are divided by 0.95 so that the relative fitness of A/A becomes 0.95/0.95 = 1, the relative fitness of A/a becomes 0.80/0.95 = 0.84, and that of a/a becomes 0.55/0.95 = 0.58.
The selection coefficient (s) is simply 1-W. Just as W reflects the chances of an organisms reproductive success, so does ‘s’ reflect the chances of its reproductive failure due to selection? They are simply two sides of the same coin.
III. Directional Selection:
A principal pattern followed by selection in natural populations is that of directional selection. Its effect is to eliminate or to reduce the reproductive potential of particular phenotypes in a population.
Specifically, if the variance in a particular trait is normally distributed in a population before directional selection begins to operate, it might become markedly skewed in one direction afterwards.
Directional selection in its extreme form eliminates systematically the recessive homozygotes (a/a) from the population. In such cases the relative fitness of a/a is set at 0 and its selection coefficient, therefore, becomes 1- 0 = I. If we assume, for simplicity, that A/A and A/a have identical fitnesses and that no other alleles at the locus are involved, then by definition A/A and A/a each have fitness of 1 and selection coefficients of 0.
Further, when directional selection remains in its incomplete form against homozygous recessives, then selection coefficients is less than 1 and the relative fitness W of the homozygous recessives is 1 – s, a number greater than zero.
IV. Artificial Selection:
The artificial selection is simply a selection programme devised by humans rather than managed by natural situations. The process of enriching a population for a given trait normally occurs far more rapidly under the artificial situation, since all but the desired phenotypes can be prevented from reproducing.
Countless experiments have shown that it is possible to select for just about any trait present in a population, a major limitation being the heritability of the trait. Strains with particular shapes, sizes, behaviour patterns, temperature optima, sexual preferences, and soon have been selected.
S. Spiegelman and colleagues have even subjected isolated RNA molecules from RNA phage ɸβ to artificial selection and they reported that a rigorous selection for one trait can lead to the drastic exclusion of many other traits.
V. Significance of the Heteroygote:
When the frequency of an abnormal recessive gene becomes very low, most affected offspring (aa) will come from matings of two heterozygous carriers (Aa). For example, in the human population, the vast majority of newly arising albino individuals (aa) in a given generation (more than 99 per cent of them) will come from normally pigmented heterozygous parents.
Detrimental recessive genes in a population are obviously harboured mostly in the heterozygous state. The frequency of heterozygous carriers is many times greater than the frequency of homozygous individuals afflicted with the trait. Thus, an extremely rare disorder such as alkaptonuria (blackening of urine) occurs in 1 in one million persons.
This detrimental gene, however, is earned in the hidden state by 1 out of 500 persons. There are two thousand as many genetic earners of alkaptonuria. For another recessive trait, cystic fibrosis, 1 out of 1,000 individuals is affected with this homozygous trait. One out of 16 persons is a carrier of cystic fibrosis.
Genetic load (concealed variability) and price of evolution:
Genetic load can be defined as the relative deficiency in viability or fecundity or both viability and fecundity as compared to that which would occur if the population were all of the most fit phenotype.
Thus, genetic load is the difference between the actual fitness and an optimum fitness, this difference being caused by the presence of suboptimal genetic variants, concealed or not, within the population.
In fact mutants arise in any species and many or most of them are deleterious. Some of these mutants may persist in the populations, i.e., one or several generations intervene between the origin and the populations of any species, including the best adapted and most successful ones, carry burdens or loads of genetic defects. One can distinguish between expressed (overt) or concealed genetic loads.
The former comprises individuals with diseases, malformations, or constitutional weaknesses that are manifestly genetic. Concealed genetic burdens are due to recessive genes or gene complies carried in heterozygous conditions in individuals who are themselves healthy or “normal”. When homozygous, however, these genes too become lethal.
Types of genetic loads:
According to Fransworth (1988), the genetic loads are of the following three types:
1. Mutational load:
It refers to the deleterious effects on a population caused by recurrent mutation.
2. Segregational load:
It arises when the heterozygote is the favoured genotype. The formation of heterozygotes is necessarily accompanied by the generation of the less well-adapted homozygous genotypes whose lowered viability and fecundity cause the segregational load.
Segregational load is, thus, the loss in average fitness of a population caused by the segregation of the less fit homozygous genotypes. Because selection acts to preserve the heterozygous genotype, neither allele, even if one or both are lethal, is eliminated from the population.
As an example, in malarial regions of Africa the loss or lowered fecundity of HbS and HbA homozygotes as compared to heterozygotes (HbA HbS) constitute the segregational load for that locus.
3. Substitutional or transient load:
It occurs when a change in the environment results in directional selection in favour of a previously neutral or unfavourable allele whose effects in the altered environment now confer greater fitness. The substitutional load, thus, consists of the relative number of deaths and lack of birth which the population must allow in order to progress from the original new state of fitness.
Substitutional load is transient in that it occurs only during the replacement of one allele by another. For example, in the United States where falciparum malaria is absent, HbA homozygotes possess the best adapted phenotype, and directional selection in favour of the normal allele is occurring.
As a result, the lowered fecundity and viability of HbS/HbA heterozygotes and HbS homozygotes, as compared to normal individuals contribute a transient substitutional load to the black population of the United States.
The substitutional genetic load has been called the price of evolution and J.B.S. Haldane (1957) has made some theoretical calculations of the extent of this price in terms of reduced fitness or loss of individuals.
He has estimated that a selection intensity of 0.1, approximately 300 generations will be required to replace a gene at a cost in members lost by death or sterility of 30 times the average number of individuals present in a single generation. Thus, according to these calculations, approximately 10 per cent of a population would be lost per generation if gene replacement occurred over a time span of 300 generations.
Because of the enormous amount of protein, and, therefore, genetic polymorphism recently revealed at the molecular level by immunological and electrophoretic methods, these calculations pose a dilemma.
Evolution requires the concurrent replacement of many genes, and to the substitutional load borne by a population must be added to the mutational load as well as segregational load contributed by stabilizing selection.
It would appear that no population could withstand such a drain on its members, especially if the effects of individual subvital genes were independent and, therefore, multiplicative. The problem, then, is how are these numerous genie polymorphisms maintained without increasing a genetic load so heavy as to cause the extinction of a population?
The following two hypotheses have been proposed to resolve this problem:
1. Selection hypothesis of genetic load:
This hypothesis proposes that observed polymorphisms are each due to heterozygote advantage and are maintained in a population by various types of stabilizing or balancing selection.
Examples include balanced polymorphism, such as sickle cell heterozygotes, as well as cases where one or another allele or chromosomal inversion favoured in different seasons, regions or stages of life cycle or in different sexes. Frequency- dependent selection also serves to maintain polymorphism, and at equilibrium, the alleles involved are selectively neutral, contributing no genetic load.
Supporters of the selection theory suggest that the primary cause of the extensive heterozygosity observed in nature is heterosis at hundreds or even thousands of gene loci, the cumulative effects conferring fitness.
In 1967, various workers such as Sved, King, Milkman and others have proposed that the genetic load should not be estimated on the assumption that each locus contributes to this load independently and that the individual effects are multiplicative.
In view of the known interaction between genes, they suggest that the values of the sums of these effects exhibit a normal distribution in a population. Since selection scans the entire organism and not single gene loci, only those individuals that fall below a certain threshold of fitness, that is, those with fewer heterotic loci than a minimum number required for average fitness, are collected from the population.
As a result, members of a population are divided into two classes, those above and those below a given threshold and under these circumstances it is estimated that perhaps 1,000 selectively maintained polymorphisms, each conferring an advantage to heterozygotes of around 1 percent, could persist in a population with only minimal effects as genetic load.
2. Neutralist theory of genetic load:
This theory was proposed and developed by J.F. Crow, K. Kimura, T. Ohta, and T.H. Jukes during 1968 and 1983. These workers suggest that the majority of thousands of genie polymorphisms discovered at the biochemical level have no significant effect on fitness and, therefore, selectively neutral, making no contribution to the genetic load.
They propose that adaptation does occur through natural selection, but that selection acts primarily to remove deleterious mutations from a population. Proponents of the neutralist theory indicate that while mutations which result in the substitution in a protein of a compatible amino acid are detectable by sophisticated methodology, they may not be detected by natural selection because the function of the protein will remain essentially unchanged.
Under these circumstances, the frequencies of polymorphic genes conferring no advantage or disadvantage will be determined primarily by random genetic drift rather than by selective processes.
Genetic load in human populations:
Estimates of the genetic load in the human population have been based principally on the incidence of defective offspring from marriages of close relatives (consanguineous marriages). It can be safely stated that every human individual contains at least one newly mutated gene.
It can also be accepted that any crop of gametes contains, in addition to one or more mutations of recent origin, at least 10 mutant genes that arose in the individuals of preceding generations and which have accumulated in the population.
The average person is said to harbour four concealed lethal genes, each of which, if homozygous, is capable of causing death between birth and maturity. The most conservative estimates place the incidence of deformities to detrimental mutant genes in the vicinity of 2 per 1,000 births.
Essay # 2. Mutation:
Mutation is an evolutionary agent and mutability is a required property of the genetic material if evolution is to occur. One can visualise several ways that mutation might bring about evolutionary changes. Mutation may be highly directed at a particular locus such that allele a1 is selectively driven to the a2 form.
Alternatively, the mutation process might be random but with time the a2 form would come to predominate over a1. Finally, mutation might simply provide a population with new alleles (new mutational “currency”) on which any and all evolutionary agents (natural selection, for example) can act.
Essay # 3. Meiotic Drive and Migration Pressure:
Besides directional selection (natural and artificial) and mutation pressure, meiotic drive and migration pressure are two agents that can shift gene frequencies in a population out of a Hardy- Weinberg equilibrium.
I. Meiotic Drive:
The Hardy-Weinberg concept of an equilibrium population assumes that allele, will segregate in a 1: 1 fashion at meiosis and that all gametes in the pool have an equal probability of fertilizing one another.
Clearly, if gametes of genotype a1 have greater success in fertilization than gametes of genotype a2 the frequency of a1 in the population should increase as a2 decreases. Such a mode of bringing about potential evolutionary change is in many respects a form of natural selection but it is called meiotic drive.
Normal segregation ratios can be biased by a number of factors. The most extensively studied example is the segregation distorter (SD) locus in D. melanogaster: a +/SD heterozygous male transmits to his offspring many SD alleles than + alleles. A high frequency of SD might, therefore, be expected in natural populations, but the allele is, in fact, uncommon, its frequency being less than 0.1.
Some additional selective pressures, thus, appear to counteract meiotic drive at this particular locus presumably by agents that act against SD-carrying flies. The existence of meiotic drive is not easy to demonstrate experimentally. However, its specific role in evolution is still un-assessed.
Migration is similar to mutation in the sense that it adds or removes alleles and thereby changes allelic frequencies. Human populations are frequently affected by migration.
Assume there are two populations, natives and migrants, both containing alleles A and a at the A locus, but at different frequencies (pN and and qN) versus pM and qM). Assume that a group of migrants joins the native population and that this group of migrants makes up a fraction m (e.g., 0.2) of the new conglomerate population.
Thus, the old residents, or natives, will make up a proportionate fraction (1-m; e.g., 0.8) of the combined population. The conglomerate a-allele frequency qc, will be the weighted average of the allelic frequencies of the natives and migrants (the allelic frequency weighted- multiplied-by their proportions)-
qc = m qM + (1 – m) qN … (1)
qc = qN + m (qM – qN) … (2)
The change in allelic frequency, a, from before to after the migration event is
Δq = qc – qN = [qN + m (qM – qN)] – qN … (3)
Δq = m (qM – qN) … (4)
The conclusions one can draw from this model are intuitive. Migration can upset the Hardy- Weinberg equilibrium. Allelic frequencies in a population under the influence of migration will not change if either the size of the migrant group drops to zero (m, the proportion of the conglomerate made up of migrants, drops to zero) or the allelic frequencies in the migrant and resident groups become identical.
This migration model can be used to determine the degree to which alleles from one population have entered another population. It can analyse the allele interactions in any two populations can, for example, analyse the amount of admixture of alleles from Mongol populations with eastern European populations to explain the relatively high levels of blood type B in certain European populations (for this one has to make the relatively unrealistic assumption that each of these groups is homogenous).
The calculations are also based on a change happening all in one generation, which did not happen. Blood type and other loci can be used to determine allelic frequencies in western European, eastern European, and Mongol populations.
One can rearrange equation 2 to solve for m; the proportion of migrants:
m = qc – qN/qm – qN … (5)
From one sample, we find that the B allele is 0.10 in Western Europe, taken as the resident or native population 0.12 in Eastern Europe, the conglomerate population (qc); and 0.21 in Mongols, the migrants (qM).
Substituting these values into equation 5 gives a value for m of 0.18. That is, given the stated assumptions, 18% of the alleles in the eastern European population were brought in by genetic mixture with Mongols.
When a migrant group first joins a native group, before genetic mixing (mating) takes place, the Hardy-Weinberg equilibrium of the conglomerate population is disturbed, even though both groups are themselves in Hardy-Weinberg proportions.
A decrease will occur in heterozygotes in conglomerate population as compared to what we would predic from the allelic frequencies of that population (the average allelic frequency of the two groups). This is a phenomenon of sub-division, called Wahlund effect.
The reason this happens is because the relative proportions of heterozygotes increase as intermediate allelic frequencies. As allelic frequencies rise above or fall below 0.5, the relative proportion of heterozygotes decreases.
In a conglomerate population, the allelic frequencies will be intermediate between the value of the two subgroups because of averaging. This generally means the predicted proportion of heterozygotes will be higher than the actual average-proportion of heterozygotes in two subgroups.
An example is worked out in Table 47.6. Assume that the two subgroups each make up 50% of the conglomerate population. In subgroup 1, p = 0.1 and q = 0.9; in subgroup 2, p = 0.9 and g = 0.1. Each subgroup will have 18% heterozygotes. The average (0.18 + 0.18)/2 = 0.18, is the proportion of heterozygotes actually in the population.
However, the conglomerate allelic frequencies are p – 0.5 and q = 0.5, leading to the expectation that 50% of the population will be heterozygotes. Hence the observed frequency of heterozygotes is lower than the expected frequency (i.e., the Wahlund effect).
When most population geneticists sample a population and find a deficiency of heterozygotes, they first think of inbreeding and then of subdivision, the Wahlund effect.
Essay # 4. Random Genetic Drift:
Random fluctuation in allele frequencies, called genetic drift, also occurs in breeding populations. The effect of genetic drift is negligible in large populations but in small breeding populations all the limited number of progeny might be of the same type with respect to certain gene pairs because of chance alone.
Random Genetic Drift
Should this happen, fixation or homozygosity will have occurred at the locus concerned. Fixation is defined as gene frequency reaching p = 1.00 or q = 1.00. Chance fluctuations may or may not lead to fixation.
Essay # 5. Founder Principle:
When a few individuals or a small group migrate from a main population, only a limited portion of the parental gene pool is carried away. In the small migrant group, some genes may be absent or occur in such low frequency that they may be easily lost.
The unique frequency of genes that arise in population derived from small bands of colonisers or “founders”, has been called the founder effect or founder principle. This principle was proposed in 1956 by Harvard evolutionist, Ernst Mayr.
The founder principle essentially emphasises the conditions or circumstances that support the operation of Sewall Wright’s genetic drift. For example, North American Indian tribes, for the most part, lack the gene IB that governs type B blood. However, in Asia, the ancestral home of the American Indians, the IB gene is widespread.
The ancestral population of Mongoloids that migrated across the Bering Strait to North America might have been very small. Accordingly, the possibility exists that none of the prehistoric immigrants happened to be of blood group B. It is also likely that a few individuals of the migrant band did carry the IB gene but they failed to leave descendants.
Evolutionary geneticists interpret this peculiar feature in terms of genetic drift. Most of the North American Indians possess only blood group O, or stated another way, contain only the blood allele i. With few exceptions, the North American Indian tribes have lost not only blood group allele IB but also the allele that controls type A blood (IA). The loss of both alleles, IA and IB by sheer chance perhaps defies credibility.
Indeed, many modern students of evolution are convinced that some strong selective force led to the rapid elimination of the IA and IB genes in the American Indian populations. If this is true, it would provide an impressive example of the action of natural selection in modifying the frequencies of the genes in a population.
It is a form of genetic drift (i.e., changes in allelic frequency due to chance factors) that occurs when a population is reduced in size (population crash) and later expands in numbers (population flush). The enlarged population that results may have gene frequencies that are distinctly different from those before the bottleneck.
Thus, bottleneck effect may counter the effects of previous selection for a short period of time — an interval during which previously favourable mutations may be lost and deleterious mutations may be fixed.
For example, it seems likely that at least some populations began with only a few “Adams” and “Eves” carrying genotypes that may have differed greatly in frequency from their parental population.
Thus, the relatively high incidences of some genes such as achromatopsia (A visual defect marked by total colour blindness in which colours of the spectrum are seen as tones of white-gray-black) among the Pingelapese, are difficult to explain except as founding accidents, since they do not impart any possible advantage on either homozygous or heterozygous carriers.