Contents
One of the most pressing Biology Topics of our time is the conservation of endangered species and habitats.
Genetic Drift and Estimation of Effective Population Size
Genetic drift is defined as “Changes in gene frequencies when small groups of individuals are separated from or leave a larger population”.
Genetic drift is a kind of genetic change in populations caused by stochastic processes rather than by selection, mutation, or immigration. The three forces of evolution, mutation, selection, and migration have one important quality in common, they act in a directional fashion to change gene frequencies progressively from one value to another. When unopposed, these forces can lead to the fixation of one allele and elimination of all others, or when balanced, they can lead to equilibrium between two or more alleles. In addition to these directional forces, there are also changes that have no predictable constancy from generation to generation. One of the most important of such non-directional forces arises from a variable sampling of the gene pool of each generation and is known as random genetic drift.
Genetic drift is caused by limited population size. But if the population size is large, there is always a strong chance of obtaining good samples of the genes of previous generations. Therefore, a small sample of genes may deviate widely from the gene frequency of the previous generation. In flipping a coin, we usually get two alternative outcomes as head and tail. If the coin is unbiased the frequency of two alternatives becomes 50 : 50. But in tossing a coin 4 times or 6 times we may not get the desired outcome and the results deviate from the expected frequency of outcome. Such deviation may be large compared to tossing the same coin 100 times.
However, all these deviations are considered sampling errors. When the number of tossing is more error will be less compared to the condition when the experiment is conducted for less number of times. Based on this example we may also consider the results of random mating in the natural population. In the case of sexual breeding the male and female gametes unite at random and when the two types of gametes will be many by frequency, the chance of union of the varieties of gametes due to different alleles may show closeness towards expectation. But, if the number of individuals participating in random mating, is less i.e., the population is small, deviation from the expectation becomes large, and change in allelic frequency by this mechanism is called as genetic drift. Therefore, the genetic error is nothing but the sampling error which is increased in small populations.
An example, regarding this, may be given in the following manner, let us consider that, the alleles of a gene namely, A and a are present in a population by a distinct frequency. In the population, the heterozygote (Aa) mating will produce three genotypes AA, Aa, and aa with the proportion \(\frac{1}{4}: \frac{1}{2}: \frac{1}{4}\). If it is a human population due to the practice of population control, we never invite more progeny into the family. In certain families, the number of progeny may be one or two and there we may find only AA or Aa or aa genotypes. If the progeny becomes AA only it means from the family a gene is completely abolished. Considering such conditions in several populations, we may find the unexpected frequency of the alleles in the population which may result in alteration of gene frequencies.
How the genetic drifts affect the allelic frequencies in a population may also be realized in another way. By natural phenomenon, the gene frequency may be altered to some extent which may be determined by variance (v). The value of v is determined by pq/N (p and q are frequencies of the two alleles and N = number of individuals in the population). From this formula, it appears that when N is more v becomes small, and when N is small v becomes large. In the diploid population as the individuals carry a pair of genes together then v is expressed by \(\frac{pq}{2N}\) considering the frequency of two alleles, i.e., A and a as 0.5 and 0.5 respectively we get variance in allelic frequency equal to \(\frac{0.5 \times 0.5}{2 \times 50}=\frac{0.25}{100}\) = 0.0025 (when number of individual in the population is 50), in contrast when p = 0.8 and q = 0.2 variances in allelic frequency will be \(\frac{0.8 \times 0.2}{2 \times 50}=\frac{0.16}{100}\) = 0.0016.
If the population size becomes 10 while considering the same genotypic frequency the variance become in case 1 = \(\frac{0.5 \times 0.5}{2 \times 10}=\frac{0.25}{20}\) = 0.0125 and in case 2 = \(\frac{0.8 \times 0.2}{2 \times 10}=\frac{0.16}{20}\) = 0.008
In both cases, variance is increased by many times. In terms of Hardy Weinberg’s principle, the effective population size is determined by the number of individuals who may contribute alleles to the next generation. Therefore, genetic drift should also be considered in terms of population size. Different factors such as sex ratio, variation in reproductive success, variation in population size, the age structure of the population, and random mating together are considered for calculating the influence of genetic drift.
The Situations in which Genetic Drift becomes Apparent
Genetic drift is the way to change gene frequency in the population. When the number of individuals in a population is decreased, simply due to sampling error, gene frequency is altered. Such a change in gene frequency does not depend on the adaptability or inadaptability of a gene in the environment. Several situations in which a population faces genetic drift are Population bottleneck, Founder effect, and Non-random mating such as inbreeding and assortative mating.
A. Population Bottleneck
In a natural environment, if the population of an organism is depleted severely due to a certain reason, it is called a population bottleneck. Due to this event number of partners for reproductive activities is also decreased when its effect is genetic drift. The progeny population developed in a depleted population becomes devoid of much genetic variability. Not only .that some recessive but deleterious genes get a scope to achieve homozygosity for their expression. Such a situation further promotes the reduction of population size and this condition is much favourable for change in gene frequency. Population bottleneck may be represented by the following diagram.
One example of a population bottleneck may be given as under. Upto 1890 for the purpose of collecting oil from seals, northern elephant seals would have been hunted to a great extent. As a result of this, the number of seals came to only 20 by 1890 and this was a condition for the extinction of this animal. But due to some special measures taken afterward, in 2005 the number of this animal reached to about 20000. The harem polygynous mating system was introduced for increasing the population of seals. The system means that one male may perform mating with many females. The result of such mating has promoted a decrease in genetic variability in the present northern seal population. Due to the attainment of homozygosity of some deleterious genes of the immune system, the seals faced some serious diseases. About 40% of young seals died of a skin disease. A similar condition has also been observed in the case of the population of leopards. In the past few days, a population bottleneck occurred in the cheetah population. The population of leopards now shows very little genetic variability.
B. Founder Effect
When from a larger population some individuals are isolated, establish a new founding habitat in an isolated place, and some genetic variability becomes evident in the newly established population. It is known as the founder effect. Ernst Mayer (1942) first discussed this founder effect with great emphasis. The variation that becomes prominent in the new population due to the founder effect makes it prominently different from the original population and as a consequence of this, the new population may be evolved into a new species in the course of time. The founded new small population may be represented by the following diagram.
In the case of a new isolated population, because of their limited number of individuals, the near relations become involved in mating, and as a result, the chance of developing homozygosity of the deleterious recessive gene is increased and the individual having such homozygosity faces detrimental effects. In many cases, the affected individuals become deprived of natural selection. If a population is small how it may face deleterious effects that may be highlighted through the following examples.
In Eastern Pennsylvania, a religious group called Ames formed a colony taking only 200 individuals. Because the population is small enough, consanguinous marriage (Marriage between relatives) became inevitable. As a result of this homozygosity, many deleterious recessive genes would have developed promoting many bad effects in the population.
One such bad genetic disorder is Ellis-Van Creveld syn-drome when the affected person is dwarf with polydactyly and abnormality of teeth and nails. Some affected persons develop an aperture in the interauricular septum. One couple among the Ames carried the defect in the population during 1744. Afterward, through the forerunners of the king family, the defective condition spread in many individuals in this population. Among the Ames, this disease could be detected in 43 individuals.
When the defect was detected only in 50 individuals of the world population in 100 years. Among the Ames, some other genetic disorders are pyruvate kinase deficiency, hemophilia, and immune disorders. During 1991-1992 Ames’s population was severely affected by Rubella infection. It is, therefore, easily realized how the population may be greatly affected by genetic disorders if it is sufficiently small.
New founding small populations become detached from the principal large populations. The small populations simply genetic drift, may develop variations. In the course of time, these small populations become isolated from each other and face reproductive isolation. The populations thus developed appear as several new species in the course of time. Species formed in this way are called allopatric species and the process is called allopatric speciation.
C. Non-random Mating
The stability of a species population depends on some conditions when the allelic frequency remains constant and genotypic frequencies maintain equilibrium. If due to some reason gene frequency is altered, the genotypic frequencies face disequilibrium and it promotes the path of evolution. Non-random mating may be one cause of changing the allelic frequency in a population. In non-random mating, the partners of sexual reproduction cannot mate at random. Non-random mating may also be called mating based on choice and this may also cause a change in allelic frequency distorting Hardy Weinberg equilibrium. Non-random mating may be of two types Assortative mating and Inbreeding.
1. Assortative Mating:
Assortative mating is often found in the human population when a man prefers to mate with a partner of his choice. In most cases, a phenotypic feature becomes the index under choice. Tall males prefer tall women as partners, a man of fair complexion prefers a fair woman, a dwarf fellow prefers a dwarf partner, deaf and dumb prefers another deaf and dumb, and so on and so forth. All these examples come under assortative mating. However, assortative mating may again be of two types namely Positive assortative mating and Negative assortative mating.
(a) Positive Assortative Mating:
When a sexual partner having some special attribute prefers a mate having a similar attribute, it may be categorized as positive assortative mating. Suppose, a man is albino, no female with normal skin colour prefers him to marry. Therefore, in most cases, an albino male marries an albino woman. A similar marriage between a tall man and a tall woman or marriage between two dwarfs belongs to positive assortative mating.
For persons with similar attributes as mates with identical attributes, the probable progeny types may be indicated in the following checkerboard. However, the attainment of homozygosity for a character may be the result of the progeny in most of cases.
| Probable Mating Type | Positive Assortative Mating | ||
| Probable Genotype of Progeny | |||
| AA | Aa | Aa | |
| AA × AA | 4 | ||
| Aa × Aa | 1 | 2 | 1 |
| aa × aa | 4 | ||
| Total | 5 (42%) | 2 (17%) | 5 (42%) |
Based on the above checkerboard it may be projected that A is a dominant gene and its recessive allele is a. Therefore, for genotype AA or Aa, the dominant character is expressed, whereas genotype aa produces the recessive feature. Only homozygote matings (AA × AA or aa × aa) produce homozygosity in the progeny population whereas random mating may keep a balance of the gene frequency in the population, but assortative mating of this nature is contradictory to genotypic equilibrium if random mating would occur what may be the genotypic distribution in the next generation may be indicated in the following checkerboard.
Genotype Distribution in Case of Random Mating:
| Probable Mating Type | Random Mating | ||
| Probable Genotype of Progeny | |||
| AA | Aa | Aa | |
| AA × AA | 4 | ||
| AA × Aa | 2 | 2 | |
| AA × aa | 4 | ||
| Aa × AA | 2 | 2 | |
| Aa × Aa | 1 | 2 | 1 |
| Aa × aa | 2 | 2 | |
| aa × AA | 4 | ||
| aa × Aa | 2 | 2 | |
| aa × aa | 4 | ||
| Total | 9 (25%) | 18 (50%) | 9 (25%) |
(b) Negative Assortative Mating:
When in the population mating occurs between individuals having opposite characters it is known as negative assortative mating. In the human population, such type of mating is rare. One tall and thin male usually does not prefer to marry a dwarf and fatty woman. Suppose in the human population there are two alleles A and a, when a is recessive to A. For these two alleles, three genotypes are possible in the population. In consideration of genotypes, mating between dissimilar genotypes will be negative assortative mating. The following checkerboard indicates the generation of possible genotypes in the next generation in case of different assortative matings. It is to be noted that altogether with three genotypes six types of negative assortative matings are possible and as a result of these negative assortative matings frequency of heterozygote production is more which is contradictory to positive assortative mating.
Genotype Distribution in Case of Negative Assortative Mating:
| Probable Mating Type | Negative Assortative Mating | ||
| Probable Genotype of Progeny | |||
| AA | Aa | Aa | |
| AA × Aa | 2 | 2 | |
| AA × aa | 4 | ||
| Aa × AA | 2 | 2 | |
| Aa × aa | 2 | 2 | |
| aa × AA | 4 | ||
| aa × Aa | 2 | 2 | |
| Total | 4 (17%) | 16 (67%) | 4 (17%) |
2. Inbreeding:
Inbreeding occurs when mating happens between near relatives. In one aspect inbreeding is one type of positive assortative mating. Because in this type of mating, partners possess many identical features. In this type of mating homozygosity of the genes increases significantly. When the mating partners are heterozygous for some deleterious and autosomal genes, the mating may bring the deleterious recessive genes in homozygous condition and the harmful character is expressed. This incident becomes a cause of misery and pressure to the family. Albinism in humans is caused due to homozygous condition of an autosomal recessive gene.
The albino individual cannot produce melanin in their skin and the albinos are very prone to skin cancer. If two albino individuals being a near relationship marry, they produce all albino children. Usually, recessive deleterious genes remain suppressed in heterozygous conditions. If two relatives carry some deleterious genes in heterozygous conditions, they do not express the disorder or genetic defect. But if the related individual marries due to inbreeding due to achievement of homozygous condition in the progeny, defective character or abnormality may be expressed. In many cases of inbreeding such type of incidence could be noted.
Effects of Genetic Drift
1. Change in Gene Frequency:
The allelic frequencies of a gene undergo changes at random. This change may lead to either increase or decrease of any of the alleles.
2. Fixation of Alleles:
The change in gene frequency may result in homozygosity for one allele. Thereby a population means only the homozygote individual when only one allele would be fixed in the population. This event is called fixation and results in a reduction in genetic variation. Population bottleneck in the northern elephant seals thus has reduced genetic variation in the population.
3. Population Divergence
Due to genetic drift genetically diverse population may arise with time. Because genetic drift is random and the change will not occur at the same pace in all populations. All these effects occur simultaneously but changes in allelic frequency and fixation of the allele may be observed in the same population. On the contrary genetic divergence is expressed in a number of different populations.
Migration
Migration refers to the movement of individuals from the main population to a nearby population. Through this exercise, the genes flow from one population to the other and this results in a change in gene frequency. In this phenomenon when one population loses some of the genes from the total pool, the other population gains having the newly migrated individual of a species. Therefore, through gene flow in case of migration gene frequency is altered. This is one of the causes of genetic instability or a state of disequilibrium. With the help of the migration of human beings from the mainland to a nearby island, the impact of gene flow or migration may be explained.
Let us consider, from a mainland habitat the individuals migrate at a rate per generation. In that case, the resident population indicates the existing inhabitants by the figure (1 – m), while the proportion of the migrating individual is m. If the frequency of a particular allele, say a in two populations is qm and q, the frequency of allele a becomes say q’. Therefore, q’ = (1 – m)q + mqm = q – m(q – qm)
Therefore, change in gene frequency, Dq = q’ – q = {q – m(q – qm)} – q = -m(q – qm)
Hence, due to migration, the frequency of the particular gene will be increased in the population of the island. In case migration continues in successive order in each generation, it will be difficult to maintain the genotypic equilibrium in the population.
Isolating Mechanism
In evolution, the population is considered a unit for evolution. With the change of gene frequency in a population, its identity is changed in consideration of other populations of the same species. In this way, a new population is evolved from the pre-existing species. A species represents a group of interbreeding individuals that are reproductively isolated from another such group. In the evolutionary process, the origin of a new species is the result of evolution. In this process development of variation due to mutation, gene frequency change, due to genetic drift, gene flow, etc. are important.
However, these changes may occur due to isolating mechanism in which a large population is fragmented into groups and groups of a species becomes reproductively isolated. In Hardy-Weinberg’s principle, it has been mentioned that random mating is a phenomenon to keep the gene frequency unchanged. The isolating mechanism is a hindrance to the stability of gene frequency. In that perspective, isolation may be a process of evolution.
Types of Isolating Mechanisms
The situation due to which two species or groups of organisms are prevented from sexual reproduction is called isolating mechanism. The member of one species normally does not mate with the member of another species. If they do so and produce offspring they would be infertile. If species arise by splitting of one ancestral species that might be possible by isolation. This isolation may be of different types as under.
On the whole, the isolating mechanism may be divided into two major heads as premating isolating mechanism and the post-mating isolating mechanism. Each of them is again categorized into several heads.
(a) Premating Isolating Mechanism:
Isolation causes failure to mating.
- Temporal isolation: In this case, individuals cannot mate because they remain active at different times of the year.
- Ecological isolation: Individuals with different ecological habitats cannot mate.
- Behavioural isolation: Individuals of different species cannot meet because of their difference in sexual behaviour.
- Mechanical isolation: When an individual due to morphological incompatibility cannot transfer the sperms resulting in failure to mating is called mechanical isolation.
(b) Post-Mating Isolation:
In this case, mating may occur but no fruitful results after mating may appear. Post-mating isolation may again be of different types:
- Gametic incompatibility: Gametes from 2 species are incompatible and cannot produce form a zygote.
- Zygote mortality: After mating zygote may appear but it cannot show development further.
- Hybrid inviability: Hybrids cannot survive.
- Hybrid sterility: Hybrids of two species are sterile.
- Hybrid breakdown: In this case, first-generation hybrids may be viable and fertile, but next-generation hybrids become infertile.