Causes and changes which bring down frequency – mutation, isolation, migration, selection, inbreeding and genetic drift.

According to Hardy-Weinberg Equilibrium law, the relative frequency of alleles in the population remains constant from generation to generation in a population of sexually reproducing organisms when:

1. The population is large enough so that accident of sampling may be ignored
2. Mating takes place at random;
3. Mutation does not take place or if it does, the rate is same in both directions
4. All the members of the population survive and have equal reproduction rates

I. Mutations:

These are characterized by:

• (i) These are sudden, large and inheritable changes in the genetic material.
• (ii) Mutations are random (indiscriminate) and occur in all directions.
• (iii) Most mutations are harmful or neutral. It is estimated that only one out of 1,000 mutations is useful.
• (iv) Rate of mutation is very low, i.e. one per million or one per several million genie loci. But rate of mutation is sufficient to produce considerable genetic variability.
• (v) Certain mutations are preadaptive and appear even without exposure to a specific environment. These express and become advantageous only when after exposure to new environment which only selects the preadaptive mutations that occurred earlier.

On the basis of their origin, mutations are of two types

Spontaneous and Induced mutations.

Significance of mutations:

• (a) Mutations create and maintain variations within a population.
• (b) These also introduce new genes and alleles in a gene pool
• (c) Accumulation of mutations over a number of generations may lead to speciation.

II. Isolation

Isolation means segregation of different populations into smaller units by certain mechanism so as to prevent interbreeding among individuals.

1. Geographical isolation

When the populations are separated by a geographical barrier, such as river, sea, mountain, deserts and for aquatic animals land, they are physically prevented from interbreeding. Such populations are termed as allopatric and are forced to evolve independently and accumulate genetic differences. Geographical isolation may be different for different species.

eg. Visa by countries leads to isolation

2. Reproductive isolation

It is the property of individuals that prevents interbreeding in populations that are actually sympatric (living in the same area).

eg. Gotra isolation

Classification of reproductive isolating mechanisms 

A. Premating mechanisms

They prevent interspecific crosses in sympatric populations.

1. Seasonal isolation: Also called temporal isolation, in which potential mates do not come in contact with each other because of differences in breeding seasons.

2. Habitat isolation: Also called Ecological isolation, in which also potential mates do not meet each other due to differences in habitats, requirements of food, space, climate etc.

3. Ethological isolation: It is a behavioral isolation, in which potential mates meet but cannot mate, due to differences in courtship displays or other specific signals.

4. Mechanical isolation: In this case the above isolating mechanisms are not present and therefore mating is attempted but is not successful due to mechanical problems.

B. Postmating mechanisms

In case premating mechanisms fail to prevent mating then several postmating mechanisms prevent the success of mating and hybridization.

1. Gamete mortality: Mating and sperm transfer takes place but egg is not fertilized.

2. Zygote mortality: Egg is fertilized but the zygote dies.

3. Zygote inviability: Zygote develops and hybrid is produced but is physically weak and inviable due to physiological disturbances in the body.

4. Hybrid sterility: Hybrid is viable, physically strong and physiologically sound but is sterile due to differences in chromosomes and different gene arrangements.

Significance of isolating mechanism

• Wasteful courtship is avoided. If isolating mechanisms are distinct and specific only individuals of the same species indulge in courtship.
• Isolating mechanism protects gene pool of a species and prevents hybridization.
• It prevents wastage of gametes and energy.
• A weak isolating mechanism leads to production of new species through hybridization.
• Absence of isolating mechanism leads to production of new species by instant speciation.
• Geographical isolation followed by reproductive isolation ultimately leads to production of new species.
• Isolating mechanisms protect the identity of a species, which all species fiercely guard.

III. Migration

Usually some migration-emigration (moving out of some individuals out of a population) or immigration (entry of some members of a population into another population of same species) occurs between the populations.

Immigration results in the addition of new alleles into the existing gene pool and changes the allele frequencies. Degree of changes in allele frequencies depends upon the differences between the genotypes of immigrants and native population.

If there is no much genetic differences, then entry of a small number of migrants will not change the allele frequencies much. However, if the populations are genetically quite different, a small amount of immigration can result in large changes in allele frequencies.

If the migrating individuals interbreed with the members of local population, called hybridization, these may bring many new alleles into the local gene pool of the host population. This is called gene migration. If the inter specific hybrids are fertile, then these may initiate a new trend in evolution which lead to formation of new species.

This addition or removal of alleles when individuals enter or leave a population from another locality is called gene flow. Unrestricted gene flow decreases the differences between the gene pools and reduces the distinctiveness between different populations.

IV. Selection

The process by which comparatively better adapted individuals out of a heterogeneous population are favoured by the nature over the less adapted individuals is called natural selection.

Mechanism:

The process of natural selection operates through differential reproduction.

It means that those individuals, which are best adapted to the environment, survive longer and reproduce at a higher rate and produce more offsprings than those which are less adapted.

So the formers contribute proportionately greater percentage of genes to the gene pool of next generation while less adapted individuals produce fewer offsprings. If differential reproduction continues for a number of generations, then the genes of those individuals which produce more offsprings will become predominant in the gene pool of the population .

So natural selection of Neo-Darwinism acts as a creative force and operates through comparative reproductive success. Accumulation of such variations leads to the origin of a new species.

Types of Natural selection:

1. Stabilizing or balancing selection:

It leads to the elimination of organisms having overspecialized characters and maintains homogenous population which is genetically constant. It favours the average or normal phenotypes, while eliminates the individuals with extreme expressions. In this, more individuals acquire mean character value.

It reduces variation but does not change the mean value. It results very slow rate of evolution. If we draw a graphical curve of population, it is bell-shaped. The bell-shaped curve narrows due to elimination of extreme variants (Fig. 7.49).

Example:

Sickle-cell anaemia in human beings

2. Directional or Progressive selection:

In this selection, the population changes towards one particular direction along with change in environment. As environment is undergoing a continuous change, the organisms having acquired new characters survive and others are eliminated gradually.

In this, individuals at one extreme (less adapted) are eliminated while individuals at other extreme (more adapted) are favoured. This produces more and more adapted individuals in the population when such a selection operates for many generations. In this type of selection, more individuals acquire value other than mean character value.

3. Disruptive selection:

It is a type of natural selection which favours extreme expressions of certain traits to increase variance in a population. It breaks a homogeneous population into many adaptive forms. It results in balanced polymorphism.

In this type of selection, more individuals acquire peripheral character value at both ends of the distribution curve. This kind of selection is rare and eliminates most of the members with mean expression so producing two peaks in the distribution of a trait .

Selection can act in the opposite direction by increasing fitness. For some autosomal recessive disorders there is evidence that heterozygotes show a slight increase in biological fitness as compared with unaffected homozygotes. This is referred to as heterozygote advantage.

eg. sickle- cell disease in which affected homozygotes have severe anemia and often show persistent ill-health. Heterozygotes are relatively immune to infection with Plasmodium falciparum malaria because if their red blood cells are invaded by the parasite they undergo sickling and are rapidly destroyed. In areas in which this form of malaria is endemic, carriers of sickle-cell anemia, who are described as having sickle-cell trait, are at a biological advantage as compared with
unaffected homozygotes.

Selection in heterozygotes is found in  maternal-foetal incompatibility (Erythroblastosis fetalis) as is observed for the allele R (Rh blood group), and also for other blood group genes (Rh-ABO incompatibility).

V. Inbreeding

If an individual mates with a relative, the offspring may be homozygous for a copy of an allele which is identical by descent from one of the ancestors. Inbreeding results in the existence of deleterious recessive alleles in most populations.

Inbreeding causes deterioration of most of the characters. eg fertility tends to decrease,  general size also decrease.

This phenomenon of deterioration on inbreeding is known as inbreeding depression.

In human societies where some families have a lot of wealth, or where a bridal dowry is paid, inbreeding is common.  Examples – European royal families.

It is believed that Neanderthal became extinct due to less variation due to inbreeding.

VI. Genetic Drift:

It is the random change in the frequency of alleles occurring by chance fluctuations. It is characterized by:

(i) It is a binomial sampling error of the gene pool, i.e. that alleles which form the gene pool of the next generation are a sample of the alleles of present population.

(ii) Genetic drift always influences frequencies of alleles and is inversely proportional to the size of population. So genetic drift is most important in very small populations in which there are increased chances of inbreeding which increases the frequency of individuals homozygous for recessive alleles, many of which maybe deleterious.

(iii) Genetic drift occurs when a small group separates from a larger population and may not have all the alleles or may differ from the parental population in the frequencies of certain genes. This explains for the difference between island populations and mainland population.

(iv) In a small population, a chance event (e.g. snow storm) may increase the frequency of a character having little adaptive value.

(v) Genetic drift can also operate through founder effect. In this, genetic drift can cause dramatic changes in the allele frequencies in a population derived from small groups of colonisers, called founders, to a new habitat.

e.g. evolution of locomotary organs in primates is probably derived from  initial founders.

In a large population the numbers of children produced by individuals with different genotypes, assuming no alteration in fitness for any particular genotype will tend to balance out, so that gene frequencies will remain stable. However, in a small population it is possible that by random statistical fluctuation one allele could be transmitted to a high proportion of offspring by chance, resulting in marked changes in allele frequency from one generation to the next, so that Hardy-Weinberg equilibrium is disturbed.

VII. Population bottleneck

It is reduction in allele frequencies caused by drastic reduction in population size called population crash e.g. decrease in cheetah population in Africa due to over-hunting. As the given gene pool is limited, population bottleneck often prevents the species to reestablish its former richness so new population has a much restricted gene pool than the larger parent population.