HARDY -WEINBERG PRINCIPLE 

Five basic processes affect the Hardy Weinberg equilibrium and cause variations at genetic level. These are:

(i) Mutation

(ii) Gene migration

(iii) Genetic drift

(iv) Recombination

(v) Natural selection

The Hardy-Weinberg principle states that the proportions of different alleles will stay the same in a large population if mating occurs at random and the above mentioned forces are absent.

In algebraic terms, the Hardy-Weinberg principle is written as an equation.

Its form is what is known as a binomial expansion.

For a gene with two alternative alleles, called A and a, the frequency of allele A can be expressed as p and that of alternative allele a as q, because these are only two alleles, p + q must always be equal to one. The equation looks like this

For example, if q is the frequency of the allele a, then the Hardy-Weinberg equation states that q2 = percentage individuals homozygous for allele a say 16%.

q2 = 0.16, q = 0.4

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Factors of Evolutionary Change

(i) Mutation (ii) Migration (iii) Genetic Drift

(iv) Recombination (v) Natural Selection

(I) MUTATION

Replica Plate Experiment of Lederberg and Lederberg

1. Mutations are random (indiscriminate) with respect to the adaptive needs of organisms.

2. Most mutations are harmful or with no effect (neutral) on their bearer.

3. Mutation rates are very slow.

The Lederberg Replica Plating Experiment, a beautiful example of the genetic basis of a particular adaptation was demonstrated in bacteria by an ingeneous method devised by Joshua Lederberg and Esther Lederberg.

E.coli bacteria are usually grown in the laboratory by plating dilute suspensions of bacterial cells on semi-solid agar plates.

After a period of growth, discrete colonies appear on the agar plates.

Each of these colonies originates from a single bacterium through a large number of cell divisions.

The Lederbergs inoculated bacteria on an agar plate and obtained a 'master plate' containing several bacterial colonies.

They, then created several replicas of this master plate by a simple procedure.

A sterile velvet disc, mounted on a wooden block, was gently pressed on the master plate.

Some bacteria from each colony adhered to the velvet.

By pressing this velvet on to new agar plates, they obtained exact replicas of the master plate, because the few bacteria transferred by the velvet formed colonies on the new agar plates.

However, when they attempted to make replicas using plates containing an antibiotic such as penicillin, most colonies found on the master plate did not grow on the replica plates.

The few colonies that did grow were obviously resistant to penicillin.

How did the bacteria acquire the ability to grow in a new environment (here, agar medium, containing penicillin)? In other words, what was the origin of this adaptation?

A Lamarckian interpretation of this adaptation would have been that penicillin somehow induced a change in one or more bacteria, enabling them to grow in the presence of penicillin.

A Darwinian view is that there were, in the original suspension of bacteria from which the master plates were prepared, a few bacteria carrying mutant genes which conferred on them the ability to survive the action of penicillin and form colonies.

These mutations, which had arisen by chance, and not induced by penicillin, were present only is small numbers in the original suspension.

Lederberg's experiment provided evidence that mutations are actually preadaptive.

These kinds of mutations are regarded as advantageous mutations.

They appear without exposure to the environment.

Actually, the preadaptive mutations express themselves only after exposure to the new environment to which the organisms are to adapt themselves.

The new environment does not induce the formation, it only selects the preadaptive mutations that occurred earlier.

(II) MIGRATION

Migration, defined in genetic terms as the movement of individuals from one population into another, can be a powerful force in upsetting the genetic stability of natural populations.

If the characteristics of the newly arrived animal differ from those already there, the genetic composition of the receiving population may be altered, if the newly arrived individual or individuals can adapt to survive in the new area and mate successfully.

Gene pool : A total collection of all genes and its allele in a population is called gene pool. Thus, gene pool will have all genotypes i.e., genes of the organisms.

Gene flow: If genes are exchanged between two different populations of a species, it is gene flow.

(III) GENETIC DRIFT I SEWALL WRIGHT EFFECT I NON-DIRECTIONAL FACTOR

Natural selection is not the only force responsible to bring about changes in gene frequencies. There is the role of chance or Genetic Drift also.

Genetic Drift causes the change in gene frequency by chance in a small population.

In a small population, the one individual alleles of a gene are represented by a few individuals in a population.

These alleles will be lost if these the individuals fail to reproduce.

Allele frequencies appear to change randomly, as if the frequencies were drifting, Janes thus, a random loss of alleles in small population is Genetic Drift.

A series of small populations that are isolated from one another may come to differ strongly as a result of Genetic Drift.

Genetic Drift has two ramifications are described below.

1. Bottle neck effect:

It is the decrease in genetic variability in a population, e.g., cheetah population in Africa decreased due to hunting.

Their decreased numbers have limited cheetahs genetic variability, with serious consequences.

The present cheetah population is susceptible to a number of fatal diseases.

If any of these diseases attacks the cheetah population, the path of extinction of cheetah cannot be reversed.

2. Founder's effect:

When one or a few individuals are dispersed and become the founders of a new, isolated population at some distance from their place of origin, the alleles that they carry are of special significance.

Even if these alleles are rare in the source population, they will be a significant fraction of the new population's genetic endowment.

This effect by which rare alleles and combinations of alleles may be enhanced in new populations -is called the founder's effect.

The founders effect is particularly important in the evolution of organisms on islands, such as Galapagos Islands which Darwin visited.

Most of the kinds of organisms that occur in such areas were probably derived from one or a few initial founders.

Fixation of new mutations:

Genetic drift fixes new alleles, genes that arise by mutation, from time to time and eliminate the original gene, thereby changing the genetic make up of small population.

(IV) RECOMBINATION

Gene recombination is also an important source of variations.

It occurs during crossing over at the time of meiosis, free assortment (selection) of genes at the time of gamete formation, random union of gametes at the time of fertilization and even chromosomal aberrations.

They cause reshuffling of gene recombinations which provide new combinations of existing genes and alleles.

This is the entity of gene recombination.

Gene recombination can occur not only between genes but also within genes resulting in the formation of a new allele.

Since it adds new alleles and combination of alleles to the gene pool, it is an important process during evolution which causes variations.

(V) NATURAL SELECTION

It causes allele frequencies of a population to change. Depending upon which traits are favoured, natural selection can produce different results.

Forms of Selection: There are three kinds of natural selections

Stabilizing Selection (Normalizing selection) :

When selection acts to eliminate both extremes from an array of phenotypes, the frequency of the intermediate type which is already the most common, is increased.

Directional Selection (Progressive selection) :

When selection acts to eliminate one extreme from an array of phenotypes, the genes determining this extreme become less frequent in the population. The industrial melanism is peppered moth, Biston betularia provides good example of directional selection from nature.

Disruptive Selection (Diversifying selection) :

In some situations, selection acts to eliminate, rather than favour, the intermediate type. The individuals at both the extremes are favoured.

Examples of Natural Selection-Industrial Melanism

First studied by R.A. Fisher and E.B. Ford and in recent time by H.B.D. Kettlewell.

One of the most striking examples, which demonstrates the action of natural selection, is the industrial melanism in England.

The peppered moth Biston betularia, with a dull grey colour or white was abundant in England before the Industrial Revolution.

A black coloured form of the same moth (melanic, a dominant mutant differing in a single gene), carbonaria, was very rare.

Within a couple of hundred years, however, the proportion of carbona ria increased to almost 90 per cent.

The moths rest on tree trunks.

Before the Industrial Revolution, the tree trunks used to be covered with grey coloured lichen.

The dull grey moth easily blended with this background, while the black moth stood out conspicuously, and was therefore more susceptible to predation by birds.

With the advent of the Industrial Revolution, large-scale burning of coal became common.

The enormous amount of smoke produced resulted in the deposition of particulate matter on tree trunks, turning them black.

As a result, the grey moths now became more conspicuous than the black variety, and hence more susceptible to predation.

The frequency of black coloured moths in the population therefore increased.

Gradual replacement of coal by oil and electricity, as well as the improved methods of controlling soot production, reduced the soot desposition on the trees.

Conditions then became more suitable for the survival of grey moths, consequently their frequency once again increased.

Thus, reduction in pollution is now correlated with reverse evolution.

Industrial melanism, as this phenomenon is called, is thus a particularly interesting example which clearly brings out the action of natural selection.

This has been observed in about 70 different species of moths, and in several other European countries as well.

This understanding is supported by the fact that in areas where industrialisation did not occur e.g., in rural areas, the count of melanic moths was low.

This showed that in a mixed population, those that can better-adapt, survive and increase in population size.

Remember that no variant is completely wiped out.

Similarly, excess use of herbicides, pesticides, etc., has only resulted in selection of resistant varieties in a much lesser time scale.

This is also true for microbes against which we employ antibiotics or drugs against eukaryotic organisms/cell.

Hence, resistant organisms/cells are appearing in a time scale of months or years and not centuries.

These are examples of evolution by anthropogenic action.

This also tells us that evolution is not a direct process in the sense of determinism.

It is a stochastic process based on chance events in nature and chance mutation in the organisms.

Change In Genotypic Frequencies

If the alleles for grey and black colours are denoted by G and B, the genotypes of the moths would be GG, GB and BB.

Since B is dominant, GB and BB will be black.

Due to greater predation by birds on the black (melanic) phenotype the proportion of B in the population was maintained at a much lower value than G.

Resistance of Mosquitoes to Pesticides

Mosquitoes have always been a major health hazard, especially as they are responsible for the spread of diseases such as malaria and filaria.

When DDT was first introduced to control mosquitoes, it was tremendously successful; most mosquitoes were sensitive to DDT and were therefore killed.

However, DDT has now become ineffective against mosquitoes.

This is explained as follows:

In the original population of mosquitoes, some individuals were resistant to DDT.

However, in the absence of DDT, such resistant individuals were few because they had no advantage over the DDT-sensitive mosquitoes.

However, when DDT was used on a large-scale, only the resistant genotypes were able to survive and reproduce.

As a result, over a period of time, almost the entire population came to consist of the resistant type, which made DDT quite ineffective.

Evolution is thus a change in gene frequencies in the population in response to changes in the environment-in this case the introduction of DDT.

The principle of natural selection thus helps us to understand, why such chemical insecticides would remain useful only for a limited time.

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Plants Growing Around Mines

A few plants are now known to grow on the tailings or refuse around mines. Professor A.D. Bradshaw studied one such grass, the bent grass Agrostis tenuis growing on tailings of lead mines in Wales, U.K.

He took some of these and planted them in soil from a pasture nearby.

Similarly, he transplanted live Agrostis plants from the pasture to the lead-rich soil.

The bent grass from the mine soil grew very slowly on normal pasture soil.

The one from the pasture, on the other hand, could not survive in the lead-rich soil.

A very small percentage (three out of sixty), however, could grow in the soil rich in lead.

These were undoubtedly the kind, from which the race of bent grass capable of growing in lead-rich mine soil evolved originally.

Plants tolerant to selenium such as Astragalus and Haplopappus have been reported from the U.S.A.

These plants are not only capable of growing in seleniferous soils, but require selenium as an essential element.

In our country, Professor Y.D. Tyagi discovered populations of Impatiens balsamina growing around Zwar zinc mines in Udaipur, Rajasthan.

The presence of such plants, which have evolved metal tolerance, can indicate the occurrence of specific metal deposits.

Such plants are called bioindicator plants.

Sickle Cell Anaemia Is an example of balancing selection.

(i) In few RBCs, 1-2% became sickle shaped during lack of oxygen.

(ii) The heterozygotes (HbA / HbS), who have one copy of sickle cell allele, coupled with one normal allele are better survivors in the areas where malaria is endemic; because the malarial parasite spends a part of the life cycle in the RBC; if they enter into the RBC which are sickle shaped, they will die.

(iii) The women who are heterozygote have higher fertility; that's why natural selection has not eliminated the allele.

(iv) The loss of deleterious recessive genes through deaths of homozygotes (HbS / HbS) is being balanced by the gain resulting from successful reproduction by heterozygotes in malaria prone areas. For this reason, the selection is called balancing selection.

(v) Heterozygotes enjoy some resistance to malaria, so they survive the malarial parasite more successfully than either normal or sickle cell homozygotes.

ARTIFICIAL SELECTION

Some genetic variability is always present in a population.

Some alleles make organisms better adapted to the environment, and thus make them more successful in survival and reproduction.

As a result, the frequency of such alleles in a population gradually increases.

This is called selection; these alleles are thus 'selected' over the other alleles.

This process operating in natural populations is therefore called 'Natural Selection'.

The process of natural selection, acting on variability inherent in the population, over millions of years, has given rise to the great diversity we see in the biological world.

Variation among breeds of domestic pigeons.

Ancestry of different breeds can be traced to wild rock pigeon. (Artificial Selection)

Man has been using a similar process for improving the qualities of domesticated plants and animals for centuries.

Plant-breeding and animal-breeding are very similar to the action of natural selection, the difference being that the role of nature is played by man.

The criteria for selection are based on human interests.

Cabbage, Cauliflower, Kohlrabi are descendants
of a common ancestor, colewort (Artificial Selection)

To obtain cows with high milk yield, the dairy scientists monitor milk production of a large number of cows.

Only the calves produced by cows which are high-yielders, are chosen to breed and form the next generation.

When this process is repeated (i.e., artificial selection is applied) for many generations, a population of cows with high milk yield is obtained.

Here, the work of selection is done by man.

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Mimicry: It is a kind of adaptation. The term mimicry was introduced in Biology by Bates (1862). It is defined as "the resemblance of one organism to another or to any natural object for the purpose of concealment, protection or for some other advantages. The organism which exhibits mimicry is called a mimic. The organism or object which is mimicked or imitated is called a model.

Bateslan Mimicry : It is a form of mimicry in which an edible species resembles an inedible one.

Mullerian Mimicry : When two or more inedible or unpalatable species resemble each other the mimicry is termed Mullerian mimicry.

Both Batesian and Mullerian mimicries are two forms of protective mimicries.