Natural selection operates in two ways:

1 First way: Natural selection at individual level: One way looks at characteristics of individual animals. For example, a bird acquires an adaptation through natural selection. This adaptation permits it to feed more efficiently on butterflies. This trait may be physical characteristics (e.g., bill shape) or inherited behaviors. It shows that natural selection acts the living organisms.

2 Second way: Natural selection at population level: It indicates that selection operates on the genes. Birds and butterflies are not permanent. They die. But their genes are immortal. The result of natural selection (and evolution) shows that how many common and rare genes are present in a group. They interbreed and share genes. The individuals of the same species that occupy a given area at the same time and share a unique set of genes is called population.

Therefore, there are new definitions of organic evolution.

• The change in frequency of alleles in a population is called evolution. The abundance of a particular allele in relation to the sum of all alleles at that locus in a population is called frequency of an allele.

  • The organic evolution can also be defined in another way. The change in the total genetic makeup of a population (the gene pool) is called evolution. The total genes of a trait in a population are called gene pool.


The combination of the principles of population genetics and Darwinian evolutionary theory is called modern synthesis. Modern synthesis comes from the study of genetics. It explains why variations exist among individuals. It also explains how they pass to the future generations. Some variations give an advantage to individuals. Therefore, nature selects them through natural selection. Thus genetic variations are important in evolution. A large number of genetic variations can be produced in population. Even the simple principles of inheritance of Gregor Mendel provide for remarkable variation. Crossing over, multiple alleles, and mutations increase number of variation. Therefore, no two individuals are genetically identical. Chance produces different combinations of genes. Therefore, some Individuals are better able to survive and reproduce in a given environment than others.


Evolution is central to biology. Evolution always occurs in a particular population Sometimes, the rate of evolution is slow and sometimes it is rapid. But there are some conditions in which evolution does not occur at all. The theories of population genetics explain how evolution can be stopped. The study of the genetic events in gene pools is called population genetics.

THE HARDY-WEINBERG THEOREM                          •

English mathematician Godfrey H. Hardy and German physician Wilhelm Weinberg independently derived a mathematical model in 1908. This model explains what happens to the frequency of alleles in a population over time. Their combined ideas became known as the Hardy-Weinberg theorem. It states that “the mixing of alleles at meiosis and their recombination do not alter the frequencies of the alleles in future generations, if certain assumptions are met” It can be stated in another way. If certain assumptions are met, evolution will not occur because the allelic frequencies will not change from generation to generation, even though the specific mixing of alleles in individuals may vary.

The assumptions of the Hardy-Weinberg theorem are as follows:

1. The population size must be large. Gene frequency does not change by chance in large population size.

  • 2. Mating within the population must•be random. Every individual must have an equal chance to mate with any other Individual in the population. In non random mating, some individuals can more reproduce than others.Then natural selection may occur. Therefore, mating should be random.3 Individuals cannot migrate into, or out of, the population. Migration may introduce new genes into the gene pool. It can add or delete copies of existing genes.4 Mutations must not occur. Or mutational equilibrium must exist. Some mutations take place from wild type allele to a mutant form. Some mutations occur from the mutant form back to the wild type. If both these mutations are balanced then mutational equilibrium is established. Thus no new genes are introduced into the population from this source.

    These assumptions stop the change in allelic frequencies. Therefore, evolution dies not occur. These assumptions are restrictive. A real population cannot meet thhse conditions. Therefore, evolution occurs in most populations.

    However, Hardy-Weinberg theorem provide a useful theoretical framework. The changes in gene frequencies in populations can be examined by this theorem.


    EVOLUTION  is not a creative work force. It is not working for the progress. Evolution helps some individuals in a population to survive. They reproduce more effectively than others in the population. It causes changes in gene frequencies. S. e factors do not follow Hardy-Weinberg assumptions. Therefore, gene frequencies are changed from one generation to the next. It causes evolution.



    Th change in gene frequencies by chance is called genetic drift. Chance pla s an important role in.maintaining a gene in a population. Chance becomes mo e significant in a smaller population. Chance encounters reproductive ind iduals. Thus it promotes reproduction. Some traits of a population do not su ive. But they come in the gametes by chance involved in fertilization. Chance en ts influence the frequencies of genes in populations. It causes genetic drift. Th; gene frequencies are changing independently of natural selection. Th refore, genetic drift is called neutral selection.

    Th process of genetic drift is similar to flipping a thin coin. There is equal cha ce of getting a head or a tail. There are almost 50:50 ratios of heads and tail in large number of tosses. The ratios are disproportionate in only 10 tosses. The ratio may be a 7 heads and 3 tails. Similarly, there are two equally adaptive allel s. One of these two alleles incorporated into a gamete by chance. The efore, it is transferred into a second generation. Thus both alleles have equ I chance.

    But eiosis causes unusual proportions of alleles in gamete in a small population like tossing a coin. Suppose both alleles have equal fitness. These chance eve ts may increase or decreases frequency of a particular allele.

    Inbreeding is also common in small populations. Genetic drift and inbreeding reduce genetic variation within a population.

    Suppose mutation introduces a new allele into a population. That allele is less adaptive than existing alleles. Genetic drift may help the new allele to establish in the population. Or the new allele may be lost due to genetic drift. Thus Hardy-Weinberg equilibrium does not occur due to gerptic drift in small populations.

    Two special cases of genetic drift influenced the genetic makeup of some populations. These are:

    1. Founder effect

    Genetic drift in a new colony is called founder effect. For example, a few individuals from a parental population colonize new habitats. They do not carry all the gene pool of their parental population.


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    The founding individuals develop new colony. It has a distinctive genetic makeup It has far less variation than the larger population.

    This form of genetic drift is called founder effect. The Dunkers (a tribe) of eastern Pennsylvania is an example of founder effect. They emigrated from Germany to the United States early in the eighteenth century. They did not marry outside their sect.due to religious reasons. Certain traits like ABC blood type were examined in their population. It shows that they have very different gene frequencies from the Dunker populations of Germany. It is believed that genetic drift or chance removed certain genes from the founding individuals of the Pennsylvania Dunker popubtion. Therefore, next generations of the Dunkers have different gene frequencies than the German Dunkers.

    1. Bottle neck effect

    A genetic drift on a population which has been drastically reduced by disasters is called bottle neck effect. For example



Fig: Bottle neck effect (Human activities on Cheetah)

(a)   Cheetah populations in South and East Africa are endangered. Their depleted populations have reduced genetic diversity. If their population is restored, they have only remnant of the original population’s gene pool. It occurs due to bottleneck effect.

(b)   Northern elephant seal is also an example of bottle neck effect. They were hunted to near extinction in the late 1800s. Law was enacted in 1922 to protect the seal. Now their population is greater than 100,000 individuals. But they have low genetic variability in the population.

The effects of bottlenecks are controversial. The traditional interpretation is that bottle neck effect decreases genetic diversity of populations. This population cannot withstand environmental stress. Therefore, they become extinct. If one population has high genetic diversity, its individuals have combination of genes These genes help them to withstand environmental changes. Recent evidence indicates the population of cheetah has high genetic diversity. But lions and spotted hyenas prey on cheetah cubs. Therefore, they are becoming extinct However, most evolutionary biologists agree that over evolutionary time frames and high genetic diversity reduces the chance of extinction.


Changes in gene frequency due to migration of individuals are called gene flow. The Hardy-Weinberg theorem assumes. that there is no immigration and emigration. Immigration means that no individuals enter a population from the outside. Emigration means that no individuals leave a population. Immigration or emigration upsets the Hardy-Weinberg equilibrium. They cause changes in gene frequency (evolution). Some natural populations do not have significant gene flow. But most populations have significant gene flow.


Changes in the structure of genes and chromosomes are called mutations. The Hardy-Weinberg theorem assumes that no mutations occur. Or mutational equilibrium exists in the populations, However mutations are a fact of life Mutations are the origin of all new genes. They are source of variation. These variations may prove adaptive for an animal. Mutation counters the loss of genetic material from natural selection and genetic drift. It increases the chance of variations in the population. These variations allow a group to survive in future environmental shocks. Mutations are always random. All mutations are not • useful. Organisms cannot filter harmful genetic changes from advantageous changes. The effects of mutations vary enormously. Most mutations are deleterious. Some mutations are neutral. Some mutations are harmful in one environment. But help,an organism to survive in another environment.

Mutational equilibrium

A case in which Mutation from the wild-type allele to a mutant form is balanced by mutation from the mutant back to the wild type is called mutational equilibrium. It gives that same allelic frequency as was without mutation. However mutational equilibrium rarely exists. The measure of the tendency for gene frequencies to change through mutation is called mutation pressure.

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