Objectives :-

1. Theories of evolution

• Lamarck’s theory of evolution
• Darwinian theory of evolution
• Industrial melanism
• Chemical resistance
• Mutation theory of evolution

Theory of evolution ( Biological Evolution)

Lamarck’s theory of evolution or lamarckism :-

According to lamarck’s evolution of life forms occurred due to use and disuse of organs.

Example :- giraffes initially did not have a long necks. But to access leaves on tall trees,
They had to adapt by elongating their necks. By passing this acquired character to succeeding generation, they acquired long necks,
This theory is more accepted.

Darwinian Theory of Evolution :-

Charles darwin based on his observation during a sea voyage around the world in the ship H.M.S.beagle, concluded the following :-

• Varying degree of similarities can be observed between existing life forms and those that existed million years ago.
• Any population has built in variation in characteristics which adopt it better to environment.

Survival of fittest :-

The characteristics which enable some populations or individuals to survive better in natural conditions ( climate , food , physical factors ).

Natural selection :-

Those populations which are better fit in an environment will be selected by nature and will survive more.
Natural selection is based on following factual observations :-

1. Limited natural resources
2. Stable population size except seasonal fluctuations.
3. Varying character of member of population.
4. Most of variations are inherited.

The two key concept of darwinian theory are :-
Branching descent ( adaptive radiation ) and natural selection.

Industrial melanism :-

In England, before industrialization, white winged moths were more in number than dark winged moths.
But after industrialization,dark winged both became more in number than white winged moths.
This is because during industrialization, the tree trunks covered by white, lichens became dark due to deposition of dust and coal particles.
As a result, white winged moths could be easily picked up by predators from the dark background and dark winged moths survived.

Chemical resistance :-

• Excessive use of herbicides and pesticides has resulted in evolution of resistant varieties of microbes in much lesser time scale.
• As a result pathogenic bacteria are appearing is very short period.

Mutation theory of evolution.

• Hugo de vries based on his work on evening primrose :-
• According to him , evolution is caused by sudden large different in population i,e, mutation and not the minor variations as per drawing.
• He believed that mutation caused speciation and called it saltation or single step large mutilation.
• Mutations are random and directionless in corporate to small directional variations as per darwin.

Hardy weinberg princlple :-

This principle states that allelic frequencies in a population are stable and remain constand from generation to generation i, e, gwnw pool ( total number of genes and their alleles in a population ) is constand. This is called entic equilibrium or hardy- weinberg equilibrium.

hence , the formula is p2 + 2pq + q2 which binomial expansion of (p+q)2 which can be applied to any population to find out the gene frequency.

Factors affecting hardy- Weinberg principle are–

Gene flow –

• The transfer of section of population to another place resulting in a change in gene frequencies in both old and new population is called gene flow.
• New genes and alleles are added to new population which genetically different but can interbreed.

Genetic drift –

• The random change in gene frequency occurs by chance is called genetic drift.
• Sometimes,the change in allelic frequency is so different in the new population, that they become a different species and the original drifted population becomes founders hence the effect is called founder effect.

Mutation-

• The spontaneous change in the genetic makeup of an individual is called mutation.
• Pre-existing advantageous mutations when selected will result in observation of new phenotypes and over few generations this would result in

Genetic recombination –

• Exchange of genes between non sister chromatids of homologous chromosomes during gametogenesis is called genetic.
• Variation due to recombination during gametogenesis,or due to gene, flow or genetic drift results in changes frequency of genes and alleles in future generation.

CONT…..

Natural selection –

• The process by which better adapted individuals with useful variations are selected by nature and leave greater number of progenies is called natural selection.
• Natural selection can lead to-
• Stabilizing selection – here, more individuals acquire mean character value.
• Directional change – here, more individuals acquire other than the mean character value.
• Disruptive selection – here, more individuals acquire peripheral character value at both ends of the distribution curve.