CBSE Class 12 Biology Evolution Notes Set B

ORIGIN OF LIFE
• Evolution (Gk. e – out, volvere – to roll) is unrolling or unfolding of nature that brings about an orderly change from one form or condition to another resulting in descendants becoming different from ancestors.
• Many theories were put forward to explain the origin of life.
• The origin of life is considered a unique trend in the history of universe.
• Most accepted theory to explain the origin of universe is the Big-Bang theory which was proposed by Abbe Lemaitre in 1931 according to which universe had an explosive beginning.
• Universe originated about 15 billion years ago by a big bang (thermonuclear explosion) of a dense entity. The universe expanded and hence the temperature came down. Hydrogen fused into progressively heavier atoms of different kinds of elements found today.
• Panspermia theory was proposed by Richter in 1865 according to which life came from outside in the form of spores.
• Theory of spontaneous generation believes that under certain conditions nonliving substances gave rise to living beings spontaneously. It was supported by many old scholars like Thales, Anaximander, Aristotle, Plato, Epicurus, etc. This theory was challenged by Francesco Redi, Spallanzani and Pasteur.
• The modern hypothesis of origin of life was formulated by Haeckel. This idea was elaborated in the chemical theory (in 1920s) by two workers independently : a Russian biochemist A.I. Oparin and an English biologist J.B.S. Haldane. It was summarised by Oparin in his book “The Origin of Life”.

• Oparin and Haldane state that :
1. Spontaneous generation of life under the present environmental conditions is not possible.
2. Earth’s surface and atmosphere during the first billion years of its existence were radically different from today’s conditions.
3. Earth’s initial atmosphere was a reducing one.
• The Oparin-Haldane theory (also called protobiogenesis) was experimentally supported by Stanley Miller in 1953.
• He created similar conditions in a laboratory scale. He created electric discharge in a closed flask containing CH₄, H₂, NH₃ and water vapour at 800°C. He observed formation of amino acids. In similar experiments others observed, formation of sugars, nitrogen bases, pigment and fats. With this limited evidence, the first part of the conjectured story, i.e., chemical evolution was more or less accepted.
• The synthesis of carbohydrates, fats and amino acids and other complex organic substances probably occurred in sea, which had been described by Haldane as ‘The hot dilute soup’. The formation of protein molecule is considered a land mark in the origin of life. 

Microsphere
The large organic molecules which were synthesised abiotically on primitive earth later came together, and due to intermolecular attraction, they formed large colloidal aggregates. Such water bound aggregates have been named microspheres by Sydney Fox. 
Coacervate is an aggregate of macromolecules, such as proteins, lipids, and nucleic acids, that form a stable colloid unit with properties that resemble living matter.
• The first living organisms originated among organic molecules and in oxygen free atmosphere (reducing atmosphere). They were anaerobes capable of respiration in the absenceof oxygen. They depended on the existing organic molecules for their nutrition and thus they were heterotrophs.
• When the supply of existing organic molecules was exhausted, some of the heterotrophs might have evolved into autotrophs.
• The prokaryotes gradually modified to adapt themselves according to new conditions. They developed a true nucleus and other specialised cell organelles. Thus free-living eukaryotic cell -like organisms originated in the ancient ocean presumably about 1.5 billion years ago.
• Primitive eukaryotes led to the evolution of protists, plants, fungi and animals. Life was present on earth about 3.9 billion years ago.

THEORIES OF EVOLUTION
To understand and explain the process of evolution various theories have been put forward by various scientists such as Lamarck, Darwin etc.

Neo-Lamarckism
• It is the modified form of Lamarckism supported by a number of scientists viz Spencer, Cope ,  Richard, Wells, Lawrence Nageli, etc. 

• According to it, the acquired characters which become incorporated in the germplasm are heritable and accumulate generation after generation resulting in the origin of new species.
• Neo-Lamarckism explains that germ cells may be affected by environment either directly or indirectly (i.e., through somatic cells).

Modern Synthetic Theory
Dobzhansky (1937) in his book ‘Genetics and Origin of Species’ provided the initial basis of synthetic theory. ‘Modern Synthetic Theory of Evolution’ was designated by Huxley in 1942. According to synthetic theory there are five basic factors involved in the process of organic evolution.

I. Variations :
Evolution occurs through the accumulation of genetic variations in the gene pool of population over long periods of time. The change in genes occurs in many ways such as mutations, genetic drift, gene migration, gene recombination and hybridisation.
(i) Mutations : Gene mutation is a random change in the base sequence of a gene. 

The mutated gene may give rise to a new protein or may fail to produce any. This may change the phenotype (trait) . Gene variation results in change in gene frequency.

(ii) Genetic drifi : The theory of genetic drift was developed by geneticist Sewall Wright in 1930. It is also called as Sewall Wright Effect or scattering of variability. Genetic drift refers to chance elimination of the genes of certain traits, independent of gene’s useful or harmful effect when a section of population migrates or dies of natural calamity. It alters the gene frequency of the remaining population. Hence genetic drift is a mechanism of evolution that acts in concert with natural selection to change species characteristics over time. Two important examples of genetic drift are bottleneck effect and founder effect.
Bottle neck phenomenon : The yearly or seasonal phenomenon of cyclic fluctuation in population density causing periodic squeezing of some of the genes in a gene pool in random fashion is called bottle neck phenomenon.
Founder effect : When a few individuals or a small group of individuals from some large population invades a new or isolated geographical region, they become the founders. These founders carry on a limited portion of the parental gene pool. The descendants of the founder isolates in new area and tend to have allele ratios similar to the founders rather than to the source population.

(iii) Gene migration (Gene flow) : If the migrating individuals interbreed with the members of local population, these may bring many new alleles into the local gene pool of the host population. This is called gene migration. This addition or removal of alleles when individuals enter or leave a population from another locality is called gene flow.

(iv) Gene recombination : It occurs due to the dual parentage, independent assortment of chromosomes, crossing over during meiosis, random fusion of gametes and formation of new alleles.

(v) Hybridisation: It is the crossing of organisms which are genetically different in one or more traits (characters). It helps in intermingling of genes of different groups of the same variety, species and sometimes different species.

II. Inheritance of variation :
Organisms possessing hereditary characteristics that are helpful, either in the animal’s environment or in some other environment, are favoured in the struggle for existence. Thus, the offspring are able to benefit from the advantageous characteristics of their parents.

III. Natural selection :
It is differential reproduction which means some members of population have traits (genes) that enable them to grow up and reproduce at a higher rate and leave more surviving offsprings in the next generation than others, i.e., they are selected by nature. Based upon different organism-environment relationships, following different kinds of natural selections have been recognised.
Stabilising selection or Balancing selection: It acts in the absence of large scale environmental change or directional changes for long period. It favours an intermediate form and eliminates the extreme variants.
Directional selection or Progressive selection: It produces a regular change within a population in one direction in respect to certain characteristics. It favours the phenotype which is non-average or extreme and then pushes the phenotype of the population in that direction. 
Disruptive selection or Diversifying selection: It is just the opposite of stabilising selection i.e., the extremes have more adaptable phenotypes than the average ones. Consequently, the original population is disrupted into two or more separate groups that later evolve into new species. If disruptive selection results in many new species then it is termed as adaptive radiation. This kind of selection is rare.

Industrial melanism :
Industrial melanism can be explained briey as follows:
1. The peppered moth existed in two strains (forms) : light coloured (white) and melanic (black).
2. In the past, bark of trees were covered by whitish lichens, so white moths escaped unnoticed from predatory birds.
3. After industrialisation, barks got covered by smoke, so the white moths were selectively picked up by birds.
4. But black moths escaped unnoticed so they managed to survive, resulting in more population of black moths and less population of white moths.
5. Thus, industrial melanism supports evolution by natural selection. 

IV. Reproductive isolation :
It refers to the mechanisms which check the populations of two different species from interbreeding. The reproductive isolation, thus preserves the integrity of a species by checking hybridisation. It may, however, lead to the origin of new species by accumulation of genetic variations in a population. Reproductive isolation, thus, lets evolution to occur.

V. Speciation :
The population of a species separated by geographical and physiological barriers, accumulate different genetic differences due to mutations, recombination, hybridisation, genetic dri and natural selection. These populations, therefore, undergo morphological and genetic differentiation, and eventually reproductive isolation, forming new species. 

EVIDENCES OF EVOLUTION
• Homology is the similarity between organs of different animals based on common ancestry or common embryonic origin and built on the same fundamental pattern, but perform varied functions and have different appearance. E.g., the flipper of a seal, wing of a bat, forelimb of  a mole, front leg of horse and the arm of a man look very different, perform different functions, but exhibit the same structural plan including same bones.
• Study of homologous structures illustrates the occurrence of adaptive radiation (divergent evolution).
• Adaptive radiation represents evolution of new forms in several directions from the common ancestral type (divergence).
• The analogous organs have almost similar appearance and perform the same function but these develop in totally different groups and are totally different in their basic structure and developmental origin. E.g., wing of a butterfly, bird, pterodactyl and bat serve the same purpose of uplifting the body in the air, but their basic structure is totally different.
• Study of analogous structures illustrates the occurrence of convergent evolution. 
• Homology in the embryos of closely related vertebrates, indicates evolutionary relationship of the adult vertebrates.
• Haeckel formulated the “Recapitulation theory or Biogenetic Law” regarding this. This theory says that “ontogeny recapitulates phylogeny”, i.e., life history recapitulates evolutionary history. This means an organism repeats its ancestral history during its development. For example, in the development of frog a fish-like tailed larva is formed, which swims with the tail and respires by gills. This indicates that the frog has been evolved from a fish-like ancestor.
• Living organisms possessing characters of two different groups of organisms are known as the connecting links. Examples – Viruses : between non-living and living, Euglena: between plants and animals, Peripatus : between annelida and arthropoda, Balanoglossus : between nonchordates and chordates, Chimaera : between cartilaginous and bony fishes. They give a hint about evolution of one group from the other. On the other hand, fossils that show combined features of two groups are termed missing links, e.g., Archaeopteryx.
• Similarities in the biochemical composition, reactions and physiological activities are the most convincing evidences of common ancestry. Aspects of biochemistry that indicate biochemical affnity are metabolic processes, enzymes, hormones, blood and lymph, blood proteins, blood groups and molecular homology.
• The direct evidence of organic evolution comes from the study of fossils. The term fossil refers to the petrified remains or impressions of organisms that lived in past and got preserved in the sedimentary rocks. These include bones, teeth, shells and other hard parts of animal or plant body, and also impressions or imprints left by previous organisms in the soft mud or the moulds and casts of entire organisms.
• Palaeontology is the study of past life based on fossil records. Their study reveals the existence of life in past and illustrates the course of evolution of plants and animals.
• Fossils can be arranged in chronological sequence according to their age only when their correct age is determined. The age of fossils is determined by radioactive dating technique.
• Geological time is a chronological order or history of evolution based upon the study of fossils. It has been divided into eras, periods and epochs. The fossil studies have given evidences of several mass extinctions.

HARDY-WEINBERG PRINCIPLE
• The genetic equilibrium is defined as– “The relative frequencies of various kinds of genes in a large and randomly mating sexual panmictic population tend to remain constant from generation to generation in the absence of mutation, selection and gene flow.” This is called Hardy-Weinberg principle or Hardy-Weinberg equilibrium.
• In a population at equilibrium, for a locus with two alleles, D and d having frequencies of p and q, respectively, the genotype frequencies are: DD = p², Dd = 2pq, and dd = q².
• The two formulae are :
p²+ 2pq + q² = 1,   p + q = 1
where,
p = Frequency of the dominant allele in the population.
q = Frequency of the recessive allele in the population.
p²= Percentage of homozygous dominant individuals.
q² = Percentage of homozygous recessive individuals.
2pq = Percentage of heterozygous individuals.
1 = Sum total of all the allelic frequencies.
• Hardy-Weinberg’s law describes a theoretical situation in which a population is undergoing no evolutionary change.

Salient features of Hardy-Weinberg principle:
(i) Random mating,
(ii) Large population size,
(iii) Biparental mode of reproduction,
(iv) Homogeneous age structure.
• The gene frequency will remain static only in the absence of evolutionary forces like mutations,  selection, genetic drift and migration. 

The basic timeline of a 4.6 billion year old Earth, with approximate dates: 
1. 3.6 billion y X ears of simple cells (prokaryotes),
2. 3.4 billion years of stromatolites demonstrating photosynthesis,
3. 2 billion years of complex cells (eukaryotes),
4. 1 billion year of multicellular life,
5. 600 million years of simple animals,
6. 570 million years of arthropods (ancestors of insects, arachnids and crustaceans),
7. 550 million years of complex animals,
8. 500 million years of fish and protoamphibians,
9. 475 million years of land plants,
10. 400 million years of insects and seeds,
11. 360 million years of amphibians,
12. 300 million years of reptiles,
13. 200 million years of mammals,
14. 150 million years of birds,
15. 130 million years of owers,
16. 65 million years since the dinosaurs died out,
17. 2.5 million years since the appearance of the genus Homo,
18. 25,000 years since the appearance of modern man.

HUMAN EVOLUTION
Human evolution, or anthropogenesis, is the part of biological evolution concerning the emergence of Homo sapiens as a distinct species from other hominids, great apes and placental mammals.

Summary of Human Phylogeny