Ch-7 evolution

The Origin of Life and Evidences of Evolution:

1. The study of history of life forms on earth is called evolutionary biology.
2. Evolution is a process that results in heritable changes in population spread over many
generations leading to diversity of organisms on earth.
3. Origin of life is considered a unique event in the history of universe.
(i) The Universe
(a) It is very old-almost 20 billion years ago. It contains huge galaxies.
(b) Galaxies contain stars and clouds of gas and dust.
(c) The origin of universe is explained by Big Bang theory.
(d) The Big Bang theory states that a huge explosion occurred, the universe
expanded, temperature came down and hydrogen and helium were formed later.
The galaxies were then formed due to condensation of gases under gravitation.
(ii) The earth was supposed to have been formed about 4.5 billion years back in
the solar system of the milky way galaxy.
(a) Water vapour, methane, carbon dioxide and ammonia released from
molten masses covered the surface.
(b) UV rays from the sun broke up water molecule into hydrogen and
oxygen and lighter hydrogen escaped.
(c) Oxygen combined with ammonia and methane to form water, carbon
dioxide and others.
(d) Ozone layer formed, as it cooled, the water vapour fell as rain to fill
depression and form oceans.
(e) Life appeared 500 million (about 4 billion years back) years after the
formation of earth.
4. Theories of origin of life were given by different thinkers and
scientists.
(i) Theory of special creation states that God created life by his divine act
of creation.
(ii) Theory of panspermia/cosmozoic theory, given by early Greek thinkers
states that the spores or panspermia came from outer space and
developed into living forms.
(iii) Theory of spontaneous generation states that life originated from
decaying and rotting matter like straw, mud, etc.
(a) Louis Pasteur rejected the theory of spontaneous generation and
demonstrated that life came from pre-existing life.
(b) In his experiment, he kept killed yeast cells in pre-sterilised flask and
another flask open into air. The life did not evolved in the former but new
living organisms evolved in the second flask.
(iv) Theory of chemical evolution or Oparin-Haldane theory states that life
originated from pre-existing non-living organic molecules and that
formation of life was preceded by chemical evolution.
The conditions on the earth that favoured chemical evolution were very
high temperature, volcanic storms and reducing atmosphere that
contained CH4,NH3, water vapour, etc.
5. Miller’s experiment provided experimental evidence for chemical
evolution.
(i) The experiment was carried out by SL Miller and HC Urey in 1953.
(ii) He took a closed flask containing CH 4,H2,NH3 and water vapour at 800°C
and created electric discharge. These conditions were similar to those in
primitive atmosphere.
(iii) After a week, formation of amino acids were observed. Complex
molecules like sugars, nitrogen bases, pigments and fats were seen in the
flask by other scientist.
(iv) Analysis of the meteorite also revealed the presence of similar
compounds.
(v) Chemical evolution of life was more or less accepted.

7. Evidences of evolution come from

(i) Palaeontology (ii) Comparative anatomy and morphology
(iii) Biochemical/Physiology (iv) Biogeography
(v) Embryology
(i) Palaeontology is the study of fossils. The fossils are the remains of past
organisms
preserved in sedimentary rocks
(a) Rocks form sediments and a cross-section of earth’s crust indicates the
arrangement of sediments one over the other during the long history of
earth.
(b) Different aged rock sediments contain fossils of different life forms,
who died during the formation of the particular sediment,
(c) Some organisms appear similar to modern organisms. They represent
extinct organisms like dinosaurs.
(d) A study of fossils in different sedimentary layers indicates the
geological period in which they existed.
(e) The study showed that life forms varied over time and certain life
forms are restricted to certain
(a) The organs with same structural design and origin but different
functions are called homologous organs. Examples are forelimbs of some
animals like whales, bats and cheetah have similar anatomical structure,
such as humerus, radius, ulna, carpals, metacarpals and phalanges.
(b) Homology in organ indicates common ancestry.
(c) Other examples of homology are vertebrate hearts or brains. In plants
also, thorns and tendrils of Bougainvillea and Cucurbita represent
homology.
 (d) Homology is based on divergent evolution. The same structure
developed along different directions due to adaptations to different needs.
The condition is called divergent evolution.
II. (a) Organs which are anatomically different but functionally similar are
called analogous organs. For example, wings of butterfly and birds.
(b) Analogy refers to a situation exactly opposite to homology.
(c) Analogous organs are a result of convergent evolution. It is the
evolution in which different structures evolve for same function and hence,
have similarity.
(d) Other examples of analogy are eyes of Octopus and mammals; flippers
of penguins and dolphins. In plants, sweet potato (root modification) and
potato (stem modification).

III. Vestigial organs like homologous organs provide evidences for organic evolution.
These are degenerate, non-functional and rudimentary organs to the possessor, while
correspond to fully developed and functional organs of related organisms.
(a) There are about 90 vestigial organs in the human body. Same of them are tail bone
(coccyx), wisdom teeth, nictitating membrane, vermiform appendix, etc.
(b) Some examples from other animals are hip girdles and bones of the hind limbs in some
whales and certain snakes and wings of flightless birds.
Biochemical Evidences
(a) The metabolic processes in organisms are similar with same new materials and end
products. For example, energy released by oxidation is stored in ATP which then powers the
energy requiring process.
(b) Molecular homology is the similarity among animals at the molecular level.
For example, human DNA differs in only 1.8% of its base pairs from chimpanzee DNA and
there is no difference between the two in the amino acid sequence for the protein
cytochrome-c.
(iv) Biogeographical evidences The species restricted to a region develop unique features.
Also, species present in far separated regions show similarity of ancestry.
This can be explained with the help of following processes:
I. Adaptive radiation is an evolutionary process in which an ancestral stock gives rise to
new species adapted to new habitats and new ways of life. Examples are (0 Darwin’s finches
These were small black birds, which Darwin observed in Galapagos island.
(a) He observed many varieties of finches in the same island.
(b) All varieties of finches had evolved from original seed-eating finches.
(c) There was alternation in beaks enabling some to become insectivorous and some
vegetarian.

(ii) Marsupials of Australia A number of marsupials, different from each other evolved from
an ancestral stock, all within the Australian island continent.
II. Parallel evolution refers to independent development of similar characters in two animal
groups of common ancestry living in similar habitats of different continents. Examples are
Marsupial mammals in Australia show parallel evolution as they have evolved from placental
mammals. All these closely resemble and look similar to a corresponding marsupial.
Few examples are mentioned in the table.
III. Convergent evolution is development of similar adaptive functional structures in
unrelated groups of organisms. Examples are:
(i) Wings of insect, bird and bat.
(ii) Spiny anteater and scaly anteater belong to different orders of class-Mammalia. They
have acquired similar adaptations for food, e.g. leg ants, termites and insects.
(v) Embryological evidences Study of comparative embryology shows common patterns of
development.
(a) The principles of embryonic development were given by Von Baer.
(b) Ernst Haeckel propounded The theory of recapitulation or Biogenetic law which states
that an individual organism in its development (ontogeny) tends to repeat the stages passed
through by its ancestors (phylogeny), i.e. ontogeny recapitulates phylogeny.
(c) This means that the life history of an animal reflects its evolutionary history.
For example, during the life history, frog’s tadpole larva resembles fishes, the ancestors of
amphibia.
The presence of gill clefts in all vertebrate embryos including human provides a strong
evidence in support of organic evolution.
(vi) Anthropogenic evidences Excess use of herbicides, pesticides, etc has resulted in
selection of resistant varieties in a lesser time scale. This is also true for microbes against
which antibiotics or drugs have been used. All these evidences tell us that ‘Elvolution is a
stochastic process based on chance events in nature and chance mutation in the organisms’.

Lamarck theory of evolution: (theory of inheritance of acquired characters)

Lamarck who is a French Naturalist stated that the evolution of life forms
occurs due to the use and forbearance of organs. ● He demonstrated this
technique using giraffes as an example. ● He insisted that giraffes
developed elongated necks in an attempt to forage leaves on tall trees.
Therefore, this character was acquired based on a need to adapt and
survive ● This acquired character was passed to succeeding generations. ●
Giraffes slowly develop long necks over many years. MECHANISM OF
EVOLUTION: ● Hugo de Vries managed to plant evening primrose. Hugo de
Vries gave the various ideas of mutations. ● The mutation is the difference
appearing quickly in a group.

HARDY – WEINBERG PRINCIPLE:

● For a given population the frequency of occurrence of alleles of a particular gene present
on a specific locus can be calculated. • This frequency is usually fixed and remains the same
throughout different generations. • Hardy-Weinberg principle expressed the same using
algebraic equations. This is called the Hardy-Weinberg Principle. • Hardy-Weinberg principle
states that allele frequencies are constant in a population and they are constant from
generation to generation. • The gene pool remains constant. Which is called genetic
equilibrium. The principle can be represented mathematically as follows: • (p + q)2 = p2 +
2pq + q2 = 1.
• p and q represent the individual allele frequencies. • Therefore, p2 = frequency of
homozygous condition represented by p And q2 = frequency of homozygous alleles
represented by q And pq = frequency of the heterozygous condition • Change in the genetic
equilibrium (Hardy Weinberg equilibrium) can then be translated as collection or change in
inequalities which causes evolution.

• Five factors are known to affect Hardy-Weinberg equilibrium:

Gene migration or gene flow.
▪ Genetic recombination
▪ Natural selection.
▪ Genetic drift.
▪ Mutation.
● Gene migration: When a segment of the population transfers to another place gene
frequencies will vary in the original as well as in the new population. New genes /alleles will
be added to the new population and the same is lost from the old population.
● Gene flow: When gene migration happens usually it is called gene flow.
● Genetic drift: Alter in quality recurrence that happens due to arbitrary occasion or by
chance.
● Founder effect: Sometimes the alteration in allelic frequency is so radical that in the new
population species and the variants form a different species. The first migrated population
from which the variations emerged gets to be founder species and this impact is called the
founder effect

Operation of natural selection on different traits: Natural selection can lead to :

Stabilization: In which more individuals acquire mean character value.
Directional changes: Occurs when a large number of individuals acquire features other than
the average character value.
Disruption: more numerous individuals obtain peripheral character value at both extremes
of the distribution curve.
ORIGIN AND EVOLUTION OF MAN: Around 15 million years ago, primates such as
Dryopithecus existed. These animals seemed to be comparable to chimpanzees and gorillas
in their looks and walking.
● Ramapithecus was more comparable to man while Dryopithecus was more comparative to
gorillas. ● Some fossils of bones that look like human bones have been found in Ethiopia and
Tanzania. ● Two mya, some australopithecines used to live. Most likely experienced in East
African grasslands. ● They employed weapons made up for hunting.
They originally had a vegetarian diet.
● The first human-like organism was the hominid and was called Homo habilis. Brain
capacity of the hominid was 650 – 800 cc. Hominid also remained on a plant-based diet.
Fossils found in Java 1891 appeared to be of the subsequent stage that is Homo erectus.
Homo erectus developed about 1.5 mya. Homo erectus Had a large brain with a capacity of
900 ccs. They Probably were non-vegetarians & ate meat.
• Neanderthal man: The brain size of a neanderthal man was around 1400 cc. They resided
in eastern & central Asia usually between 1, 00,000-40,000 years back. o Neanderthal man
developed use of animal hides to protect their body. Neanderthal man buried their dead
members.
Homo sapiens: They originated in Africa and then relocated to various continents and they
developed different races. Through the ice age 75,000-10,000 years ago, modern Homo
sapiens emerged. Prehistoric cave art was produced around 18,000 years ago. o Agriculture
developed around 10,000 years back and human settlement started after