EVOLUTION

CLASS XII
Chapter 07
• Evolution is the change in the characteristics of a species over several generations over time in response
to their changing environment.
• ORIGIN OF UNIVERSE
• 20 billion years old
• Huge clusters of galaxies comprise the universe.
• Galaxies contain stars and clouds of gas and dust
• Big Bang theory - explain to us the origin of universe.
• Due to singular huge explosion, The universe expanded and hence, the temperature came down. Hydrogen
and Helium formed sometime later. The gases condensed under gravitation and formed the galaxies of the
present day universe.
• ORIGIN OF EARTH
• 4.5 billion years back
• There was no atmosphere on early earth
• Water vapour, methane, carbon dioxide and ammonia covered the surface.
• UV rays from the sun broke up water into Hydrogen and Oxygen
• H2 escaped.
• Oxygen combined with ammonia and methane to form water, CO2 and other gases.
• ozone layer was formed
• As it cooled, the water vapor fell as rain, to fill all the depressions and form oceans.
ORIGIN OF LIFE (THEORIES)
• four billion years back.
• THEORY OF SPECIAL CREATION: god created life by his divine act of creation
• Panspermia/ cosmozoic theory: life comes from other planets. Early Greek thinker thoughts that unit of life is
called spores transferred from other planets.
• Theory of spontaneous generation: life comes out of dead and decaying matters like straw and mud. Louis
Pasture experimentally proved that life arises only from pre-existing life. Spontaneous theory of origin of life is
dismissed after that.
• Oparin and Haldane Theory/ Theory of chemical evolution:
• life could have come from pre-existing non-living organic molecules like RNA and protein etc.
• formation of diverse organic molecules from inorganic constituents.
• At that time condition on earth were- high temperature, volcanic eruption, reducing atmosphere
containing CH4and NH3.
 Urey-Miller’s Experiment:
• EXPERIMENTAL SETUP:
• created similar earth’s primitive conditions in a laboratory scale
• closed flask containing CH4 , H2 , NH3 and water vapour at 800°C.
• created electric discharge
• OBSERVATIONS:
• He observed formation of amino acids. In similar experiments others
observed, formation of sugars, nitrogen bases, pigment and fats.
• CONCLUSIONS
• Experimental evidence of chemical evolution
• Showed that first non-cellular forms of life could have originated 3
billion years back.
• Showed that non cellular biomolecules exist in the form of RNA,
Protein and Polysaccharide
• The first cellular form of life did not possibly originate till about
2000 million years ago. These were probably single-cells.
EVIDENCES FOR EVOLUTION
• PALEONTOLOGICAL EVIDENCE
• Fossils are remains of hard parts of life-forms found in rocks.
• Different-aged rock sediments contain fossils of different life-forms
who probably died during the formation of the particular sediment.
• A study of fossils in different sedimentary layers indicates the
geological period in which they existed.
• The study showed that life-forms varied over time and certain life
forms are restricted to certain geological time spans.
• EMBRYOLOGICAL SUPPORT FOR EVOLUTION
• Ernst Heckel
• certain features during embryonic stage common to all vertebrates
that are absent in adult.
• embryos of all vertebrates including human develop a row of
vestigial gill slit just behind the head but it is a functional organ only
in fish and not found in any other adult vertebrates.
• this proposal was disapproved on careful study performed by Karl
Ernst von Baer
• COMPARATIVE ANATOMY AND MORPHOLOGY
• Homologous organs-
• those organs that perform different function but have similar origin and structure are called
homologous organs.
• For example human, cheetah, bat and whales share similarities in pattern of bones of forelimbs
although these forelimbs perform different functions in these animals.
• thorn and tendrils of Bougainvillea and Cucurbita represent homology.
• similar structure developed along different directions due to adaptation of different needs.
• This is called divergent evolution.

• Analogous structures-
• they are not anatomically similar organs but perform similar function.
• For example eyes of mammals and octopus or flippers of penguin and dolphins.
• Sweet potato (root modification) and potato (stem modification) is another example for analogy.
• This is due to similar habitat that resulted in similar adaptive features in different groups of organisms.
• This type of evolution is called convergent evolution.
• ANTHROPOGENIC ACTION
• 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.

• BIOCHEMICAL SIMILARITIES
• similarities in proteins and genes performing a given function among diverse organisms give clues to
common ancestry.
ADAPTIVE RADIATION / Branching descent
• Darwin went to Galapagos Islands and observed an amazing
diversity of creatures.
• Adaptive radiation is the evolutionary process by which many
species originate from one species in an area and radiate to
different species on other geographical areas.

• Example:
• Darwin Finches:
• observed many varieties of finches in the same island
• Evolved from original seed-eating finches
• with altered beaks enabling them to become insectivorous
and vegetarian finches
• Marsupials of Australia
• Placental mammals in Australia (convergent evolution)
 BIOLOGICAL EVOLUTION
• LAMARCK THEORY:
• Lamarck had said that evolution of life form had occurred
but driven by use and disuse of organs.
• Example: giraffe evolve their neck by foraging leaves on
tall trees and had to adapt by elongation of their necks.
• This theory is no more accepted
• DARWIN THEORY
• Ship- H.M.S Baegle
• the nature select for fittest and fitness is based on characteristics which are inherited.
• Some organisms are better adopted to survive in otherwise hostile environment.
• Fitness is the end result of the ability to adopt and get selected by nature.
• he asserted that variations, which are heritable and which make resource utilisation better for few (adapted to
habitat better) will enable only those to reproduce and leave more progeny. Hence for a period of time, over
many generations, survivors will leave more progeny and there would be a change in population characteristic and
hence new forms appear to arise
• The rate of appearance of new forms is linked to the life cycle or the life span.
• Branching descent and natural selection are the two key concepts of Darwinian Theory of Evolution
• Natural selection is based on certain observations
• natural resources are limited
• competition for resources
• populations are stable in size except for seasonal fluctuation
• members of a population vary in characteristics
INDUSTRIAL MELANISM
• In England, before industrialization, white winged moth were more
in number but after industrialization dark winged moth became
more in number
• The peppered moth Biston betularia F.typica is a light-coloured
species with dark patches, that help them to camouflage against the
lichens on the barks of the trees.
• F.carbonaria which is a sub-species of F.typica had undergone some
mutation and changed to a dark-coloured moth having light-
coloured patches.
• In the polluted areas, the light-coloured species could not protect
themselves from predators. This was because the lichen population
had reduced due to pollution and the light-coloured species were
easily visible on the tree barks. This replaced the F.typica species
with the F.carbonaria species.
Stabilization: large
number of individual
acquire mean character.
Directional change:
large number of
individual acquire value
other than mean
character.
Disruption: large
number of individual
acquire peripheral
character value at both
ends of the curve.
MUTATION THEORY / MECHANISM OF
EVOLUTION
• Hugo deVries based on his work on evening primrose brought forth the idea of mutations
• large difference arising suddenly in a population.
• mutation caused speciation and hence called it saltation (single step large mutation)
According to Hugo deVries According to Darwin
Major variation Minor variation
Mutations are random Mutations are non random
Mutations are directionless Mutations are directional
HARDY- WEINBERG PRINCIPLE
This principle says that :
• allele frequencies in a population are stable and is constant from generation to generation.
• The gene pool (total genes and their alleles in a population) remains a constant. This is called
genetic equilibrium.
• Sum total of all the allelic frequencies is 1 (p+q=1)
• p 2+2pq+q2=1 where p and q are frequency of allele A and a
• This is a binomial expansion of (p+q)2=1
• FACTORS AFFECTING THE PRINCIPLE
• Gene migration or gene flow: migration of individual. When gene migration occurs many times it
is called gene flow.
• Genetic drift: changes occurring in the frequency by chance.
• Founder effect: Sometimes the change in allele frequency is so different in the new sample of
population that they become a different species. The original drifted population becomes
founders
• Mutation: Advantageous mutations lead to new phenotypes and over few generations results in
speciation
• Genetic recombination during gametogenesis results in new phenotypes due to variation.
• Natural selection: heritable variation that enables survival of the fittest
A BRIEF ACCOUNT OF EVOLUTION
• 2000 million years ago (mya) the first cellular forms of life appeared
• Some of these cells had the ability to release O2
• EVOLUTION OF PLANT
• first organisms that invaded land were plants
• Sea weeds and few plants existed probably around 320 mya
• EVOLUTION OF ANIMALS
• 500 mya, invertebrates were formed
• Jawless fish evolved around 350 mya
• Fish with stout and strong fins could move on land and go back to water.
• a fish caught in South Africa happened to be a Coelacanth which was thought to be extinct. These animals
called lobefins evolved into the first amphibians. These were ancestors of modern day frogs and
salamanders
• amphibians evolved into reptiles around 200 mya
• Some of these land reptiles went back into water to evolve into fish like reptiles a (e.g. Ichthyosaurs). The
biggest of them, i.e., Tyrannosaurus rex
• About 65 mya, the dinosaurs suddenly disappeared from the earth.
 ORIGIN AND EVOLUTION OF MAN
Human Ancestors Origin Features
Dryopithecus 20-25 mya ape-like, hairy, arms and legs were of same length, large brain, ate soft
fruits and leaves
Ramapithecus 15 mya More man like, walked more erect, teeth like modern man
Australopithecus 3-4 mya Few fossils discovered in Ethiopia and Tanzania, man like primates, 4 feet
tall, walked upright, ate fruit, hunted with stone weapons, brain capacity
400-600cc
Homo habilis 2 mya fossils discovered in East African grasslands, first human-like being, brain
capacity 650-800cc, did not eat meat
Homo erectus 1.5 mya Fossils discovered in Java, large brain around 900cc, probably ate meat

Homo sapiens 1,00,000-40,000 Neanderthal man, Fossils discovered in east and central Asia, brain capacity
years back 1400cc, used hides to protect their body and buried their dead

modern Homo sapiens 75,000-10,000 Pre-historic cave art, Agriculture, human civilisation started
years ago