CLASS 12

BIOLOGY REVISION NOTES

EVOLUTION
What Is Evolution?

Evolution is the study of the different forms of life, its characteristics and
the changes over several generations. It is the scientific theory used by
biologists to study the genetic variation in a population.

There are many theories about how life originated on the planet Earth.
The theory of evolution is based on the idea that all species are correlated
and progressively vary with time.

Darwin’s Theory of Evolution

Charles Robert Darwin, an English naturalist, geologist, and biologist, is

well known for his theory of evolution and the process of natural
selection. He gave the theory of evolution, which is known as ‘Darwin’s
theory of natural selection’, and also published a book on “Origin of
Species by Natural Selection”.

The main features of Darwin’s Theory are as follows,

All organisms reproduce and multiply enormously.

No two individuals are alike. They are different from each other either in
size, shape, behaviour, etc.

Some traits are consistently passed on from their parent to the offspring.

The rate of reproduction varies in all living species. Some reproduce more
and some minimum.

A Brief Account of Evolution

According to several theories of evolution:

The first cellular forms of life appeared on Earth about 2000 million years
ago.

Later single-celled organisms, multicellular forms, and invertebrates were

formed and became active.

Jawless fish evolved, and different organisms started to invade from water
to land. Later, amphibians and reptiles, some of them viviparous
mammals, came into existence.

About 23.03 to 5.333 million years ago, evolved primates that resembled
today’s gorillas and chimpanzees. During the ice age between 75,000-
10,000 years ago, modern Homo sapiens arose.

Evidence for Evolution

It indicates the paleontological evidence, which denotes that life forms

have arisen at various instances in the Earth’s history, which covers
fossils, rocks from sediments, etc.

Ernst Heckel proposed embryological support for evidence which was

based upon the observation of some traits during the embryonic stages
that persisted in all vertebrates that are not found in adults.

The proposal was disapproved by Karl Ernst von Baer as he noted that
embryos do not go through the adult phases of other animals.

Divergent evolution is used to refer to the animals having the same

structure developed along different directions as a result of adaptations
to different needs. The structures are referred to as homologous, where
homology represents common ancestry. Example – The tendrils and thorns
of Cucurbita and Bougainvillea indicate homology.

Convergent evolution refers to different structures evolving for the same

function, thereby having similarities. This is why analogous structures are
a result of convergent evolution. Examples of analogy – Flippers of
Dolphins and Penguins.

It was indicated that entities in a mixed population could adapt better,

survive and cause an increase in the size of the population. But none of
the variants got wiped out completely.

Examples of evolution by anthropogenic action are the appearance of

resistant cells/entities in a time span of months or years as opposed to
centuries.

Through this evidence, it was also indicated that evolution is not a

directed process in the context of determinism; rather is a stochastic
process basis the chance events in nature and chance mutations in
entities.

Adaptive Radiation

It is referred to the process of evolution of various species in particular

geographical regions right from the point and radiating literally to other
geographical areas. Examples are Australian marsupials and Darwin’s
finches.

Entities are better adapted to survive in an otherwise hostile environment

as the adaptive ability is inherited, it being a genetic basis, whereas
fitness is the outcome of the ability to adapt and hence getting selected
by nature.

Two key concepts of the Darwinian theory of evolution are branching

descent and natural selection.

Hardy-Weinberg Principle

This principle states that allele frequencies in a population are stable and
are constant from generation to generation; the gene pool remains
constant. This is referred to as genetic equilibrium, where the sum total of
all allelic frequencies is 1.

In a diploid, let p and q represent the frequency of alleles A and a. The

probability that an allele A with a frequency of p appears on both the
chromosomes of a diploid individual is simply the product of the
probabilities.

Five factors affecting the Hardy-Weinberg principle are – gene flow,

genetic drift, genetic recombination, mutation and natural selection

When the original drifted population becomes founders, the effect is

known as the founder effect. Operation of natural selection on different
traits:

Natural selection can lead to :

o Stabilization: In which more individuals acquire mean character value.

o Directional changes: Occurs when a large number of individuals acquire

features

other than the average character value.

o Disruption: more numerous individuals obtain peripheral character value

at both
extremes of the distribution curve.

A BRIEF ACCOUNT OF EVOLUTION:

Approximately 2000 million years ago (mya) the first forms of life
appeared on

earth. They were cellular

● Certain cellular shapes created the capacity to photosynthesize and in

this way

discharge O2. The climate gradually got to be wealthy in oxygen. This in

turn

advanced the improvement and advancement of more high-impact shapes

of

living beings.

● Slowly and gradually the single-cell organisms started to form multi-

cellular life
forms.

● Around 500 mya invertebrates were formed.

● The first fish evolved from invertebrates around 350 mya. They were
probably

jawless fish. ● At around 320 mya seaweeds and few plants evolved and
existed.

● The coelacanth is a lobe-finned fish that was discovered in South Africa.

The

coelacanth is considered to have developed into the starting amphibious

animals

that lived on both land and water. The amphibians were the ancestors of
modernday frogs and salamanders.

● Amphibians steadily evolved into reptiles.

● Eggs of reptiles do not dry up in sun unlike eggs of amphibians.

● Giant ferns such as Pteridophytes present on land fell and got buried in
the soil.

● Some of the reptiles retracted back into aquatic conditions to evolve

into reptiles

like fish (probably 200 million years ago)

▪ The most prominent land reptiles, dinosaurs.

o The largest dinosaur was Tyrannosaurus rex, he was about 20 feet in

height

and had enormous dangerous teeth like daggers.

o Around 65 mya, the dinosaurs quickly encountered mass extinction from

the earth. Some of these dinosaurs evolved into birds.

o The earliest mammals were little and shrew-like. They had tiny sized

fossils.

o Mammals developed to be viviparous. This preserved their future

blooming

embryos inside their mother’s body.

o Pouched mammals of Australia persisted because of a shortage of

competition from any other mammals. That lack of competition results

from Continental Drift

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.

Class XII Biology www.vedantu.com 20

● They originally had a vegetarian diet.

● The first human-like organism was the hominid and was called Homo
habilis.

o Brain capacity of the hominid was 650 – 800 cc.

o 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. o Homo erectus Had a
large brain
with a capacity of 900 ccs. o They Probably were non-vegetarians & ate
meat.

• Neanderthal man:

o The brain size of a neanderthal man was around 1400 cc. They resided
in

easernt& central Asia usually between 1, 00,000-40,000 years back.

o Neanderthal man developed use of animal hides to protect their body.

o Neanderthal man buried their dead members.

• Homo sapiens:

o They originated in Africa and then relocated to various continents and

they

developed different races.

o Through the ice age 75,000-10,000 years ago, modern Homo sapiens
emerged.

o Prehistoric cave art was produced around 18,000 years ago.

o Agriculture developed around 10,000 years back and human settlement

started