NEET Biology Class 12 Evolution Notes

Evolution Notes

Evolution is an orderly change from one form to another. Evolutionary Biology is the study of the evolutionary history of life forms.

Origin Of Life

Big Bang Theory states that universe originated about 20 billion years ago by a singular huge explosion.

• The earth was formed about 4.5 billion years ago. There was no atmosphere on early earth. Water vapour, CH₄, CO_2, and NH₃ released from molten mass covered the surface.
• The UV rays from the sun broke up water into H₂ and O₂. Oxygen combined with NH₃ and CH₄ to form water, CO₂ etc. The ozone layer was formed. As it cooled, the water vapour fell as rain to form oceans. Life appeared almost four billion years ago.

Read And Learn More: NEET Biology Class 12 Notes

Theories Of Origin Of Life

1. Theory of spontaneous generation (Abiogenesis): It
   states that, life came out of decaying and rotting matter like straw, mud etc.
   • Louis Pasteur disproved this theory. He demonstrated that life comes only from pre-existing life. He showed that life did not come from killed yeast in a closed pre-sterilized flask. But in an opened flask, life (microbes) appeared.
2. Biogenesis: Proposed by Francisco Redi, Spallanzani and Louis Pasteur. It states that, life originates from pre¬existing life. But it does not explain origin of first life.
3. Cosmic theory (Theory of Panspermia): It states that the units of life (spores) were transferred to different planets including Earth.
4. Theory of special creation: It states that living and nonliving was created by some supernatural power (God).
5. Theory of chemical evolution: Proposed by Oparin and Haldane. It states that, the first form of life was originated from non-living inorganic and organic molecules such as CH₄, NH₃, H₂O, sugars, proteins, nucleic acids etc. i.e. “Abiogenesis first, but biogenesis ever since”.

Urey-Miller experiment: Harold Urey and Stanley Miller experimentally proved theory of chemical evolution. They created a condition like that of primitive earth (i.e. high temperature, volcanic storms, reducing atmosphere with CH₄, NH₃, H₂O, H₁ etc.).

• They made electric discharge in a closed flask containing CH₄, NH₃, H₂ and water vapour at 800^o As a result, some amino acids are formed. In similar experiments, others observed the formation of sugars, nitrogen bases, pigment and fats.
• The first non-cellular forms of life originated 3 billion years ago. They were self-replicating metabolic capsule containing RNA, proteins, Polysaccharides etc.

Evidence For Evolution

1. Paleontological evidences

• Paleontology: It is the study of fossils.
• Fossils are remnants of life forms found in rocks (earth’s crust). Fossils are written documents of evolution.

Significance of fossils:

1. To study phytogeny (evolutionary history or race history). For example, Horse evolution.
2. To study the connecting link between two groups of organisms. Example, Archaeopteryx.
3. To study extinct animals. Example, Dinosaurs.
4. To study about the geological period by analysing fossils in different sedimentary rock layers. The study showed that life forms varied over time and certain life forms are restricted to certain geological time spans.

2. Morphological and Anatomical evidence: Comparative anatomy and morphology shows that different forms of animals have some common structural features. This can be explained as follows

1. Homologous organs: Homologous organs are organs that have fundamental similarities in structure and origin but different functions. This phenomenon is called Homology.
   • Homologous organs Example, the Human hand, Whale’s flippers, Bat’s wing and Cheetah’s foot. These forelimbs have different functions but similar anatomical structures such as bones (for example, humerus, radius, ulna, carpals, metacarpals and phalanges).
   • Homology is also seen in the heart, brain etc.
   • Homology in plants: For example, Thorns of Bougainvillea and tendrils of Cucurbita.
   • The origin of homologous organs is due to Divergent evolution. It is the evolution by which related species become less similar to survive and adapt in different environmental condition.
   • Homology indicates common ancestry.
2. Analogous organs: These are the organs having similar functions but different structures and origins. This phenomenon is called Analogy. Example,
   • Wings of insects (formed of a thin flap of chitin) and wings of birds (modified forelimbs).
   • Eyes of Octopus (retina from skin) and mammals (retina from embryonic brain).
   • Flipper of Penguins and Dolphins.
   • Sweet potato (modified root) and Potato (modified stem).
   • Trachea of insects (from ectoderm) and lungs of vertebrates (from endoderm).

Origin of analogous organs is due to Convergent evolution. It is the evolution by which unrelated species become more similar to survive and adapt in similar environmental conditions.

3. Adaptive radiation (Biogeographical evidence)
Adaptive radiation (evolution by adaptation) is the evolution of different species in a geographical area starting from a point.

Adaptive radiation Example:

• Darwin’s finches in Galapagos Islands.
• Australian marsupials (Marsupial radiation).
• Placental mammals in Australia.

When more than one adaptive radiation appears in an isolated geographical area, it results in convergent evolution. For example, Australian Marsupials and Placental mammals.

4. Biochemical evidence: Organisms show similarities in proteins, genes, other biomolecules and metabolism. It indicates common ancestry.

5. Embryological evidence

• Proposed by Ernst Haeckel.
• He observed that all vertebrate embryos have some common features that are absent in adult.
• Embryological evidence For Example, all vertebrate embryos (including humans) develop vestigial gill slits just behind the head. However, it is functional only in fish and not found in other adult vertebrates.
• However, Karl Ernst von Baer rejected this proposal. He noted that embryos never pass through the adult stages of other animals.

6. Evidence for evolution by natural selection: Natural selection is the process by which the organisms that are best suited for their environment survive and reproduce. Some evidence are given below.

Industrial melanism: In England, before industrialization (1850s), there were more white-winged moths (Biston betularia) on trees than dark-winged or melanised moths (Biston carbonaria). After industrialization (1920), more dark-winged moths and less white-winged moths were developed.

Embryological evidences Reason:

• Before industrialization: There was white lichens covered the trees. In that background, white-winged moths survived but dark-winged moths were picked out by predators.
• After industrialization: The tree trunks became dark due to industrial smoke and soot. No growth of lichens. Under this condition, the white-winged moth did not survive because the predators identified them easily. Dark-winged moth survived because of suitable dark background.

Natural selection by anthropogenic action: It is the development of resistant varieties in organisms due to human action. Example, Excessive use of herbicides, pesticides, antibiotics or drugs etc.

Theories Of Biological Evolution

Lamarckism (Theory of Inheritance of Acquired characters): It is proposed by Lamarck. It states that evolution of life forms occurred by the inheritance of acquired characters. Acquired characters develop by the use and disuse of organs.

• Evolution by use of organs: Long neck of giraffe is due to continuous elongation to forage leaves on trees. This acquired character was inherited to succeeding generations.
• Evolution by disuse: Disappearance of limbs in snakes.

This theory was eliminated out because it is proved that the characters are inherited only through genes.

Darwinism (Theory of Natural selection)

• Proposed by Charles Darwin.
• It was based on observations during a sea voyage in a sail ship called H.M.S. Beagle.
• Alfred Wallace (a naturalist worked in Malay Archepelago) had also come to similar conclusions.
• Work of Thomas Malthus on populations influenced Darwin.

Darwinism is based on 2 key concepts:

• Branching descent: It explains that all organisms are modified descendants of previous life forms.
• Natural selection: Consider a bacterial colony A growing on a given medium. If the medium composition is changed, only a part of the population can survive under new condition. This variant population (B) outgrows the others and appears as a new species, i.e. B is better than A under new conditions. Thus, nature selects for fitness.

Natural selection is based on the following facts:

• Heritable minor variations: It is either beneficial or harmful to the organisms.
• Overproduction: Population size grows exponentially due to maximum reproduction (For example, bacterial population).
• Limited natural resources: Resources are not increased in accordance with the population size.
• Struggle for existence: It is the competition among organisms for resources so that population size is limited.
• Survival of the fittest: In struggle for existence, organisms with beneficial variations can utilize resources better. Hence, they survive and reproduce. This is called
• Survival of the fittest. It leads to a change in population characteristics and new forms appear.
• Darwin ignored about origin of variation and mechanism of evolution or speciation.

Mechanism Of Evolution

• Hugo de Vries proposed the Mutation Theory of evolution.
• He conducted experiments on Oenothera lamarckiana (evening primrose) and believed that evolution takes place through mutation and not by minor variation.
• Darwinian variation is minor, slow and directional. Due to this, gradual evolution occurs.
• Mutation is sudden, random and directionless variation. Here, speciation is by saltation (single step, large mutation).
• Mutation is the origin of variation for evolution.

Hardy Weinberg Principle

It states that allele frequencies in a population are stable and constant from generation to generation in the absence of other evolutionary influences.

The gene pool (total genes and their alleles in a population) remains constant. This is called genetic equilibrium (Hardy-Weinberg equilibrium).

Sum total of all the allelic frequencies = 1

In a diploid, p and q are the frequencies of alleles A and a respectively.

The frequency of AA = p²

The frequency of aa = q²

The frequency of Aa = 2pq

Hence p² + 2pq + q² = 1 [binomial expansion of (p+q)²]

Change of frequency of alleles in a population causes disturbance in genetic equilibrium. This is due to evolution.

Factors affecting Hardy-Weinberg equilibrium

1. Gene migration: Gene flow from one population to another. Here gene frequencies change in both populations. Gene flow occurs if migration happens multiple times.
2. Genetic drift: The accidental gene flow causing change in frequency. Sometimes, the change in frequency is so different in the new sample of population that they become a different species. The original drifted population becomes founders and the effect is called the founder effect.
3. Mutation: Mutations result in formation of new phenotypes. Over few generations, this leads to speciation.
4. Genetic recombination: Reshuffling of gene combinations during crossing over resulting in genetic variation.
5. Natural selection: It is 3 types.

1. Stabilizing selection: Here, more individuals acquire mean character value and variation is reduced. For example, consider the body size of organisms.

2. Directional selection: Here, individuals of one extreme are more favoured.

3. Directional selection: Here, individuals of both extreme are more favoured.

A Brief Account Of Evolution

The geological time scale includes 4 eras: Proterozoic, Palaeozoic, Mesozoic and Cenozoic.

1. Proterozoic era: 2500 – 541 million years ago (mya)

• The first cellular forms of life appeared (2000 mya).
• Some of the cells had the ability to release O₂ as the light reaction in photosynthesis.
• Single-celled organisms became multicellular organisms.

2. Palaeozoic era (540 – 252 mya)

• It has 6 periods: Cambrian (540 – 490 mya), Ordovician (490 – 443 mya), Silurian (425 mya), Devonian (405 mya), Carboniferous (360 mya) and Permian (285 mya).
• 500 mya: Invertebrates were formed.
• 450 mya: First land organisms (plants) appeared.
• 400 mya: Arthropods invaded the land.
• 350 mya: Jawless fishes were evolved. Fishes with stout and strong fins could move on land and go back to water.
• In 1938, a Coelacanth fish (lobefins) was caught in South
• Africa which was thought to be extinct. This fish was evolved into the first amphibians (ancestors of modern-day frogs and salamanders).
• 320 mya: Seaweeds and few plants were existed.
• Amphibians evolved to reptiles. They lay thick-shelled eggs (do not dry up in sun).
• Giant ferns (Pteridophytes) were present but they all fell to form coal deposits slowly.

3. Mesozoic era (252 – 66 mya) – Age of reptiles

• It has 3 periods: Triassic (230 mya), Jurassic (208 mya) and Cretaceous (144 mya).
• 200 mya: Some of the land reptiles went back into water to evolve into fish-like reptiles (E.g. Ichthyosaurs).
• The land reptiles were dinosaurs (Tyrannosaurus rex, Triceratops, Pteranodon, Stegosaurus, Brachiosaurus etc.) T. rex was the largest dinosaur (20 feet in height, huge fearsome dagger-like teeth).
• First toothed birds were emerged.

4. Cenozoic era (66 – 0 mya)

• Age of Mammals and Angiosperms.
• It has 2 periods: Tertiary (66 mya) and Quaternary (2 mya – Age of man).
• 65 mya: Dinosaurs suddenly disappeared. Some say climatic changes killed them. Some say most of them evolved into birds.
• First mammals were shrew-like. Their fossils are small-sized.
• In South America, there were mammals resembling horses, hippopotamuses, bears, rabbits etc. Due to continental drift, when South America joined North America, these animals were overridden by North American fauna.
• Due to continental drift, Australian marsupials survived because of a lack of competition from any other mammals.

Origin And Evolution Of Man

• 15 mya: Dryopithecus and Ramapithecus. Hairy. Walked like gorillas and chimpanzee.
• Dryopithecus: ape-like.
• Ramapithecus: man-like.
• 3-4 mya: Man-like primates. Height up to 4 feet. Fossils of man-like bones found in Ethiopia and Tanzania.
• 2 mya: Australopithecus. Lived in East African grasslands. Hunted with stone weapons. Ate fruits.
  Homo habilis: First human-like being (hominid).
  Brain capacity: 650-800 cc. Did not eat meat.
• 1.5 mya: Homo erectus (Java man). Large brain (900 cc). Ate meat.
• 1 lakh – 40,000 yrs ago: Homo neanderthalensis (Neanderthal man).
• Brain capacity: 1400 cc. Lived in East and Central Asia. Used hides to protect their body. Buried their dead.
• 75,000 – 10,000 yrs ago (ice age): Homo sapiens (Modern man). Prehistoric cave art developed about 18,000 years ago. Agriculture and settlements: 10,000 years ago.