EvolutionClass 12 Biology Notes

Evolution

Evolutionary Biology is the scientific study of the history of life forms on our planet. To understand evolution, which is the process of change in living organisms over millions of years, we must first look at the origin of the universe, stars, and Earth itself. This is the grand story of how life began and diversified on Earth.

Origin of Life

The Universe and Earth

The story of life begins with the story of the universe.

• The Big Bang Theory: This is the most accepted explanation for the origin of the universe. It proposes that about 13.8 billion years ago, the universe began with a singular, massive explosion.
  • The universe expanded and cooled down.
  • Hydrogen and Helium were the first atoms to form.
  • Over time, gravity pulled these gases together to form the galaxies we see today.
• Formation of Earth: Our planet, Earth, formed about 4.5 billion years ago in the Milky Way galaxy.
  • Early Earth had no atmosphere.
  • The surface was a molten mass that released gases like water vapour ($H_2O$), methane ($CH_4$), carbon dioxide ($CO_2$), and ammonia ($NH_3$).
  • UV rays from the sun split water into Hydrogen and Oxygen. The lighter Hydrogen ($H_2$) escaped into space.
  • Oxygen combined with ammonia and methane to form more water, $CO_2$, and other gases.
  • This led to the formation of the ozone layer.
  • As the Earth cooled, the water vapour condensed and fell as rain, filling depressions to form oceans.

Theories on the Origin of Life

Life itself appeared about 500 million years after Earth's formation, meaning around 4 billion years ago. But how did it start?

• Panspermia: This idea, supported by some early Greek thinkers and modern astronomers, suggests that life came to Earth from outer space in the form of "spores."
• Theory of Spontaneous Generation: For a long time, people believed that life could arise from non-living, decaying matter like straw or mud.
  • This theory was disproven by Louis Pasteur. Through careful experiments with pre-sterilised flasks, he demonstrated that life comes only from pre-existing life. Organisms did not grow in a sealed, sterilised flask, but they did appear in a flask left open to the air.
• Chemical Evolution: Proposed by Oparin of Russia and Haldane of England, this is the most accepted theory today. It states that the first life forms originated from pre-existing, non-living organic molecules like RNA and proteins.
  • This process, called abiogenesis, suggests that life was preceded by the formation of diverse organic molecules from inorganic constituents on early Earth.
  • The conditions on early Earth were: high temperatures, volcanic storms, and a reducing atmosphere (containing gases like $CH_4$ and $NH_3$ but no free oxygen).

To test the theory of chemical evolution, American scientist S.L. Miller created a laboratory setup simulating the conditions of early Earth.

• He filled a closed flask with methane ($CH_4$), hydrogen ($H_2$), ammonia ($NH_3$), and water vapour ($H_2O$) at $800^{\circ}\text{C}$.
• He created an electric discharge to simulate lightning.
• After a week, he observed the formation of amino acids, which are the building blocks of proteins.
• Similar experiments by others produced sugars, nitrogen bases, pigments, and fats. Analysis of meteorites also showed similar compounds, suggesting these processes can occur elsewhere in space. This experiment provided strong evidence for chemical evolution.

From Molecules to Cells

• The first non-cellular forms of life may have appeared 3 billion years ago. These were likely giant self-replicating molecules like RNA and proteins, enclosed in a capsule.
• The first cellular forms of life probably did not originate until about 2 billion years ago. These were likely single-celled and lived in water.

Evolution of Life Forms - A Theory

While chemical evolution explains the origin of the first life, the theory of organic evolution explains how this simple life diversified into the complex forms we see today.

• Theory of Special Creation: This conventional religious belief states that all life was created as it is, diversity has always been the same, and the Earth is only about 4000 years old. These ideas were challenged in the 19th century.
• Darwin's Theory of Evolution: Based on his observations during a voyage on the H.M.S. Beagle, Charles Darwin proposed a scientific theory of evolution. His key ideas are:
  • Existing life forms share similarities with each other and with life forms that existed millions of years ago.
  • Life forms have gradually evolved over time, with new species arising and others going extinct.
  • The mechanism for this change is natural selection.

Natural Selection

Darwin's theory of natural selection is based on two key concepts: branching descent (all life is related through common ancestors) and natural selection (the mechanism driving evolution).

1. Variation: Within any population, individuals have built-in variations in their characteristics.
2. Competition: Natural resources are limited, leading to a "struggle for existence."
3. Survival of the Fittest: Individuals with characteristics that make them better suited (or "fitter") to their environment are more likely to survive and reproduce.
4. Reproductive Fitness: According to Darwin, fitness is ultimately about the ability to reproduce and leave more offspring.
5. Inheritance: These advantageous traits are inherited by the next generation. Over many generations, the population's characteristics change, and new forms appear.

What are the Evidences for Evolution?

Multiple lines of evidence from different fields of science support the theory of evolution.

Paleontological Evidence (Fossils)

• Fossils are the preserved remains or impressions of hard parts of life-forms found in rocks.
• Rocks are formed in layers (sediments) over time. A study of fossils in different sedimentary layers reveals the geological period in which they existed.
• This shows that life forms have changed over time, and certain species are restricted to specific geological timespans (e.g., dinosaurs).

Comparative Anatomy and Morphology

This involves comparing the structures of different organisms.

• Homologous Structures: These are structures that have a similar anatomical plan and origin but are adapted for different functions.

  • Cause: Divergent evolution, where the same basic structure evolves in different directions in different lineages.
  • Significance: Indicates a common ancestry.
  • Examples:
    • The forelimbs of mammals like whales, bats, cheetahs, and humans all share the same bone pattern (humerus, radius, ulna, etc.) but are used for swimming, flying, running, and grasping, respectively.
    • The thorn of Bougainvillea and the tendril of Cucurbita are both modified stems but serve different functions (protection vs. support).

• Analogous Structures: These are structures that have different anatomical designs but perform a similar function.

  • Cause: Convergent evolution, where different structures evolve for the same function due to similar environmental pressures.
  • Significance: Does not indicate close common ancestry.
  • Examples:
    • The wings of a butterfly and a bird are both used for flight but have very different structures.
    • The eye of an octopus and the eye of a mammal.
    • The flippers of penguins and dolphins.
    • Sweet potato (a root modification) and potato (a stem modification) are both used for food storage.

Biochemical Evidence

Similarities in proteins and genes that perform a specific function across diverse organisms provide strong evidence for common ancestry.

Artificial Selection

Humans have been selectively breeding plants and animals for agriculture, sport, or companionship for centuries. This has created diverse breeds (e.g., different dog breeds) that are still part of the same species. If humans can create such diversity in a few hundred years, it is plausible that nature could do so over millions of years.

Natural Selection in Action: Industrial Melanism

A classic example of evolution observed in England:

• Before industrialisation (1850s): Tree trunks were covered in light-coloured lichens. White-winged moths were well-camouflaged and numerous, while dark-winged (melanised) moths were easily spotted by predators and were rare.
• After industrialisation (1920s): Industrial pollution killed the lichens and covered tree trunks with dark soot. Now, the dark-winged moths were camouflaged and became numerous, while the white-winged moths were easily spotted and became rare.
• Conclusion: This shows that the environment exerted a selection pressure (predation), and the moths that were better adapted (camouflaged) survived and reproduced more.

What is Adaptive Radiation?

Adaptive radiation is the evolutionary process where a single ancestral species evolves into a variety of different species, each adapted to a different habitat or way of life, within a given geographical area.

• He realised that many varieties of finches existed on the islands.
• He proposed that they all evolved from an original seed-eating ancestor.
• Over time, different groups developed altered beaks that enabled them to specialise in different food sources, becoming insectivorous or vegetarian. This is a classic example of adaptive radiation.

Australian Marsupials Another prime example is the marsupials of Australia. A single ancestral stock evolved into a wide variety of marsupials, including the Tasmanian wolf, tiger cat, koala, and kangaroo, each adapted to a different niche on the island continent.

When more than one adaptive radiation occurs in an isolated geographical area, it can lead to convergent evolution. For example, placental mammals in Australia underwent their own adaptive radiation, producing species that look remarkably similar to corresponding marsupials (e.g., Placental Wolf and Tasmanian Wolf-marsupial).

Biological Evolution

The core of Darwinian theory is that evolution occurs through natural selection acting on heritable variations.

• Lamarck's Theory: Before Darwin, Jean-Baptiste Lamarck proposed that evolution was driven by the use and disuse of organs and the inheritance of acquired characteristics. His famous example was that giraffes got their long necks by stretching to reach tall trees, and this acquired trait was passed on. This theory is no longer accepted.
• Malthus's Influence: Darwin was influenced by Thomas Malthus, who noted that populations could grow exponentially, but resources are limited. This leads to competition, a key element of natural selection.

Mechanism of Evolution

Darwin understood that variation was key, but he didn't know its source.

• Darwin's View: Evolution is gradual, driven by small, directional, heritable variations.
• Hugo de Vries's View: Based on his work with evening primrose, de Vries proposed the idea of mutations—large, sudden differences that arise in a population. He believed that these large, random, and directionless mutations, which he called saltation, were the true cause of speciation, not Darwin's gradual variations.

Modern genetics shows that both mutation and variation are crucial for evolution.

Hardy-Weinberg Principle

This principle provides a mathematical baseline to measure evolution. It states that allele frequencies in a population will remain constant from generation to generation if certain conditions are met. This state of non-evolution is called genetic equilibrium.

The principle is expressed by the equation: $p^2 + 2pq + q^2 = 1$ Where:

• $p$ is the frequency of the dominant allele (e.g., A)
• $q$ is the frequency of the recessive allele (e.g., a)
• $p^2$ is the frequency of homozygous dominant individuals (AA)
• $q^2$ is the frequency of homozygous recessive individuals (aa)
• $2pq$ is the frequency of heterozygous individuals (Aa)

If the measured frequencies in a population deviate from the values predicted by this equation, it means the population is evolving.

Five factors can disrupt Hardy-Weinberg equilibrium and cause evolution:

1. Gene Flow (or Gene Migration): The movement of genes from one population to another.
2. Genetic Drift: Random, chance changes in allele frequencies, which have a greater effect in small populations. The founder effect is a special case where a new colony is started by a few members of the original population.
3. Mutation: The ultimate source of new alleles.
4. Genetic Recombination: Shuffling of existing alleles during meiosis.
5. Natural Selection: Differential survival and reproduction based on heritable traits.

Natural selection can act on a population in three ways:

• Stabilising Selection: Favors the average phenotype and selects against extreme variations.
• Directional Selection: Favors one extreme phenotype over others, causing the population's average to shift in one direction.
• Disruptive Selection: Favors individuals at both extremes of the phenotype range and selects against the average.

A Brief Account of Evolution

Here is a simplified timeline of major evolutionary events:

• ~2000 mya (million years ago): First cellular life forms appear. Some evolve the ability to release oxygen through a process similar to photosynthesis.
• ~500 mya: Invertebrates are formed and active.
• ~350 mya: Jawless fish evolve.
• ~320 mya: Seaweeds and a few plants exist.
• ~350 mya: Lobe-finned fish (like the Coelacanth, once thought extinct) develop stout fins, allowing them to move on land and evolve into the first amphibians.
• Amphibians evolve into reptiles, which lay thick-shelled eggs that don't dry out on land.
• The Age of Reptiles: For the next 200 million years, reptiles, including the dinosaurs, dominate the Earth. Giant ferns (pteridophytes) from this era formed today's coal deposits.
• ~65 mya: The dinosaurs suddenly disappear. This may have been due to climate change or a catastrophic event. Some may have evolved into birds.
• Rise of Mammals: The first mammals were small, shrew-like, and viviparous (giving birth to live young). When the reptiles declined, mammals took over.
• Continental drift played a role in evolution. The isolation of Australia allowed its unique pouched mammals (marsupials) to survive without competition from placental mammals.

Origin and Evolution of Man

The evolution of humans is one of the most fascinating stories.

• ~15 mya: Primates named Dryopithecus (more ape-like) and Ramapithecus (more man-like) existed.
• ~3-4 mya: Man-like primates walked upright in eastern Africa. They were not taller than 4 feet.
• ~2 mya: Australopithecines lived in East African grasslands. They hunted with stone weapons but were primarily fruit-eaters.
• Homo habilis: Considered the first "human-like" hominid. They had a brain capacity of 650-800cc and likely did not eat meat.
• ~1.5 mya: Homo erectus (e.g., "Java Man") appeared. They had a larger brain (around 900cc) and probably ate meat.
• 100,000 - 40,000 years ago: The Neanderthal man lived in Europe and Asia. They had a large brain size (1400cc), used hides for clothing, and buried their dead.
• Homo sapiens (modern humans) arose in Africa and migrated across continents.
• 75,000 - 10,000 years ago: During the ice age, modern Homo sapiens arose.
• ~18,000 years ago: Pre-historic cave art developed (e.g., at Bhimbetka rock shelter in India).
• ~10,000 years ago: Agriculture began, leading to the start of human settlements and civilisation.