Unit 6: Evolution

Origin of Life

The study of the origin of life deals with how life began on Earth billions of years ago. Earth formed approximately 4.5 billion years ago, and early conditions were extremely hostile.

Early Earth Conditions

Early Earth had no atmosphere initially. Water vapor, methane, carbon dioxide, and ammonia released from molten mass covered the surface. The ultraviolet rays from the sun broke up water into hydrogen and oxygen. Lighter hydrogen escaped. Oxygen combined with ammonia and methane to form water, carbon dioxide, and others. The ozone layer was not yet formed. UV rays split water molecules, and the temperature cooled, causing water vapor to fall as rain, filling depressions and forming oceans. Life appeared almost four billion years ago, about 500 million years after the formation of Earth.

Theories of Origin of Life

• Theory of Special Creation: Suggests that life was created by some supernatural power.
• Panspermia: Units of life called spores or panspermia were transferred to different planets including Earth.
• Theory of Chemical Evolution: Proposes that life originated from pre-existing non-living organic molecules (e.g., RNA, protein, etc.) and that formation of life was preceded by chemical evolution.

Life originated from pre-existing non-living organic molecules and biochemical evolution preceded biological evolution.

Experimental Proof of Chemical Evolution

S.L. Miller in 1953 created conditions similar to those of primitive Earth in a laboratory scale. He created an electric discharge in a closed flask containing methane, hydrogen, ammonia, and water vapor at 800 degrees Celsius. He observed the formation of amino acids. Similar experiments by others showed the formation of sugars, nitrogen bases, pigments, and fats. This experimental evidence strongly supported the chemical evolution theory of life.

Evolution of Life Forms - A Theory

The conventional understanding of evolution was limited by religious beliefs until scientific observations began. Charles Darwin and Alfred Wallace transformed this understanding.

Darwin's Observations and Theory

Charles Darwin concluded that existing life forms share similarities to varying degrees not only among themselves but also with life forms that existed millions of years ago. There has been gradual decline and extinction of some life forms, while new forms arose at different periods in the history of Earth. Any population has built-in variations in characteristics, and those characteristics which enable some to survive better in natural conditions (fitness) would outbreed others that are less adapted to survive.

Key Concepts

• Fitness: According to Darwin, fitness refers ultimately and only to reproductive fitness. Those who are better fit leave more progeny than others and are selected by nature (Natural Selection).
• Branching Descent and Natural Selection: These two key concepts of Darwinian Theory explain how new species arise and why certain traits persist.

Evidences for Evolution

Various scientific disciplines provide empirical proof supporting the occurrence of evolution on Earth.

1. Paleontological Evidence (Fossils)

Fossils are remains of hard parts of life-forms found in rocks. Rocks form crusts of stratified rocks, and different-aged rock sediments contain fossils of different life-forms who died during the formation of the particular sediment. A study of fossils in different sedimentary layers indicates the geological period in which they existed.

2. Comparative Anatomy and Morphology

Comparative anatomy and morphology show similarities and differences among organisms of today and those that existed years ago.

• Homologous Organs: Organs that are similar in anatomical structure and origin but perform different functions due to adaptation to different needs. Example: Forelimbs of whales, bats, cheetahs, and humans. This represents divergent evolution.
• Analogous Organs: Organs that are similar in function and appearance but anatomically different and do not share a common ancestral origin. Example: Wings of butterflies and birds, or eyes of an octopus and mammals. This represents convergent evolution.

3. Biochemical Evidences

Similarities in proteins and genes performing a given function among diverse organisms give clues to common ancestry. These biochemical similarities point to the same shared genetic machinery across living systems.

4. Natural Selection in Action

Industrial melanism is a classic observational evidence of natural selection:

Important Observation: Predators will prey on a moth that is mismatched with its background. Evolution is not a direct directed process in the sense of determinism, but it is a stochastic process based on chance events in nature and mutation.

Adaptive Radiation

Adaptive radiation is a process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats).

Examples of Adaptive Radiation

• Darwin's Finches: In the Galapagos Islands, Darwin observed small black birds later called Darwin's finches. They had many varieties on the same island, all evolving from an original seed-eating bird ancestor. They adapted into insectivorous and vegetarian finches with different beak modifications.
• Australian Marsupials: A number of marsupials, each differing from the other, evolved from an ancestral stock, all within the Australian island continent.

When more than one adaptive radiation appeared to have occurred in an isolated geographical area (representing different habitats), one can call this convergent evolution (e.g., Australian marsupials and placental mammals showing parallel adaptations like placental wolf and Tasmanian wolf marsupial).

Biological Evolution

The essence of Darwinian Theory about evolution is natural selection. Rate of appearance of new forms is linked to the life cycle or life span. For instance, microbes that multiply fast can generate thousands of new generations in a short time, resulting in new phenotypes much faster than organisms with long life cycles.

Branching descent and natural selection are the two core pillars of the theory of natural selection.

Mechanism-of-Evolution

While Darwin talked about selection of variations, he did not explicitly clarify the exact source of these variations.

Hugo de Vries and Mutation Theory

Hugo de Vries worked on evening primrose and put forward the idea of mutations. Mutations are random, directionless, and large differences arising suddenly in a population. De Vries believed mutation caused evolution and called it saltation (single-step large mutation). Unlike Darwin's gradual accumulation of small, directional variations, mutation theory proposed sudden, distinct leaps.

Hardy-Weinberg Principle

The Hardy-Weinberg Principle addresses allele frequencies in a population under certain stable conditions.

Core Principle

In a given population, allele frequencies (gene frequencies) are stable and remain 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 allelic frequencies is 1.

Algebraically, for a gene with two alleles, A and a:

p squared + 2pq + q squared = 1

Where:

• p: frequency of allele A
• q: frequency of allele a
• p squared: frequency of homozygous dominant genotype (AA)
• 2pq: frequency of heterozygous genotype (Aa)
• q squared: frequency of homozygous recessive genotype (aa)

Factors Affecting Hardy-Weinberg Equilibrium

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

1. Gene migration or gene flow: When migration of a section of population takes place to another place, gene frequencies change in both the original and new populations.
2. Genetic drift: If the change in allele frequency occurs by chance, it is called genetic drift. Sometimes the change in allele frequency is so different in the new sample of population that they become a different species (Founder effect).
3. Mutation: Random alterations in genetic sequences that introduce new alleles.
4. Genetic recombination: Shuffling of gene combinations during gametogenesis and sexual reproduction.
5. Natural selection: Differential survival and reproduction of organisms with advantageous phenotypes, leading to directional, disruptive, or stabilizing shifts in allele frequencies.

A Brief Account of Evolution

A chronological timeline helps understand the history of life forms on Earth:

• Approx. 2000 million years ago (mya): First cellular forms of life appeared.
• Approx. 500 mya: Invertebrates were formed and active.
• Approx. 350 mya: Jawless fish probably evolved.
• Approx. 320 mya: Seaweeds and few plants existed. Land plants were first seen before animals.
• Approx. 350 mya: Lobefins (stout and strong finned fishes) could move on land and go back to water. These were ancestors of modern day frogs and salamanders (amphibians).
• Carboniferous period: Amphibians evolved into reptiles. They lay thick-shelled eggs which do not dry up in sun unlike amphibian eggs. Giant ferns (pteridophytes) were present but fell to form coal deposits.
• Mesozoic era: Age of reptiles. Dinosaurs were dominant. About 65 mya, dinosaurs suddenly disappeared from Earth.
• First mammals: Small and shrew-like. Mammals were viviparous and protected their unborn inside mother's body.

Origin and Evolution of Man

Human evolution is traced through fossil records showing structural and behavioral changes over millions of years.

Key Stages of Primate and Human Evolution

Exam Note: Remember key brain capacity volumes and chronological sequencing for hominid stages to score well in objective and short-answer questions.