Evolution

Learning Outcomes:

1. Understanding the origins of life and its evolution on Earth.
2. Comprehending various scientific theories regarding life’s origin.
3. Identifying the evidence supporting evolutionary theory.
4. Grasping the role of natural selection in biological evolution.
5. Understanding mechanisms such as genetic drift, mutation, and gene flow in evolution.
6. Learning the evolutionary history of man and key milestones in life’s diversification.

Origin of Life

The origin of life is an essential and singular event in the history of the universe. The universe is vast, approximately 20 billion years old, and composed of clusters of galaxies, each containing stars, gas clouds, and dust. Earth is a mere speck in the grand scale of the universe. According to the Big Bang Theory, a massive explosion resulted in the formation of the universe. Following this explosion, the universe expanded, cooled, and hydrogen and helium formed. These gases condensed to create galaxies, including the Milky Way, and later, Earth, which came into existence approximately 4.5 billion years ago. Initially, there was no atmosphere on Earth. Water vapor, methane, carbon dioxide, and ammonia surrounded the molten surface of the planet. UV rays from the sun split water molecules, and over time, oceans formed as the planet cooled.

Life is estimated to have originated about 4 billion years ago. Theories on the origin of life include:

1. Panspermia Hypothesis: Suggests that life, or at least the building blocks of life, came from outer space in the form of spores.
2. Spontaneous Generation Theory: Early thinkers proposed that life originated from decaying matter. This was later refuted by Louis Pasteur, who demonstrated that life comes from pre-existing life, thus dismissing the theory of spontaneous generation.

Oparin and Haldane hypothesized that the first form of life emerged from non-living organic molecules through chemical evolution. This was supported by Miller’s experiment in 1953, where amino acids were synthesized by simulating early Earth’s atmosphere. The first non-cellular forms of life, likely giant molecules of RNA, proteins, and polysaccharides, might have appeared around 3 billion years ago. The first cellular forms appeared around 2 billion years ago, and these early organisms thrived in water.

Evolution of Life Forms – A Theory

The theory of special creation holds that all life forms were created as they are today and that Earth is only about 4000 years old. This idea was challenged during the 19th century by Charles Darwin. Darwin’s observations during his voyage on the H.M.S. Beagle led him to conclude that all living forms, to varying degrees, share similarities. He recognized natural selection as a key mechanism in evolution: organisms better adapted to their environment tend to survive and reproduce, leading to the gradual evolution of life forms. Fitness, in this context, refers to reproductive fitness, meaning those best suited to the environment leave more progeny, thus increasing their chances of survival.

Alfred Wallace, a naturalist who worked in the Malay Archipelago, independently reached similar conclusions around the same time as Darwin. Darwin’s ideas paved the way for understanding the gradual evolution of species over millions of years.

Evidences for Evolution

There are multiple lines of evidence supporting the theory of evolution:

1. Fossil Record: Fossils provide a historical record of life on Earth. They show that life forms have changed over time, with some species going extinct and new ones arising. Paleontology provides a timeline for these changes.
2. Embryology: Similarities in embryonic development among vertebrates suggest common ancestry. Although Ernst Haeckel’s embryological support for evolution was discredited, the concept of shared developmental pathways holds significance.
3. Comparative Anatomy: Similarities in the anatomical structure of organisms suggest a common ancestry. For instance, the forelimbs of whales, bats, cheetahs, and humans show similar bone structures despite differing functions, a phenomenon known as homology.
4. Analogy: Structures that serve similar functions but arise from different evolutionary paths, like the wings of birds and butterflies, are examples of analogous structures. These highlight convergent evolution where different species evolve similar traits due to similar environmental pressures.

Important Note: Homology reflects divergent evolution, whereas analogy reflects convergent evolution.

5. Biochemistry: The similarities in DNA sequences, proteins, and metabolic processes across species point to a common ancestor.
6. Artificial Selection: Human intervention in breeding animals and plants has led to significant variation within species over relatively short periods, demonstrating that selection can cause noticeable changes over time.

Industrial Melanism

One notable example of evolution by natural selection is industrial melanism. In the mid-1800s, white-winged moths were more common than their dark-winged counterparts in England. However, after industrialization, dark-winged moths became more prevalent as tree bark darkened due to soot. Predators could more easily spot the white-winged moths, leading to their decline. This is a clear example of how environmental changes influence natural selection.

Adaptive Radiation

Adaptive radiation occurs when a single species evolves into multiple species, each adapted to different environments or ecological niches. Darwin’s finches from the Galapagos Islands provide an excellent example. These finches evolved from a common ancestor into various species with different beak shapes, allowing them to exploit different food sources.

Another example is Australian marsupials. A single ancestral species gave rise to a variety of marsupials, each adapted to a specific niche on the Australian continent. This process, when occurring in different geographical areas and habitats, is referred to as convergent evolution.

Biological Evolution

The essence of Darwinian evolution lies in the concepts of branching descent and natural selection. Evolution begins when organisms with variations in their metabolic capabilities arise. Over time, individuals with beneficial variations survive and reproduce, passing these traits to future generations. This process can be observed in microorganisms that rapidly evolve into new species. For larger animals, which have longer lifespans, this process takes much longer.

Concept Note: Adaptive fitness is based on heritable characteristics, which form the basis of natural selection.

Lamarck’s theory, which proposed that organisms evolve through the use and disuse of organs, has been largely discredited. Instead, Darwinian natural selection focuses on the survival and reproduction of individuals best suited to their environment.

Important Note: Natural selection is not deterministic; it operates through random chance and the appearance of mutations.

Mechanism of Evolution

Hardy-Weinberg Principle: This principle states that allele frequencies in a population remain stable over time unless acted upon by external forces. Five factors can disrupt this equilibrium:

1. Gene Flow: Movement of genes between populations.
2. Genetic Drift: Random changes in allele frequencies in small populations.
3. Mutation: Introduction of new alleles through genetic changes.
4. Genetic Recombination: Shuffling of alleles during sexual reproduction.
5. Natural Selection: Differential survival and reproduction based on fitness.

In a diploid organism, allele frequencies can be calculated using the Hardy-Weinberg equation:
[ p^2 + 2pq + q^2 = 1 ]

Where:

• ( p^2 ) represents the frequency of homozygous dominant individuals,
• ( 2pq ) represents the frequency of heterozygous individuals,
• ( q^2 ) represents the frequency of homozygous recessive individuals.

When allele frequencies deviate from this equation, it indicates that evolution is occurring. Founder effect and bottleneck effect are two examples of genetic drift, where isolated populations evolve differently from the original population.

Concept Note: Evolution is not a smooth, linear process; it often occurs in response to environmental pressures and chance events.

A Brief Account of Evolution

The first cellular forms of life appeared around 2 billion years ago, and life has continuously evolved since. Single-celled organisms gave rise to multicellular life, and by 500 million years ago (mya), invertebrates and jawless fish appeared. Plants began colonizing land around 320 mya, and amphibians evolved into reptiles.

Reptiles, including dinosaurs, dominated the Earth for nearly 200 million years. The Cretaceous-Paleogene extinction event, about 65 mya, wiped out the dinosaurs, possibly due to climatic changes. Small reptiles survived and eventually gave rise to birds.

Mammals evolved around 200 mya and diversified significantly after the extinction of the dinosaurs. The evolution of mammals, such as whales, dolphins, and seals, demonstrates the adaptability of life.

Origin and Evolution of Man

Human evolution is one of the most fascinating stories in biology. Around 15 mya, primates like Dryopithecus and Ramapithecus roamed the Earth. Australopithecines, who lived about 2 mya, are among the earliest human-like beings. The first members of the genus **

Homo**, *Homo habilis*, had a brain capacity of *650-800 cc* and lived in East Africa. Homo erectus, with a brain capacity of 900 cc, evolved about 1.5 mya. By 100,000-40,000 years ago, Neanderthals lived in Europe and Asia, using tools and developing early cultural practices.

Modern humans, Homo sapiens, arose in Africa and eventually spread across the globe. Their evolution was marked by the development of language and self-consciousness, allowing them to build complex societies.

Important Note: Evolution is not just a biological process; it also includes the development of culture and intelligence.

Table: Evolutionary Timeline

MCQ:

Which of the following is NOT an example of divergent evolution?
a) Wings of birds and butterflies
b) Forelimbs of whales and humans
c) Leaves of cactus and pea plants
d) Heart structure in mammals and birds
Answer: a) Wings of birds and butterflies