Evolution Tnpsc Notes

Evolution is a fundamental concept in biology that refers to the process of change in all forms of life over generations. It explains how species of living organisms have changed over time and diversified into the multitude of forms we observe today. The theory of evolution, as proposed by Charles Darwin in the mid-19th century, is one of the central organizing principles in biology.

Key components of the theory of evolution include:

Descent with Modification

Living organisms are related through common ancestry. Over time, species change, and new species arise from existing ones. This process of descent with modification is driven by the accumulation of small, heritable variations in populations over successive generations.

Example of Descent with Modification

  1. Darwin’s Finches:
    • The finches of the Galápagos Islands, studied by Charles Darwin, provide a classic example of adaptive radiation and descent with modification. Different species of finches on the islands have evolved diverse beak shapes and sizes adapted to different types of food sources, such as seeds, insects, or nectar.
  2. Peppered Moths:
    • The peppered moth (Biston betularia) in England experienced a well-documented example of industrial melanism during the 19th and early 20th centuries. Prior to industrialization, light-colored moths were more common. As pollution darkened tree trunks, darker moths became more prevalent, illustrating how natural selection favored individuals with better camouflage against the altered environment.
  3. Cichlid Fish in African Lakes:
    • Cichlid fish in African lakes, such as Lake Malawi and Lake Victoria, have undergone rapid speciation and adaptive radiation. Different species of cichlids have evolved diverse body shapes, colors, and feeding strategies in response to the varied ecological niches within the lakes. This demonstrates descent with modification as a result of environmental pressures.
  4. Elephant Evolution:
    • The evolutionary history of elephants illustrates descent with modification over millions of years. The ancient relatives of elephants were smaller, more diverse mammals. Over time, these ancestors gave rise to larger and more specialized forms, eventually leading to the modern elephants we see today, including the African elephant (Loxodonta africana) and the Asian elephant (Elephas maximus).
  5. Adaptive Radiation in Hawaiian Honeycreepers:
    • The Hawaiian honeycreepers are a group of birds that evolved through adaptive radiation in the isolated Hawaiian Islands. A single ancestral finch-like species diversified into a variety of forms with different beak shapes, sizes, and colorations, each adapted to exploit specific ecological niches on the islands.
Descent with Modification
Descent with Modification

Natural Selection

The mechanism driving evolution is natural selection. It is the process by which organisms with advantageous traits for their environment have a better chance of surviving and reproducing, passing those favorable traits to their offspring. Over time, this leads to the prevalence of these beneficial traits in a population.

Examples of Natural Selection

  1. Peppered Moths (Biston betularia):
    • During the Industrial Revolution in England, the population of light-colored peppered moths was prevalent because they were well-camouflaged against lichen-covered tree trunks. With industrialization, tree trunks became darker due to pollution, making dark-colored moths less visible to predators. As a result, the frequency of dark-colored moths increased through natural selection.
  2. Darwin’s Finches (Geospiza spp.):
    • On the Galápagos Islands, different species of finches evolved varied beak shapes and sizes based on the availability of different types of seeds and food sources. Natural selection favored finches with beaks adapted to the specific types of seeds found on their respective islands, leading to the development of diverse beak structures.
  3. Giraffe Neck Length:
    • The giraffe’s long neck is often cited as an example of natural selection. Giraffes with longer necks have an advantage in reaching higher branches for food, especially during times of scarcity. Over time, natural selection favored giraffes with longer necks, contributing to the evolution of this characteristic in the population.
  4. Antibiotic Resistance in Bacteria:
    • Bacterial populations can evolve resistance to antibiotics through natural selection. When antibiotics are used, susceptible bacteria are killed, but any resistant individuals survive and reproduce. With continued antibiotic use, the frequency of resistant bacteria increases, demonstrating how natural selection acts on the heritable variation in the bacterial population.
  5. Camouflage in Prey Animals:
    • Many prey animals have evolved coloration and patterns that provide effective camouflage against their natural environments. For example, stick insects resemble twigs, and some species of moths mimic the appearance of tree bark. Natural selection favors individuals with better camouflage, as they are more likely to survive and avoid predation.


As a result of natural selection, populations of organisms become adapted to their specific environments. Adaptations are traits or characteristics that enhance an organism’s chances of survival and reproduction in a particular environment.

Examples of Adaptation

  1. Camouflage in Chameleons:
    • Chameleons are known for their ability to change the color of their skin to match their surroundings. This camouflage helps them avoid predators and sneak up on prey. Specialized cells called chromatophores in their skin allow them to alter their appearance by adjusting pigments.
  2. Mimicry in Viceroy Butterflies:
    • The viceroy butterfly exhibits Batesian mimicry, where it resembles the toxic monarch butterfly. Predators that have learned to avoid the toxic monarch also avoid the viceroy, even though it is not toxic. This mimicry provides protection from predation.
  3. Echolocation in Bats:
    • Many species of bats have evolved echolocation as an adaptation for navigating and hunting in the dark. Bats emit high-frequency sound waves and use the echoes to determine the location, size, shape, and even the texture of objects around them. This adaptation is crucial for their nocturnal lifestyle.
  4. Migration in Birds:
    • Many bird species have developed the ability to migrate over long distances to find food, avoid harsh weather, or breed in more favorable conditions. The ability to cover vast distances allows them to exploit different resources at different times of the year.
  5. Aquatic Adaptations in Whales:
    • Whales have evolved various adaptations for their aquatic lifestyle. Examples include streamlined bodies for efficient swimming, a layer of blubber for insulation and buoyancy, and specialized limbs modified into flippers for steering. The blowhole on the top of their heads allows them to breathe while staying mostly submerged.


Over extended periods, the accumulation of genetic changes can lead to the divergence of populations to the point where they become distinct species. This process is known as speciation.

Examples of Speciation

  1. Darwin’s Finches (Geospiza spp.):
    • The finches on the Galápagos Islands provide a classic example of speciation. Different species of finches evolved with distinct beak shapes and sizes, adapted to the specific types of seeds available on each island. This adaptive radiation led to the formation of multiple finch species from a common ancestor.
  2. Hawaiian Honeycreepers:
    • The Hawaiian honeycreepers underwent adaptive radiation on the Hawaiian Islands, resulting in diverse species with different beak shapes, sizes, and colorations. Over time, they adapted to various ecological niches on the islands, leading to speciation.
  3. East African Rift Cichlids:
    • Cichlid fish in the East African Rift Lakes (e.g., Lake Malawi, Lake Victoria) have undergone rapid speciation. Different species adapted to specific habitats within the lakes, and divergent selection pressures led to the development of unique color patterns, body shapes, and behaviors among different cichlid populations.
  4. Ring Species – Larus gulls:
    • The Larus gulls, particularly the Herring Gull complex, provide an example of a ring species. Different populations of gulls interbreed where their ranges overlap, but as you move around the ring, the populations become reproductively isolated. This gradual isolation can lead to the formation of distinct species.
  5. Apple Maggot Fly (Rhagoletis pomonella):
    • The apple maggot fly is an example of sympatric speciation. Originally feeding on hawthorn fruit, some populations of these flies shifted to apple trees after their introduction. Over time, reproductive isolation occurred between the apple-feeding and hawthorn-feeding populations, leading to the formation of distinct species.

Common Ancestry

All living organisms share a common ancestry. The diversity of life on Earth is the result of branching events in the evolutionary tree, where different species have emerged over time.

Examples of Common Ancestry

  1. Tetrapods (Vertebrates with Four Limbs):
    • Tetrapods include amphibians, reptiles, birds, and mammals. Despite their diverse forms and habitats, these animals share a common ancestor that had four limbs. The structure of limbs may vary (e.g., wings of birds, flippers of whales), but the common origin is evident in their underlying skeletal similarities.
  2. Pentadactyl Limb:
    • The pentadactyl limb, characterized by five digits, is a common feature in many vertebrates. This limb structure is present in mammals (including humans), birds, amphibians, and some reptiles. The shared ancestry is indicated by the similar arrangement of bones in the limb.
  3. Homologous Structures in Mammals:
    • Mammals, including humans, share numerous homologous structures that trace back to a common mammalian ancestor. Examples include the presence of mammary glands, hair, three middle ear bones, and a placenta in many mammalian species.
  4. Common Ancestor of Whales and Hippos:
    • Molecular and fossil evidence indicates that whales and hippos share a common ancestor. Despite the vast differences in appearance and lifestyle, the genetic similarities and fossil records support their evolutionary connection.
  5. Shared Ancestry of Insects:
    • Insects, the largest group of animals on Earth, share a common ancestry. Their evolutionary history is marked by diverse forms and adaptations, yet the underlying genetic and anatomical similarities point to a common origin for this incredibly diverse group.

Evolution is supported by a wealth of scientific evidence, including fossil records, comparative anatomy, molecular biology, and observations of natural selection in action. The modern understanding of evolution integrates these various lines of evidence and continues to be a cornerstone of biological science.

Darwin’s Theory of Evolution

Charles Darwin’s theory of evolution, outlined in his seminal work “On the Origin of Species,” revolutionized our understanding of how species change over time. Darwin’s theory is based on a few key principles:

  1. Descent with Modification: Darwin proposed that all living organisms are connected through a process of descent with modification. Over generations, organisms undergo changes, or modifications, in their traits. These modifications accumulate, leading to the divergence of different species from a common ancestor.
  2. Natural Selection: Perhaps the most famous aspect of Darwin’s theory, natural selection is the mechanism driving the process of evolution. It involves the differential survival and reproduction of organisms based on their inherited traits. Those individuals with traits better suited to their environment are more likely to survive and reproduce, passing on their advantageous traits to their offspring. Over time, this process leads to the adaptation of populations to their specific ecological niches.
  3. Variation and Heritability: Darwin recognized the existence of natural variation within populations. Individuals within a population exhibit differences in traits and some of these variations are heritable, meaning they can be passed on to the next generation through genetic inheritance.
  4. Overproduction and Struggle for Existence: Populations tend to produce more offspring than their environment can support. This leads to competition for resources and a struggle for existence. Only those individuals with traits that provide a survival advantage are more likely to live long enough to reproduce.
  5. Gradualism: Darwin proposed that evolution occurs gradually over long periods of time. Small, incremental changes accumulate over generations, leading to significant differences between species.
  6. Common Ancestry: Darwin suggested that all living organisms share a common ancestry. The diversity of life can be explained by a branching pattern of descent, where different species arose from a common ancestral population.

While some aspects of Darwin’s original theory have been refined and expanded upon in light of new scientific discoveries (such as the role of genetics), the core principles of natural selection and descent with modification remain central to the modern understanding of evolution. Darwin’s theory provides a unifying framework for explaining the patterns of biodiversity observed in the natural world.

Evidence for Evolution

The theory of evolution is supported by a wealth of evidence from various scientific disciplines. Here are some key types of evidence that strongly support the idea of evolution:

  1. Fossil Record:
    • Fossils provide a record of past life on Earth, showing the existence of extinct species and the progression of life over time.
    • Transitional fossils, which exhibit characteristics of both ancestral and descendant species, provide direct evidence of evolutionary transitions.
  2. Comparative Anatomy:
    • Homologous structures are anatomical features shared by different species due to common ancestry. For example, the bones in the limbs of vertebrates have a similar structure, indicating a common evolutionary origin.
    • Analogous structures perform similar functions but have different evolutionary origins. This suggests adaptation to similar environments rather than common ancestry.
  3. Comparative Embryology:
    • Similarities in the embryonic development of different species support the idea of common ancestry. Many organisms exhibit comparable embryonic stages, emphasizing their shared evolutionary history.
  4. Molecular Biology:
    • DNA and protein sequences provide molecular evidence for evolutionary relationships. The more closely related species are, the more similar their genetic material.
    • Molecular clock analysis, which examines the rate of genetic mutations over time, supports the timing of evolutionary events and divergence between species.
  5. Biogeography:
    • The distribution of species around the world reflects historical patterns of migration and evolutionary history. For example, marsupials are primarily found in Australia, reflecting the continent’s isolation and unique evolutionary trajectory.
  6. Vestigial Structures:
    • Vestigial structures are remnants of ancestral features that no longer serve a purpose in the organism’s current form. For instance, the human appendix is considered a vestigial structure, suggesting an evolutionary history involving a structure with a previous function.
  7. Experimental Evidence:
    • Laboratory experiments and observations of natural populations provide real-time evidence of evolutionary processes, such as natural selection in action.
    • The study of antibiotic resistance in bacteria and the evolution of pesticide resistance in insects are examples of observable evolutionary changes.
  8. Convergent Evolution:
    • Convergent evolution occurs when unrelated species develop similar traits due to adapting to similar environmental challenges. This phenomenon supports the idea that natural selection can lead to similar solutions in different lineages.

The convergence of evidence from multiple disciplines strongly supports the theory of evolution. While the details and mechanisms have been refined over time, the overarching concept of descent with modification through natural selection remains a cornerstone of modern biology.

Hardy-Weinberg Principle

The Hardy-Weinberg principle, also known as the Hardy-Weinberg equilibrium or law, is a fundamental concept in population genetics. It describes the theoretical conditions under which the genetic composition of a population will remain constant from generation to generation in the absence of disturbing influences. This principle was independently formulated by G. H. Hardy and Wilhelm Weinberg in 1908 and 1909, respectively.

The Hardy-Weinberg equilibrium is based on several key assumptions:

  1. Large Population Size: The population is assumed to be infinitely large or, at the very least, sufficiently large that random sampling errors are negligible.
  2. No Mutation: There are no new genetic variations introduced into the population through mutation.
  3. No Migration: The population is closed, meaning there is no migration into or out of the population.
  4. Random Mating: Individuals in the population mate randomly with respect to their genotype. In other words, there is no preferential selection of mates based on genetic traits.
  5. No Selection: There is no natural selection acting on the population. All genotypes have equal fitness, meaning they contribute equally to the next generation.

Under these conditions, the frequencies of alleles and genotypes in the population will remain constant from generation to generation. The Hardy-Weinberg equilibrium can be expressed mathematically through the following equations:



  • p2p2 represents the frequency of the homozygous dominant genotype (AA).
  • 2pq2pq represents the frequency of the heterozygous genotype (Aa).
  • q2q2 represents the frequency of the homozygous recessive genotype (aa).
  • pp is the frequency of the dominant allele (A).
  • qq is the frequency of the recessive allele (a).

The Hardy-Weinberg principle is a useful tool for understanding the genetic dynamics of populations and providing a baseline against which observed genetic changes can be compared. Deviations from the Hardy-Weinberg equilibrium may indicate the presence of evolutionary forces such as mutation, migration, selection, or non-random mating in a population.

* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.