UNIT 6 THEORIES OF ORGANIC EVOLUTION REVISION NOTES

 Introduction to Biological Evolution

Biological Evolution:

• Definition: Evolution means 'change.' In the context of organisms, it refers to "Descent with modification" or "Continuity of life with constant modifications."
• Origin of Term: Herbert Spencer (1857) used the term evolution to describe the development of more complex life forms from simpler ones.

Key Concepts:

1. Organic Evolution (Bio Evolution):

   • Development of complex and organized living beings from simpler, less organized ancestors.
   • Accumulation of gradual modifications over millions of years.

2. Adaptability and Adaptations:

   • Environmental conditions are constantly changing.
   • Organisms adapt to these changes, leading to the origin of new species.
   • New species are better adapted and more organized than their ancestors.

3. Diversification and Speciation:

   • Members of a species adapt to different environments, evolving along divergent lines.
   • All present-day species share a common ancestor at some point in their evolution.
   • Migration to varied geographical areas leads to adaptations and the formation of new species.

4. Comparative Anatomy and Morphology:

• Study of similarities and differences among current organisms and those from the past.
• Helps in understanding common ancestry and evolutionary relationships.

Patterns of Evolution

1. Divergent Evolution:

• Occurs when a population becomes isolated and evolves into a new species due to exposure to different selective pressures.
• Evidence: Homologous structures (similar anatomical structures with different functions).
• Example: Whales, bats, cheetahs, and humans all have similar bone patterns in their forelimbs despite different functions.

2. Convergent Evolution:

• Occurs when unrelated species occupy the same environment and develop similar adaptations due to similar selective pressures.
• Evidence: Analogous structures (different anatomical structures with similar functions).
• Example: Wings of butterflies and birds look similar and perform the same function but are anatomically different.

3. Coevolution:

• Mutual evolutionary adaptations of two interacting species.
• Example: Pollinator-plant relationships where insects, birds, or bats ensure the reproductive success of flowers while feeding on nectar.

4. Parallel Evolution:

• Describes two related species that have made similar evolutionary adaptations after diverging from a common ancestor.
• Example: North American wolf and the Tasmanian wolf (a marsupial) evolved similarly despite being geographically separated.

5. Adaptive Radiation:

• Emergence of numerous species from a single common ancestor in response to different ecological niches.
• Example: Darwin's finches on the Galapagos Islands evolved into 14 species with different beak shapes adapted for different diets.

Theories of Evolution

Evolutionary theories provide scientific explanations for the process of organic evolution. The key theories include Lamarckism, Darwinism, Mutation Theory, and the Synthetic Theory of Evolution.

1. Lamarckism

• Proponent: Jean Baptiste Pierre Antoine de Monet Lamarck (1744-1829), a French naturalist.
• Publication: Philosophic Zoologique (1809).
• Other Names: Theory of Organic Evolution, Inheritance of Acquired Characters.
• Main Points:
• Complexity: Organic matter inherently tends to reach higher levels of complexity.
• Environmental Influence: The environment changes, creating new needs for organisms.
• Use and Disuse: Continuous use of a structure enhances it; disuse leads to atrophy.
• Acquired Characters: New structures developed in response to environmental changes are acquired characters.
• Inheritance: Acquired characters are transmitted to the next generations, leading to the evolution of new species.

Key Points of Lamarckism:

• Change Due to Environment: Organisms adapt to environmental changes.
• Development of New Structures: Response to new needs leads to new structures.
• Transmission of Traits: Acquired traits are inherited by offspring.

Assumptions of Lamarckism

• Internal Urge for Growth: Living organisms and their parts have a natural tendency to increase in size due to an internal life force.
• Development of New Organs: New organs or characteristics develop to meet the needs created by new wants and are maintained as acquired characters.
• Use and Disuse: The development and use of organs are proportional to their activities. Continuous use enhances organs, while disuse leads to their atrophy.
• Inheritance of Acquired Characters: Traits acquired during an organism's lifetime are preserved and transmitted to the next generation.

Propositions of Lamarckism

• Internal Urge: Change of habits can initiate the formation of new organs or modify existing structures due to an internal life force.
• Inheritance of Acquired Characters: Environmental responses lead to new adaptive characters in an organism, which are then passed on to future generations, resulting in evolutionary changes.
• New Needs as a Reaction to the Environment: The environment influences the form and habits of organisms, leading to changes in their external and internal structures.
• Use and Disuse: Continuous use of an organ enhances its development, while disuse leads to degeneration. Examples include the long neck of giraffes and the limblessness of snakes.

Examples Supporting Lamarckism

• Long Neck of Giraffes: Originally deer-like animals stretched their necks to reach higher leaves, resulting in longer necks over generations.
• Limblessness in Snakes: Ancestors of snakes had limbs, but as they adapted to a burrowing lifestyle, they gradually lost their limbs.
• Webbed Feet in Aquatic Birds: Birds like ducks developed webbed feet to adapt to swimming, arising from terrestrial ancestors.
• Flightlessness in Birds: Birds like the kiwi evolved from flying ancestors, losing their ability to fly due to reduced use of wings and feathers.
• Biceps in Blacksmiths: Constant use of hands for heavy hammering led to well-developed biceps in blacksmiths.
• Vestigial Organs in Humans: Examples include the appendix, post-anal tail, and trace of the nictitating membrane.

Criticism of Lamarckism

Major Criticisms

• Inheritance of Acquired Characters:
  • Disproved by August Weismann's experiments on mice, showing that cutting tails for over 20 generations did not result in tailless offspring.
  • Weismann differentiated between somatoplasm (somatic cells) and germplasm (germ cells), establishing that changes in somatic cells are not inherited, only changes in germplasm are.
• Impairment vs. Improvement:
  • Individuals who use certain organs extensively (e.g., eyes for reading) often develop impairments rather than improvements.
• Desire for New Organs:
  • The idea that new organs develop based on need was questioned, e.g., humans desiring to fly have not developed wings.

Neo-Lamarckism

Key Points

• Adaptation Recognition: Acknowledged adaptation to the environment as crucial in evolution.
• Modification and Acceptance: Followers like Cope, Spencer, Packard, and Kammerer modified Lamarckism, making it more acceptable.
• Somatic and Germ Cells: Neo-Lamarckians suggested that environmental adaptations affecting somatic cells can be inherited.
• Experimental Evidence:
  • Proteus anguineus: This cave-dwelling amphibian developed black skin and normal eyes when exposed to light, and these traits were inherited.
  • Rats Experiment: Rats placed on a rotating table developed dizziness, which was also seen in their offspring.

Differences from Lamarckism

• Direct Environmental Effect: Stressed the direct impact of environmental changes on organisms.
• Inheritance Focus: Only modifications affecting germ cells or somatic cells that give rise to germ cells are inherited.
• Reduction of Internal Forces: Did not emphasize internal vital force, appetency, and use/disuse of organs as primary factors.

Summary Points

• Criticism of Lamarckism: Focused on disproving inheritance of acquired characters and questioning the role of desire and use/disuse in evolution.
• Neo-Lamarckism: Improved Lamarckism by focusing on the inheritance of environmentally induced changes affecting somatic cells that can influence germ cells.

Darwinism

Charles Robert Darwin (1809-1882):

• English naturalist appointed as a Naturalist on H.M.S. Beagle (1831-1836).
• Explored fauna and flora of various continents and islands, notably the Galapagos Islands.
• Developed the theory of evolution by natural selection, published in “On the Origin of Species” (1859).

Influences on Darwin:

1. T.R. Malthus' Essay (1798):
   • "On the Principles of Populations" - Populations grow geometrically, while food sources grow arithmetically.
2. Sir Charles Lyell's "Principles of Geology":
   • Gradualism: Earth changes slowly and gradually.
   • Uniformitarianism: Fundamental laws operate the same way today as in the past.
3. Alfred Russel Wallace’s Paper:
   • "On the Tendency of Varieties to Depart from Original Types."

Key Points of Darwinism:

• Darwin summarized his theory in a joint paper in 1858 to the Linnean Society.
• Detailed publication in "On the Origin of Species by Natural Selection" (1859).

Core Concepts of Darwin's Theory

1. Variation:

   • Individuals within a species show variation in physical characteristics.

2. Inheritance:

   • Some traits are heritable and passed on to offspring.

3. High Rate of Population Growth:

   • More offspring are produced than can survive due to limited resources.

4. Differential Survival and Reproduction:

   • Individuals with advantageous traits are more likely to survive and reproduce.

5. Adaptation:

   • Over time, favorable traits become more common in the population.

Summary of Darwin's Theory

Natural Selection Process:

• Struggle for Existence:
  • Due to limited resources, organisms compete for survival.
• Survival of the Fittest:
  • Individuals with beneficial adaptations are more likely to survive.
• Reproductive Success:
  • These individuals are more likely to reproduce, passing on advantageous traits.
• Gradual Change:
• Over generations, populations evolve with these favorable adaptations.

Darwin's Theory of Natural Selection

Principles of Natural Selection:

1. Over Production:

   • Organisms tend to increase their population geometrically.
   • More offspring are produced than can survive and reproduce.
   • Population remains constant as many offspring die before reaching reproductive age.
   • Limited resources (food, space) don't increase at the same rate as the population.

2. Variations:

• Variations exist in all organisms; they can be harmful or useful.
• Useful variations (favorable) are inherited and enhance survival.
• Harmful variations make organisms unfit in the struggle for existence.
• Beneficial variations are selected by nature and become the raw material for evolution.

1. Struggle for Existence:

   • Organisms reproduce in large numbers, but resources are limited.
   • Types of Struggle:
     • Intraspecific: Within the same species (food, mating).
     • Interspecific: Between different species (food).
     • Environmental: Against environmental factors (food scarcity, extreme weather).

2. Natural Selection (Survival of the Fittest):

   • Organisms with beneficial variations survive; less fit ones are eliminated.
   • Proposed by Herbert Spencer, variations that enhance reproductive success are favored.
   • Differential reproductive success ensures only those best adapted reproduce and pass on traits.

3. Origin of New Species:

   • Favourable variations accumulate over generations, leading to new species.
   • Evolution is gradual, with species evolving due to cumulative effect of small, favorable variations.

Experimental Verification - Industrial Melanism:

• Example: Peppered moth (Biston betularia) in England.
• Pre-Industrial Revolution: Grey moths camouflaged against light tree trunks, avoiding predation.
• Post-Industrial Revolution: Increased pollution darkened tree barks with soot.
• Grey moths became easily visible to predators; their numbers decreased.
• Dark moths (melanic) camouflaged better against blackened trees; their numbers increased.
• Result: Natural selection favored melanic moths, demonstrating evolution in response to environmental changes (industrial melanism).

Mutation Theory and Modern Synthetic Theory of Evolution

Mutation Theory (Hugo de Vries, 1900)

• Definition:

  • Evolution driven by mutations, not natural selection.
  • Mutations: Spontaneous genetic changes leading to new species.

• Key Points:

  • Sudden Origin: New species originate suddenly without visible preparation.
  • Discontinuous Variations: Hereditary changes are often discontinuous, aligning with Mendelian laws.
  • Large Mutations: Evolution is effective if large mutations are selected.
  • Opposition: Biometricians like Karl Pearson argued natural selection on small differences is key.

• Development of Population Genetics:

  • Hardy-Weinberg Equilibrium (1908): Laid mathematical foundation.
  • Key Figures: Th. Dobzhansky, R. A. Fisher, S. Wright, J.B.S. Haldane.

Modern Synthetic Theory of Evolution

• Integration of Genetics and Natural Selection:

  • Population Focus: Emphasis on genetic variation within populations.
  • Contributors: Theodosius Dobzhansky, Ernst Mayr, Julian Huxley, George G. Simpson, Bernhard Rensch, G. Ledyard Stebbins.

• Key Aspects:

  • Mutations: Random genetic changes provide evolutionary fuel.
  • Other Factors: Migration, founder effect, random genetic drift, and hybridization.
  • Biological Species Concept (Mayr, 1942): Defines species based on reproductive isolation.
  • Speciation (Dobzhansky): Evolutionary step where forms become incapable of interbreeding.

• Mechanisms:

  • Gradual Evolution: Small genetic changes and recombination ordered by natural selection.
  • Macro-Evolutionary Processes: Consistent with genetic mechanisms.