ROUTERA

Chapter 7 Evolution

Class 12th Biology Chapter hots

1. Explain the concept of evolution with reference to natural selection. How does it contribute to the survival of species?

Answer:
Evolution:

  • Evolution refers to the gradual change in the genetic composition of a population over generations.
  • Charles Darwin proposed natural selection as a mechanism driving evolution, where organisms with traits better suited to their environment survive and reproduce, passing on these beneficial traits to future generations.

Natural Selection:

  • Variation: Within a population, individuals exhibit genetic variations in traits.
  • Struggle for Existence: Organisms compete for limited resources.
  • Survival of the Fittest: Organisms with advantageous traits (e.g., camouflage, speed) are more likely to survive and reproduce.
  • Adaptation: Over time, these traits become more common, leading to the adaptation of the species to its environment.
  • Evolutionary Significance: This process results in a population that is better adapted to the environment, enhancing the survival of the species.

2. Discuss the different types of selection in natural selection with suitable examples.

Answer:
Types of Selection:

  1. Directional Selection:

    • Occurs when individuals with traits at one end of the phenotypic spectrum are favored.
    • Example: In a population of giraffes, those with longer necks are more likely to reach food high in trees and survive, leading to a population with generally longer necks over time.

  2. Stabilizing Selection:

    • Favors individuals with traits near the population average, reducing variation.
    • Example: Human birth weight; babies of average weight have higher survival rates than those with very low or very high birth weights.

  3. Disruptive Selection:

    • Favors individuals at both extremes of the phenotypic spectrum, leading to increased variation.
    • Example: In a population of birds, individuals with either very small or very large beaks may be favored if they can access different food sources, whereas medium-sized beaks may be less efficient.

3. Explain the Hardy-Weinberg equilibrium principle. How is it used to measure evolutionary changes?

Answer:
Hardy-Weinberg Equilibrium Principle:

  • The Hardy-Weinberg principle states that allele frequencies in a population remain constant from generation to generation in the absence of evolutionary influences.
  • For equilibrium to be maintained, the following conditions must be met:
    1. No mutations.
    2. Random mating.
    3. Large population size.
    4. No migration (gene flow).
    5. No natural selection.

Equations:

  • p + q = 1 (where p is the frequency of the dominant allele and q is the frequency of the recessive allele).
  • p² + 2pq + q² = 1 (where p² is the frequency of homozygous dominant, 2pq is the frequency of heterozygous, and q² is the frequency of homozygous recessive individuals).

Use in Measuring Evolution:

  • If allele frequencies deviate from the equilibrium, it indicates that one or more evolutionary forces are acting on the population, suggesting that evolution is occurring.

4. Discuss the concept of genetic drift and explain how it leads to evolution.

Answer:
Genetic Drift:

  • Genetic drift is the random fluctuation in allele frequencies in a small population. Unlike natural selection, genetic drift does not involve the survival of the fittest but occurs due to chance events.
  • Bottleneck Effect: A drastic reduction in population size due to environmental events (e.g., natural disasters) reduces genetic diversity, and the gene pool may become unrepresentative of the original population.
  • Founder Effect: When a small group of individuals breaks off from a larger population to form a new population, the genetic diversity of the new population may be limited to that of the founders, leading to evolutionary changes.

Evolutionary Significance:

  • Genetic drift can lead to a loss of genetic variation and may cause certain traits to become fixed or lost in a population, influencing evolutionary pathways.

5. How does the molecular basis of evolution support the theory of common descent?

Answer:
Molecular Evidence:

  • The molecular basis of evolution includes similarities in the genetic code, proteins, and DNA sequences across different species.
  • Common Descent: The concept that all living organisms share a common ancestor. This is supported by the following:
    1. Universal Genetic Code: All organisms use the same genetic code, suggesting a common origin.
    2. Homologous Genes: Genes with similar sequences and functions found in different species imply shared ancestry.
    3. Phylogenetic Trees: Molecular data, such as DNA sequences, can be used to construct phylogenetic trees that show evolutionary relationships between species, supporting the idea of common descent.

6. Explain the role of mutations in evolution. How do mutations contribute to genetic variation?

Answer:
Mutations:

  • Mutations are changes in the DNA sequence that can introduce new genetic variations in a population.
  • Types of Mutations:
    1. Point mutations (single base change),
    2. Insertions (addition of nucleotides),
    3. Deletions (removal of nucleotides).
  • Effect on Genetic Variation:
    • Mutations are the primary source of new alleles, creating genetic diversity within a population.
    • While most mutations are neutral or harmful, some may confer advantages in certain environments, contributing to evolutionary changes over time.
    • Example: A mutation in the hemoglobin gene can lead to sickle-cell anemia, but it also confers resistance to malaria in heterozygous individuals, demonstrating the adaptive role of mutations.

7. What is the concept of adaptive radiation? Provide examples to support your explanation.

Answer:
Adaptive Radiation:

  • Adaptive radiation refers to the rapid diversification of a single ancestral species into a wide variety of forms adapted to different environments.
  • Mechanism: Occurs when a species colonizes a new habitat with varied ecological niches, leading to the evolution of distinct traits that enable survival in these niches.

Examples:

  1. Darwin’s Finches: In the Galápagos Islands, finches with different beak shapes evolved from a common ancestor to adapt to various food sources.
  2. Mammals after the extinction of dinosaurs: Following the extinction of dinosaurs, mammals diversified into various forms, occupying niches that were previously filled by dinosaurs.

8. How do vestigial organs provide evidence for evolution?

Answer:
Vestigial Organs:

  • Vestigial organs are structures that have lost their original function through evolution. They provide evidence of common ancestry and evolutionary change.
  • Examples:
    1. Human appendix: A small, non-functional organ that is believed to be a remnant of a larger cecum used for digesting plant material in ancestors.
    2. Pelvic bones in whales and snakes: These are remnants of pelvic structures used by their terrestrial ancestors, supporting the theory of descent with modification.

9. Describe the evidence of evolution from embryology and how it supports the theory of common ancestry.

Answer:
Embryological Evidence:

  • Embryonic development in different species shows striking similarities in early stages, suggesting a common origin.
  • Example:
    1. Vertebrate embryos (fish, amphibians, reptiles, birds, and mammals) exhibit similar features, such as pharyngeal pouches (gill slits), indicating a shared ancestry.
    2. The presence of tail-like structures in human embryos, which are eventually absorbed, points to evolutionary development from an ancestor with a tail.

10. Discuss the concept of speciation. What are the different modes of speciation?

Answer:
Speciation:

  • Speciation is the process by which one species splits into two or more distinct species.
  • Modes of Speciation:
    1. Allopatric Speciation: Occurs when a population is geographically isolated, leading to reproductive isolation and divergence due to natural selection and genetic drift.
    2. Sympatric Speciation: Occurs within the same geographical area, often due to ecological or behavioral differences, leading to reproductive isolation without geographical separation.
    3. Parapatric Speciation: Occurs when populations are adjacent, with some gene flow, but ecological differences drive divergence.

11. Explain the concept of stabilizing selection with a relevant example.

Answer:
Stabilizing Selection:

  • Stabilizing selection favors individuals with intermediate traits and reduces the extremes of the phenotypic distribution, maintaining the status quo of the population.
  • Example:
    • Human birth weight: Babies with an intermediate birth weight (neither too small nor too large) have higher survival rates than those with extreme weights, leading to the stabilization of this trait in the population.

12. How does the concept of gene flow contribute to evolutionary change?

Answer:
Gene Flow:

  • Gene flow is the transfer of genetic material between populations due to migration or interbreeding.
  • Impact on Evolution:
    • Increases genetic diversity within a population, making it more adaptable to environmental changes.
    • Reduces genetic differences between populations, potentially preventing speciation.

13. How do the fossil records support the theory of evolution?

Answer:
Fossil Records:

  • Fossils provide direct evidence of organisms that lived in the past, allowing scientists to observe changes in species over time.
  • Key Observations:
    1. Transitional Fossils: Fossils that show intermediate stages between two evolutionary stages, such as Archaeopteryx (linking dinosaurs and birds).
    2. Fossil Stratigraphy: Older fossils are found in deeper strata, showing a chronological progression of life forms.

14. Describe the evolutionary significance of sexual reproduction.

Answer:
Sexual Reproduction:

  • Sexual reproduction increases genetic variation in a population, which is critical for evolution.
  • Through meiosis and recombination, sexual reproduction produces genetically diverse offspring, enhancing the chances of survival in a changing environment.

15. How does co-evolution occur? Explain with an example.

Answer:
Co-evolution:

  • Co-evolution occurs when two or more species influence each other’s evolution due to their close ecological interactions.
  • Example:
    1. Flowering plants and pollinators: Flowers evolve traits such as color and scent to attract specific pollinators (bees, birds), and pollinators evolve behaviors that enhance their interaction with flowers.

16. Explain the significance of mutations in evolution.

Answer:
Mutations:

  • Mutations introduce new genetic variations, which are essential for the evolutionary process. While most mutations are neutral or harmful, some can confer advantages under certain environmental conditions, driving natural selection.

17. Discuss the role of geographic isolation in the process of speciation.

Answer:
Geographic Isolation:

  • Geographic isolation occurs when populations are separated by physical barriers (mountains, rivers, oceans), preventing gene flow.
  • Over time, isolated populations accumulate genetic differences, leading to the development of new species. This process is called allopatric speciation.

18. Explain how artificial selection provides insight into the process of evolution.

Answer:
Artificial Selection:

  • Artificial selection is the process by which humans breed organisms for specific traits.
  • It provides a clear example of how selection pressures can drive evolutionary change in a short time.
  • Example: The breeding of dogs, where humans select for specific traits like size, coat color, and behavior.

19. What is the role of environmental factors in shaping evolutionary processes?

Answer:
Environmental Factors:

  • Environmental factors, such as climate, food availability, predators, and habitat, influence the survival and reproduction of organisms, guiding the direction of natural selection.
  • Example: Dark-colored moths in industrial areas have a higher survival rate due to camouflage, while light-colored moths are more vulnerable to predation.

20. How does the concept of convergent evolution challenge the traditional understanding of evolutionary pathways?

Answer:
Convergent Evolution:

  • Convergent evolution occurs when organisms from different evolutionary lineages independently evolve similar traits due to similar selective pressures.
  • Example: The wings of birds, bats, and insects are a result of convergent evolution. Despite different evolutionary ancestors, these organisms developed wings as an adaptation to flight.

21. Explain how adaptive radiation and divergent evolution are related.

Answer:
Adaptive Radiation and Divergent Evolution:

  • Both processes involve the evolution of diverse species from a common ancestor, but adaptive radiation occurs when a single species rapidly diversifies to fill different ecological niches, while divergent evolution involves species that, while sharing a common ancestor, evolve different traits due to different environmental pressures.

22. How does reproductive isolation lead to speciation?

Answer:
Reproductive Isolation:

  • Reproductive isolation prevents interbreeding between two populations, allowing them to evolve independently. Over time, this can lead to the formation of new species.
  • Mechanisms: Pre-zygotic (e.g., behavioral differences, temporal isolation) and post-zygotic (e.g., hybrid sterility) mechanisms ensure reproductive isolation.

23. Discuss the evolutionary significance of polyploidy in plants.

Answer:
Polyploidy:

  • Polyploidy refers to organisms that have more than two sets of chromosomes.
  • In plants, polyploidy can lead to the formation of new species as it creates genetic isolation from diploid populations. Polyploid plants are often more vigorous and can outcompete diploid relatives.

24. What is the concept of genetic variation and its importance in the process of evolution?

Answer:
Genetic Variation:

  • Genetic variation refers to the differences in DNA sequences among individuals in a population.
  • It is critical for evolution because it provides the raw material for natural selection to act upon. Without genetic variation, adaptation to changing environments would be impossible.

25. How do the principles of evolution explain the observed similarities and differences among living organisms?

Answer:
Evolutionary Principles:

  • The principles of evolution, such as common descent and descent with modification, explain the similarities and differences among living organisms.
  • Organisms share common ancestors and evolve over time, leading to both shared features (due to common ancestry) and differences (due to adaptation to different environments).