I. Introduction
- Evolution: The gradual change in living organisms over generations. This means that organisms today are different from their ancestors, and these changes have happened slowly over vast stretches of time.
II. Early Ideas of Evolution
- Before Charles Darwin, most people believed that all living things were created in their present form and remained unchanged. This concept of a fixed and unchanging world dominated for centuries.
III. Theories of Evolution
A. Lamarck’s Theory of Inheritance of Acquired Characters
- Key Points:
- Use and Disuse: Organs used extensively become stronger, while disused organs degenerate. (Think of a blacksmith’s strong arms from constant use.)
- Inheritance of Acquired Characters: Traits acquired during an organism’s lifetime are passed on to offspring. (Imagine a blacksmith’s children inheriting his strong arms.)
- Example (Misconception): Lamarck used giraffes as an example. He believed that stretching their necks to reach higher leaves caused their offspring to inherit longer necks. This is incorrect.
- Limitations:
- There is no scientific evidence to support the inheritance of acquired characters. Traits acquired during life (like strong muscles) don’t change the genetic makeup of an organism.
B. Darwin’s Theory of Natural Selection
- Key Points:
- Variation: Individuals within a species show variations in heritable traits. These variations can be in size, color, beak shape, etc.
- Overproduction: Organisms produce more offspring than the environment can support. Not all offspring will survive to reproduce.
- Struggle for Existence: Competition exists for resources like food, water, and shelter. This leads to a struggle for survival among individuals.
- Survival of the Fittest: Individuals with traits best suited to the environment are more likely to survive and reproduce. “Fittest” doesn’t mean strongest; it means best suited for the environment.
- Natural Selection: Traits that enhance survival and reproduction become more common in a population over generations. Offspring inherit these beneficial traits from their parents.
- Descent with Modification: Species arise from a common ancestor through gradual modification over generations. Natural selection acts on existing variations, leading to new adaptations and eventually new species.
- Example (Peppered Moths): During the Industrial Revolution, pollution darkened tree trunks in England. Light-colored peppered moths became easy prey for birds. Dark-colored moths, better camouflaged on the dark trunks, survived and reproduced more. Over generations, the population of dark-colored moths increased due to natural selection.
IV. Evidence for Evolution
A. Fossil Evidence
- Fossils are the preserved remains of ancient organisms. They provide a record of past life forms and show changes in body structures over time. For example, fossils of dinosaurs show they were very different from any living animals today.
B. Comparative Anatomy
- Studying the body structures of related organisms reveals similarities and differences.
- Homologous Structures: Similar structures in different organisms that have a common origin, even if they serve different functions. (e.g., the forelimbs of humans, bats, and whales have the same basic bone structure, suggesting a common ancestor.)
- Analogous Structures: Structures in different organisms that have a similar function but different origins. (e.g., the wings of birds and insects serve the same purpose of flight, but they have different bone structures.) These similarities and differences support the idea of a common ancestor and evolution.
C. Comparative Embryology
- The early stages of development (embryos) of many related organisms share remarkable similarities. This suggests a common ancestor from which they all diverged. For example, fish, frogs, birds, and humans all have a similar basic body plan in their early embryonic stages.
D. Biogeography
- The distribution of plants and animals across continents can be explained by continental drift and evolution. Similar organisms found on continents that were once joined (e.g., South America and Africa) suggest they evolved from a common ancestor before the continents drifted apart.
V. Conclusion
Evolution is the unifying theory in biology that explains the diversity of life on Earth. It helps us understand how different species arose from common ancestors through gradual changes over vast stretches of time. The theory of evolution by natural selection, supported by various lines of evidence, provides a powerful explanation for the incredible variety of life forms we see today.
Evolution (CBSE Class 10th) Questions
1. What is evolution?
- Answer: The gradual change in living organisms over generations.
2. Briefly describe the concept that existed before Darwin regarding living things.
- Answer: Before Darwin, most people believed that all living things were created in their present form and remained unchanged.
3. Who proposed the theory of Inheritance of Acquired Characters?
- Answer: Jean-Baptiste Lamarck
4. What is the main idea behind Lamarck’s theory of Use and Disuse?
- Answer: Organs used extensively become stronger, while disused organs degenerate.
5. Explain why Lamarck’s theory of Inheritance of Acquired Characters is not widely accepted.
- Answer: There is no scientific evidence to support the idea that traits acquired during an organism’s lifetime can be passed on to offspring.
6. What is variation according to Darwin’s theory of Natural Selection?
- Answer: Individuals within a species show differences in heritable traits.
7. How does overproduction in a population relate to natural selection?
- Answer: Since organisms produce more offspring than the environment can support, there’s competition for resources, leading to a struggle for survival.
8. Explain the concept of “survival of the fittest” in natural selection.
- Answer: Individuals with traits best suited to the environment are more likely to survive and reproduce. “Fittest” doesn’t mean strongest; it means best adapted for the environment.
9. What is natural selection according to Darwin?
- Answer: Traits that enhance survival and reproduction become more common in a population over generations as these traits are passed on to offspring.
10. Define descent with modification.
- Answer: Species arise from a common ancestor through gradual modification over generations due to natural selection acting on existing variations.
11. Give an example to illustrate the concept of natural selection.
- Answer: The peppered moth case during the Industrial Revolution is a classic example. Dark-colored moths, better camouflaged on polluted trees, survived and reproduced more, leading to a population shift.
12. What kind of information do fossils provide about evolution?
- Answer: Fossils are preserved remains of ancient organisms. They show changes in body structures over time, providing a record of past life forms and supporting the idea of evolution.
13. Differentiate between homologous and analogous structures with an example of each.
- Answer: Homologous structures have a common origin but may serve different functions (e.g., forelimbs of humans, bats, and whales). Analogous structures have a similar function but different origins (e.g., wings of birds and insects).
14. How does comparative embryology support the theory of evolution?
- Answer: The early developmental stages (embryos) of many related organisms share remarkable similarities, suggesting a common ancestor from which they all diverged.
15. Explain how the distribution of plants and animals across continents can be evidence for evolution.
- Answer: Similar organisms found on continents that were once joined (e.g., South America and Africa) suggest they evolved from a common ancestor before the continents drifted apart, supporting the concept of continental drift and evolution.
16. Distinguish between evolution and spontaneous generation.
- Answer: Evolution is gradual change over generations, while spontaneous generation is the outdated idea that life arises from non-living matter.
17. What is the role of mutations in evolution? (Although not covered in detail in Class 10th, mutations can be introduced as a source of variation.)
- Answer: Mutations are changes in the genetic makeup of an organism. They can introduce new variations that may be acted upon by natural selection, potentially leading to evolution.
18. How does antibiotic resistance in bacteria demonstrate evolution?
- Answer: When exposed to antibiotics, some bacteria develop resistance due to mutations. These resistant bacteria survive and reproduce, while the non-resistant ones die. Over time, the population becomes dominated by resistant bacteria, showing an evolutionary change.
19. Explain the concept of artificial selection.
- Answer: Artificial selection is the human-driven process of breeding organisms with desired traits. This can lead to the development of new varieties of plants and animals, but it’s different from natural selection as humans are making the selection choices.
20. Why is evolution considered the unifying theory in biology?
- Answer: Evolution provides a framework for understanding the diversity of life on Earth. It explains how different species arose from common ancestors through gradual changes over time, connecting all living things through a shared history.
