Lamarck's theory of acquired characteristics proposed that organisms evolve by passing on traits gained during their lifetime. He believed giraffes' necks lengthened as they stretched to reach leaves, and that these changes were inherited by offspring.

Though flawed, Lamarck's ideas were groundbreaking. He recognized species change over time and environmental influences on organisms. His theory sparked debates and research, paving the way for Darwin's natural selection and modern evolutionary biology.

Lamarck's Theory of Acquired Characteristics

Lamarck's theory of acquired characteristics

• Theory of organic progression proposes organisms become more complex over time with simple forms arising through spontaneous generation
• Use and disuse of organs suggests frequent use strengthens and enlarges organs while disuse leads to weakening and shrinkage
• Inheritance of acquired characteristics posits changes acquired during an organism's lifetime pass to offspring
• Environmental influence drives organisms to adapt through physical changes
• Directed variation occurs in response to needs, not randomly

Examples of Lamarckian inheritance

• Giraffe's long neck developed gradually over generations from stretching to reach higher leaves
• Blacksmith's muscular arms developed through constant use pass on to children
• Blind cave-dwelling animals' eyes degenerate due to disuse in darkness resulting in offspring born with reduced eyesight
• Webbed feet in aquatic birds developed through spreading toes while swimming inherited by subsequent generations
• Snake's elongated body resulted from slithering and not using limbs gradually reduced over time

Strengths vs weaknesses of Lamarckian theory

• Strengths
  • Recognized species change over time
  • Proposed mechanism for evolutionary change
  • Acknowledged environmental influence on organisms
• Weaknesses
  • Lack of genetic understanding led to misconceptions about inheritance
  • Directed evolution incorrectly assumed changes occur in response to needs
  • Failed to explain vestigial structures (appendix)
  • Unable to account for complex adaptations (eye)
  • Lacked empirical evidence to support inheritance of acquired traits
• Conflicts with modern evolutionary theory
  • Natural selection drives evolution instead of need-based changes
  • Random genetic mutations create variation rather than directed changes
  • Population-level changes occur rather than individual adaptations

Historical impact of Lamarck's ideas

• Precursor to Darwin's theory of evolution introduced concept of species change over time
• Catalyzed evolutionary debates stimulating scientific discussions on mechanisms of evolution
• Influenced other scientists inspiring further research and alternative theories
• Contributed to establishing evolution as a scientific discipline
• Impacted philosophy and social sciences influencing ideas about human progress
• Neo-Lamarckism revived Lamarckian ideas in late 19th and early 20th centuries
• Epigenetics emerged as modern field studying heritable changes not caused by DNA sequence alterations
• Reflected scientific understanding and limitations of his time providing historical context for evolutionary thought