Lamarckism: Theory of Inheritance

Lamarckism

• A theory of evolution based on the principle that physical changes in organisms during their lifetime can be transmitted to their offspring.
• Proposed by Jean-Baptiste de Monet Lamarck in 1809 in Philosophie Zoologique.
• The theory describes a two-part mechanism where change is gradually introduced and passed through generations.
• It is also known as the theory of transformation.
• The theory states that all physical changes occurring in an individual's lifetime are inherited by its offspring.

Acquired Characteristics

• An acquired character is produced by the organism's behavior, usually as a response to the environment.
• Ancestral giraffes stretched their necks to reach leaves, and weightlifters develop muscles as examples
• Modern genetics disproves the inheritance of acquired biological characters and that genetic differences are due to random mutations.

Four Postulates of Lamarckism

• Living organisms and their parts tend to increase in size continuously due to internal forces.
• New organs form in response to a new need.
• Continuous use of an organ leads to its development, while disuse leads to degeneration.
• Changes acquired during an individual's lifetime are inherited.
• Aquatic birds developing webbed feet by stretching the skin, limbs disappearing in snakes due to disuse, and the long necks of giraffes are examples.
• Reduced muscles of the external ear in humans compared to animals like dogs and rabbits as another example.

Use and Disuse

• Constant straining to reach high branches could lead to an elongated neck in giraffes.
• Straining toes to swim could lead to elongated, webbed toes in water birds.
• Disuse leads to trait reduction, wings of penguins.

Lamarckian Inheritance

• Acquired traits are passed down to offspring.
• Giraffes with acquired long necks would have offspring with long necks, instead of the shorter necks they were born with.
• Lamarckian inheritance has been disproven by hereditary genetics.
• Evolutionary change occurs gradually and constantly and species start simple and move towards complexity

Criticism of Lamarckism

• Lamarck's theory was subject to severe criticism; two scientists, Cuvier and Weismann, were great critics.

Objections to Lamarckism

• The tendency to increase in size is not universal; there are instances of size reduction.
• New organs do not develop in response to need.
• Changes acquired during an organism's lifetime cannot be inherited.
• Somatic changes are non-heritable, while germplasm changes are inheritable.
• August Weismann removed the tails of mice for 22 generations, but offspring still had tails.
• Mendel's law of inheritance disproved Lamarck's theory.

Neo-Lamarckism

• A modification of Lamarck's theory to align with modern knowledge.
• It stresses the direct effect of changed environments on organisms.
• Only modifications influencing germ cells are transferred to the next generation.
• Adaptations are universal and happen through the relationship of structure, function, and environment.
• Environmental changes alter habits, leading to new structures that become inheritable traits.

Neo-Lamarckian Experiments - McDougall

• McDougall tried to prove that learning is an inheritable acquired character through experiments on rats.
• Rats were trained to escape a 'T' shaped tank with a lighted exit that gave electric shocks and a dark exit with cheese.
• Over generations, rats made fewer mistakes and learned faster, suggesting learning is an acquired character.
• However, later studies found technical mistakes, and the experiment could not be replicated.

Sumner

• Temperature-related changes in mice showed that those in warmer conditions developed larger ears and longer tails, which were claimed to be inherited.
• Similar claims for inherited acquired traits were made by Lindsey, Guyer, Smith, and Kammerer, but critics found technical mistakes.
• The controversy over the inheritance of acquired characters continues, but Lamarck's theory has not been proven correct.