Lamarck’s Theory of Evolution (Lamarckism)

Lamarck’s Theory of Evolution (Lamarckism) – Inheritance of Acquired Characters

Principle:  Newly developed traits (useful or unuseful organs) during the lifetime of an organism passed to the next generation.

Jean-Baptiste Lamarck was a French naturalist who proposed one of the earliest theories of evolution in the early 19th century, prior to Charles Darwin’s theory of “Natural selection”. Lamarck’s theory, often referred to as “Lamarckism” or “The Inheritance of Acquired Characteristics,” postulates that species evolve through the inheritance of acquired traits. Traits are the novel modifications of a changing environment. The environment does not cause change, it triggers and causes the need for change to lead to survival successfully. The new changes may acquire new organs or may lose already existing organs.

Q: What are the two laws of Lamarckism? Causes, exolanations and Examples?

1. The Law of Use and Disuse:

• All organisms had an inner temperament, what Lamarck called a “sentiment Interieur” which caused the performance of actions sufficient to meet the needs (survival) created by a changing environment.
• He proposed that when an organism used a particular organ or body part more frequently, it would become more developed and larger. Ex: Stretched neck and increased length of forelegs of Zirafee to catch the leaves and young branches of large trees for food (figure 1.1).
• Conversely, if an organism did not use a specific organ, it would gradually weaken and eventually disappear. Ex: Disappearance of the tail in apes and man (man evolved from the ancestors of apes, the primates like animals, which consist of the tail as balance organs. In man, while standing and walking on two legs, the forelegs develop as hands, which are the balancing organs (balancing from tail to hands). Another example is the adaptations in five fingers in ruminants which two fingers developed as hoofs and the other three remained as vestigial (not used fingers) (Figure 1.2).

  

2. Inheritance of Acquired Characteristics:

• Lamarck’s most famous idea was that the changes an organism acquired during its lifetime would be passed on to its offspring.
• For example, if a giraffe stretched its neck to reach leaves higher in the trees, it would develop a longer neck over time. This acquired longer neck would then be passed on to its offspring.

3. The Drive for Perfection:

• Lamarck proposed that organisms have an inherent drive to become more complex and perfect over time.
• He believed that this drive was guided by an internal force that led to organisms becoming better adapted to their environments.

4. Continuous Change:

• According to Lamarck, evolution was a continuous and gradual process, occurring over long periods of time.
• He did not propose any mechanism for rapid evolutionary change or speciation.

Critiques and Problems with Lamarckism:

Lamarck’s theory was supported by Hebertspencer and other scientists but bitterly opposed by August Weismann (1883) who propagated “Germplasm Theory”.

@ What are the criticisms of “Lamarck’s Theory”?

1. Lack of Mechanism: Lamarck’s theory lacked a mechanism for how acquired characteristics would be inherited. Yet, later, Darwin’s theory of natural selection provided a clear mechanism for the process of evolution.
2. Inconsistent with Genetic Inheritance: Lamarck’s theory did not account for the role of genetics in inheritance. We now know that traits are primarily passed down through genes, not acquired characteristics.
3. No Evidence for the Inheritance of Acquired Traits: Numerous experiments conducted since Lamarck’s time have failed to provide convincing evidence for the inheritance of acquired characteristics.
4. Insufficient Explanation for Adaptation: Lamarck’s idea that organisms have an inherent drive towards perfection did not explain how organisms become better adapted to their environments, which is a key aspect of evolutionary theory.

Q: Why and how did the Germplasm theory of August Weismann strongly oppose the “Lamarckism”?

 

Inheritance of Acquire characters vs. Germplasm theory (a strong criticism from August Weismann)

 

August Weismann postulated the “Germplasm Theory” which disproves Lamarck’s Theory of evolution. According to Weismann, living organisms are composed of two distinct parts: the germplasm (sex cells) and the soma (the body cells). Only sex cells contain the hereditary material and are involved in reproduction, and whatever the characteristics in the germ cells could be transferred to the next generation, but not the characteristics of body cells.

Experiment on the Rats:

1. Weismann removed the tails of mice, starting with the first generation (parental mice) and continuing the same amputation experiments for the next  22 generations.
2. He observed that, despite tail amputation in the somatic cells of the parents, the offspring continued to develop with tails intact without any changes in the tail shape or length.

Experiment on the Amphibians: Weismann conducted experiments with amphibians, such as salamanders and newts, to demonstrate that cutting off their limbs did not lead to offspring with fewer or extra limbs. This further emphasized that acquired traits resulting from injury or mutilation were not inherited by the next generation, as Lamarckism suggested.

Some other examples shown to disprove the Lamarckism:

1. If a man lost his hand in an accident, Could he give birth to a baby with no hands? Apparently, not.
2. The women of Harappa and Sindhu civilizations wore jwellery on the ears and nose on prickled apertures, which is continuing today! If Inheritance of acquired characters is true, why don’t the females not have the apertures on the ears and nose? not possible.

From the results of experiments and shown examples, Weismann emphasized the “Germplasm” was the only material-determined character in the next generation, but not Lamarck’s “body cell characters”.

Neo-Lamarckism supports Lamarckism with some modifications:

Neo-Lamarckism is a modified or updated version of Lamarckism. Herbert Spencer, Ernest Heckel, McDougall (1938)etc., supported the “Inheritance of acquired Characters” with little changes from their research contributions. According to this, “adaptations” in somatic or body organs are universal in nature and happen through the causal relationship of structure, function and environment. Due to the changes in the environment, the habits and lifestyle of the organism get altered. Neo-Lamarckists may point to epigenetic mechanisms as a potential bridge between “acquired traits and heredity”. Epigenetic changes can alter gene expression without altering the underlying DNA sequence, and some of these changes can be influenced by environmental factors. Thus gradually the organism acquires new structures. The newly obtained character gradually becomes an inheritable trait. In view of the Neo-Lamarckists, August Weismann made a mistake in that, he cut the “useful Organ, the tail” in the rat during his experiments, where the tail is used as a balancing organ, and changes direction while the rat moving and running. An important question from Lamarckism and Neo-Lamarckism to Weimann is “How could an organism lose “Useful Organs?”. If the tail is a “useless organ”, however, automatically the rat could have lost it gradually! In another case, women with holes in their noses and ears to wear the jwellery. The question to Weismann was “What is the use of holes and wearing jwellery on noses and ears for successful survival? Here the women had’nt need to wear jwellery compulsory. Then how could this change occupy the population? Apparently not!  Lamarckism was focused on “whether the organ is Useful or Useless”, and the useful organs developed while the useless organs disappeared gradually! Although Lamarckism and neo-Lamarckism face several challenges and criticisms, their ideas are still a subject of debate and research within the scientific community.