Lamarckism – Postulates and Evidences

Introduction:

Evolution states that distinct types of plants, animals, and other living organisms on Earth have their origin in pre-existing life forms. It is a variation in the inherited characteristics (traits) of biological populations over successive generations. These traits are the expressions of genes that are passed on from parents to offspring in the course of reproduction.

Distinct characteristics exist due to mutations, genetic recombination, and other sources of genetic variation.

Evolution takes place when evolutionary processes such as natural selection and genetic drift act on these variations. The process of evolution has given rise to biodiversity at every level of biological organization.

In simple terms, evolution can be defined as change over time.

Explanation:

Various scientists and biologists have put forward their theories of evolution in their time. Some of these theories are given below:

1. Lamarckism
2. Darwinism
3. Mutation theory
4. Neo-Darwinism

Lamarckism:

Lamarckism was the first theory of evolution which was proposed by Jean-Baptiste de Monet Lamarck in the year 1744-1829. Although the theory was brought to notice in 1801, his famous book “Philo­sophic Zoologique” was published in 1809, in which his theory is discussed in detail.

Jean Baptiste Lamarck (1744–1829) was a French biologist and one of the first to propose that organisms change through time. Lamarck had found numerous lines of ancestry, each in chronological order of older to younger fossils that leads to a living species. He accomplished this by comparing living species with fossil forms of his time.

Lamarck’s theory is based on the principle of inheritance of acquired characters. It states that living things pass acquired characteristics to their offspring. His theory was discredited by most geneticists after the 1930s.

Acquired Characters:

Acquired characteristics are changes developed within the body of an organism in response to the changes occurring in the environment, or in the functioning of organs, in their own lifetime to fulfill their new needs.

Examples of acquired traits include muscle development in athletes, skills like painting, swimming, disability due to an accident, etc.

Postulates of Lamarckism:

Lamarck’s theory contains four main postulates:

1. Effect of environment and new needs.
2. Use and disuse of organs.
3. Organisms are driven to greater complexity.
4. Inheritance of acquired characters.

1. Effect of Environment and New Needs

He stated that the environment influences all organisms. Every organism is found in some kind of environment.

Even a small change in environmental factors like light, temperature, medium, food, air, etc., or migration of animals lead to the origin of new needs in living organisms, especially animals. This leads to the rise of new needs, which in turn produces new structures and changes the habits or behavior of the organisms.

2. Use and Disuse of Organs:

Lamarck said that the organs that are used extensively would become stronger and larger, whereas organs that are not in use will deteriorate.

The new habits may involve a higher use of some organs to meet the new needs, and the disuse or lesser use of some other organs which are useless in new conditions. This use and disuse of organs highly affect the form, structure and functioning of the organs.

According to Lamarck, if an organ is used continuously, it will become more efficient, while the continued disuse of some other organs will lead to its deterioration and ultimate disappearance. So, Lamarckism is also known as the theory of the use and disuse of organs.

An organism acquires certain new traits due to direct or indirect environmental effects during its own life span and is called Acquired or adaptive characteristics.

3. Organisms Driven to Greater Complexity:

Lamarck also thought that evolution takes place as organisms have an innate drive to become more complicated. He stated that as organisms became adapted to their surroundings, they became increasingly complex from the simpler forms.

4. Inheritance of Acquired Characters:

He stated that characteristics acquired by an individual during its lifetime are inherited by their offspring as well.

To prove this, he explained with the example of a blacksmith. A blacksmith has strong arms because of the nature of their work. He proposed that the children of the blacksmith would inherit the development of strong muscles.

Speciation: Lamarck concluded that in every generation, new traits are acquired and transmitted to the next generation. Due to this, new traits accumulate generation after generation, and after a number of generations, a new species is formed.

Evidence Supporting Lamarckism:

1. Elephants: Phylogenetic studies of elephants, horses, and other animals show that all these organisms increase in size in their evolution from simple to complex forms.
2. Giraffe: Lamarck explained his theory with the example of the giraffe. He said that the ancestors of the present-day long-necked giraffe looked like deer with small necks and forelimbs. The gradual elongation of the neck and forelimbs of the giraffe was in response to the deficiency of surface vegetation on the barren ground in dry deserts of Africa. Eventually, these body parts were elongated so as to eat the leaves on the tree branches.

This is an example of the effect of extra use and elongation of certain organs.

3. Snakes: The present-day limbless snakes with long slender bodies developed from their limbed ancestors. According to Lamarck, this happened due to prolonged disuse of their limbs and stretching of the body to go with the creeping mode of movement and fossorial mode of living due to the concern of larger and more powerful mammals. This is an example of disuse and degeneration of some organs.

4. Aquatic birds: Ducks, geese, and other aquatic birds evolved from their terrestrial ancestors by acquiring characteristics such as reduced wings because of continuous disuse and the development of webs between their toes for wading. These changes were induced by a lack of food on land and intense competition. It demonstrates both extra use (skin between the toes) and disuse (wings) of organs.

5. Flightless birds: Flightless birds like ostrich and emu must have evolved from flying ancestors as a result of the continued disuse of wings. This happened because these birds were found in well-protected areas with plenty of food.

6. Horses: The ancestors of the modern horse were short-legged with more functional digits and used to live in areas having soft ground. They gradually migrated to live in areas with dry ground. This change in habitat was accompanied by an increase in the length of legs and a decrease in functional digits so as to run fast over hard ground.

Drawbacks of Lamarckism:

Lamarck’s theory was disproved by a German biologist, August Weismann, who proposed the theory of continuity of germplasm in 1892.

His theory states that environmental factors affect only somatic cells and not germ cells. He stated that variations produced in somatic cells (somatoplasm) are not inherited, while variations produced in germ cells (germplasm) are inherited to the next generation.

As a result, the acquired characters must get lost with the death of an organism, so these would have no role in evolution.

Weismann cut the tails of many rats for about 22 generations and allowed them to breed, but tailless rats were never born.

Pavlov was a Russian physiologist who trained mice to come for food on hearing a bell. He mentioned that this training is not inherited and was necessary for every generation.

Mendel’s laws of inheritance also object to Lamarck’s theory of inheritance of acquired characters.

Similarly, the boring of the pinna of the external ear and nose in Indian women, the small-sized feet of Chinese women, etc. are not transmitted from one generation to another generation.

Eyes that are constantly being used develop defects instead of being improved. Similarly, the size of the heart does not increase generation after generation though it is used continuously. The presence of weak muscles in the son of a wrestler was also not explained by Lamarck.

There are several examples in which there is a reduction in the size of organs, e.g., among angiosperms, shrubs, and herbs that have evolved from the trees.

So, Lamarckism was rejected.