In this article we will discuss about:- 1. Meaning of Lamarckism 2. Lamarckism — Misunderstood 3. Neo-Lamarckism Theories 4. Baldwin Effect and Neo-Lamarckism 5. Experimental Review.

Contents:

  1. Meaning of Lamarckism
  2. Lamarckism — Misunderstood
  3. Neo-Lamarckism Theories
  4. Baldwin Effect and Neo-Lamarckism
  5. Experimental Review

1. Meaning of Lamarckism:

Lamarck in his book Philosophic Zoologique (1809) conveyed the greatest theory of evolution of his time as a break-through of evolutionism. The full name of this evolutionist is ‘Jean Baptiste Pierre Antonie de Monet, Chevalier de Lamarck’.

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He wrote his belief in the form of soft inheri­tance and emphasized speciation by sponta­neous generation and gradualness of evolu­tion. It is for the first time, the importance of behaviour and environment were recognised in the course of evolution.

He also showed courage to include man in the evolutionary stream. Lamarck is considered as the ‘first evolu­tionist’ by Mayr (1982). He was neglected for a long time. In 1859 Lamarck was redis­covered and his ideas were coined as “Lamarckism”.

First Law of Lamarck (Law of Use and Disuse):

“In every animal…, the more frequent and sustained use of every organ gradually strengthens, develops and enlarges that organ, and gives it a strength proportional to the length of time it has been used; while the constant disuse of such an organ impercep­tibly weakens and deteriorates it, progres­sively diminishing its faculties until finally disappears”.

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Second Law of Lamarck {Inheritance of Acquired Characters (Soft Inheritance)}:

“Everything which nature has caused individuals to acquire or loose as a result of the influence of environmental conditions to which their race has been exposed over a long period of time — consequently, as a result of the effects caused either by the extended use (or disuse) of a particular organ – (all these) is conveyed by generations to new individuals descending therefrom, provided that the changes acquired are com­mon to both sexes, or to those which produce the young”.

Lamarck’s concept of evolution can be summarized sequentially as follows:

(i) Complex from Simple:

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Complex organisms have been developed from simpler forms.

(ii) Appearance of Adaptive Characters:

During the course of evolution organ­isms become adapted to the condi­tions in which they live.

(iii) Continuous Spontaneous Generation:

Replacement of simplest organisms continuously taking place due to spontaneous generation.

(iv) Use and disuse:

During lifetime of an individual the effects of use and disuse of organs either enlarges or reduces, respectively. Continuous use elaborates an organ while continuous disuse incurs the reduction or dege­neration of a definite organ.

Explanation:

(a) Water birds in their effort to swim developed webbed toes,

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(b) The long neck of giraffe was established as it was needed for taking the leaves of the tall trees, and

(c) the cave dwelling ani­mals have lost their eyes through disuse in the darkness of the cave.

(v) Inheritance of Acquired Characters:

The changes that take place in the body of an individual during its life time due to environmental influences or due to use and disuse are termed as “acquired characters”. Such acquired characters are preserved by the repro­duction and inherited to the next generation, producing a new type of individual with better adaptive com­binations.

2. Lamarckism — Misunderstood:

i. Lamarck stressed on the ‘need’ of an organism to the changed environment (‘besoin’). But hasty readers ascribed Lamarck’s theory as a ‘theory of Volition’. They mistranslated the term ‘besoin’ into ‘want’ instead of ‘need’.

The critics also “neglected Lamarck’s careful­ly developed chain of causations from needs → to efforts → to physiological excitations → to the stimulation of growth → to the production of struc­tures” (Mayr, 1982). According to Lamarck, the evolutionary force was not teleological but materialistic.

ii. The misunderstanding about Lamarck’s paradigm is that the idea of a direct induc­tion of new characters by the environ­ment. Lamarck himself rejected this inter­pretations by saying, “Whatever the envi­ronment may do, it does not work any direct modification whatever is the shape and organisation of animals”.

3. Neo-Lamarckian Theories:

The term ‘Neo-Lamarckism” was pro­posed by A. S. Packard in 1884. To oppose Darwinism, several theories were combined and coined as Neo-Lamarckism. This con­cept shared with Lamarckism in two aspects — one is that, the evolution is “vertical” and the other is that, the acquired characters of an individual can be “inherited” (soft inheri­tance).

But this concept ignores the most fundamental component of Lamarckism, i.e., there is a finalistic element in evolution lea­ding phyletic lines of organisms to even greater perfection.

Therefore, according to Mayr ‘Neo- Lamarckism was much a theory of inheri­tance than a theory of evolution’.

Geofroysin, one of the Neo Lamarckian theories, ascribes evolutionary change to the direct influence of the environment.

‘Psycho-Lamarckism” was proposed by Panly (c.f. Hamberger, 1980) which empha­sized mental forces and compared with Lamarck’s “efforts” and “consciousness” to satisfy “needs”. Freud (c.f. Sulloway, 1998) proposed that any mental or psychological development in a generation is hereditarily transmitted to the next generation.

The most important aspect that the Neo- Lamarckians had ignored is the contribution of minimizations and re-combinations as the genetic vehicle of evolution.

4. Baldwin Effect and Neo-Lamarckism:

Baldwin and Morgan tried to bridge up between genetic concept and Lamarckism, and proposed that:

(i) Organisms adapt themselves in the new environment by non-genetic mechanism.

(ii) Random micro-mutations threw up new alleles which produce the required developmental modifications.

(iii) Natural selection increases the fre­quency of new alleles by which the developmental modifications, which was originally acquired, become an inherited one.

5. Experimental Review of Lamarckism:

Different components of Lamarckism were critically reviewed by different authors through experimental supports. The experi­ments with advanced scientific knowledge were mostly performed in the twentieth cen­tury. Some of these experiments are described here under the following headings.

A. Genetic Assimilation of Acquired Cha­racters:

Waddington (1957) performed ‘ two experiments which are as follows:

(i) A laboratory population of Drosophila was treated with heat shock. Some flies produced offspring’s with only one of their wings cross-vein-less. These one cross-vein-less winged flies when inter breed, produced complete­ly cross-vein-less winged offspring’s even when not exposed to heat-shocks.

(ii) Drosophila larvae, exposed to high concentration of salt in their diet, gen­erate large sized anal plates. The pop­ulation of flies produced after several generations in this medium had both a higher salt tolerance and larger anal plates. They retained this phenotype.

Explanation:

The term “genetic assimi­lation” is given to such processes by which the originally acquired characters may be converted into the hereditary ones acting for several generations on population.

From the above experiments it can be deduced that:

(i) The genetic variation was present in the original population, but remained sup­pressed in the environment due to lack of dominance, epistacy, etc.,

(ii) When a stress was added in the environment, the charac­ters which were at sub-threshold i.e., those characters were not exposed, become supra-threshold (characters phenotypically expressed), and

(iii) The selection made the new characters hereditary.

B. Somatic Selection Hypothesis for the Inheritance of Acquired Characters:

August Weisman, the German critic of Lamarckism, removed the tails from mice for many generations. But still the mice in the last generation were born with tails as long as those of the mice of the first generation. He concluded that disuse of tail could not affect its inheritance. The concluding remark from his experiment is known as “Weismann’s doctrine”.

In contravention of this doctrine, Steele (1979) has proposed a genetic mechanism, known as the ‘somatic selection hypothesis’, where by acquired somatic genetic modifica­tions selected in response to environmental changes, may become inherited.

This theory states that clonally selected somatic genetic variants will express these variations so that information (probably mRNA) may be captured by endogenous retroviruses. Which in-turn, will transmit the information across the tissue barriers parti­tioning the soma from the germ-line and infect the germ cells.

In the germ cells the mRNA of retroviruses will be reversely trans­cribed to form DNA. This reverse transcrip­tion is facilitated by RNA-dependent DNA- polymerase (reverse transcriptase). This DNA will be integrated into the appropriate position in the germ-line DNA (Steele et al., 1984). The above hypothesis is best illustra­ted by consideration of the immune system (Steele, 1984).

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