Theories of Organic Evolution

The scanned notes state that the process of organic evolution has been explained by different scientists through different theories.

Lamarckism

According to page 1 of the scan, it was one of the first theories of evolution and was published by Jean Baptiste Lamarck in 1809 in the book Zoological Philosophy. The notes state that Lamarck’s idea of evolution was based on the observation that changes in the environment lead to changed patterns in species.

The conclusions of Lamarckism are summarized in four points:

Objections / Criticisms to Lamarckism

Page 2 lists the following objections to Lamarckism:

• The source states: “Mutations are not inherited.”
• Eyes that are blind continuously and constantly develop defects instead of being improved, contrary to what the theory of use and disuse of organs would suggest.
• The source says that the strongest blow to Lamarck’s theory came from Weismann, described there as the greatest opponent of Lamarckism.
• Lamarckism explains the improvement or change of a character but not its initiation.

Darwinism – Theory of Natural Selection

The notes identify Charles Darwin (1809–1882) and state that his theory of natural selection was published in 1839. They also state that Darwin published his findings in the book The Origin of Species in 1859, describing its publication as an important event in biological history.

The elements of Darwin’s theory are given as follows:

I. Overproduction

Examples provided on page 3 include:

II. Struggle for Existence

Due to overproduction, there is a struggle for existence. Individuals face both intraspecific and interspecific competition for basic requirements.

The notes also state that organisms struggle against natural calamities and environmental disasters such as:

Only a few organisms are able to obtain the basic requirements. A large number fail to survive or reproduce. As a result, population size remains limited or checked and the number of individuals in a population remains nearly constant.

III. Natural Selection – Survival of the Fittest

The source explains that organisms show variations. Some variations prove beneficial when facing environmental calamities. Individuals with favourable variations have a better chance of survival, while others die.

The notes describe nature as the selecting power that selects the best-fitted individuals and rejects those that are poorly developed. This process is called natural selection.

IV. Variation and Heredity

The offspring of the same parents differ from one another. The notes state that such variations arise over numerous generations.

The source states that variations are important for evolution because some of them may help individuals survive in a particular type of environment while others may not.

V. Origin of Species

According to page 4, favourable variations of the fittest animals are inherited by offspring and accumulate in their progenies.

After several generations over a long period of time, organisms may become so different from individuals of the original species that they constitute a new species.

Objections / Criticisms to Darwin’s Theory of Natural Selection

• It does not explain the causes of variations.
• The source says Darwin considered all variations heritable, whereas it states that only germinal variations are inherited and not somatic variations.
• The theory of natural selection does not give an adequate explanation of the occurrence of vestigial organs generation after generation.
• Darwinism explained the survival of the fittest but not the arrival of the fittest.
• The notes state that natural selection did not explain overspecialization of particular structures which in some cases led to extinction of the race.

Neo-Darwinism – Modern Synthetic Theory of Evolution

The notes mention neo-Darwinians such as Wallace, Haeckel, Weismann and Mendel. According to the source, they gave a very large role to natural selection in evolution.

The scan contrasts Darwin’s view with Neo-Darwinism as follows: Darwin is presented as believing that adaptation results mainly from a single source, natural selection, whereas Neo-Darwinism considers adaptation to result from multiple forces, with natural selection being one of those forces.

The source further states that the theory was later modified by scientists such as Fisher, Huxley, Haldane and Hugo de Vries into the modern synthetic theory of evolution.

The supplied notes include the following concepts:

I. Mutation

The notes state that mutation was first proposed by Hugo de Vries. According to him, new species are formed due to discontinuous variations caused by mutation.

Types of Mutation

II. Genetic Recombination

As a result, new recombinants with new genotypes are produced from already existing genes.

Page 6 states that genetic recombination occurs through:

1. Crossing over during prophase I
2. Independent assortment (selection) of genes at the time of gamete formation
3. Random union of gametes at the time of fertilization

III. Natural Selection in Neo-Darwinism

The source repeats the central natural-selection idea: organisms face continuous competition or struggle, the stronger survive while weaker individuals are eliminated, and favourable variations provide a better chance of survival under environmental challenges.

Nature is described as selecting the best-fitted individuals and rejecting those that are poorly developed. The process is called natural selection.

IV. Isolation

The notes state that such isolation prevents interbreeding among organisms of different species, producing reproductive isolation.

Isolation is described as one of the most significant factors responsible for evolution.

V. Genetic Drift