Biology Today — January 2018

Table : Observations and inferences of principle of natural selection

Observations Inferences

(i) All species have such great potential of

fertility that their population size would

increase exponentially if all individuals

that were born reproduced successfully.

—

(ii) Most populations are normally stable in

size, except for seasonal fluctuations. —

(iii) Natural resources are limited. (a) Production of more individuals than the environment can support leads

to a struggle for existence among individuals of a population, with only

a fraction of offspring surviving each generation.

(iv) Individuals of a population vary extensively

in their characteristics, no two individuals

are exactly alike.

—

(v) Much of this variation is heritable. (b) Survival in the struggle for existence is not random, but depends in part on

the hereditary constitution of the surviving individuals. Those individuals

whose inherited characteristics fit them best in their environment are

likely to leave more offspring than less fit individuals.

(c) The unequal ability of individuals to survive and reproduce will lead

to a gradual change in a population with favourable characteristics

accumulating over the generations.

HUGO DE VRIES’ MUTATION THEORY

• Hugo de Vries, a Dutch botanist proposed the mutation theory of evolution, based on the observations of the experiments
  conducted on Oenothera lamarckiana (evening primrose).

Salient Features of Mutation Theory

• Mutations or (discontinuous variations) are the raw material of evolution.


• Mutations appear all of a sudden and become operational immediately.


• Unlike Darwin’s continuous variations or fluctuations, mutations do not revolve around the mean or normal character of the
  species.


• The same type of mutations can appear in a number of individuals of a species.


• All mutations are inheritable.


• Mutations appear in all conceivable directions.


• Useful mutations are selected by nature whereas lethal mutations are eliminated. However, useless and less harmful ones can
  persist in the progeny.

Evidences against theory

• Natural mutations are not common.


• Most of the mutations are negative or retrogressive.


• Mutation theory does not explain development of mimicry.


• Mutations are generally recessive.

Evidences in favour of theory

• Mutations have genetic basis and are therefore, heritable.


• It explains both progressive and retrogressive evolution.


• Mutations have given rise to new varieties, e.g., Ancon sheep,
  hornless cattle, hairless cats, etc.

Evidences of Mutation Theory