Principles of darwinism, Biology



  • All organisms have enormous fertility and they multiply in geometric ratio. Examples of over production and rapid multiplication are as follows -
  • A female rabbit gives birth to six young ones in one litter and produces four litters in a single year. If all rabbits were survived and multiplied at this rate, their number would be very large after sometime.
  • Elephant is considered as slowest breeder, which matures at the age of 30 years and alive for about 90 years age.
  • Each female gives rise to about six offsprings and if all survived in 750 years, a single pair would produce about 19 million elephants.


  • Populations tend to increase in size until the environment supports no further increase and equilibrium is reached.
  • The population fluctuates around this equilibrium. Hence population sizes generally remain approximately con- stant over a period of time related to length of animal's life cycle.
  • The competitions between individuals for environment resources create a struggle for existence. It is three types:
  • Intraspecific : A struggle between the individual of the same species for similar necessities like food, shelter breeding place, light, water etc. is referred to as intra-specific struggle. e.g., Cannabolism.
  • Interspecific : It is a struggle that takes place between the individuals of different species for similar require- ments. E.g., Hunter-prey interaction.
  • Extraspecific or environmental conditions: The natural forces and environmental conditions like excess of mois- ture or drought; heat or cold, earthquakes and volcanic eruptions wipe out thousands of organisms of a species.


  • No two organisms are exactly alike.
  • Under a given set of environment conditions certain individual of a species posses variations tending to adjust them very advantageously to external conditions, whereas other individuals of the same species possess less favourable variations and thus are at a disadvantage in relation to the same set of environment factors.
  • Variations are small morphological, physiological and behavioristic differences among the individuals.


  • In the struggle for existence, individuals with favourable or useful variations would be survivors in the competitions, while those with less favourable variation would die.
  • This is called as survival of fittest by natural selection i.e., nature selects only those individuals of a particular species that are capable of surviving in the changed conditions.


  • This is the basis for formation of new species. Hence, he belived evolution as slow gradual and long time process.


Ultimately the Natural Selection is based on five observations and three inferences as suggested by Ernst Mayr (1982) in the form of following table -

OBSERVATION                                                                                                   INFERENCES


Great potential to reproduce results

(a) High number of individuals cause struggle for existence in which only few survive


in population increase exponentially


Most populations stable in size except


for seasonal variation


Limited Natural resources

(b) Survival depends upon the hereditary constitution of individual. Those fit as per the environment generate more offsprings





There is variation among individuals

(c) Favourable characteristic in a population will accumulate gradually through generation


to a large extent



Much of these variations are inheritable


Population become well adapted to their local environments through natural selection.

Natural selection is the differential success in reproduction and its product is adaptation of organisms to their environment; it is the result of interaction between environment and the variability.

Posted Date: 10/9/2012 1:18:31 AM | Location : United States

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