PRINCIPLES OF DARWINISM -
1. OVERPRODUCTION (PRODIGALITY) -
- 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.
2. STRUGGLE FOR EXISTENCE -
- 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.
3. VARIATIONS AMONG INDIVIDUALS -
- 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.
4. SURVIVAL OF FITTEST -
- 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.
5. NATURAL SELECTION -
- This is the basis for formation of new species. Hence, he belived evolution as slow gradual and long time process.
6. PRINCIPLE OF NATURAL SELECTION -
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
|
1.
|
Great potential to reproduce results
|
(a) High number of individuals cause struggle for existence in which only few survive
|
|
|
in population increase exponentially
|
|
2.
|
Most populations stable in size except
|
|
|
for seasonal variation
|
|
3.
|
Limited Natural resources
|
(b) Survival depends upon the hereditary constitution of individual. Those fit as per the environment generate more offsprings
|
|
|
|
|
|
4.
|
There is variation among individuals
|
(c) Favourable characteristic in a population will accumulate gradually through generation
|
|
|
to a large extent
|
|
|
5.
|
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.