DARWINISM: The main concept of Darwinian theory is natural selection. He presented his theory after Lamarck’s theory. Thomas Malthus essay on ‘populations’ influenced Darwin to develop his theory of evolution.
According to Darwinism, heritable variations and utilization of resources for few enables the organism to adapt to the environment and leave more progeny.
Darwin said that the ‘appearance’ of new life forms is due to the accumulation of variations from one generation to another.
Darwin also talked about the rate (speed) of the appearance of new forms. He said that the appearance of new forms is linked to the life span of that organism. For example, the microscopic microbes have shorter life spans so they divide faster and become millions of individuals within hours. Hence microbes have more chances to evolve faster. But if we talk about any other organism like fish or fowl, they will take millions of years to evolve because their life spans are much longer (i.e., in years) than microbes.
To explain evolution, Darwin talked about two things
1) Branching descent
2) Natural selection
Branching descent explains the ‘pattern’ of evolution and natural selection explains the ‘mechanism’ of evolution. Branching descent and natural selection are the two key concepts of Darwinian theory of evolution.
Branching descent: He said that evolution is a branching process. With time, new life forms appear, and these new life forms arise as ‘branches’ from the previously existing life forms. Hence branching descent points towards the concept of common ancestry.
Natural selection: He emphasized that the selection of ‘useful variations’ by nature is the main (but not exclusive) mechanism for evolution to occur.