How have scientists changed their mind about Darwin's theory of natural selection?

Darwin's theory was formulated during the Enlightenment. This philosophical movement was characterized by a desire to explain everything by natural cause eliminating any reference to supernatural explanations..

Darwin's theory was designed to explain how the complexity of life as observed could have occurred by natural causes. The type of changes occurring in the present were extrapolated backwards to explain how life could have occurred by accident.

This was in line with the current theories of geology as proposed by James Hutton and Charles Lyell, that the present is the key to the past.

Darwin's theory was depended on ideas that have been called into question since his theory was formulated. One idea was spontaneous generation. It was widely believed in Darwin's time that nonorganic material could change spontaneously into living things.
This idea was disproved by the experiments of Louis Pastour.

Today the idea of biogenesis has replaced spontaneous generation. However there is no consensus or coherent theory of how abiogensis found have occurred.

The other idea is that there is an infinite possibility of variation in the cell. Mendel's laws of genetics had not been discovered or widely known at the time Darwin proposed his theory. When the laws of genetics were widely known, it called Darwin's theory into question.

DeVries theory of mutation seemed to solve the problem. However current information theory and 100 years of experimentation have brought the idea that mutations can create new inventions or information into question.

Darwin's theory or Neo Darwinian theory remains the best explanation of how the complexity of how life could have occurred by natural causes. The assumption of natural cause remains a philosophical position not supported by empirical science.

The theory definitely influenced biologists for a long time. Before its publication species concept was static. Moreover, evolution of one species from an ancestral one defied God's role in creation of human.

The main aspect of Darwinism-- 'descent with modification' would never go out of vogue.

So let us briefly note what was Darwin's theory of evolution. Once we know that we can comment.

1. Living organisms produce progeny in very high rate
2. Food and space for living organisms are limited

So there is struggle for existence, due to intraspecific and interspecific competition. Moreover living organisms must also face and survive environmental calamities.

1. Organisms carry different variations
2. Variations could be helpful or harmful

There is survival of fittest, i.e. organisms carrying more of favorable variations would survive in higher rates.

1. Organisms pass on favorable variations to next generation
2.

Natural selection of only favourable variations through ages

Eventually there will be appearance of new morphology, i.e. evolution of a new species from ancestral species.

Now, let me discuss some of its pitfalls.

• Darwin's theory could not explain origin of variations. He also overlooked Mendel's papers and thus had no idea about the mechanism of inheritence of variations in next progeny.

(Much later De Vries explained origin of variation by mutation.)

• Darwin thought variations are either favorable or not favorable. He could not assess the fact that most variations are neither helpful nor harmful for survival, but the potentiality of such variations may change when the environment starts changing.

(For example lungs that appeared in a fish; in the beginning it neither helped nor did it harm: but when drought like conditions prevailed on earth, such fishes survived and eventually became an ancestral evolutionary stock of all terrestrial tetrapods.)

• Darwin's definition of 'fittest' do not match with the concept of biological fitness.

  Longer life and production of viable offspring in more number are criteria of biological fitness. This will ensure amplification of favorable variations in progeny population.

• Favorable variations actually accumulate in gene pool, hence frequency of alleles, controlling such characters, gradually increase. Darwin talked about individual organisms, but evolution is studied at the level of population.
• Many evolutionists posed a relevant question: is it theory of natural selection or should it be called theory of natural rejection ; because nature actively rejects unfit organisms like those who are sterile, or others with more of unfavorable variations.
• Darwin thought slow but steady selection of favorable variations and continuous rejection of unfavorable variations would change morphology of progeny organisms and thus organisms gradually evolve into new species. Species is no longer defined on the basis of morphological characters.

Speciation really occurs when members of a subpopulation fail to interbreed with members of its ancestral population. Hence, subpopulation/deme is regarded as an evolutionary unit.

Evolutionary scientists today acknowledge few more evolutionary forces other than the natural selection. Most important is genetic drift.