His theory of evolution by natural selection, now the unifying theory of the life sciences, explained where all of the astonishingly diverse kinds of living things came from and how they became exquisitely adapted to their particular environments. Darwin demonstrated that the difference between humans and other animals is one of degree not kind. Darwin’s writings produced profound reactions in geology, zoology, taxonomy, botany, palaeontology, philosophy, anthropology, psychology, literature and theology, many of which are still ongoing.
Darwin presented compelling evidence for evolution in On the Origin of Species  and, since his time, the case has become overwhelming. Countless fossil discoveries allow us to trace the evolution of today’s organisms from earlier forms. DNA sequencing has confirmed beyond any doubt that all living creatures share a common origin. Innumerable examples of evolution in action can be seen all around us, from the pollution-matching peppered moth to fast-changing viruses such as HIV and H5N1 bird flu. Evolution is as firmly established a scientific fact as the roundness of the Earth.
There are two fundamental mechanisms of evolution. Natural selection is only one of them. The less realised mechanism is the random genetic drift. The anti-evolutionists fail to recognise genetic drift. Darwin did not know about it either.
Genetic drift is the random change in the genetic composition of a population due to chance events. It all boils down to how the two alleles, one inherited from each parent, get affected. The smaller a population, the bigger the role of genetic drift.
Evolution does not always increase the chances of a species’ survival. Mutations, which provide the vital mechanism for natural selection, need to be helpful.
Detrimental mutations are a common phenomenon. Species may not survive if such mutations happen rapidly and if they keep accumulating.
Again, if the rate at which mutations happens is slow, then a species may not be able to cope with changing environmental conditions.
The ‘lowly’ sponge and the jellyfish with a few simple cell types have persisted from the Precambrian, and have changed very little. It is the same with fungi, mosses, sharks, and horseshoe crabs.
But the fact is that complex brains — and sophisticated cognition — have evolved from simpler brains multiple times independently in separate lineages.
This can be seen in molluscs such as octopuses; in bony fishes such as goldfish and, separately again, in cartilagenous fishes such as sharks and manta rays; and in reptiles and birds.
On his birthday today let us applaud the contribution of this great scientist to humanity.