Although Darwin talked about heritable variation, he did not know exactly how heritable characters arose and were passed on from one generation to another. This is because Darwin was not aware of the principles of genetics developed by Mendel a few years before the publication of his book "Origin of Species".
Incorporation of Mendelian genetics into Darwinian theory later by evolutionary biologists led to the emergence of Neo-Darwinism. Further refinements in the light of advances in population genetics and other areas of biology led to the modern synthetic theory of evolution. It is to Darwin’s credit that his basic theory of evolution by natural selection finds support even in the most recent developments in molecular biology.
With progress in various fields of Biology, the theory of evolution by Natural Selection became more and more acceptable.
The unit of evolution, in the modern synthetic theory of evolution, is the population. It is the population which evolves and not the individual. Variation occurs at the genetic level through mutation and sexual reproduction in the "gene pool" of the population (gene pool means all the different genes in a population of individuals). Natural Selection causes greater reproduction of the variant genes having adaptive advantage.
Evolution at the level of the hereditary material or genes that is the gene pool of a population is termed microevolution. Populations of a species differ due to microevolution. Macroevolution or adaptive radiation is the evolution and diversification at the level of species and genera. For example, Dinosaurs evolved as runners, fliers, swimmers due to macroevolution or adaptative radiation.
Although Darwin and many scientists of his time were convinced that natural selection is the right mechanism by which species evolve, they could not prove it experimentally or otherwise. It was felt that since any visible modification in a species evolves slowly over a long time, the effects of natural selection cannot be demonstrated easily. But, now we know that it can certainly be demonstrated.
1. Industrial melanism in peppered moth (Biston betularia)
Peppered moth is a common moth in England and it occurs in two varietiesa light-coloured variety called typica and a dark-coloured carbonaria. When these moths rest on the light-coloured trunks of trees, the typica moths get nicely camouflaged and cannot be spotted by birds that catch and eat them. But the carbonaria moths being dark against the light background of the bark stand out and are be easily located by the birds and captured. Because of this, the carbonaria moths suffered more predation and therefore always remained in very low numbers in the population.
Then the industrial revolution in England in the mid 19th century brought in many coal-based industries and the resulting soot started depositing on the trunks of trees on the countryside. Soon after, scientists noticed that the numbers of typica moths started going down drastically while those of the carbonaria forms increased.
How did it happen? The soot deposits made the bark black and against the dark background, the carbonaria forms now had the advantage of camouflage whereas the typica moths became more and more vulnerable to bird predation. Consequently, the carbonaria form increased in numbers while typica numbers decreased. This is natural selection in action.
In the industrialized England the dark carbonariavariety of the moth had the selective advantage because they escaped from bird predation more often and lived to leave more offspring for the next generation. Only the best adapted live and leave their genes in the next generation.
2. Evolution of insecticide-resistant mosquitoes
In our desperate efforts to control pest and disease-carrying vector insects, we have been spraying pesticides like DDT in large and larger quantities, but have not been able to eliminate them. When we spray a poisonous chemical on a population of mosquitoes, surely many of them die. But in any population there is variation for resistance and a few genetically resistant individuals survive the spraying.
They breed and produce resistant offspring. In the next generation the proportion of pesticide-resistant individuals is higher. As spraying practice continues, the entire population becomes resistant in a few generations and thus a genetically distinct variety of mosquito has evolved on whom DDT has no effect.