Natural selection for the first time was used by Darwin. He talked about the similarities and also the dissimilarities among the organisms. These dissimilarities are also known as variation. Natural selection is the way by which nature selects suitable and the fittest organism for evolution. There are some challenges to the organisms in nature. These challenges can be harsh climates, fighting for food, natural disasters like floods, lightning, earthquakes, etc. All organisms possess some kind of characteristics, these characteristics can be a game-changer for some organisms in the course of evolution. This is the basic natural selection definition. We will learn more about Darwin's theory of natural selection, types of natural selection, and the meaning of Darwinism in the below paragraph.
Theory of Natural Selection
Darwin and Wallace worked closely to understand natural selection meaning and have a proper understanding of evolution by natural selection. When there are differences in the reproduction of individuals because of differences in their phenotypes then natural selection takes place. Variations in organisms are caused by mutations or sexual reproduction. Some variations are helpful for the organisms whereas some are not. The variations that can help the organism withstand the harsh environmental conditions are favoured by nature and progress in the course of evolution. The changes in the genomes of organisms can be made when they interact with nature. Generations of this interaction with the environment result in genotype changes and these genotypic changes are reflected in the phenotype of an organism. The way these changes give an advantage to the organism is the sole basis of natural selection which we learned in the natural selection definition.
Darwin Natural Selection
The main concept behind Darwin natural selection is what we read in the theory of natural selection. He presented his theory after Lamarck’s theory. The main points of the Darwin natural selection theory were as follows:
All the successful organisms have high biotic potential or reproductive rate. The organisms produce a large number of offspring that can survive, for example, mice produce dozens of mice at one time. A rabbit produces six young ones in a litter and there are four litters in a year for the rabbit. A rabbit starts producing at the age of six months. Not all, but only some individuals will survive, reach adulthood, and those which reach adulthood reproduce at different rates, and this phenomenon is known as differential reproduction.
Darwin studied the flora and fauna of Galapagos island. It is a chain of islands on the west coast of America. Amazing types of creatures were seen by him there. He found that there were many varieties of small blackbirds in these islands which differed mainly in the shape of beaks and feeding habits. These varieties of small blackbirds were later called Darwin finches. After analyzing them he realized that all of these varieties evolved on the same island itself and were not from separate areas. He gave a conclusion that these varieties have evolved from the original seed-eating ancestor bird which was present in South America. Some of them flew to these islands and according to the needs of different environments, they developed different shapes of beaks and feeding habits. Some became fruit-eating, some insect-eating, some insect-eating, etc.
Antibiotics or Drug Resistance
We continuously use drugs and antibiotics against microbes. Over course of time, some microbes developed resistance against these drugs by bringing changes or modifications in their structures. Those who became resistant survived and hence were selected by nature. Hence this is also an example of natural selection and explanation of the content we read in the above paragraph of the theory of natural selection.
Before the emergence of industrialization in England, white-winged moths were more in number than dark-winged or melanized moths. But after industrialization, in 1920, the condition was reversed. It was seen that in the post-industrial period the number of dark-winged moths increased more than the white-winged moths. This was because before industrialization no air pollution was there so the tree trunks were lighter in colour as no deposition of smoke and soot took place on tree trunks. Under this condition, it was difficult for the predator to spot the white-winged moth against the light background of tree trunks but easy to spot the dark-coloured moth. Hence, the birds killed and ate the dark-winged moths resulting in a decrease in the number of dark-winged moths. But after industrialization tables turned and now as smoke and soot deposited on trees so it was easy to spot the white-winged moths as compared to the dark-winged moths and hence the population of dark-winged moths increased because of natural selection. Natural selection brings evolution but the other species are not completely wiped out.
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Figure: Industrial melanism
Causes of Variation
The mutation is the ultimate source of variation
At the next level is recombination.
The intermingling of two widely separated populations.
Weakness of Darwinism
He was not able to explain the cause of discontinuous variations observed by himself in nature and the mode of transmission of variants to the next generation. In 1868, he gave a theory of “Theory of Pangenesis” which stated that every organ of the body produces minute hereditary particles, called pangenes or gemmules and they are carried through the blood into the organs.