In the mid 19th century, Charles Darwin (1809 – 1882), formulated the scientific theory of evolution by natural selection, published in his book “On the origin of species” in 1859.
Darwin’s idea were inspired by the observations that he had made during a sea voyage in a sail ship called H.M.S Beagle round the world, from 1831 to 1836.
Natural selection is the process whereby organisms better adapted to their environment tend to survive and produce more offspring. It is a key mechanism of evolution which involves the change in heritable traits of a population over time.
The concept of fitness is central to natural selection i.e., individuals that are more “fit” have better potentials for survival. Herbert Spencer (1820 – 1903) coined the phrase “survival of the fittest”, in his book “Principles of biology” in 1864, after reading Darwin’s book “On the origin of species”.
Alfred Russel Wallace (1823 – 1913) best known for independently conceiving the theory of evolution through natural selection by working in Malay Archepelago. The concept of natural selection was published by Darwin and Wallace in a “Joint presentation of papers” (1858).
Five basic concepts of Darwinism
- Rapid multiplication or overproduction
- Limited resources
- Natural selection
- Speciation (new species formation)
Types of selections
- Stabilizing or balancing selection: It reduces the variations and maintains the mean value in a population, thereby preventing the evolutionary change. For example, selection against homozygous sickle-cell sufferers, and the selection against the standard HgbA homozygotes by malaria.
- Directional or progressive selection: Population changes towards a particular direction, thereby disturbing the mean value in a population. For example, evolution of DDT resistant mosquitoes and industrial melanism (described below).
- Disruptive or diversifying selection: Favors both extremes in a population thereby eliminating most individuals of mean values. It leads to development of two different populations. For example, sexual dimorphism.
Classical example of natural selection: Industrial melanism
The classical example of natural selection is provided by the response of a peppered moth Biston betularia, which is found in all parts of England.
Industrial melanism is an adaptation where the moths living in the industrial areas developed melanin pigment to hide themselves from their predators.
Before the industrial revolution, most of the peppered moths in the UK were white and very few were melanic (black). After their habitats become polluted with soot from the coal-fired industries, the white moths were selectively picked up predators.
On the other hand, dark-colored moths were camouflaged very well by the blackened trees and in turn their population rapidly increased.
Evidence from biogeographical distribution
Darwin studied the climatic conditions, flora and fauna of Galapagos islands, during his voyage around the world.
He noticed nearly 20 related varieties of small birds in these islands which differed mainly in the shape and size of their beaks. These birds are now called Darwin’s finches.
The first clear explanation for these different varieties is allopatric speciation or geographical speciation. It occurs when biological populations of the same species become isolated from each other to an extent that creates hindrance in genetic interchange.
Other observations of natural selection
- Resistance of insects to pesticides.
- Antibiotic resistance in bacteria.
- Heavy metal resistance in plants.
- Transient genetic polymorphism.
- Production of new varieties through artificial selection.
Limitations of natural selection
- Small variations which are not essential also inherited.
- According to this theory, vestigial organs should not be present.
- Evolution of complex internal structures and organs are not explained.
- No clear explanation for the causes and origin of variations.
- No explanation for evolution of terrestrial animals from aquatic animals.