Explain Darwin's theory of natural selection?

Darwin Theory of natural selection was proposed by Charles Darwin in 1858. Darwin believed all plants and animals had evolved from a few common ancestors by means of natural selection. Plants and animals produce many offspring, but some of the young die before they can become parents. According to Darwin's theory, natural selection determines which members of a species die prematurely and which ones survive and reproduce.

All living things must compete for a limited supply of food, water, space, and other necessities. The individual plants and animals whose variations are best adapted to conditions have an advantage in this struggle. These organisms, on average, tend to leave a larger number of offspring than other members of their group. As a result, the proportion of the group sharing the traits of the best-adapted organisms increases from generation to generation. Scientists use the term fitness to refer to the ability of an organism to reproduce. For this reason, natural selection is often called the "survival of the fittest."

For natural selection to operate, two biological conditions must be met. First, the individuals of a population must differ in their hereditary characteristics. Human beings, for example, vary in almost every aspect of their appearance, including height, weight, and eye colour. People also differ in less-obvious features, such as brain size, thickness of bones, and amount of fat in the blood. These differences have some genetic basis.

The second requirement for natural selection is that some of the inherited differences must affect chances for survival and reproduction. When this occurs, the fittest individuals will pass on more copies of their genes to future generations than will other individuals. Over time, a species accumulates genes that increase its ability to survive and reproduce in its environment.

There are several types of natural selection. They include (1) directional selection, (2) stabilizing selection, and (3) sexual selection.

Directional selection produces new features that help a species adapt to its environment. This type of selection is what most people think of as natural selection.

Stabilizing selection occurs if a species is already well adapted to its environment. In such cases, the individuals with average characteristics leave the most offspring, and individuals that differ most from the average leave fewest. Unlike directional selection, stabilizing selection eliminates extreme characteristics, reducing the amount of variation in a population. Stabilizing selection may actually be the most common type of natural selection.

Sexual selection occurs primarily among animals. Adults of many species prefer mates who display certain behaviors or have certain external features. Sexual selection explains, for example, why males of many bird species have more colorful feathers than the females.