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Natural Selection and Adaptations

Class 8
May 19, 2023


Natural selection occurs when organisms with traits best suited to their environments survive and reproduce successfully. This process is sometimes called the “survival of the fittest.”

Natural selection is one of the effective mechanisms of evolutionary change and is responsible for the evolution of adaptive features. It is a force that causes groups of organisms to change over time.


How Does Natural Selection Work?

Natural selection depends on the environment. It does not select the superior trait but determines the beneficial features for survival and reproduction in a specific area and condition. Characteristics that are useful in a particular habitat may be harmful to another. For example, this deer mouse can blend in a forest but will be easy prey in desert areas.

Deer mouse blending in its environment

Natural selection acts only on the existing heritable traits and not on the acquired characteristics. This heritable variation serves as a starting material for natural selection to act upon. The heritable variations can result in random changes or mutations in the genes. The random mutations of genes result in new variants of traits that are heritable.

Whereas acquired traits are the changes developed within the body of an organism in response to the changes occurring in the environment, or in the functioning of organs, in their own life to fulfill their new needs.


For example, the muscular strength of a wrestler is an acquired trait and cannot be passed on to the next generation.

Who Proposed Natural Selection?

The process of natural selection was first described by Charles Darwin in 1859 in his book “On the Origin of Species.” Darwin was an English naturalist of the 19th century whose ideas helped explain how the many varied species on Earth could be descended from a single ancestral species.

The Process of Natural Selection:

Natural selection is a mechanism that occurs whenever certain conditions (size, color, etc.] exist. This mechanism explains how populations evolve (undergo descent with modification) to become better suited to their environments over time.

The mechanism of natural selection involves the following steps:

First, more individuals are born than can survive. Organisms are capable of producing more offspring than their environments can support. This results in competition for the limited resources in every generation.


Second, there is a heritable variation in the population. For example, we know that offspring vary in traits such as size, color, etc. These variations are inherited from parents to offspring.

The sources of genetic variation include mutations, the formation of the gametes, and sexual reproduction. In addition, genetic variation is necessary for evolution by natural selection to occur.

Third, some individuals have traits their environment favors, i.e., such features favor survival and successful reproduction.

Natural selection

For example, in the above image, natural selection favors the trait that produces brown color in beetles, and as a result, their number increases over time.

The process of natural selection

Example of Natural Selection:

Populations keep evolving as they become better adjusted to their surroundings. People can change if their environment changes.

Traits that were once well suited to the old environment may not work well in the new environment. Due to this, new adaptations may evolve. This can be explained by the example of industrial melanism and peppered moths.

Before industrialization, the grey-white-winged moths (Biston betularia) outnumbered the black-winged moths (Biston carbonara) in Britain. These moths are nocturnal and rest on tree trunks during the day. As a result, white-winged moths can blend in well with lichen-covered trees, allowing them to avoid predatory birds.

On the other hand, the black-winged moths resting on lichen-covered tree trunks were easy prey for the predatory birds, and so their number was reduced. During the industrial revolution, numerous industries emerged in Great Britain. The industries emitted sooty smoke that killed the lichens and turned the tree black due to pollution.

This change benefitted the black-winged moth as it blended well with the black tree trunks, and their number increased while the white-winged moth became the prey for predatory birds resulting in a decline in their population.

Industrial melanism


The inherited traits that favor an organism’s survival and reproduction in a specific environment are called adaptations. Adaptations only spread through a population in future generations if natural selection favors them.

Variations or adaptations that benefit an organism’s survival and reproduction increase over time in the population.


Darwin postulated that if groups of organisms having a common ancestor are isolated from other organisms of the same species, natural selection acts on them to become different over generations. As a result, the separated groups adapt to different environmental conditions, and this process can create a new species.

With enough time, this could explain the large number of species on Earth today.

Natural Selection – Increase or Decrease in Specific Traits

Let us see – How does natural selection increase or decrease specific traits in a population?

Darwin gave the example of the Galapagos finches to explain his theory. He stated that the different finch species on other islands evolved from a common ancestor. The variations were developed due to the other environmental conditions of other islands.

Darwin also stated that natural selection results in the increase or decrease of specific traits in a population. For example, the medium ground finch has a comprehensive, robust beak to crack open big, hard seeds. However, a variation is seen in the size of the nib in the finch population.

During drought, when there is less food availability, the beak size increases in the population. When there I enough rainfall and abundant food available, the size of the beak tends not to grow in the population.

Thus, we can say that natural selection leads to the increase and decrease of certain traits in a population.

Variation in beak size depending on the environmental conditions
Mathematical Representation and Increase and Decrease of Traits


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