Evolution of Life Forms | Biology Class 12 - NEET PDF Download

Introduction

Evolution entails the gradual alteration of heritable traits within a biological population across successive generations and extended periods. A population, comprising individuals of the same species residing in a common area and capable of interbreeding, serves as the focal point for these transformative changes.

Theories of Evolution

Various theories have been proposed by different religious groups, philosophers, and scientists, adding depth and intrigue to the subject and sparking curiosity about ourselves and the world.

The Theory of Special Creation, a traditional religious viewpoint, comprises three key tenets:

• All present-day living organisms were created in their current form.
• The diversity among populations has remained constant throughout history and will persist unchanged in the future.
• Earth's inception is believed to have occurred approximately 4000 years ago.

Darwin’s theory of Evolution

• Charles Darwin, a renowned English naturalist, biologist, and geologist, is celebrated for his contributions to evolutionary biology.
• His theory stems from observations made during a global sea voyage aboard the H.M.S. Beagle.
• Darwin emphasized the similarities shared among species.
• All extant life forms exhibit varying degrees of resemblance to one another.
• This resemblance extends not only to present species but also to those that existed millions of years ago.

Darwin’s Theory of Natural Selection

The concept of natural selection centers on the survival of the fittest, widely embraced as a theory explaining how nature selects organisms that are most adapted or reproductively fit. Charles Darwin introduced this theory in his seminal work "On the Origin of Species," published on November 24, 1859.

Natural selection elucidates how variations in a population's traits enable organisms or populations to thrive in their natural environment, including factors such as climate, food availability, and physical conditions. Darwin's emphasis on fitness in his book primarily refers to reproductive fitness, as organisms better suited to their environment tend to produce more offspring compared to those less suited. Consequently, organisms with greater fitness stand a higher chance of being selected by nature.

Example of Natural Selection

Industrial Melanism is a famous example of natural selection. 1850 was the time when England did not have industrialization. The trees of England were populated by two major species of moths; white-winged and dark-winged moths.

Before the onset of Industrial Melanism, the environment hosted both white-winged and dark-winged moths, with the white-winged variety consistently outnumbering the dark-winged counterparts. The white moths enjoyed an advantage due to their lighter coloration, blending effectively with tree trunks for camouflage. Conversely, the dark-colored moths were more conspicuous to predators, leading to higher mortality rates and a decline in their population.

Following the industrialization of England from the 1850s onwards, pollution levels escalated, depositing pollutants onto tree bark. This environmental change favored the dark-winged moths, as their coloration now allowed them to blend seamlessly with the darker background. However, the white-winged moths faced adversity in this altered environment, becoming more visible targets for predators. Consequently, there was a significant decline in the population of white-winged moths.

How does natural selection work in the example of industrial melanism?

Before industrialization, white-winged moths thrived due to their natural ability to blend into their environment. However, the advent of industrial melanism shifted the balance in favor of dark-winged moths. Pollution darkened tree bark, providing a more favorable habitat for dark-winged moths to camouflage effectively. This environmental change resulted in the natural selection of dark-winged moths as the dominant moth variant.

Alfred Wallace supported Darwin’s theory

Alfred Wallace, a renowned naturalist, conducted his research in the Malay Archipelago and independently proposed a theory that mirrored Darwin's, providing further validation for Darwin's ideas. Wallace's theory posited that all present-day species share common ancestors and exhibit similarities. These ancestral forms persisted throughout different geological epochs, periods, and eras. He highlighted the parallelism between Earth's geological history and the evolutionary history of life forms.

Both Darwin's and Wallace's theories converge on the understanding that Earth is billions of years old, and ancestral organisms displayed characteristics akin to those seen in modern organisms.