Organic Evolution | Lucent for GK - UPSC PDF Download

Introduction

Organic evolution occurs when organisms undergo changes in shape and behavior between generations. This concept applies not only to individual organisms but also to the evolution of entire populations over time. Two prominent theories of organic evolution have shaped our understanding of this phenomenon.

Darwin's Theory of Evolution

Charles Darwin's theory of evolution, expounded in his groundbreaking book "On the Origin of Species" published in 1859, introduced the concept of "Descent with modification." According to Darwin, the complexity of present-day animals and plants is a result of their development from simpler ancestors through a series of events. He proposed that changes in inherited physical and behavioral features, driven by natural selection, lead to the adaptation of species to their changing environments. This adaptation enhances the survival chances of their offspring.

Darwin's theory finds support from various scientific fields such as genetics, geology, paleontology, and developmental biology. The core tenets of the theory can be summarized as follows:

• All life on Earth is interconnected and related.
• The diversity of life arises from improvements in populations through natural selection.
• Natural selection favors certain characteristics in an environment, leading to the concept of "survival of the fittest."

In one of his remarkable illustrations, Darwin speculated about the transformation of a land mammal into a whale through natural selection. He used the example of North American black bears, known for their swimming technique of catching prey with their mouths open. This hypothetical scenario was later confirmed by scientific evidence, revealing the role of natural selection in the origin of whales.

Lamarckian Theory of Evolution

Jean-Baptiste Lamarck's Lamarckism, proposed in 1801, was the first theory of evolution put forth. Lamarckism encompasses several propositions that contribute to our understanding of organic evolution:

• The constant rise in internal vitality leads to the growth of living organisms and their components.
• Environmental changes influence living creatures, causing them to change and develop new requirements.
• New habits are formed in response to these requirements, which modify the structural characteristics of entities.
• The inheritance of acquired features transfers the qualities developed due to the above factors to subsequent generations.
• Lamarckism finds support in various examples, such as webbed toes in waterbirds, the evolutionary history of giraffes, limb loss in reptiles, flightless birds, flat-bodied fishes, and cave-dwelling organisms.

Evidence for Organic Evolution

Multiple lines of evidence from diverse scientific disciplines support the concept of organic evolution. These evidences reinforce the interconnectedness of all living organisms and their shared ancestry.

• Anatomy: Similar physical characteristics among different species suggest a common ancestor (homologous structures).
• Molecular Biology: The genetic code and DNA reveal the common ancestry of all living things. Comparisons of DNA sequences can determine the degree of relatedness between species.
• Biogeography: The global distribution of creatures and the unique characteristics of species on islands provide insights into the interplay between evolution and geological changes.
• Fossils: Extinct species documented through fossils provide a historical record that connects them to current species.
• Direct Observation: Organisms with short life cycles, such as pesticide-resistant insects, offer tangible evidence of small-scale evolution occurring within our lifetime.

Modern Interpretation: Contemporary Evolutionary Synthesis

Following Darwin's contributions, the integration of genetics and his theory led to the formation of contemporary evolutionary synthesis. This modern interpretation takes into account the understanding of how genes encode specific biological and behavioral features, which can be inherited and tested. Genetic changes at the DNA level, known as mutations, serve as raw material for evolution. While most mutations are neutral or harmful, some rare cases result in positive changes. Over time, these changes aggregate and lead to the emergence of entirely new species, exemplifying macroevolution.