What Is Theory of Evolution by Natural Selection?


Jane Flores

The theory of evolution by natural selection is a fundamental concept in biology that explains how species change over time. This theory was first proposed by Charles Darwin in the mid-19th century and has since become widely accepted as the best explanation for the diversity of life on Earth.

The Basics

At its core, the theory of evolution by natural selection suggests that organisms that are better adapted to their environment are more likely to survive and reproduce. Over time, this process results in changes to a species’ genetic makeup, which can lead to the development of new traits and eventually new species.

Natural Selection

The key mechanism driving this process is natural selection. Natural selection occurs when certain traits give an organism an advantage in surviving and reproducing in its environment.

For example, a bird with a longer beak may be better able to reach food sources than a bird with a shorter beak. Over time, more birds with longer beaks will survive and reproduce, passing on their advantageous trait to their offspring.


This gradual process of change is called adaptation. Over generations, small changes accumulate until they result in significant differences between populations. For example, a population of birds living on an island with hard-to-reach food sources may eventually evolve longer beaks than their mainland counterparts.

Common Ancestry

Another important aspect of evolution by natural selection is the idea of common ancestry. All living organisms share a common ancestor at some point in the past. Even seemingly disparate organisms like bacteria and humans share many similarities at the genetic level.

Evidence for Evolution

There is a wealth of evidence supporting the theory of evolution by natural selection. Fossil records show how species have changed over time, while molecular biology has revealed many shared genetic traits between different organisms.

Transitional Fossils

Transitional fossils are particularly compelling evidence for evolution. These are fossils of organisms that show characteristics of both their ancestral and descendant species. For example, the famous Archaeopteryx fossil shows a combination of reptile and bird features, providing strong evidence for the evolution of birds from reptiles.

Homologous Structures

Homologous structures are another line of evidence for evolution. These are structures that have different functions but share a similar underlying anatomy. For example, the bones in the wings of bats and birds have very similar structures despite being used for different purposes.


The theory of evolution by natural selection has revolutionized our understanding of biology and the natural world. By explaining how and why species change over time, it has shed light on the interconnectedness of all living things and provided insights into some of life’s greatest mysteries.