The incredible diversity of life on Earth, from the simplest bacteria to the most complex mammals, is the result of a process that has been unfolding for billions of years: evolution. The core mechanism driving this process, natural selection, was famously proposed by Charles Darwin. I find his theory to be one of the most powerful and unifying ideas in all of science, as it explains how species originate and adapt to their environments.
Table of Contents
This guide will explore the key principles of Darwin’s theory and review the major lines of evidence that scientists use to support it—some of which were used by Darwin himself, and others that have emerged with modern technology.
🔬 The Principle of Natural Selection
Darwin’s theory of evolution by natural selection is built on a few key observations that I find both simple and profound:
- Variation: Individuals within a population are not identical; they have heritable variations in their traits.
- Competition: Organisms produce more offspring than can survive, leading to a struggle for existence for limited resources.
- Adaptation: Some individuals have traits that make them better suited to their environment. These individuals are more likely to survive and reproduce.
- Selection: Over many generations, these favorable traits become more common in the population, leading to the evolution of the species.
In essence, nature ‘selects’ for the traits that provide a survival and reproductive advantage in a particular environment.
🦴 Evidence from Anatomy and Fossils
One of the classic lines of evidence for evolution comes from comparing the anatomy of different species. When I look at the forelimbs of a human, a whale, a bat, and a dog, they are used for very different functions. However, their underlying bone structure is remarkably similar. These are known as homologous structures, and their similarity suggests that these species all inherited this basic bone pattern from a common ancestor.
The fossil record provides another powerful line of evidence. Fossils document the existence of now-extinct species and show a progression of forms over millions of years. For instance, the fossil record clearly shows the evolutionary transition of land mammals into the ancestors of modern whales.
🧬 Evidence from Molecular Biology
With modern technology, we now have what I consider to be the most compelling evidence for evolution: molecular biology. At the most fundamental level, all living things share the same genetic material (DNA), the same genetic code, and the same basic processes for reading genes and building proteins. This universal molecular toolkit points to a shared ancestry for all life on Earth.
We can also compare the DNA sequences of specific genes between different species. The basic idea is that two species have a similar gene because they inherited it from a common ancestor. For example, the gene for the hormone insulin is found in humans, cows, and chickens. By comparing the DNA sequences, we find that human and chimpanzee insulin are about 98% identical, while human and chicken insulin are only about 64% identical. This molecular data strongly supports the idea that humans are more closely related to chimpanzees than to chickens, confirming patterns first suggested by anatomical and fossil evidence.
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