Chapter: Natural Selection 

Understanding where modern organisms come from has long been a subject of human inquiry. Throughout history, various thinkers have proposed explanations based on philosophy, religion, and, more recently, scientific observation. This chapter traces the development of evolutionary thought, beginning with early philosophical ideas and culminating in Darwin’s theory of natural selection and the modern evidence supporting it.

Early Philosophical Views: Plato and Aristotle

The earliest known written ideas about the origin and diversity of life can be traced to Ancient Greece. The philosopher Plato argued that all organisms were created in a perfect form by a divine creator. According to his view, each organism was an imperfect representation of an ideal "type," and variation within a species was insignificant. He maintained that species were unchanging and eternal, reflecting the unchanging nature of their creator.

Aristotle, Plato’s student, shared the belief that species were fixed and unchanging. However, Aristotle introduced a hierarchical framework, ordering life forms based on their perceived complexity and size. This "Great Chain of Being" placed humans at the top, followed by increasingly simple organisms. His system would dominate Western thought well into the 18th century, reinforcing the idea that life forms existed in a static, ranked order.

Lamarck and the Inheritance of Acquired Characteristics

A major departure from typological thinking came with Jean-Baptiste de Lamarck, who was the first to formally propose a theory of evolution. While he retained Aristotle’s idea of a hierarchical chain of life, Lamarck argued that species change through time and progress toward greater complexity. According to Lamarck, simple organisms are continually generated at the bottom of this chain and evolve into more complex forms.

Crucially, Lamarck offered a mechanism for evolutionary change: the inheritance of acquired characteristics. He proposed that an organism’s traits change during its lifetime in response to environmental challenges, and these changes are then passed on to offspring. For instance, Lamarck believed that if a giraffe stretched its neck to reach higher leaves, its offspring would inherit a slightly longer neck. Over generations, these small changes would accumulate.

He extended this concept to humans as well—suggesting, for example, that if a person were to lift weights throughout life, their children might be born with more muscular builds. Though this view is scientifically inaccurate, it marked a significant step by proposing a testable hypothesis about how species change over time.

Lamarck was also among the first to suggest that fitness, or the ability of an organism to survive and reproduce, played a role in evolution. However, his explanation for giraffe neck length still rested on the idea that individuals acquired traits during life rather than inheriting natural variation.

Charles Darwin and the Foundations of Evolutionary Biology

When Charles Darwin began his scientific career, geologists were just starting to study and interpret the fossil record. Darwin was struck by fossils of species that clearly no longer existed, which contradicted the idea of immutable life forms. Moreover, fossils of the same species were being discovered on different continents—South America, Africa, and Australia—despite the impossibility of those organisms crossing oceans.

These observations coincided with the work of Alfred Wegener, who proposed that Earth's continents had once been joined, later drifting apart. This idea challenged the religious belief, dominant at the time, that Earth was only a few thousand years old. Motivated by these puzzles, Darwin set out on a journey to understand the origin of species.

Darwin’s Finches and the Galápagos Islands

At the age of 24, Darwin secured a position as the naturalist aboard the HMS Beagle, a British ship set to circumnavigate the globe. During this voyage, Darwin spent several weeks exploring the Galápagos Islands, located 600 miles from the South American mainland.

Darwin collected many animal specimens, but it was the finches that captured his attention. Although similar in appearance, the finches on different islands exhibited distinct beak shapes adapted to different food sources. Darwin inferred that all these species likely descended from a common ancestor and that natural selection acting on beak shape had led to speciation. This observation became foundational to his theory.

Population Thinking vs. Typological Thinking

Darwin introduced the revolutionary idea of population thinking, contrasting sharply with the typological thinking of Plato and Aristotle. While typological thinking dismissed variation as meaningless, Darwin recognized that variation within populations was the key to understanding evolution. Instead of evolving as individuals, Darwin argued that entire populations change over time as certain traits become more or less common.

Natural Selection

Darwin’s most important contribution is the theory of natural selection. He proposed that in any population, individuals vary in traits such as physical characteristics, behaviors, and abilities. These differences affect an individual’s fitness, or success in surviving and reproducing.

For example, Darwin explained giraffes' long necks as resulting from natural variation in neck length. Individuals with longer necks were more successful at reaching high vegetation and therefore had higher fitness. Over time, these traits became more common in the population.

Alfred Wallace: The Forgotten Co-Author

Although Darwin is often solely credited with the theory of natural selection, Alfred Russel Wallace played a critical role. Wallace, a younger contemporary of Darwin, independently developed a theory of speciation through natural pressures such as droughts, floods, and other environmental events.

In 1858, Wallace sent a letter to Darwin outlining his theory. The similarities were so striking that Darwin was prompted to publish On the Origin of Species, a manuscript he had withheld for nearly 25 years due to concerns about religious backlash. Together, Darwin and Wallace co-authored a short paper presenting the theory of natural selection.

Wallace’s contribution lacked the extensive evidence and data Darwin had amassed, but his work remains an essential part of evolutionary biology’s history.

The Darwin-Wallace Theory of Evolution

The Darwin-Wallace theory fundamentally challenged centuries of belief. Unlike Plato and Aristotle, they argued that species are not fixed but change through time. Unlike Lamarck, they claimed that evolution is not linear or progressive, always moving toward complexity.

Most importantly, their theory was scientific, not just philosophical. It could generate testable predictions through the scientific method, a major advancement in biology.

Descent with Modification

Darwin preferred the phrase descent with modification to describe evolution, avoiding the implication of purposeful direction associated with the word "evolution." The phrase emphasizes two key ideas:

1. All species share common ancestry.

2. Species change gradually over time through inherited modifications.

These principles are now foundational to biology.

The Four Postulates of Natural Selection

Darwin’s theory can be summarized in four key postulates:

1. Variation exists among individuals within a population.

2. Some of these traits are heritable and passed from parent to offspring.

3. More offspring are produced than can survive, leading to competition.

4. Individuals with advantageous traits are more likely to survive and reproduce, increasing those traits in the population.

Although Darwin didn’t understand the genetic basis for heredity, this would later be explained through the work of Gregor Mendel, the father of genetics.

Fossil Evidence for Evolution

Radiometric dating of meteorites suggests that Earth is approximately 4.6 billion years old, and life has existed for about 3.5 billion years. Fossil evidence supports the claim that over 99% of all species that ever lived are now extinct.

Darwin predicted that if his theory were correct, there should be transitional fossils showing gradual changes between ancestral and modern forms. These intermediate fossils have been discovered, such as in the evolution of the horse and the transition from land-dwelling mammals to whales.

Whale Evolution

Fossils show a gradual shift from terrestrial to aquatic life in early whale ancestors. Intermediate forms with partial limb structures support the hypothesis that whales evolved from land mammals.

Vestigial Traits

Vestigial structures are another line of evidence for evolution. These are reduced or non-functional remnants of features that served a purpose in ancestral species. For example, whales possess tiny pelvic bones that resemble leg bones, suggesting a four-legged ancestor.

Evidence of Evolution in Real Time

Though evolution is slow in long-lived species, it can be observed in microorganisms. MRSA, a drug-resistant strain of Staphylococcus aureus, evolved in response to the overuse of antibiotics. Initially easy to treat, S. aureus developed resistance due to the survival and reproduction of bacteria with natural resistance genes.

This case emphasizes the importance of using antibiotics correctly—only when prescribed, and for the full course of treatment.

Homologies: Clues to Common Ancestry

Homologies are similarities inherited from a common ancestor. These include:

• Structural homology: Vertebrate limbs (e.g., in humans, whales, and birds) share a common bone structure.

• Developmental homology: Embryos of vertebrates show striking similarities—humans, for example, have gill slits and tails during early development.

• Genetic homology: Species with similar DNA sequences are more closely related evolutionarily.

Peppered Moths: Natural Selection in Action

The peppered moth (Biston betularia) in northern Europe illustrates natural selection. Before the Industrial Revolution, light-colored moths blended into lichen-covered oak trees. As soot darkened the trees, dark-colored moths gained a survival advantage.

Over time, dark-colored variants became more common due to increased fitness in the polluted environment. This well-documented example supports Darwin’s predictions.

What Evolution Is—and What It Isn’t

Understanding evolution also requires understanding what it is not:

• Evolution is not goal-oriented; there is no plan or "perfect form" being pursued.

• Evolution does not occur in individuals; it occurs in populations over generations.

• Evolution is not progressive; it does not necessarily favor complexity. In fact, some species lose complex traits (e.g., whale hind limbs) if they no longer offer a fitness advantage.