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 attempt to make sense of life’s diversity by arranging organisms in a linear hierarchy from simple to complex. 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.
Figure 1. Plato, Aristotle, and Typological Thinking. Plato and Aristotle viewed species as fixed and unchanging. Their typological approach treated variation within species as unimportant and helped reinforce a static view of life.
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.
Figure 2. Lamarck and the Inheritance of Acquired Characteristics. Lamarck proposed that species change through time and that traits acquired during life could be inherited. Although this mechanism is inaccurate, it was an important early evolutionary hypothesis.
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.
Figure 3. Charles Darwin and Fossil Observations. Fossils of extinct organisms and similar fossil forms on separated continents helped challenge the idea that species were fixed and encouraged Darwin to investigate how species originate.
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.
Figure 4. Darwin’s Finches and the Galápagos Islands. Darwin observed that Galápagos finches had different beak shapes suited to different food sources. These patterns helped support the idea that natural selection can contribute to speciation from a common ancestor.
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.
Figure 5. Population Thinking vs. Typological Thinking. Typological thinking treats variation as unimportant, while population thinking recognizes variation as the raw material for evolution. Populations evolve as traits become more or less common over generations.
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.
Figure 6. Natural Selection. Natural selection occurs when individuals with heritable traits survive or reproduce more successfully than others. Over generations, advantageous traits can become more common in a 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.
Figure 7. Alfred Russel Wallace. Alfred Russel Wallace independently developed a theory of evolution through natural selection and shared his ideas with Darwin. Their joint work helped establish natural selection as a scientific explanation for evolution.
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.
Figure 8. The Darwin-Wallace Theory of Evolution. Darwin and Wallace argued that species change through time by natural selection. Their theory replaced fixed and progressive views of life with a branching, testable scientific explanation.
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:
All species share common ancestry.
Species change gradually over time through inherited modifications.
These principles are now foundational to biology.
Figure 9. Descent with Modification. Descent with modification describes how species share common ancestry while changing gradually through inherited modifications. This concept emphasizes branching relationships rather than purposeful progress.
The Four Postulates of Natural Selection
Darwin’s theory can be summarized in four key postulates:
Variation exists among individuals within a population.
Some of these traits are heritable and passed from parent to offspring.
More offspring are produced than can survive, leading to competition.
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.
Figure 10. The Four Postulates of Natural Selection. Natural selection requires variation, heritability, overproduction of offspring, and differential survival or reproduction. Together, these conditions can cause trait frequencies to change across generations.
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.
Figure 11. Fossil Evidence for Evolution. Fossils provide evidence that many species have gone extinct and that lineages change through time. Transitional fossils show intermediate features between ancestral and later forms.
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.
Figure 12. Whale Evolution. Fossils show that whales evolved from land-dwelling mammal ancestors. Transitional forms preserve intermediate features, including partial limb structures, that document the shift toward aquatic life.
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.
Figure 13. Vestigial Traits. Vestigial structures are reduced remnants of features that served functions in ancestral species. Whale pelvic bones provide evidence that whales descended from ancestors with hind limbs.
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.
Figure 14. Evidence of Evolution in Real Time: MRSA. Antibiotic resistance evolves when resistant bacteria survive treatment and reproduce. Over time, resistance genes become more common in the bacterial population.
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.
Figure 15. Homologies: Clues to Common Ancestry. Homologies are similarities inherited from a common ancestor. Structural, developmental, and genetic homologies help reveal evolutionary relationships among species.
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.
Figure 16. Peppered Moths: Natural Selection in Action. Peppered moth color frequencies changed as industrial pollution altered tree bark color. Moths with better camouflage survived predation more often and became more common.
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.
