Charles Darwin, an English naturalist and biologist, reshaped understanding of life on Earth in the 19th century. He addressed how the immense variety of living organisms, with their distinct characteristics, came to exist. When many believed species were fixed, Darwin proposed a dynamic explanation for the diversity of traits observed in nature. His insights offered a new framework for comprehending relationships among all forms of life.
The Foundation: Variation Within Species
Darwin observed that individuals within a species are not identical; they vary naturally in their traits. This inherent variability was a cornerstone of his theory. For instance, on the Galápagos Islands, Darwin noted finches displayed varying beak sizes and shapes, while tortoises had different shell patterns by island. These differences provided the raw material for environmental forces. He understood these variations were passed to offspring, though he didn’t know the inheritance mechanism.
The Engine of Change: Natural Selection
Darwin’s explanation for how trait differences lead to evolutionary change focused on natural selection. He proposed organisms produce more offspring than their environment can support, leading to a “struggle for existence” due to limited resources. Influenced by Thomas Malthus, he noted not all individuals survive to reproduce. Within this struggle, individuals possess heritable variations, meaning traits are passed to offspring.
Individuals with traits providing an advantage in their environment are more likely to survive and reproduce. For example, a finch with a beak better suited to cracking available seeds would survive better. This “differential survival and reproduction” makes advantageous traits more common over generations. Over time, this accumulation of beneficial traits adapts populations to their surroundings. Natural selection is not a random process; it consistently favors traits that enhance survival and reproduction.
From Small Shifts to New Species: Descent with Modification
Natural selection, over time, leads to what Darwin termed “descent with modification.” This idea suggests all life forms are connected through a common ancestor and diversified through accumulated changes. As populations adapt via natural selection, variations accumulate. Over many generations, these changes can differentiate groups, forming new species.
This divergence from a common ancestor creates a branching pattern of life, often visualized as a tree, with new species splitting off. For example, populations of an ancestral species might separate geographically, facing different environmental pressures. Natural selection would then favor different traits in each isolated group, evolving independently. Eventually, these accumulated differences could make them reproductively isolated, becoming distinct species.
Darwin’s Supporting Evidence
Darwin marshaled various lines of evidence to support his theory of how traits change and diversify.
Artificial Selection
Darwin drew an analogy to artificial selection, observing how human breeders select for desirable traits in plants and animals. This demonstrated that significant trait changes could occur over generations through selective breeding, providing a model for natural processes.
Geographic Distribution
The geographic distribution of species also provided evidence. Darwin noted that species on isolated islands, like the Galápagos, were unique but resembled those on the nearest mainland. This suggested diversification from common ancestors that had migrated.
Comparative Anatomy
Comparative anatomy revealed similarities in the skeletal structures of different animals, like bone patterns in mammal, bird, and reptile limbs. Such homologous structures suggested a shared ancestry, with modifications for different functions.
Fossil Record
Though incomplete in his era, the fossil record offered glimpses of past life forms and implied a succession through geological time. This supported the idea of gradual change and common descent, consistent with his theory.