Evolution is the change in heritable traits within a population over successive generations. This process provides a framework for understanding the biological world, which presents a paradox: Life is simultaneously unified, sharing deep similarities, and vastly diverse, with millions of distinct species. The theory of evolution resolves this contradiction by proposing that life arose once and has since been continuously modified and branched out. Exploring shared ancestry, adaptation, and the splitting of lineages reveals how a single biological origin led to the immense variety of life observed today.
The Basis of Unity: Common Ancestry
The profound similarities observed across all forms of life, from bacteria to blue whales, point directly to a single, ancient common ancestor. This shared heritage is evident at the most basic molecular and cellular levels. All known organisms use the same universal genetic code, where specific three-base sequences in DNA and RNA code for the same amino acids, demonstrating a common genetic language inherited from the earliest life forms.
This unity extends to core biochemical machinery that powers the cell. Processes like glycolysis, which extracts energy from glucose, and the use of Adenosine Triphosphate (ATP) as the primary energy currency are conserved across all domains of life. The fundamental structure of DNA, RNA, and the ribosome also remains highly similar in all species. These molecular resemblances would be highly improbable if life had originated multiple times independently.
Anatomical comparisons further illustrate this shared history through homologous structures. The forelimbs of mammals, such as the human arm, the cat’s leg, the whale’s flipper, and the bat’s wing, all contain the same arrangement of bones (humerus, radius, ulna, carpals, metacarpals, and phalanges) despite serving vastly different functions. The presence of this identical underlying structure is best explained by descent from a common ancestor whose limb plan was modified over time to suit diverse environments.
The Engine of Diversity: Natural Selection
While common ancestry explains life’s unity, natural selection drives its diversity by promoting adaptation. This mechanism operates based on observable components, beginning with the fact that individuals within any population exhibit natural, heritable variation in their traits, such as size, color, or behavior. Offspring tend to resemble their parents because traits are passed down through genes.
A population typically produces more offspring than the environment can support, leading to a “struggle for existence” where resources are limited. In this environment, individuals with traits that provide even a slight advantage in survival or reproduction—a concept known as differential reproductive success—are more likely to pass their genes to the next generation. For instance, a slightly thicker coat in a cold climate or a more efficient enzyme for digesting a local food source would be favored.
Over many generations, this differential success causes advantageous traits to become more common in the population, leading to adaptation. As different populations inhabit diverse local environments, they face different selective pressures, causing their trait distributions to diverge over time. The adaptation of various finch species on the Galápagos Islands to different food sources, resulting in distinct beak shapes, is a classic illustration of how local selection pressures generate unique traits and promote diversity.
Creating New Branches: The Process of Speciation
The process of speciation is what transforms continuous variation within a lineage into the distinct, separate branches of the tree of life. Biologists define a species primarily through the biological species concept, which states that a species is a group of populations whose members can interbreed and produce viable, fertile offspring. The formation of a new species requires the evolution of reproductive isolation, which prevents gene flow between groups.
Allopatric speciation is the most common mechanism for this separation, beginning when a physical barrier splits a single population into two isolated groups. A new mountain range, lava flow, or river can serve as such a barrier, preventing the exchange of genes between the separated populations. Once isolated, the two groups are subjected to different selective pressures and accumulate independent random genetic changes, such as mutations and genetic drift.
As genetic differences build up, the two populations diverge to the point where they can no longer interbreed, even if the geographic barrier is later removed. This reproductive isolation can manifest as prezygotic barriers, which prevent mating or fertilization, or postzygotic barriers, which result in sterile or non-viable hybrid offspring. The formation of these new species is the source of the planet’s biological richness.
Synthesis: Descent with Modification and the Tree of Life
The integration of common ancestry, natural selection, and speciation explains life’s unity and diversity through “descent with modification.” This phrase, used by Charles Darwin, describes evolution as a branching process where species change over time from a shared ancestor. Unity is represented by the deep root and trunk of the “Tree of Life,” symbolizing the single origin and the shared molecular and cellular machinery inherited by all organisms.
Diversity is represented by the branches extending from this trunk. Each branching point represents a speciation event where a lineage split, and the subsequent length of the branch shows the accumulation of modifications through adaptation. Natural selection fine-tunes organisms to their specific ecological niches, causing the traits of different branches to become specialized and distinct.
This synthesis illustrates that unity and diversity are two inseparable outcomes of the same continuous process operating over geological time. The shared characteristics are the inherited legacy of descent, while the unique characteristics are the result of modification and adaptation to new environments. The resulting Tree of Life is a physical representation of this history, with closely related species sharing more recent common ancestors and exhibiting greater genetic similarity.