Evolution is the process of change in the heritable characteristics of biological populations over successive generations. The evidence for these changes comes from many different fields of study, which scientists classify into several major categories. Each category provides a unique and independent line of reasoning, and when combined, they create a detailed picture of the history and ongoing processes of life on Earth.
Evidence from Anatomy and Embryology
The study of anatomy, which involves comparing the physical structures of different organisms, provides evidence of shared ancestry. These comparisons reveal that many animals, while living in different ways, are built upon the same underlying plans.
Homologous structures are features shared by related species because they have been inherited from a common ancestor. A classic example is the forelimb of mammals. Although a human arm, a cat’s leg, a whale’s flipper, and a bat’s wing are used for vastly different functions—grasping, walking, swimming, and flying—they all possess the same fundamental bone structure.
In contrast to homologous structures, analogous structures are features that serve similar functions but do not share a common ancestral origin. These structures arise when different species independently evolve similar solutions to the same environmental challenges, a process known as convergent evolution. For instance, the wing of a bird and the wing of an insect both enable flight, but they are constructed very differently and evolved independently. Bird wings are modified forelimbs with bones, while insect wings are extensions of their exoskeleton.
Some organisms retain vestigial structures, which are reduced in size and have little or no discernible function in the modern organism. These are remnants of features that served a purpose in an ancestor. The human appendix and the pelvic bones found in some whale species are prime examples. The pelvic bones in whales are leftovers from their four-legged, land-dwelling ancestors.
Embryology, the study of the development of organisms from embryo to adult form, reveals further evidence. In their early stages of development, the embryos of different vertebrates, such as fish, reptiles, birds, and humans, exhibit similarities. For example, all vertebrate embryos, including humans, have gill slits and a tail at some point in their early development. In fish, the gill slits develop into gills, but in most terrestrial vertebrates, they are modified for other purposes, such as parts of the jaw and inner ear in humans.
Evidence from the Fossil Record
The fossil record acts as a historical archive, preserving the remains or traces of past life in layers of rock. This record provides a tangible timeline of evolution, showing how organisms have changed over vast stretches of geological time.
A principle used to interpret the fossil record is the law of superposition, which states that in undisturbed layers of sedimentary rock, the oldest layers are at the bottom and the youngest are at the top. This chronological sequencing allows paleontologists to establish the relative age of fossils. Fossils found in deeper strata represent earlier forms of life, while those closer to the surface are more recent.
Among the finds within the fossil record are transitional fossils, which exhibit traits common to both an ancestral group and its more derived descendant group. These fossils capture evolutionary snapshots of change, providing direct evidence of the links between different major groups of organisms.
An example of a transitional fossil is Archaeopteryx, which lived about 150 million years ago. Its fossilized skeletons display a mosaic of features found in both non-avian dinosaurs and modern birds. Archaeopteryx had dinosaur-like characteristics such as a jaw with sharp teeth, a long bony tail, and three clawed fingers on its hands. At the same time, it possessed distinctly bird-like features, including well-developed feathers structured for flight. This combination makes Archaeopteryx a clear bridge between these two groups.
Evidence from Geographic Distribution
The study of how life is distributed across the planet, a field known as biogeography, offers evidence for evolution. The patterns of where organisms are found and not found are best explained by the theory of evolution in conjunction with major geological events like continental drift.
Island biogeography provides examples of evolution in action. Islands are often home to many endemic species—species found nowhere else—which have evolved in isolation. The finches of the Galápagos Islands, famously studied by Charles Darwin, are a primary example. A single ancestral finch species from the South American mainland colonized the islands millions of years ago. Over time, populations on different islands adapted to unique local conditions, leading to the diversification into more than a dozen distinct species, each with a specialized beak.
The history of Earth’s continents also plays a role in the distribution of organisms. Broad groups of organisms that evolved before the breakup of the supercontinent Pangaea are found worldwide. In contrast, groups that evolved after the continents separated have more localized distributions. The prevalence of marsupial mammals in Australia is a classic case. After Australia drifted and became isolated, its marsupial populations evolved without competition from the placental mammals that came to dominate elsewhere.
Evidence from Molecular Biology
Modern molecular biology has provided detailed and quantifiable evidence for evolution. By examining the fundamental molecules of life, like DNA and proteins, scientists can uncover relationships that might not be apparent from anatomy alone.
The universal genetic code is a piece of evidence for evolution. Virtually all life on Earth uses the same molecular system to store and translate genetic information. Information is encoded in DNA using the same four nucleotide bases, and this information is transcribed into RNA and then translated into proteins using the same codons to specify the same amino acids.
Comparing the DNA sequences of different species offers a direct way to measure their evolutionary relatedness. The degree of similarity between the DNA of two species is a reflection of how recently they shared a common ancestor. For instance, comparisons of the human and chimpanzee genomes reveal a high degree of similarity. Their DNA sequences are often cited as being between 98% and 99% identical for protein-coding regions, indicating a very recent common ancestor.
Similar comparisons can be made with proteins, which are built from the instructions in DNA. Homologous proteins, such as cytochrome c, are found in a wide range of species. Cytochrome c is a protein involved in cellular respiration, and its amino acid sequence has changed over evolutionary time due to mutations. By counting the number of amino acid differences between the cytochrome c of different species, scientists can infer their evolutionary relationships. This method acts as a “molecular clock,” providing estimates for how long ago species diverged from one another.
Evidence from Direct Observation
Evolution is not just a process that occurred in the distant past; it is an ongoing phenomenon that can be observed directly. In populations with short generation times, such as bacteria and insects, evolutionary changes can be witnessed over a matter of years or even months.
A modern example is the evolution of antibiotic resistance in bacteria. When a population of bacteria is exposed to an antibiotic, most are killed, but individuals with a random mutation that confers resistance may survive. These resistant bacteria then reproduce, passing the trait to their offspring. Over time, and with the continued use of the antibiotic, the resistant strain becomes dominant in the population.
A similar process is observed with pesticide resistance in insects. When a new pesticide is introduced, it is often effective at first. However, a few insects in the population may have a pre-existing genetic trait that allows them to survive the chemical application. These survivors reproduce, and the resistance gene spreads through the population. Because insects have short life cycles, this can happen very quickly, sometimes in just a few years, rendering the pesticide ineffective.
A historical example of observed evolution is the peppered moth in England during the Industrial Revolution. Before this period, most peppered moths were light-colored, camouflaging them against lichen-covered trees, while a rare, dark-colored form was easily spotted by predatory birds. As factories released soot that blackened the tree trunks, the light-colored moths became conspicuous, while the dark moths became well-camouflaged. Consequently, the frequency of the dark form increased dramatically. With the later implementation of clean-air laws, the trend reversed, and the light-colored moths once again became dominant.