Evidence for Evolution
Describe the main types of evidence for evolution: the fossil record (change over time), comparative anatomy (homologous structures), and the geographic distribution of related species
What a learner can do afterwards
- Explains how fossils form and what they can tell us about past life
- Uses the fossil record as evidence that species have changed over time
- Describes homologous structures, including the five-digit limb plan, and reduced ancestral structures as evidence for common ancestry
- Explains why island species provide evidence for evolution (e.g. Darwin's finches, Galapagos)
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Habitat change, such as flooding by a beaver dam, changes surroundings. Evolution is change in inherited variation in populations across generations. You have met natural selection, one cause of evolution. Fossils, shared body structures and the locations of related species help scientists investigate its history. Scientists can test explanations about the past using evidence without a human eyewitness. They can also observe evolutionary change in populations today.
Fossils are preserved remains or traces of past life. Burial in sediment can protect bones or shells. Minerals may fill spaces in them or replace their material. Impressions, footprints and preserved body material can also be fossils. Fossils reveal features of organisms or their activities. Most organisms leave no fossil, so the record has gaps.
In sedimentary layers that have not been overturned, lower layers were deposited before higher ones. Their fossils are generally older. Comparing fossils from different dated layers reveals changes in the kinds and forms of organisms through time. This supports evolution. It does not mean newer organisms must be more complex, or that every older fossil was a direct ancestor of a later species.
A human arm, bat wing and whale flipper have corresponding bones in a shared arrangement, used for different tasks. These homologous structures support common ancestry. Their limb plan includes one upper-arm bone, two forearm bones, wrist bones and five digits. Pentadactyl means five digits, not five bones.
A vestigial structure has lost or reduced an ancestral function but can retain another use. Whales have reduced pelvic bones that no longer support walking. Alongside fossils and other anatomy, these remnants support ancestry from land-living mammals.
Darwin's finches include related species with different inherited beak traits. Several species can live on one island. Shared features and geographic patterns support descent from an ancestral population that reached the islands. Isolation and different foods helped populations diverge. Natural selection changed inherited traits across generations; individual birds did not reshape their beaks to meet a need.
Dated fossils reveal changes in past life. Shared anatomy, reduced ancestral structures and the distribution of related species support common ancestry.
Lesson, questions and flashcards written for this dataset, with scientific background from OpenStax Biology and Khan Academy.
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What a fossil comparison can tell us
A fossil may preserve part of an organism or a trace of its activity. Bones, shell impressions and footprints provide different information. Bones can reveal body structure. A footprint records contact with a surface. Neither gives a complete account of the organism's life.
Comparing fossils requires attention to time and place. In sedimentary layers that have not been overturned, lower layers were deposited earlier. Scientists can compare their fossils with those in younger layers. Samples from different places add further evidence.
Different forms recorded at different times support change in past life. That conclusion does not require every newer form to be larger or more complex. It also does not make every older organism a direct ancestor of a later one.
The record has gaps. Many organisms decay without leaving fossils, and preserved fossils may never be found. Scientists use the available evidence to test explanations while keeping those limits in view.
Shared structure and shared function
A bat wing and a whale flipper perform different tasks. Underneath, corresponding bones occupy similar positions. Such inherited structural similarities are called homologous structures. They help scientists investigate common ancestry.
The five-digit limb plan contains many bones. Five digits do not mean five bones altogether. The lengths and shapes of corresponding bones can differ while the inherited arrangement remains recognizable.
Sharing a task is a different observation. Insects and bats both fly, but an insect wing is not made from the forelimb bones found in a bat wing. Flight alone does not establish that both inherited wings from a recent ancestor with wings.
Scientists therefore compare the details of structures, rather than counting every resemblance as the same kind of evidence. Fossils and genetic comparisons can add further tests of an explanation about relationships.
Comparing fossils of human relatives
Researchers compare fossil teeth, skulls and limb bones with other fossils and living species. Tooth size and limb proportions can help identify similarities and differences. A fossil may combine features that make its exact relationship difficult to establish.
Human evolution was a branching history. Different human relatives lived during overlapping periods. An older fossil is not automatically our direct ancestor, and a larger brain alone does not establish a closer relationship to us.
Stone tools can provide evidence about activity. Finding tools near fossils does not, by itself, identify which species made them. Researchers must investigate the association. Preserved genetic material, when available, can add evidence about relationships.
A new discovery can change an earlier interpretation. Uncertainty about one fossil's position on a family tree is different from having no evidence for evolutionary change.
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