Evidence of Evolution

Core Concepts

The fossil record

Fossils are preserved remains or traces of ancient organisms, usually found in sedimentary rock. The record shows a pattern: older rock layers contain simpler, often extinct forms; younger layers contain more recent forms, some clearly transitional between major groups. Famous transitional fossils (commonly taught): Archaeopteryx (a feathered dinosaur with teeth — between reptiles and birds) and Tiktaalik (a fish with limb-like fins — between fish and tetrapods).

Two dating methods work together. Relative dating uses the law of superposition: in undisturbed layers, lower layers are older. Absolute (radiometric) dating measures radioactive decay: an isotope decays into a daughter product at a known rate (its half-life), so the ratio of parent to daughter gives the age. The fossil record has gaps because fossilization is rare — an organism must be buried quickly and preserved — so the absence of a fossil in a layer is not evidence of the organism's absence.

Comparative anatomy

• Homologous structures are inherited from a common ancestor, even if they now serve different functions: the forelimbs of humans, whales, bats, and horses share the same bone pattern (one bone, two bones, many small bones, digits) because they descended from a common tetrapod ancestor. Similarity from shared ancestry.
• Analogous structures serve the same function but evolved independently: the wings of birds and insects, or the streamlined bodies of dolphins and sharks. Similarity from convergent evolution — independent solutions to the same problem, not shared ancestry.
• Vestigial structures are reduced remnants of organs that were functional in ancestors: the pelvic bones of whales and snakes, the human appendix and coccyx, and (commonly taught) the wings of flightless birds such as ostriches. Their existence makes sense only if those species descended from ancestors in which the structures worked.

Embryology

Vertebrate embryos share striking early features: pharyngeal (gill-like) pouches, tails, and similar early body plans. Humans, chickens, and fish all pass through comparable embryonic stages even though the adults look nothing alike. Shared developmental programs point to shared ancestry; differences appear as development proceeds.

Biogeography

Species are distributed in patterns that fit descent with modification. Australia's marsupials (kangaroos, koalas) evolved in isolation because placental mammals never reached the continent — marsupials radiated into the roles placentals fill elsewhere. Island species resemble the nearest mainland species rather than species from similar climates elsewhere — Darwin's finches, Hawaiian honeycreepers, Galápagos tortoises. And closely related species are usually found near each other. Common ancestry plus geographic separation explains these patterns; "each species in its own place" does not.

Molecular biology

DNA and proteins carry the history of descent in their sequences. The genetic code is universal — the same codon table in bacteria, yeast, plants, and people — strong evidence of a single common ancestor. Genes shared across distantly related groups (homeobox/Hox genes controlling body plans, cytochrome c in respiration) show conserved ancestry. More closely related species have more similar sequences: compare the same gene in humans, chimpanzees, mice, and yeast, and the similarity tracks the known family tree. Pseudogenes and endogenous retroviruses are molecular fossils. Molecular clocks use the roughly steady accumulation of neutral differences to estimate when lineages split — with the caveat that rates must be calibrated against the fossil record.

Evolution observed directly

Evolution is not just history; it is observable now. Antibiotic and pesticide resistance spread within decades; Peter and Rosemary Grant's long-term studies documented beak-size changes in Galápagos finches in response to drought (commonly taught); and laboratory populations — fruit flies, and E. coli in the famous long-term evolution experiment started in 1988 (commonly taught) — have been seen to change and even acquire new abilities. These studies confirm that the mechanisms produce the change the fossil record documents over longer timescales.

Common Confusions