List the three kinds of evidence on which Darwin’s theory of natural selection rested andexplain how they helped him advance his argument.

Question

What will be an ideal response?

Answer 1

Fossils: By the early nineteenthcentury biologists in Europe realized that fossilsrepresented earlier forms of life. Darwin knew the principleof faunal (animal) succession, named by an Englishcanal engineer, William Smith (1769–1839), who noticedthat rocks of different ages preserve different assemblagesof fossils and that these assemblages succeeded each otherin regular order. Smith could not explain this, but Darwinused this evidence to support his theory of natural selection, which explains the findings: As organisms evolve,change, and go extinct, they leave behind fossils in layersrepresenting time elapsed. This demonstration proves thatorganisms change over time, rather than being created in aform that does not change.

In Darwin’s day, the fossil record was tantalizingly incomplete;today some fossil lineages are remarkably complete,such as that from the ancestral horse to the modernhorse or that from the land-living ancestors of whales totheir aquatic descendants. Darwin explained that not finding transitionalspecies must be expected, since fossilizationof any organism is extremelyrare. Organisms decomposequickly after death, and to becomefossils they must be covered in sediment,frozen, dried out, or depositedin an oxygen-free environment, assoon as possible. Only those organisms with hard bodyparts and with wide territories could have a chance to berecorded as fossils.

Two years after the publication of The Origin of Species,an important fossil was discovered in southern Germany—the skeleton of a creature called Archaeopteryx. Withfeatures intermediate between living birds and ancient reptiles,it seemed a kind of missing link, although this termis now considered outmoded and has been replaced by theterm intermediate form. About the size of a crow, Archaeopteryxhad birdlike feathers, wings, and large eyes, withreptilian teeth, clawed hands, and a long tail. This fossil confirmed Darwin’s theory in the strongest possibleway, showing that reptiles and birds shared a commonancestor. Several more of these fossils have since been found. Fossils of feathered dinosaurs have also been found,mostly in China.

2. Geographic Distribution: In considering the geographicdistribution of plants and animals, Darwin observedthat climate and environment alone do not accountfor similarity or dissimilarity of inhabitants. For example,Australia, South America, and South Africa between latitudes25 and 35 degrees all contain similar conditions bututterly dissimilar plants and animals. From this and otherobservations Darwin concluded that each species is producedin one area and then migrates from out of that areaas far as it can adapt to conditions.

3. Homologies: Homologies are similarities of form seenin plants and animals. In evolutionary biology, homologyhas come to mean any similarity that is due to shared ancestry.For instance, cats, whales, bats, and humans all have fingers, suggesting that these species are all related despitethe huge differences.

Unexpected similarities between species at the level of embryos are even more astonishing. In its early stages, a human embryo has traits found in fish, amphibians, and reptiles before developing its mammalian characteristics.Darwin explained that adaptive modifications generally areproduced in later stages of growth, leaving the early patternof development unchanged and revealing the naturalrelationships. Since Darwin, biologists have learned thatthese ancestral structures serve as organizers in the ensuingsteps of development.