Chapter Summary

1. The theory of organic evolution received its greatest impetus from Charles Darwin's On the Origin of Species.

2. Before 1900, many biologists, notably Lamarck, believed that organic evolution occurred as a result of the inheritance of acquired characteristics. This theory was discredited experimentally.

3. Evidence in support of organic evolution is drawn from fossils and from the form, ecology, geographical distributions, and relationships of living organisms, homologies, molecular structures, and analogies such as succulent form in deserts.

4. Darwin's natural selection theory is based on four principles: (1) overproduction; (2) struggle for existence; (3) variation and inheritance; and (4) survival and reproduction of the fittest.

5. The primary mechanisms of organic evolution include natural selection, mutations, migration, and genetic drift.

6. Darwin believed that evolution through natural selection was a gradual process over great periods of time. Some contemporary biologists believe that organic evolution has taken place in spurts between long periods of little change, based on evidence from the fossil record. Interpretation of the fossil record, however, can be controversial and will be debated indefinitely.

7. If new genes are produced in a freely interbreeding population, they will gradually be spread throughout the population, and the nature of the whole population will change in time. If a population is divided by a barrier, genes occurring in the one population will not spread throughout the isolated population as before. In time, because of the isolation, each new population may develop into separate species incapable of breeding with one another.

8. Reproductive isolation and hybridization play major roles in the evolution of species, especially in plants.

9. Mechanisms of organic evolution include mutations in chromosomes or genes, hybridization, introgression, polyploidy, apomixis, and reproductive isolation.

10. Opinions and convictions on origins vary, but few can deny the major impact that theories of evolution have had on modern peoples and on their concepts of life.

11. Biologists generally feel evolution is the only plausible explanation for the unity of life at the molecular and cellular level and the great diversity of life, but there is little agreement among them as to the precise pathways of evolution in the past.