Chapter 21 - Evolution of Organisms

Abstract

This chapter discusses the main ways in which one can measure evolutionary change and the major environmental factors that influence extinction and emergence of new species. Of the species that have become extinct, records are available for only about 250,000. Fortunately, in recent years, the information that has been derived from classical morphological comparisons has been considerably bolstered by biochemical information. Fossil record supplies some of the missing pieces of the evolutionary tree puzzle. Although mostly limited to existing organisms, the biochemical record has provided a wealth of quantitative information on the relatedness of species and has led to a somewhat different picture from that provided by morphological comparisons. Vertical and horizontal transfers of genetic information both have had major influences on the phylogenetic tree. Since the time of Charles Darwin, it has been conventional to think in terms of genetic transfer being confined within a species. A gene is a hereditary determinant that specifies a certain trait or traits and that behaves as a unit of inheritance occupying one (usually) contiguous region of a chromosome. Mutations are retained or eliminated from the population by the process of natural selection. Asteroid bombardments can result in sudden traumatic changes in the environment. Bombardment of the Earth by asteroids was far more common in the Earth's early history than it is today. At present, oxygen constitutes about 21% of the atmosphere. Carbon dioxide serves as the source of carbon for all organisms, even though the direct acquisition of carbon from carbon dioxide is restricted to a limited number of producer organisms.