Chapter 10. Chapter 10: The Origin and Diversification of Life on Earth

Bacteria-like prokaryotic cells were the first life forms. Fossils of these organisms, 3.4 billion years old, have been found in Australia and South Africa.

The development of a membrane enabled the separation of a group of chemicals from the environment and the evolution of cells. Later, the segregation of chemicals within cells by intracellular membranes allowed chemical reactions of different kinds to occur in different parts of the cell.

According to the biological species concept, a species is a population of organisms that can interbreed or possibly interbreed with each other under natural circumstances and cannot interbreed with organisms from other groups.

The specific epithet, the second part of the species name, indicates the species within a genus and is the narrowest classification of an organism. The specific epithet alone, however, is not used to name an organism; the same specific epithet can be used with different genus names to name different species.

The biological species concept is based on reproductive isolation, due to prezygotic or postzygotic barriers. The morphological species concept relies on physical characteristics or features. The biological species concept is applicable to plants and animals that use sexual reproduction. It is not applicable to organisms that use only asexual reproduction, or to fossilized organisms, since they are no longer living and reproducing. In both cases, the morphological species concept is applicable. There is also difficulty in applying the biological species concept to species that arise over long periods of time, to ring species such as greenish warblers, and to hybridizing species such as butterflies of the genus Heliconius.

Reproductive isolation enables organisms to have independent evolutionary fates, since there can no longer be genetic exchange between the two populations and each population can evolve differently, depending on the specific environmental conditions it encounters.

A phylogenetic tree not only shows relatedness among organisms but also allows biologists to hypothesize about the evolutionary history of life on earth. As new knowledge is discovered, new branches are added to the tree and a more complete understanding of the evolutionary history of life on earth is constructed.

Until the 1980s, evolutionary trees were constructed by comparing physical characteristics between species. Species that shared traits also shared evolutionary heritage. With advances in the late 20th century, biologists began using molecular sequencing, including DNA sequencing, in place of physical traits when constructing evolutionary trees. This method is based on the transmission of DNA from one generation to the next. Organisms that share the same genes also share evolutionary heritage.

Analogous traits are produced through convergent evolution. They do not share a common ancestry but have developed as similar traits evolved to serve similar purposes, through natural selection. Problems arise when constructing evolutionary trees based on similarity of structures. Although similar structures are often a sign of shared heritage, for analogous structures this is not the case.

Microevolution occurs as a result of changes in allele frequencies in a population; this process modifies the genetic makeup of a population simply by changing allele frequency. Macroevolution results in the development of whole new groups of species and may involve many new alleles.

The development of long, flat teeth and a more square jaw is an evolutionary innovation that enabled horses to inhabit and adapt to a wide range of environments where they could find suitable food. When an evolutionary innovation appears, adaptive radiation allows the evolution of many species.

Fossils are an invaluable source of transitional species in the past. Study of the physical features of fossilized organisms allows comparisons with other fossilized organisms and with existing organisms. More recently, genetic analysis has made it possible to examine the genetic relatedness of species, often presenting a clearer picture than anatomical comparisons alone.

Background extinctions occur at lower rates and typically result from natural selection. Mass extinctions do not result from natural selection; they are caused by extraordinary changes in the environment over relatively short periods of time. Typically, mass extinctions are global events, while background extinctions are more local.

Prior to the 1970s and 1980s, classification of organisms depended on comparisons of physical characteristics. When molecular sequencing methods became available, genetic information could be used for classification.

The biological species concept depends on the ability of organisms to reproduce sexually and produce fertile offspring. Bacteria do not reproduce sexually and cannot be defined according to the biological species concept.

Archaeal cell membranes contain polysaccharides not found in bacteria or eukaryotes, and archaeal ribosomes, enzymes, and membranes are more similar to those of eukaryotes than those of bacteria.

The domain eukarya contains single-celled and multicellular organisms with cells containing a membrane-enclosed nucleus.