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Chapter 6 Summary & Outline

How Did the Enormous Variety of Species Arise on Earth?

Darwin’s theory of natural selection posits that individuals with adaptive traits produce more offspring, so that species evolve over time. This process of natural selection favors newly arisen genes (mutations) that confer adaptive traits, including behavioral traits. By these gradual changes, all animal species arose from a common ancestor. Review Figure 6.1
Studies of the classification of animals help determine how closely related different species are. Knowing this relationship, in turn, helps us interpret similarities and differences in the behavior and structure of different species. Review Figures 6.3 and 6.4, Web Activity 6.1

Study questions: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13

Size differences in specific brain regions are sometimes related to distinctive forms of behavioral adaptation in different species, such as song repertoires or food storing. Review Figures 6.6 and 6.7
Comparative studies help us understand the evolution of the nervous system, including the human brain. They also provide a perspective for understanding species-typical behavioral adaptations. Review Box 6.2
The nervous systems of invertebrate animals range in complexity from a simple nerve net to the complex structures of molluscs. The nervous systems of certain invertebrates may provide a simplified model for understanding some aspects of vertebrate nervous systems. Review Figure 6.8, Web Activity 6.2

Study questions: 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22

The main divisions of the brain are the same in all vertebrates. Differences among these animals are largely quantitative, as reflected in the relative sizes of nerve cells and brain regions, and the amount of dendritic branching in neurons. Review Figures 6.9 and 6.10

Study questions: 23 | 24 | 25

Fossil endocasts of brains from extinct species indicate that the main result of mammalian evolution has been larger overall brain size.
The brain size of a species must be interpreted in terms of body size. As a rough rule of thumb, vertebrate brain weight is proportional to the 0.69 power of body weight. Review Figure 6.12
Some animals have larger brains and some have smaller brains than the general relation between brain and body weights predicts; that is, they differ in encephalization factor. Humans, in particular, have larger brains than their body size would predict. Review Figure 6.13
Primates have an especially large cortex relative to overall brain size. This relative enlargement of the cortex appears to have evolved because the later stages of brain development are prolonged, resulting in a disproportionately large cortex. Review Figures 6.15 and 6.16

Study questions: 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35

Several factors, including tool use, innovation, and social relationships, are thought to have driven enlargement of the primate cortex. Review Figures 6.17 and 6.18
Not only natural selection, but also sexual selection has been proposed to account for the large size of the human brain, an issue central to evolutionary psychology. Review Box 6.3
Differences between humans and their nearest evolutionary relatives, the chimpanzees, reflect not only the small differences in their genomic DNA sequences but also differences in gene expression patterns. Humans differ from other primates especially in the large number of genes expressed in the brain. Review Figures 6.21 and 6.22

Study questions: 36 | 37 | 38 | 39 | 40

Study questions: 41 | 42 | 43