Short Summary

Chapter 9, Heredity and Evolution, explains how physical traits are passed from parents to offspring and how genetic variations contribute to evolution and the formation of new species over time. Traits are transmitted through cellular DNA, and Gregor Mendel used garden pea plants, *Pisum sativum*, to demonstrate that traits are controlled by discrete factors called genes that occur in pairs and can show dominant and recessive patterns, producing characteristic ratios of 3:1 in monohybrid crosses and 9:3:3:1 in dihybrid crosses. In human beings, sex is genetically determined by the 23rd pair of chromosomes, with females having XX chromosomes and males having XY chromosomes, and the father's sperm determining the sex of the child. In some other organisms, however, sex may be determined by environmental factors, such as egg temperature in turtles, or may change during an individual's lifetime, as observed in some species of snails. Inherited traits result from changes in the DNA of germ cells that can be passed to future generations, whereas acquired traits developed during an individual's lifetime, such as increased muscle mass or weight loss, do not alter germ-cell DNA and therefore cannot normally be inherited. Speciation is the formation of new species from existing populations and can occur through factors such as reproductive isolation, geographical barriers, natural selection, and genetic drift acting on separated populations. Evolutionary relationships can be studied through homologous organs, which have the same basic structural plan but different functions, analogous organs, which have different structures but similar functions, and fossils, which provide evidence of evolutionary history, such as *Archaeopteryx*, which shows characteristics linking birds and reptiles. Humans also provide an example of artificial selection, as selective breeding transformed wild cabbage into different vegetables such as broccoli, cauliflower, and kale. Molecular evidence and phylogenetic studies indicate that all modern humans, *Homo sapiens*, belong to a single species with origins in Africa.