20.1 Genes vary in natural populations. Gene Variation Is the Raw Material of Evolution Evolution Is descent with modification The word"evolution"is widely used in the natural and so- cial sciences. It refers to how an entity-be it a social sys- tem,a gas, or a planet-changes through time. Although can be traced to Darwin's On the Origin of Species, the first modem-day okapi leaves, then passed the never act Rather, Darwin used the phrase "descent with modifica- (a) Lamarck's theory: variation is acquired. tion. "Although many more complicated definitions have en proposed, Darwins phrase probably best captures the essence of biological evolution: all species arise from other, pre-existing species. However, through time, they accumu- late differences such that ancestral and descendant species are not identical Natural Selection Is an Important Mechanism of Evolutionary Change reproduction Rather, he followed a long line of earlier philosophers and bom wuals a Darwin was not the first to propose a theory of evolution. naturalists who deduced that the many kinds of organisms have longer round us were produced by a process of evolution. U ke his predecessors, however, Darwin proposed natural Over many generations, selection as the mechanism of evolution, Natural selec- tion produces evolutionary change when in a population uccessful and pass some individuals, which possess certain inherited charac teristics, produce more surviving offspring than individu to their offspring. als lacking these characteristics. As a result, the (b)Darwins theory: variation is inherited tion will gradually come to include more and more individuals with the advantageous characteristics. In this FIGURE 20.2 way, the population evolves and becomes better adapted How did giraffes evolve a long neck? to its local circumstances Natural selection was by no means the only evolution ary mechanism proposed. A rival theory, championed by the prominent biologist Jean-Baptiste Lamarck, was that in the genetic makeup of populations. Allele frequencies evolution occurred by the inheritance of ace cquired can also change as the result of repeated mutations from characteristics. According to Lamarck, individuals one allele to another and from migrants bringing alleles passed on to offspring body and behavior changes ac- into a population. In addition, when populations are small, quired during their lives. Thus, Lamarck proposed that the frequencies of alleles can change randomly as the result ancestral giraffes with short necks tended to stretch their of chance events. Evolutionary biologists debate the rela necks to feed on tree leaves, and this extension of the tive strengths of these processes. Although no one denies neck was passed on to subsequent generations, leading to that natural selection is a powerful force leading to adaptive the long-necked giraffe(figure 20.2a). In Darwin's the hange, the importance of other processes is less certain by contrast, the variation is not created by experi ence, but is the result of preexisting genetic differences among individuals(figure 20.2b) Darwin proposed that natural selection on variants Although the efficacy of natural selection is now widely within populations leads to the evolution of different accepted, it is not the only process that can lead to changes 422 Part vI Evolutionin the genetic makeup of populations. Allele frequencies can also change as the result of repeated mutations from one allele to another and from migrants bringing alleles into a population. In addition, when populations are small, the frequencies of alleles can change randomly as the result of chance events. Evolutionary biologists debate the rela￾tive strengths of these processes. Although no one denies that natural selection is a powerful force leading to adaptive change, the importance of other processes is less certain. Darwin proposed that natural selection on variants within populations leads to the evolution of different species. 422 Part VI Evolution Gene Variation Is the Raw Material of Evolution Evolution Is Descent with Modification The word “evolution” is widely used in the natural and so￾cial sciences. It refers to how an entity—be it a social sys￾tem, a gas, or a planet—changes through time. Although development of the modern concept of evolution in biology can be traced to Darwin’s On the Origin of Species, the first five editions of this book never actually used the term! Rather, Darwin used the phrase “descent with modifica￾tion.” Although many more complicated definitions have been proposed, Darwin’s phrase probably best captures the essence of biological evolution: all species arise from other, pre-existing species. However, through time, they accumu￾late differences such that ancestral and descendant species are not identical. Natural Selection Is an Important Mechanism of Evolutionary Change Darwin was not the first to propose a theory of evolution. Rather, he followed a long line of earlier philosophers and naturalists who deduced that the many kinds of organisms around us were produced by a process of evolution. Un￾like his predecessors, however, Darwin proposed natural selection as the mechanism of evolution. Natural selec￾tion produces evolutionary change when in a population some individuals, which possess certain inherited charac￾teristics, produce more surviving offspring than individu￾als lacking these characteristics. As a result, the popula￾tion will gradually come to include more and more individuals with the advantageous characteristics. In this way, the population evolves and becomes better adapted to its local circumstances. Natural selection was by no means the only evolution￾ary mechanism proposed. A rival theory, championed by the prominent biologist Jean-Baptiste Lamarck, was that evolution occurred by the inheritance of acquired characteristics. According to Lamarck, individuals passed on to offspring body and behavior changes ac￾quired during their lives. Thus, Lamarck proposed that ancestral giraffes with short necks tended to stretch their necks to feed on tree leaves, and this extension of the neck was passed on to subsequent generations, leading to the long-necked giraffe (figure 20.2a). In Darwin’s the￾ory, by contrast, the variation is not created by experi￾ence, but is the result of preexisting genetic differences among individuals (figure 20.2b). Although the efficacy of natural selection is now widely accepted, it is not the only process that can lead to changes 20.1 Genes vary in natural populations. Proposed ancestor of giraffes has characteristics of modern-day okapi. The giraffe ancestor lengthened its neck by stretching to reach tree leaves, then passed the change to offspring. (a) Lamarck's theory: variation is acquired. stretching stretching reproduction reproduction reproduction reproduction Individuals are born who happen to have longer necks. Over many generations, longer-necked individuals are more successful and pass the long-neck trait on to their offspring. growth to adult growth to adult (b) Darwin's theory: variation is inherited. FIGURE 20.2 How did giraffes evolve a long neck?