The Introduction to Theory of Evolution
Newts and other salamanders can move only slowly, so they are easy prey for garter snakes. But some salamanders have evolved defensive toxic chemicals that make them less desirable as prey. The rough-skinned newt, Taricha granulosa, is a salamander that lives on the Pacific Coast of North America. Taricha sequesters in its skin a potent neurotoxin called tetrodotoxin (TTX). TTX paralyzes nerves and muscles by blocking sodium channels. Most snakes die if they eat a rough-skinned newt, but some populations of the garter snake Thamnophis sirtalis have evolved TTX-resistant sodium channels in their nerves and muscles. These snakes are able to eat the newts and survive — but the addition to their diet comes at a price. TTX-resistant snakes can crawl only slowly for several hours after eating a newt, and they never crawl as fast as nonresistant snakes. Thus, TTX-resistant snakes are more vulnerable to their own predators. Pufferfish, octopuses, tunicates, and some species of frogs also use TTX as a defensive chemical. Many other species use a variety of chemicals to defend themselves against predators and many predators have evolved resistance to those chemicals.
But production of and resistance to defensive chemicals like all other adaptations, has costs as well as benefits. Such adaptations may impose a cost in the form of speed of movement, as they do on garter snakes. They may reduce the ability of the organism to function efficiently, or they may be energetically costly to develop and maintain. That is, to improve its performance in one area, the organism must accept reduced performance in some other area—a trade-off.
Biologists try to identify and measure the tradeoffs that different adaptations impose because the nature and strength of these trade-offs influences how adaptations evolve. If there were no cost to TTX resistance, then snakes that live in places where toxic newts are rare would probably also be resistant to TTX — which they are not.
Charles Darwin’s main contribution to biology was to propose a plausible and testable hypothesis for a mechanism that could result in the adaptation of organisms to their environments. In effect, Darwin offered a mechanistic explanation for the evolution of life on Earth, the last component of the known universe that lacked such an explanation. The mechanism that Darwin proposed can explain the evolution of all forms of life, including humans. It has been difficult for many people to accept that the same processes that determined the evolutionary pathways of other species also guided human evolution, but as Darwin noted, “there is grandeur in this view of life.” In this chapter we will see how Darwin developed his ideas, and then turn to the advances in our understanding of evolutionary processes since Darwin’s time. We will discuss the genetic basis of evolution and show how genetic variation within populations is measured. We will describe the agents of evolution and show how biologists design studies to investigate them. Finally, we will discuss constraints on the pathways evolution can take. When you understand these processes, you will understand the mechanisms of evolution.
2.3.1. Charles Darwin’s Theory of Evolution
As a youth, Charles Darwin was passionately interested in natural history — the study of how different organisms carry out their lives. He briefly studied medicine at Edinburgh but he was nauseated by observing surgery conducted without anesthesia. He gave up medicine to study for a career as a clergyman of the Church of England at Cambridge University. However, he was more interested in natural history than theology, and he became a companion of scientists on the faculty, especially the botanist John Henslow. Darwin was given an unprecedented opportunity when in 1831 Henslow recommended him for a position as ship’s naturalist on the H.M.S. Beagle which was preparing for a survey voyage around the world. Whenever possible during the 5-year voyage, Darwin (who was often seasick) went ashore to observe and collect specimens of plants and animals. He noticed that the species he saw in South America differed strikingly from those of Europe. He observed that the species of the temperate regions of South America (Argentina, Chile) were more similar to those of tropical South America (Brazil) than they were to European species. When he explored the Galápagos Islands, west of Ecuador, he noted that most of its animal species were found nowhere else, but were similar to those of mainland South America, 1,000 kilometers to the east. Darwin also recognized that the animals of the archipelago differed from island to island. He postulated that some animals had dispersed from mainland South America and then evolved differently on each of the islands. When he returned to England in 1836, Darwin continued to ponder his observations. Within a decade he had developed the major features of his theory which had two major components:
- Species are not immutable; they change over time. (In other words, Darwin asserted that evolution is a historical fact that can be demonstrated to have taken place.)
- The agent that produces these changes is natural selection. Darwin wrote a long essay on natural selection and the origin of species in 1844, but, despite urging from his wife and colleagues, he was reluctant to publish it, preferring to assemble more evidence first.
Darwin’s hand was forced in 1858 when he received a letter and manuscript from another traveling naturalist, Alfred Russel Wallace, who was studying plants and animals in the East Indies. Wallace asked Darwin to evaluate the manuscript, in which Wallace proposed a theory of natural selection almost identical to Darwin’s. At first Darwin was dismayed, believing that Wallace had preempted his idea. But parts of Darwin’s 1844 essay, together with Wallace’s manuscript, were presented to the Linnaean Society of London on July 1, 1858, thereby giving credit for the idea to both men. Darwin then worked quickly to finish his own book, The Origin of Species, which was published the next year. Although both men conceived of natural selection independently, Darwin developed his ideas first, and The Origin of Species provided an enormous amount of evidence from many fields to support both the concept of natural selection and evolution itself, which is why these concepts are more closely associated with the name Darwin than Wallace. The facts that Darwin used to conceive and develop his theory of evolution by natural selection were familiar to most contemporary biologists. His unique insight was to perceive the significance of relationships among them. On September 28, 1838, Darwin happened to read An Essay on the Principle of Population by Thomas Malthus, an economist. Malthus argued that because the rate of human population growth is greater than the rate of increase in food production, unchecked growth inevitably leads to famine. Darwin recognized that populations of all species have the potential for exponential increases in numbers. To illustrate this point, he used the following example: Suppose…there are eight pairs of birds, and that only four pairs of them annually…rear only four young and that these go on rearing their young at the same rate, then at the end of seven years…there will be 2048 birds instead of the original sixteen.
Yet such rates of increase are rarely seen in nature. Therefore, Darwin reasoned that death rates in nature must also be high. Without high death rates, even the most slowly reproducing species would quickly reach enormous population sizes. Darwin also observed that, although offspring tend to resemble their parents, the offspring of most organisms are not identical to one another or to their parents. He suggested that slight variations among individuals significantly affect the chance that a given individual will survive and reproduce. Darwin called this differential survival and he called reproduction of individuals natural selection. Darwin may have used the words “natural selection” because he was familiar with the artificial selection of individuals with certain desirable traits by animal and plant breeders. Many of Darwin’s observations on the nature of variation came from domesticated plants and animals. Darwin was a pigeon breeder and he knew firsthand the astonishing diversity in color, size, form and behavior that pigeon breeders could achieve (Figure). He recognized close parallels between selection by breeders and selection in nature. As he argued in The Origin of Species, How can it be doubted, from the struggle each individual has to obtain subsistence, that any minute variation in structure, habits or instincts, adapting that individual better to the new conditions, would tell upon its vigour and health? In the struggle it would have a better chance of surviving; and those of its offspring which inherited the variation, be it ever so slight, would have a better chance. That statement, written almost 150 years ago, still stands as a good expression of the theory of evolution by natural selection. It is important to remember, as Darwin clearly understood, that individuals do not evolve; populations do. A populationis a group of individuals of a single species that live in a particular geographic area at the same time. A major consequence of the evolution of populations is that their members become adapted to the environments in which they live. The term adaptationhas two meanings in evolutionary biology. The first meaning refers to the processes by which adaptive traits are acquired—that is, the evolutionary mechanisms that produce them. We will discuss those processes in great detail in this chapter. The second meaning refers to traits that enhance the survival and reproductive success of their bearers. For example, wings are adaptations for flight, and a spider’s web is an adaptation for capturing flying insects. Biologists regard an organism as being adapted to a particular environment when they can imagine—or better still, measure the performance of—a slightly different organism that reproduces and survives less well in that environment. To understand adaptation, biologists compare the performance of individuals within or among species that differ in their traits. For example, to investigate the adaptive nature of spiders’ webs, we might try to determine the effectiveness of slightly different web structures in capturing insects. We might also measure changes in the webs of the same species in different environments. With these data, we could understand how variations in web structure influenced the survival and reproductive success of their builders. When Darwin proposed his theory of evolution by natural selection, he had no examples of evolutionary agents operating in nature. Since then many studies of the action of evolutionary agents have been conducted. Similarly, many investigations have documented changes over time in the genetic composition of a population. Darwin understood the importance of heredity for his theory but he knew nothing of the mechanisms of heredity. He devoted considerable time developing a theory of heredity but he failed in this effort. Fortunately, the rediscovery of Gregor Mendel’s publications in the early 1900s paved the way for the development of population genetics, which provides a major underpinning for Darwin’s theory. Population geneticists apply Mendel’s laws to entire populations of organisms. They also study variation within and among species to understand the processes that result in evolutionary changes in species through time. The perspective of population genetics given in this chapter which emphasizes the role of variation in characteristics of adult organisms.
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