Chapter 15 Darwin s Theory of Evolution Evolution, or change over time, is the process by which modern organisms have descended from ancient organisms. The theory of evolution consists of the following four major points: - 1. Variation exists within the genes of every species (the result of random mutation). - 2. ln a particular environment, some individuals of a species are better suited for survival and so leave more offspring (natural selection). - 3. Over time, change within species leads to the replacement of old species by new species as less successful species become extinct. - 4. Evidence from fossils and other sources indicate species now on Earth have evolved (descended) from ancestral forms that are extinct (evolution). Charles Darwin and Voyage of the Beagle - Charles Darwin struggled in school - Darwin's father wanted him to become either a doctor or a minister. - Darwin frequently spent more time outdoors than in class - He studied medicine but was repelled by surgery, which was done without anesthetics - he then changed majors at his father s wishes and completed a degree in theology - In 1831, one of his professors recommended Darwin for a post as the unpaid naturalist on a naval voyage of the HMS Beagle - At the age of 22, Darwin was off on a 5 year journey that would change his life and the views of others 1
- Darwin initially accepted the prevailing view that each species was a divine creation, unchanging and existing as it was originally created - However, traditional views could not explain the kinds and distribution of fossils - Darwin read Charles Lyell's book Principles of Geology - Darwin also saw things that he, thought could only be attributed to a process of gradual change - For example, in South America, he found fossils of extinct armadillos that closely resembled, but were not identical to, the armadillos currently living in the area. - Darwin wondered why living and fossilized species found in the same place would be similar and yet different. - The most probable explanation, he reasoned, was that one species had given rise to the other - On the Galapagos Islands Darwin found his most convincing evidence that species evolve. - Darwin noticed the plants and animals of the Galapagos Islands resemble those of the nearby coast of South America - If each species had been created independently and placed on the Galapagos Islands, why would they resemble the plants and animals of the adjacent South American coast? - Why did they not instead resemble the plants and animals of similar islands, such as those near Africa, for instance - Darwin felt that the simplest explanation was that the ancestors of Galapagos species must have migrated there from South America and changed after they arrived. - Darwin referred to such change as "descent with modification"-evolution. 2
Darwin Sought an Explanation for Evolution - Darwin returned from his voyage at the age of 27 and continued his lifelong study of plants and animals and did not publish his ideas about evolution for many years later - Malthus's Contribution - Thomas Malthus "Essay on the Principle of Population" (1798), stated that human populations have the potential to increase faster than the available food supply - Malthus stated a population grows in a geometric progression - Food supply increases at best by an arithmetic progression - populations would cover Earth's entire surface in a short time if it reproduce unchecked - This does not occur because death caused by disease, war, and famine intervenes - a population is a group of individuals that belong to the same species, live in a defined area, and breed with others in the group Natural Selection - Darwin realized that Malthus's principles applied to all species - Every organism has the potential to produce many offspring but only a limited number survive to reproduce - Darwin made a key association: Individuals that possess superior physical or behavioral attributes are more likely to survive than those that are not so well endowed - Darwin saw that by surviving, individuals have the opportunity to reproduce and pass on their favorable characteristics to offspring. - Thus, these characteristics increase in a population, and the population will gradually change. - Darwin called the process by which populations change in response to their environment natural selection - Darwin saw organisms differed from place to place because their habitats presented different challenges to, and opportunities for, survival - Each species had evolved in response to its environment - The changing of a species that results in its being better suited to its environment is called adaptation - Darwin concluded that the species of a particular place evolved from species that previously lived in the same area or that migrated from areas nearby Darwin Published - In 1858 Alfred Russel Wallace in a letter to Darwin described the idea of evolution by natural selection and asked if Darwin would help get the essay published - On July 1, 1858, Darwin and Wallace s papers were be presented at a public scientific meeting. - Darwin in 1859 had finally finished his book On the Origin of Species by Means of Natural Selection Darwin's Ideas Updated - a modern understanding of genetics have given scientists new insight into how natural selection brings about the evolution of species - 3
Natural Selection Causes Change Within Populations - Darwin's key inference was that within any population, those individuals best suited to survive and prosper in their environment will leave the most offspring - Thus, the traits of those individuals will become more common in successive generations - Scientists now know that genes are responsible for inherited traits - certain traits become more common in a population because more individuals in the population carry the genes for those traits - natural selection causes the frequency of certain genes in a population to vary over time - Mutations and the recombination of genes are constant sources of new variations for natural selection to act upon. - the term microevolution to refer to change that occurs within a species over time Isolation Leads to Species Formation - populations of the same species living in different locations tend to evolve in different directions - The condition in which two populations of the same species are separated from one another is called isolation. - As isolated populations of the same species become increasingly different the populations may no longer be able to interbreed (breed with one another) - When two populations cannot interbreed the populations are considered to be different species. - Extinction Leads to Species Replacement - events such as climatic changes and natural disasters result in some species becoming extinct, which means that they disappear permanently. - Species. that are better suited to the new conditions may replace those that become extinct - the organisms alive today are but the latest members of a long parade of life - biologists use the term macroevolution to refer to change among species over time - The replacement of the dinosaurs by mammals is an example of macroevolution. Evidence of Macroevolution - Discoveries of microscopic fossils have extended the known history of life on Earth to more than 3.5 billion years - a large body of evidence supports three major points: (1) Earth is about 4.5 billion years old; (2) organisms have inhabited Earth for most of that time; (3) all organisms living today evolved from earlier, simpler life-forms. Fossils Provide Record of Macroevolution - most direct evidence that a has occurred comes from fossils - A fossil is the preserved or mineralized remains (bone, tooth, or shell) or traces (footprint, burrow, or imprint) of an organism that lived long ago. - Fossils provide a record of Earth's past life-forms - Change over time (evolution) can be observed in fossil record. - Darwin predicted that "missing links" (intermediate forms) between the great groups of organisms would eventually be found. - For example, fossil links have been found between fishes and amphibians, between reptiles and birds, and between reptiles and mammals, making the fossil history of the vertebrates remarkably complete 4
- In the early 1990s, the discoveries of the fossilized remains of two whale ancestors, provided new links in the evolution of whales from four-legged land mammals. - Most fossils form when organisms are rapidly buried in fine sediments deposited by water, wind, or volcanic eruptions - Thus, the environments that are most conducive to fossil formation are wet lowlands, slow-moving streams, lakes, shallow seas, and areas near volcanoes that spew out volcanic ash. - What are the chances that organisms living in upland forests, mountains, grasslands, or deserts will die in just the right place to be buried in sediments? - Although the fossil record will never be complete, it contains striking evidence that macro evolution has occurred. - Paleontologists, scientists who study fossils, can determine the age of fossils fairly accurately by using radiometric dating - Radiometric dating enables paleontologists to arrange fossils in sequence from oldest to youngest. - family tree shows how organisms are related through evolution - Each branch point in a family tree indicates a common ancestor, which is a species from which two or more species diverged (separated). 5
Molecules Contain a Record of Macroevolution - if species have changed over time, then the genes that determine characteristics also should have changed - As species evolved, one change after another should have become part of their genetic instructions through mutation - If evolution has occurred, species that diverged from a common ancestor in the distant past should have more amino acid sequence differences between their proteins than do species that recently diverged - Comparing human hemoglobin (146 amino acids) with that of several species reveals these differences. - Notice that species that recently shared a common ancestor (humans and gorillas) - Comparisons of amino acid and nucleotide sequences among organisms have provided a wealth of direct evidence of successive change over time. 6
Anatomy and Development Reflect Macroevolution - Comparisons of the anatomies (structures) of different types of organisms often reveal basic similarities in body structures, even though functions may be different - the forelimbs of all vertebrates are constructed from the same basic array of bones - Such structures are said to be homologous - Homologous structures share a common ancestry. - Sometimes, bones (or other structures) are present in an organism but are reduced in size and either have no use or have a less prominent function in other, related organisms. 7
- Such structures, which are considered to be evidence of an organism's evolutionary past, are called vestigial structures. - An example of a vestigial structure is the human tailbone - The fused vertebrae that make up the tailbone are homologous to those of other vertebrate tails - The evolutionary history of organisms is also evident in their embryonic development. - Compare the development of a human embryo with that of the other vertebrate embryos - Notice that in addition to their early similarity, each embryo develops a tail, buds that become limbs, and pharyngeal pouches (which house the gills of fish and amphibians). The tail remains in most adult vertebrates. - Although vertebrate bones are homologous, limbs develop somewhat differently in each group of vertebrates - Only adult fish and immature amphibians retain pharyngeal pouches. - In humans, the tail and pharyngeal pouches are relics that disappear by the time of birth. These similarities in embryos strongly suggest that the development of all vertebrates evolved as new genetic instructions were layered on top of older ones Does Evolution Occur Spurts - most biologists have envisioned evolution as a gradual process that goes on all the time 8
- The model of evolution in which gradual change over a long period of time leads to species formation is called gradualism. - some biologists suggest that evolution occurs in spurts, separated by long periods of environmental stability in which little change in species occurs - This model of evolution, in which periods of rapid change in species are separated by periods of little or no change, is called punctuated equilibrium. - The fossil record shows that drastic environmental changes have occurred infrequently, separated by quiet periods that often last tens of millions of years. - volcanic eruptions, asteroid impacts, and ice ages have been linked to sudden and drastic changes in climates, both locally and across the entire planet. - These changes have been linked to the extinction of many groups of organisms. - habitats that were once occupied became vacant and provided opportunities for colonization by species that could rapidly adapt to the new conditions through natural selection. - the fossil record seems to provide evidence of both types of evolution. Evidence of Microevolution: Industrial Melanism Is Natural Selection at Work - An example of natural selection in action is industrial melanism (darkening of organisms in response to industrial pollution) 9
- This occurs in many insect species but the best-known case of industrial melanism involves the European peppered moth, - there are two color variations - darker moths have genes for increased production of melanin (a black pigment) - the dark variety was extremely rare until the 1850s. - Starting around 1850, however, dark peppered moths began to appear more often in heavily industrialized areas. - After 100 years, almost all of the peppered-moth populations near industrial centers were composed of dark individuals Sickle Cell Anemia Reveals Evidence of Microevolution - sickle cell anemia arises from a single nucleotide change in the gene that codes for beta- hemoglobin - sickled red blood cells tend to clog tiny arteries and is usually lethal - Heterozygous individuals, who have both a defective hemoglobin gene and a normal hemoglobin gene, make enough functional hemoglobin to keep their red blood cells healthy. - sickle cell anemia affects roughly 2 out of every 1,000 African Americans in the United States - in Central Africa where it is thought to have originated, 1 in every 100 people is homozygous for the defective allele and develops the fatal disorder - The sickle cell allele provides a definite advantage in certain environments - People who are heterozygous for the sickle cell allele are far more resistant to malaria (a leading cause of death in central Africa) than are people who are homozygous for normal hemoglobin. - While one in a hundred individuals is homozygous for the sickle cell allele and dies of anemia, one in five individuals is heterozygous for the sickle cell allele and survives malaria. - Thus, even though people who are homozygous for the sickle cell allele die, a population that lives where malaria is prevalent experiences far fewer deaths from sickle cell anemia than would occur from malaria Balancing Selection - In central Africa, natural selection affects the sickle cell allele in opposing ways: (1) it tends to eliminate the sickle cell allele because it is lethal to homozygous individuals (2) it tends to preserve the sickle cell allele because heterozygous individuals are resistant to malaria. - Selection that acts in opposite directions is called balancing selection - Balancing selection maintains both the normal hemoglobin allele and the sickle cell allele in central Africa because heterozygous individuals are the most "fit." Directional Selection - in other populations sickle cell allele is gradually being eliminated - In these areas the environment does not favor the sickle cell allele because its lethal potential is not balanced by any advantage - African Americans in the U.S. gain no advantage from being heterozygous for the sickle cell allele therefore the sickle cell allele is becoming less common among - When the frequency of a particular allele tends to move in one direction-in this case, toward elimination. Biologists call such unopposed selection directional selection. Species Formation Begins With Microevolution 10
- Species formation occurs in a series of successive stages - natural selection favors changes that increase reproductive success - microevolution continually molds and shapes a species to improve the "fit" between a species and its environment - Over time, separate populations of the same species may become very different as different kinds of changes accumulate in each isolated group - Accumulated differences between groups is called divergence - divergence leads to the formation of new species - Biologists call the process by which new species form speciation - - Forming Ecological Races Over time, populations of the same species that differ genetically because of adaptations to different living conditions become what biologists call ecological races. Overview of NATURAL SELECTION The process of natural selection depends on five main elements: 1. All species have genetic variation. 2. The environment presents many different challenges to an individual's survival. 3. Organisms tend to produce more offspring than their environment can support; thus, individuals of a species often compete with one another to survive (struggle for survival). 4. Individuals that are better able to cope with the challenges of their environment tend to leave more offspring than those less suited to the environment (survival of the fittest). 5. The characteristics of the individuals best suited to a particular environment tend to increase in a population over time. The key point is that the environment dictates the direction and extent of change. + 11