Unit 37 THE EVIDENCE FOR EVOLUTION LEARNING OBJECTIVES 1. Understand the meaning of the term evolution. 2. Learn about fossil evidence including how fossils are formed. 3. Learn how comparative anatomy can indicate evolutionary relationship. 4. Understand the meaning and significance of vestigial structures. 5. Obtain a general understanding of how embryological development can be used to indicate evolution. 6. Gain a general understanding of the use of molecular (genetic) evidence to show evolutionary relationship. 7. Learn what the term molecular clock means. INTRODUCTION Evolution in the broadest sense attempts to explain that what we see today is different from what existed in the past. Biological evolution concerns changes in living things during the history of life on earth. It explains that living things share common ancestors. Over time processes such as natural selection give rise to new species.. The theory is built upon two things: 1) natural selection can produce evolutionary change and 2) the fossil record gives evidence that change has occurred. Several types of evidence, including fossil, comparative anatomy, embryology, molecular, and genetic, are used to support the theory of evolution. Some criticisms or questions about the validity of evolution: 1. Evolution is just a theory its not a fact. a. In science theory does not mean a guess. b. Scientists often use fact to describe an observation c. Theories are explanations of natural phenomena built up logically from testable observations and hypotheses. 2. Isn t evolution just an inference? a. No one saw the evolution of one-toed horses from three-toed horses, but that does not mean that we can not be confident that the horse evolved. b. Much scientific discovery is done through indirect experimentation and observation in which inferences are made, and hypotheses generated from these inferences are tested. 492
3. If humans evolved from apes, why are there still apes? a. Humans didn t evolve from apes, but both humans and apes evolved from a common ancestor. b. Because of this they share many anatomical, genetic, biochemical and even behavioral similarities. 4. The statement that there are no connecting links in the fossil records for every change in an linage, doesn t alter the fact that there are fossils showing linkage for many of the changes and the connecting links continue to increase as we discover more fossils. a. Archaeopteryx is a fossil transitional between reptiles and birds. (Known in Darwin s time) b. Figure 37-1, from Raven and Johnsons, 6 th ed. p. 441 indicates how recently found fossils fill in some of the gaps between the hypothetical ancestor of hoofed mammals and the present day whales. (ambulates---latin ambulare = walk and cetus = whale) 1) Ambulocetus natans probably walked on land and swam by flexing its backbone and paddling with its hind limbs. 2) Rodhocetus kasrani s has reduced limbs, which probably required it to move with an up and down movement as do present day whales. Figure 37-1 Whale missing links. The recent discoveries of Ambulocetus and Rodhocetus have filled in the gaps between mesonychids, the hypothetical ancestral link between the whales and the hoofed mammals, and present-day whales. 493
FOSSIL EVIDENCE 1. Lamarck in 1809 published his theory of evolution and based much of his work on his observation that the species from lower levels of rock were more primitive or less complex than those from the higher levels. 2. Even earlier in 1799, an engineer named William Smith reported that, in undisrupted layers of rock, fossils occurred in a definite sequential order, with more modern-appearing ones closer to the top. 3. The abundance of fossils is quite surprising when one realizes that conditions must be just right to produce a fossil a. The organism must die and be buried in sediment. b. Then bone or other hard material must be mineralized. c. The surrounding sediment must become rock. d. Then the fossil must be exposed in such a way that it isn t destroyed. 3. Figure 37.2, taken from Raven and Johnson 6 th ed., p. 440 shows a timeline for the fossil record. a. This shows fossils arranged according to their geological age. b. They show evidence of successive evolutionary change. c. This is allowing more that 3.5 billion years for evolution of living things. Figure 37.2. Timeline of the history of life as revealed by the fossil record. 4. Evolution of the horse. a. Figure 37-3 from Raven and Johnson 6 th ed, p. 443 shows the evolution of the modern one toed horse from a three toed horse. b. The earliest known fossil, Hyracotherium sandrae, was about the size of a cat (60MYA). 494
c. Evolution has resulted not only in reduction of the number of toes but in size of the horse and tooth shape. d. Also there have been branches, which ended in extinction (Figure 37-4 from Campbell, Biology 6 th ed. page 481). COMPARATIVE ANATOMY 1. The inferences about common descent derived from fossils is reinforced by comparative anatomy Figure 37.3. Evolutionary changes in horses through time. 495
Figure 37-4 The branched evolution of horses. If we use a yellow highlighter to trace the sequence of fossil horses that are intermediate in form between the modern horse and its Eocene ancestor Hyracotherium, we create the illusion of a progressive trend toward larger size, reduced number of toes, and teeth modified for grazing. In fact, the modern horse is the only surviving twig of an evolutionary bush with many divergent trends. 2. Figure 37-5 from Raven and Johnson, 6 th ed., p. 439 compares the bones in the forelimb of several vertebrates. 496
Figure 37-5. Shows homology of the forelimbs of some vertebrate animals. a. The bones of these animals can be compared in every part of the body, including the limbs. b. Yet the bat flies, the mouse runs and the human writes. c. These are referred to as homologous structures, those arising from common origin. 4. Comparing the bones of the reptile ear and the bones of the human jaw and ear can be used as examples of evolution. a. The reptile jaw has several bones; whereas the human has only one. b. The other bones in the reptile jaw are homologous with the hammer and anvil of the mammalian ear. c. Paleontologists have discovered intermediate forms of mammalian-like reptiles (Therapsida) with a double jaw joint-one composed of the bones which persist in mammalian jaws, the other consisting of bones that eventually become the bones of the middle ear. 4. Vestigial structures. a. These are structures that have no apparent function b. Table 37-1 from Raven and Johnson, 6 th ed., p. 451 lists some vestigial structures of humans. 1) Tail reduced to only three to five tiny tailbones. 2) Wisdom teeth 497
3) Appendix (all that is left of a cecum) 4) Ear wiggling muscles. EMBRYOLOGY 1. Figure 37-6 from Raven and Johnson 6 th ed., p. 450 compares the early vertebrate development for organisms from fish to humans Table 21.1. Lists some vestigial structures in humans. 2. The more closely two organisms are evolutionary wise, the longer they show parallel development. 3. A wide range of organisms from fruit flies to worms to mice to humans have very similar sequences of genes that are active early in development. Figure 37-6. Showing embryos of various vertebrates at an early stage. Dark areas are gill slits or gill pouches. Also they share tails. MOLECULAR 1. Figure 37-7 from Raven and Johnson, 6 th ed., p. 452 shows the differences (similarities) in the amino acids of the polypeptide in hemoglobin of various organisms including humans. 2. All proteins do not evolve at the same rate even though they are all composed of only 20 different amino acids. 498
Figure 37-7 Shows the difference in the amino acids in the hemoglobins of the of various vertebrates. The greater the evolutionary distance the greater the number of different amino acids. 3. They change at different rates depending on the constraints imposed by their functions. a. Cytochrome c, a protein involved in energy transfer changes slowly. b. Hemoglobin changes at an intermediate rate. c. Fibrinopeptides involved in blood clotting changes much more rapidly than either hemoglobin or cytochrome c. d. Molecular clocks refer to the time it takes for a particular kind of gene or protein to evolve. 1) The clock for fibrinopeptides runs rapidly; 1% of the amino acids change in a little longer than 1 million years. 2) The clock for cytochrome c is very slow; 1% change in amino acid sequence requires 20 million years. 499
3) The clock for hemoglobin is about 5.8 million years. CONVERGENT AND DIVERGENT EVOLUTION 1. Refers to the similarities in phenotypes of plants or animals located in different geographic areas. 2. Natural selection has favored the development of similar traits so that parallel or convergent evolution (change) occurs. 3. Good example is the development of marsupials in Australia and placentals in the rest of the world (fig. 37-8 from Raven and Johnson 6 th.ed, p. 453). a. Australia separated from the other continents more than 50 MYA. b. Marsupials had developed but placentals had not. c. The marsupials of Australia resemble the placentals of the rest of the world. 500
Figure 37-8. Showing convergent evolution in the marsupials of Australia and the placental mammals in the rest of the world. With this unit we have completed our examination of the concepts of human biology. We hope that you have enjoyed the course and have acquired the basic concepts that will allow you to critically examine news stories (newspapers, radio, TV, etc.) and other information you may encounter. Even though you may not pursue a career in biology, you influence the direction of biological research because your tax dollars fund the majority of the research performed in the 501
United State. How our elected officials budget your taxes determines the research performed and the advances achieved. 502
Unit 37 OBJECTIVE QUESTIONS OVER EVIDENCE FOR EVOLUTION 1. The term theory refers to a/an (A) guess (B) educated guess (C) hypothesis (D) fact (E) none of the preceding. 2. All the other evidences of evolution are based upon (A) comparative anatomy (B) fossil records (C) embryology (D) genetic evidence. 3. (A) Charles Darwin (B) Jean-Baptiste Lamarck (C) William Smith (D) Sherwood Roland was the first to comment on fossils occurring in a definite sequential order, with more modern-appearing ones closer to the top. 4. The time table for living things has the first organisms appearing on earth about (A) 1 million (B) 100 million (C) 1 billion (D) 3.5 billion (E) 10 billion years ago. 5. The ancestor of the modern horse is believed to have (A) been smaller with three toes (B) been larger with three toes (C) been smaller with one toe (D) been larger with one toe. 6. Analogy refers to structures having (A) similar in function and having a similar evolutionary origin (B) similar in function but having a different evolutionary origin. 7. Which of the following structures of humans is not considered to be vestigial? (A) wisdom teeth (B) appendix (C) gall bladder (D) tail. 8. Vertebrates from fish to humans in an early embryological stage show evidence of (A) a tail (B) relatively large head (C) gill slits or pouches (D) two of the preceding (E) all the preceding. 9. The closer two organisms are to one another evolutionary wise, the (A) smaller (B) larger the number of similar amino acids in their hemoglobin. 10. (A) Cytochrome c (B) fibrinopeptides (C) hemoglobin changes (mutates) most rapidly. 11. (A) Convergent evolution (B) Divergent evolution (C) Both A and B is/are suggested by the similarities of marsupials in Australia and placentals in the rest of the world. DISCUSSION QUESTIONS OVER EVIDENCE FOR EVOLUTION 1. We have discussed fossil evidence, anatomical evidence, embryological evidence, and molecular evidence. Which of the evidences is the basis for all the others? 503
2. What is a theory? 3. What could prevent an organism from becoming a fossil? 4. What are some of the major evolutionary changes that have occurred in the horse? 5. What kind of evidence is used to determine the origin of the bones of the middle ear in humans? 6. How do you explain that some humans have 4 wisdom teeth and some have none? 7. What is the reason that proteins are so important in comparing relationships? CRITICAL THINKING QUESTIONS OVER EVIDENCE FOR EVOLUTION 1. How have paleontologists been able to date rocks that are millions of years old? 2. Using evolutionary principles, explain why large ears might be expected to develop in a terrestrial herbivore like a deer, but not in an aquatic mammal like a seal. CRITICAL THINKING QUESTIONS OVER EVIDENCE FOR EVOLUTION II 1. Referring back to the yucca moth and the yucca flower, how does this suggest coevolution? 2. Relate how incomplete fossil records could be used to support the hypothesis of punctuated equilibrium. 504