Biology Teach Yourself Series Topic 15: Evolution and the evidence for evolution

Transcription

1 Biology Teach Yourself Series Topic 15: Evolution and the evidence for evolution A: Level 14, 474 Flinders Street Melbourne VIC 3000 T: W: tssm.com.au E: TSSM 2011 Page 1 of 25

2 Contents Evolution... 4 Convergent evolution Review Questions... 5 Coevolution Review Questions... 6 Divergent evolution The rate of evolutionary change Review Questions... 8 Evidence for evolution Fossils Review Questions Dating fossils Review Questions Using fossils to establish details about appearance and lifestyle Review Question Problems with using the fossil record DNA hybridisation Review Question Immunology Review Question Protein comparison Biogeography Review Question Cladograms Review Questions Comparative anatomy TSSM 2011 Page 2 of 25

3 Review Questions Vestigial organs and structures Review Questions Solutions to Review Questions TSSM 2011 Page 3 of 25

4 Evolution Evolution refers to changes in the genetic composition of populations. There are several types of evolution that you need to be aware of. These include: Convergent Divergent Parallel Coevolution Convergent evolution In some cases organisms have similar features because they have evolved from a common ancestor. However, species that have come from different background can develop similar features because they live in similar environments and experience similar selection pressures. They will never become the same species, but converge on (approach) similar phenotypes from different ancestral lines. Organisms that are the result of convergent evolution have analogous features. These features fulfill the same function and appear similar because of environmental similarities, not because of relatedness. However, the similarity is only apparent at the surface; the underlying structures are completely different. An example of this is seen in sharks and dolphins. Both have a similar stream-lined shape and arrangements of fins but the shark is a fish and the dolphin is a mammal. Species A Species B Specific phenotype Species C TSSM 2011 Page 4 of 25

5 Review Questions 1. Provide another example where two or more organisms have analogous structures, but do not share a recent common ancestor. 2. When European scientists travelled to new world continents such as North and South America or Australia, they found some organisms that appeared similar to those found in their homelands. As a result they assumed that the new-found animals were closely related to those they were familiar with. Explain why making this assumption was an error. 3. The Australian marsupial mole and the European mole have many similar adaptations, such as their body shape and the arrangement of their claws. Although they appear similar they do not share a recent common ancestor. Explain why these two organisms possess these similarities. TSSM 2011 Page 5 of 25

6 Coevolution This term is used to describe cases where two or more organisms affect the evolution of the other. There are a number of different types of relationships that can cause this to occur. One of these is the predator/ prey relationship. A predator evolves structures and abilities that allow them to hunt their prey, for example teeth, claws and the ability to chase their prey. Prey evolves defensive mechanisms to protect themselves such as horns, large size, toxicity and camouflage. If a new structure or feature evolves in either the predator or the prey then a selection pressure is placed on the other, which then may evolve in response. Most symbiotic relationships such as parasitism and mutualism are considered to be examples of coevolution. Review Questions 4. Explain how coevolution could lead to an increase in biodiversity. 5. Plants can be pollinated by a variety of different organisms. A particular plant is only pollinated by bats; these plants have flowers that are not highly coloured, they only open at night and have a very strong scent that is similar to the smell of the bats. Explain why this is considered to be an example of coevolution. TSSM 2011 Page 6 of 25

7 Divergent evolution Divergent evolution is the diversification of one ancestral species into two or more species that continue to evolve and become less and less alike over time. Species that arise from divergent evolution share a common ancestor and they have features that are homologous. Homologous features have the same basic underlying structures even though they may have been modified through evolution to serve different functions. For example, the same bones are found in the flippers of seals and the wings of bats. Seals and bats are both mammals and they share a common ancestor who possessed the same bones. Adaptive radiation is an example of divergent evolution on a larger scale, where many species evolve from the same common ancestor. For example, in Australia ancestral marsupials diversified into a wide range of species. Divergent Evolution Adaptive radiation Species A Species B Species C Species D Species B Species C Species A Species D Species E Species F TSSM 2011 Page 7 of 25

8 The rate of evolutionary change There are two theories regarding the rate of evolutionary change on a large scale. Punctuated equilibrium: species stay the same for very long periods of time (called stasis) which are punctuated with very short periods of rapid change. Gradualism: two or more species gradually evolve from one parent species. If modern species evolved by gradualism then there are usually transitional forms found in the fossil record. On a smaller scale, the generation time of a species has an impact on its rate of evolution. For example, the generational time of humans is approximately 20 years, but the generational time of Drosophila, a small fly frequently used in the study of genetics, is only two to three weeks. The Drosophila population will undergo evolutionary change in response to a selection pressure at a faster rate than humans. Review Questions 6. Identify the different types of environments that would support each type of evolutionary change. TSSM 2011 Page 8 of 25

9 Evidence for evolution There are a number of techniques and sources of information that are used to support the theory of evolution. Some of these, such as the analysis of fossils, have been used for many years and others, such as DNA analysis, are more recent. As modern techniques and more accurate techniques become available some older techniques become obsolete. Evidence for evolution includes: Fossils Biogeography Immunology Protein comparison Vestigial organs DNA hybridisation Comparative anatomy Comparative embryology Fossils Fossils are the preserved traces or remains of an organism. It is from studying these that we get a great deal of information regarding the changes that have occurred over a long period of time. Conditions have to be right for fossilisation to occur. These conditions include rapid coverage and a lack of oxygen. Fossils are most often found in sedimentary rock such as sandstone and limestone. The majority of fossils are hard materials such as teeth and bones. Soft materials do not preserve well due to the impact of pressure over a long period of time and they are usually subject to decay prior to being covered. Terms that you need to be aware of regarding fossils include: Index fossils: these are fossils that are used to date the rock in which they are found. These fossils need to be abundant, easily recognisable and present for a short period of time. Transitional fossils: these are fossils of organisms that have some traits from an ancestral organism and some traits that develop in the descendant organisms. TSSM 2011 Page 9 of 25

10 Review Questions 7. Currently there have been 2 complete fossils found of organism X. Would these fossils be able to be used as index fossils? Provide a reason for your answer. 8. Explain why fossils are not found in volcanic rock. 9. Explain the value of transitional fossils. 10. Explain why fossils are commonly found in caves. 11. The first bacteria and eukaryotes appeared in the precambrian era between approximately 600 million and 5000 million years ago. Explain why there are few if any fossils from this period of time. TSSM 2011 Page 10 of 25

11 Dating fossils There are two classes of dating methods: relative dating and absolute dating. Relative dating is a comparison technique. The age of a layer of rock is dated in comparison with surrounding layers. This is referred to as stratigraphy. The assumption of this technique is that lower layers are comparatively older than the upper layers. Layer 1 Layer 2 Layer 3 Layer 2 would have fossils that are comparatively older than layer 1 and comparatively younger than layer 3. This technique can only be used to compare the ages of different layers; no assumptions can be made about the abundance of the organisms or whether there is any relationship between them. The problem with this technique is that it cannot be used to establish the actual age of layers or fossils. Absolute dating techniques are used to determine the approximate age of a specimen. The most common methods of absolute dating involve radioactive decay. Since radioactive isotopes have known half lives (the time taken for the radioactivity to decay to half its initial value), measurement of the radioactivity in a sample can easily be calculated as a function of number of years since death occurred. Radio carbon dating can be used to date organic materials that are up to about 50,000 years. This technique measures the amount of carbon-14 taken up when an organism was alive compared to the amount in a fossil. Potassium to argon dating is used to measure the amount of potassium-40 in volcanic rocks that has decayed to argon-40. This method can be used to date materials that are much older. This method starts to become reliable at about point that radio carbon dating becomes unreliable. The long half life of potassium-40 means that this technique can be used to date specimens more than 100 million years. Although the two types of absolute dating already mentioned cover all of the time frames you could be asked about in an exam, there are other types of absolute dating that you could learn about. These are summarised in the diagram below. DATING METHOD DATING RANGE MATERIALS THAT CAN BE DATED Fission tracking 5000 to more than 100 million years Glass, volcanic substances and pottery Decay of uranium 2000 to 1 million years Carbonates, shells and coral series Electron spin resonance 2000 to just under 1 million years Burnt substances, bones and teeth Tree rings 1000 to 8000 years Trees, timber in structures such as buildings and ships Thermoluminescence 1200 to just over years TSSM 2011 Page 11 of 25

12 Review Questions 12. An archeologist located a prehistoric village believed to be approximately 35,000 years old. Identify one relative dating technique and one absolute dating technique that could be used to establish the date of this village. 13. The half-life of radioactive elements can be used to determine the age of fossils. Uranium-235 has a half life of approximately 700 million years. If the rock surrounding a fossil was found to have 1/64th of the original amount of uranium-235, what would be the approximate age of the fossil? 14. The diagrams below show 2 rock strata taken from opposite cliffs surrounding a valley. Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 Layer 7 Layer 8 Layer 9 Layer 10 Section 1 Section 2 i. Identify which stratum/strata of rocks is the youngest. ii. Identify all of the pairs of strata that are the same age. TSSM 2011 Page 12 of 25

13 Using fossils to establish details about appearance and lifestyle Most of us have been to museums where we can see reconstructions of animals that are long since extinct. Details such as colouration, calls and diet are freqently included. These type of features cannot be seen in fossils. However, researchers can make inferrrences about how an animal looked, sounded and behaved from a number of sources. These include: The points where muscles and ligaments were attached to in order to ascertain its musculature. Deep groves would imply a highly muscular organism. Living related species. Features such as fur colour, sounds, and behaviour can be inferred from living relatives. Teeth and jaw structures. These can be used to infer information about the animal s diet. For example, the presence of sharp canine teeth would indicate a carnivore/predator. Review Question 15. Identify two features that you would expect to find in the fossil of a large carniverous organism. TSSM 2011 Page 13 of 25

14 Problems with using the fossil record There are a range of problems involved in using the fossil record. These include: The fossil record is scanty ; the number of fossils compared to the number of previously living organisms is small. Conditions have to be right for fossilisation to occur. A dead body needs to be rapidly placed into an environment where decomposers cannot be active. Rapid coverage to decrease the amount of oxygen available, extreme temperatures and high salt or mineral concentrations promote fossilisation. It is highly likely that there have been many species where no individuals have been fossilised. It is also likely that there are missing links evolutionary intermediate forms which we don t have a fossil record of. Even in an environment suitable for fossilisation remains can be disturbed by weather, scavengers, floods and other natural occurrences. DNA hybridisation The central idea behind this technique is that the closer the relationship between two species the greater the similarity between the base sequences in their DNA. This is due to the to a reduced time for mutations to occur. The technique is performed as follows: 1) DNA samples are taken from members of two different species. 2) Each sample is heated to cause the strands to separate into two single strands. 3) Single strands from each species are combined. 4) Hydrogen bonds reform between the two strands resulting in a hybrid double strand. The greater the similarity between the strands the greater the strength of the bond between the strands. 5) The hybrid strand is heated again to split the double strand back into two single strands again. The greater the similarity between the two strands the greater the amount of heat required to separate them. TSSM 2011 Page 14 of 25

15 Review Question 16. DNA samples were taken from several different species to determine the extent of relatedness between them. The following results were obtained. Hybrid from species A and species B: Hybrid from species A and species C: Hybrid from species A and species D: 82.4ºC 75.2ºC 79.7ºC Which species is most closely related to species A? Provide a reason for your answer. TSSM 2011 Page 15 of 25

16 Immunology Immunology is used to indirectly estimate the similarity of proteins from different species. The central idea behind this concept is that if the proteins are similar then the DNA must be similar. Organisms with very similar DNA probably shared a recent common ancestor. The technique is performed as follows: 1) Blood serum is obtained from species A and injected into species B. 2) Species B then produces antibodies that are specific for species A. 3) These antibodies are placed into a series of test tubes. 4) Proteins from a variety of species are then placed into the test tubes one per test tube. 5) If the anti species A antibodies bind to the proteins from the other species then a precipitate will be produced. The greater the similarity between species A and another species the greater the amount of precipitate. Review Question 17. In the diagram below the line indicates the extent of precipitation that occurs when blood serum from species C, D, E and F is placed into a solution containing antibodies specific to species A. C D E F Rank the degree of relatedness between species A and species C, D, E and F in order from most related to least related. TSSM 2011 Page 16 of 25

17 Protein comparison In this method the similarity between the amino acid sequences of proteins from different species is compared. Once again the greater the similarity, the more recently the two species shared a common ancestor. Some proteins are highly conserved. This means that they are very similar in all related species. Proteins with few differences between different species are the most useful in establishing their degree of relatedness. Biogeography Biogeography is the study of past and present distribution of species. By studying and comparing similarities and differences in present species, their environments and their global distribution, inferences can be made about the likely environments of extinct similar species which are now only known from the fossil record. Review Question 18. Explain how biogeography is used to provide supporting evidence for the theory of evolution. TSSM 2011 Page 17 of 25

18 Cladograms This type of diagram reflects the evolutionary relationships of organisms based on the distance from common ancestors. These diagrams are often constructed on the basis of biochemical evidence, with DNA homology being very useful; however, we can also use structural and physiological features. If there is very little difference in the DNA of two species, they are said to show evolutionary 'closeness', since there has not been sufficient time for many mutations to accumulate in their DNA. Similarly, the more different the DNA (the less homology shown), the further the evolutionary distance between the species and the longer the time since they shared a common ancestor. These diagrams are simple cladograms showing the divergence of 3 different species. The first shows species A and C diverging from the common ancestor and then species B diverging from species A later on. The second diagram shows the 3 species all diverging from a common ancestor at the same time. Species A and B should have more features in common with each other than either has in common with species C. Cladograms can be used to show when organisms shared a common ancestor or to estimate the extent of similarity between different species. For example, the cladogram below shows that the domestic dog and the gray wolf diverged from each other approximately 1.5 million years ago. TSSM 2011 Page 18 of 25

19 Another type of question commonly asked is to identify which organisms will have the most features in common. Sample question: Which pair of species are expected to have the most features in common: the red fox and the grey fox, or the bush dog and the domestic dog? The way to work out the answer to this question is to trace back the pathway of each species and work out when they shared a common ancestor. It can be useful to place a cross on each point of divergence. According to the cladogram the red fox and the grey fox shared a common ancestor approximately 9.5 million years ago and the bush dog and the domestic dog shared a common ancestor approximately 8 million years ago. Therefore the domestic dog and bush dog should have more features in common with each other than the red fox and the grey fox should. TSSM 2011 Page 19 of 25

20 Review Questions 19. Identify which species is most related to the crab eating fox. 20. One of the ways of determining ancestry it to compare the similarity of amino acid sequences of proteins in different species. The table below shows the number of differences between a human protein sequence and that of other species. ORGANISM NUMBER OF DIFFERENCES Shark 85 Dolphin 8 House Cat 21 Chimpanzee 1 Goldfish 78 Use the information above to complete the cladogram below. Write the species name into the boxes provided. Human TSSM 2011 Page 20 of 25

21 Comparative anatomy Anatomy relates to the physical structures of an organism. When comparisons are made between the anatomies of different species it can often be seen that, although the structures have evolved to fulfil different functions, these structures have the same evolutionary origins. These structures are referred to as being homologous and are associated with divergent evolution or adaptive radiation. Analogous structures are said to occur where evolution has given rise to anatomically very different structures which have similar function (e.g. birds and butterflies both have wings but one is an animal and the other an insect). This is associated with convergent evolution. Review Questions 21. A number of species are phenotypically similar. Does this mean they have to share a recent common ancestor? Provide a reason for your answer. 22. Early vertebrates all had pentadactyl limbs (which basically means 5 fingered). There are many adaptations of the pentadactyl limbs including wings, arms, flippers and forelegs. Are these adaptations an example of convergent or divergent evolution? Provide a reason to support your answer. TSSM 2011 Page 21 of 25

22 Vestigial organs and structures Vestigial structures are structures that used to be used for a specific purpose but are now no longer needed. These features tend to become smaller or less specialised as time passes. If a vestigial organ no longer has a purpose and it is not subjected to selection pressures then it will generally remain in the organism without any further major alterations. Review Questions 23. With respect to natural selection explain why an organism that originally had a specific purpose can become vestigial. 24. Some modern whales have a vestigial pelvis and femur. What information does this provide about the ancestry of whales? TSSM 2011 Page 22 of 25