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Biology 30 Module 4 Organic Variations Lesson 14 Evidences of Change Copyright: Ministry of Education, Saskatchewan May be reproduced for educational purposes Biology 30 1 Lesson 14 Biology 30 2 Lesson 14 Lesson 14 Evidences of Change Directions for completing the lesson: Text References for Suggested Readings: Read BSCS: An Ecological Approach Sections 9.1-9.5, pages 119-208; 9.8-9.9, pages 211-216 OR Nelson Biology Chapter 29, pages 678-694 Study the instructional portion of the lesson. Review the vocabulary list. Do Assignment 14. Biology 30 3 Lesson 14 Vocabulary analogous structures Charles Darwin convergent evolution divergent evolution divine concept evolution extinct fossil genesis half-life Jean Lamarck Biology 30 macroevolution microevolution natural selection paleontology Pangaea phylogenetic trees sedimentary rock tectonic plates uniformitarianism vestigial 4 Lesson 14 Lesson 14 – Evidences of Change Introduction One noticeable aspect of the development of our planet centers on the variety of physical and biological processes. Variations can be seen in the continual physical forces and their effects shaping our continental surfaces, climatic changes and the aging processes within the earth’s crust and interior. Biological changes are in evidence in changing plant and animal distributions, changing cell and body actions and changes in individual organisms over time. This lesson will focus on changes or variations as they occur in gene pools or populations over time. The term evolution could be used to describe these gene pool changes or population variations. At this point, it should be stressed that the term evolution is not the same as the Theory of Evolution. The latter states that present species evolved from ancestral forms, which could have been quite different in appearances. It also implies that the first living forms somehow arose through some natural causes present at the time. It then attempts to explain the reasons for variations occurring. This lesson will present some thoughts and information on species’ variations. It will be noticed that there has been an “evolution” in thinking over the years. At the present time, much of the human population agrees that change or evolution is taking place. Differences arise in trying to explain why or how evolution does take place. Such differences have been, and still are, largely responsible for the disagreements that arise among people or between groups on the topic. Different sciences all have a common characteristic. Each science builds a body of knowledge based on the scientific method. Collecting data, experimenting, observing and then interpreting are some of the major steps in establishing facts or theories. The ability to repeat observations or experiments by different people with the same results is a main feature in the development of a fact or theory. This does not mean that a theory is necessarily correct. Newer or more precise scientific techniques can show some theories to be inaccurate or unable to answer all the questions about a particular subject. Modified or new theories can take their place, based on the information available at a particular time. This method of gathering scientific knowledge is not the same as that of knowledge gained through religion. Religious knowledge is based on beliefs developed and held by specific groups. The differences in methods of gathering knowledge have probably been the main source of disagreements between groups on the topic of evolution. With biology being a Biology 30 5 Lesson 14 science, the material presented in this module will concentrate on data gathered through the scientific method. This does not mean that the course is trying to convince individuals to absolutely accept or reject certain kinds of information or beliefs. How individuals resolve or treat certain information or issues ultimately depends upon themselves. This lesson will look at the evidences or indications which confirm that changes do occur in both the physical and the biological aspects of planet earth. On the biological side, fossils have been, and still are, a major confirmation of evolution. In addition, they are valuable sources of information about once living organisms and the external conditions they experienced. Other sources of information about changes occurring and perhaps the relatedness of species are based on such things as: comparative anatomy, embryology, vestigial organs, biochemical similarities and others. Biology 30 6 Lesson 14 After completing this lesson you should be able to: • define the term “evolution”, as it relates to living things. • trace some of the thoughts and manners of thinking with respect to evolution from early times to the present. • discuss some of the contributions made by geologists to the ideas of evolution. • discuss the various evidences of evolution. • discuss some of the major forces affecting the earth’s crust and their effects on both physical and biotic conditions. • present the main points of the various theories on origins of organisms and evolution. • explain the relationship between genetic variation, natural selection and evolution. Biology 30 7 Lesson 14 Understanding Evolution If one were to try and find a definition of evolution from different sources, it would be quite probable that a number of different ones could be stated. Evolution (according to different references) is: • • • a process by which populations change and develop adaptations to their environments; a progressive change in gene frequencies and pools; the development of new species from pre-existing ones. Examining such definitions more closely, a comparison could be made to finding the meaning of a word like “help” from a dictionary. This one word could have a large variety of slightly different definitions for particular situations. All have essentially the same meaning or could be a part of the same general process. In the same way, the three definitions (and others) given for evolution could fit together. A population may evolve or change over time, through genetic changes and natural selection. Evolution, or evolving, can occur in a number of ways. Some species may have few changes taking place, others become extinct, and new species develop. Attitudes and Thoughts on Evolution Generalizations are often made by people based on what is observed. Early in human history, observations of many actions lacked details or understandings of what was actually taking place. People would frequently supply explanations based on what they thought was occurring. With respect to life or living things, records of human thoughts probably first began in the sixth to the fourth centuries, B.C. From this time until about the l700’s, the theory of spontaneous generation, renamed abiogenesis by Thomas Huxley, was commonly accepted. What observations lead to abiogenesis? One common observation was that people observed that maggots seemed to suddenly appear in rotting meat after several days. Today we know that this is a result of eggs that hatched, not because the meat spontaneously changed into maggots. This was proved experimentally by Francesco Redi. There were further experiments which disproved abiogenesis; these were experiments by Spallanzani and Louis Pasteur. Biology 30 8 Lesson 14 Religious beliefs were a common basis for evolution; these beliefs originate from the Old Testament. The Divine Concept put forward that a divine being made the world and all its living inhabitants in six days, with man coming last. Special Creation, in various other forms, has been part of the cultures of many different human societies. Some cultures have attributed the different physical and biotic conditions on this planet as being under the charge of different divine beings. Other theories of evolution or creation were based on the ideas of the influential philosophers such as Plato and Aristotle. A common belief, dating back prior to the 1800’s and traced to some of Aristotle’s work, centred on the “Great Chain of Being”. This belief put forward that original creation, when it occurred, was complete and perfect. Species remained as they were formed. Each species also had a permanent place in a “divine order” of things. This not only applied to species and life, but it governed other aspects. Some of these included the places or roles of specific human individuals in societies. It favoured such ideas as the divine right of kings and, that once established, social and economic positions of individuals in societies would remain unchanged. For instance, an individual born into some low-ranking position or order would remain there. These early theories were not to remain unchallenged. The early l800’s began to see more and more resistance to the belief of divine order and the world remaining static in so many of its conditions. Much of the resistance could have come from the dissatisfaction and feeling of helplessness many people were feeling in being locked into rigid social and economic frameworks. Such rigid systems offered little hope of advancement or betterment for those in low-ranking positions. At the same time, almost all sciences were also challenging some of the “old” ideas, such as the earth being the centre of the universe and the world remaining static and “divinely ordered”. Previous generalizations and conclusions, such as abiogenesis, were being disproved by experimentations and new observations. Development of a rational scientific method and new testing techniques (especially with the use of better microscopy) were important factors in this. A major force in challenging the idea of a static earth was the developing science of geology and the works of a number of prominent geologists. In l788, James Hutton published his Theory of the Earth. Among his conclusions in the publication was that the earth was continually changing. Also, that the same geological forces in action at the present time (volcanism, erosion, deposition...) had been shaping the earth over a long period of time. The emphasis on time was shifted from the immediate and relatively short time intervals, to “deep” time, where there is “no vestige of a beginning – no prospect of an end”. In l830, another geologist by the name of Charles Lyell published Principles of Geology, which emphasized Hutton’s conclusions of a changing earth. In addition, this publication expressed a theory of uniformitarianism. This related not only to geology, but made generalizations which could apply to other sciences. Biology 30 9 Lesson 14 The principles of uniformitarianism include the following: l. Natural laws are constant in time and space. 2. Scientists should attempt to explain past events through the same processes that can be seen today. 3. Most geological change occurs slowly and gradually, not through catastrophic events. The first two principles could almost be regarded as scientific attitudes or methods which could be utilized by all sciences. These could be used to explain many events or changes which have occurred or are occurring on earth. The third principle was considered not only by geologists, but was incorporated into the studies and findings of other scientists, including Charles Darwin. Darwin’s conclusions on evolution included the idea of changes to living forms taking place at gradual or uniform rates. By the mid-l800’s, individuals could see changes in many things around them and the idea of change began to include living forms more and more. Even though the idea of change or evolution was being shared by increasing numbers of people, the still unanswered questions of how changes took place and why they did, presented major difficulties. Attempts to answer these questions created divisions, which persist in varying degrees to the present time, among and within groups. Applying the concept of evolution or change to humans made the study that much more difficult. The next portion of the lesson will examine the various concepts and theories on changes in life forms. The remainder of the lesson will concentrate on the observations or evidences used by people in the past and present to confirm that life forms or species do change. Biology 30 10 Lesson 14 Origins of Life The time at which the first forms of life were created (or genesis) and just how this took place could follow one of several views. One of these was previously mentioned. Special Creation Different cultures view genesis or the creation of the first life as having been under the influence of a divine being or beings. The Old Testament and the Divine Concept puts forward that the world and all its inhabitants were created in six days, with man coming last. Other cultures stated that the many conditions, both living and nonliving, were created by different divine beings. The Scientific View Following the immediate formation of planet earth, scientists indicate that conditions would not have allowed for any living forms. Neither the atmosphere nor the surface conditions were favourable to the appearance or development of organic forms of life, however simple. Ironically, this put science in the position of trying to explain how living organisms could develop from the non-living, when they had been trying so hard to disprove spontaneous generation. The possible answer existed in the conditions prevalent at the time. Conditions then were not as they are now. Oparin, a Russian scientist, stated in 1936 that certain surface and atmospheric conditions of primitive Earth could have resulted in the formation of organic molecules. In an experiment in 1953, carried out by an American scientist (Miller), an electrical discharge through a “soup” of inorganic matter (methane, ammonia, hydrogen and water) did actually form organic molecules. These were short chains of amino acids, which are the actual building blocks of proteins. Other scientists, trying to simulate the possible conditions on earth as they may have been over 4 billion years ago, have also been able to produce organic molecules. Even though amino acids and other organic molecules have been created, no one has so far been able to produce organic matter which somehow has a genetic component and is able to carry out some of the life-sustaining actions of living cells as well as reproduction. However, some conditions or actions about 3.5 billion years ago could have combined organic matter into complete living cells. The oldest fossil cells were discovered in rock dated at approximately 3.4 billion years old. Other cells may then have developed from these. A theory, sometimes called the Heterotroph Hypothesis, centers on the idea of the first living cells being anaerobic and heterotrophic; anaerobic, because of the initial absence of oxygen and heterotrophic, because photosynthetic or chemosynthetic structures are quite complex and would probably need more time to develop. It would not seem likely that cells could have been formed having both life-sustaining Biology 30 11 Lesson 14 and photosynthetic capabilities. Heterotrophic cells could have lived on the short chain organic compounds that were being formed. The more complex autotrophs, with photosynthetic structures, could then have evolved from these. With photosynthesis, oxygen began to be produced and the Earth’s ozone layer began to be formed. With these processes going on, it is likely that the numbers of anaerobic cells or organisms would have begun declining, since oxygen would really have been “poisonous” to them. This seems to be borne out today, with anaerobes being present in relatively limited numbers and in places where there is no oxygen. In addition to Oparin’s and Miller’s theories and work on creation of life forms, there are other ideas. One indicates the possibility that the first organic molecules fell to the earth as passengers on meteorites or comets. This idea is reinforced by discoveries of organic molecules in space matter. Going a step further, there is a chance that perhaps some of the space matter actually carried living cells. Another theory contends that life could have begun around deep-sea hydrothermal vents. This theory puts forward the idea that such vents provide a steady flow of energy and inorganic nutrients. The energy and the nutrients could have, at some point, resulted in some matter becoming animate. Evolution After Creation Special Creation Subscribers to Special Creation differ in ideas on evolution. One view is that once created, organisms or species are immutable or unchanging. Species in existence appear in forms as they were created. This view presents a problem in that fossils have shown different life forms in different periods of time during earth’s history. To account for this, some believe that there could have been a number of creations, perhaps five or six, followed by catastrophes. Each of the catastrophes and special creations would have been by “divine decree” – rare events. Some references refer to this as Catastrophism. Interestingly, some scientists subscribe to a Theory of Catastrophism as well. However, rather than ascribing the catastrophes to divine influence(s), the scientists lay their causes to sudden geologic, climatic or other natural events. Another view embodies the idea of Special Creation and then sees evolution as occurring, but under a “guiding influence”. Biology 30 12 Lesson 14 The Scientific View Biological theories on evolution, after the first appearance of life, have been experiencing a progression of thoughts. Some early Greek scientists-philosophers felt that species experienced “successions” in forms. They could not explain why changes in forms took place, but did feel that the changes were in the direction of “better or more perfect” forms. Since that time, various other theories have been expressed. Some of the more prominent theories on reasons for evolution which were put forward at different times are described in the following paragraphs. Jean Lamarck Theory of Evolution In the early 1800’s, French biologist Jean Lamarck expressed his ideas on organisms or species changing from generation to generation. His descriptions and accounts of actual evolutionary processes and evidences of others were quite accurate. He was also the first to suggest that interactions between organisms and their environments were the basis of evolution. It is unfortunate that Lamarck is perhaps best known for his reasons in attempting to explain why evolution took place, which have proved to be inaccurate. His reasons for evolution were based on two main thoughts: 1. The fist thought is that the use or disuse of different parts would result in different developments of those parts over time. A common example of short-necked animals continually stretching their necks for a number of generations, resulting in giraffe-like characteristics, would illustrate this. Sometimes associated with this, is a related idea about an inner need to change. That is, a change could be initiated or brought on by some conscious desire on the part of an individual organism itself. 2. The second major thought in Lamarck’s works was the theory of acquired traits being inheritable. This expressed the idea that traits which organisms developed or acquired in their lifetimes could be passed on to their offspring. For instance, if an organism lost a limb, its future offspring could be born without that limb. This particular idea became a major failing of Lamarck’s entire work related to trying to explain how evolution took place. Later genetic studies showed that acquired body traits could not be inherited. Only changes affecting genes of gametes could be passed on. Although his proposed theory of evolution is known to be incorrect, it provoked much thought and discussion on the subject, influencing the thoughts of others, including Charles Darwin. Biology 30 13 Lesson 14 Charles Darwin’s Theory of Evolution In the mid-1800’s, biologists Alfred Wallace and Charles Darwin independently formulated similar ideas on reasons for evolution. It was acknowledged that Darwin had spent more time and work and had formulated his ideas just before Wallace actually presented his publicly. In 1859, Darwin published his ideas as the book On the Origin of Species by Means of Natural Selection. His book was primarily based on observations made of differences in organisms on scattered island chains in the Galapagos. His observations are summarized as follows: Darwin found fossils of extinct animals that appeared very similar to present day animals living in the same area (the extinct glyptodont and the modern armadillo) Plants and animals in the temperate regions of South America were more similar to those in other tropical regions of the same continent as compared to those of temperate regions elsewhere. While on the Galapagos Islands, he noticed that animals, such as the tortoise and the finch, varied slightly from island to island. This was perhaps the most critical observation that helped develop his theory. These observations formed questions in Darwin’s mind. In conjunction with his reading of Charles Lyell’s work, the Wallace-Darwin Theory of Evolution eventually centred on the idea of Natural Selection. Natural Selection: Variations or changes in populations which gave organisms better chances of survival were more likely to be passed on to future generations. Included in this idea was that change was random (or without purpose) and that natural selection was based on the selective forces of a particular environment. A random variation could be quite favourable in one particular environment and of no value or even harmful in another environment. Over a period of time and a succession of generations, groups of organisms could acquire enough different (favourable) variations that they will have formed species different from their ancestors. Darwin and Wallace were only different in their emphasis on the forces of natural selection. Wallace emphasized food supply as the major selective force. In Darwin’s view, selective forces could be quite varied, with food supply being just one of many. Darwin identified four factors that govern natural selection: 1. Organisms produce more offspring than can survive, and as a result, there is competition for limited resources. 2. Individuals within a population vary, and this variation in heritable. 3. Individuals better suited to the local conditions survive to produce offspring 4. Processes for change are slow and gradual Biology 30 14 Lesson 14 Similar to Lamarck, Darwin and Wallace were not able to explain why or how variations took place. This was one contributing factor to the resistance from some against this theory. Another objection probably arose from the viewpoint which developed about the role of natural selection. Phrases adopted by some people such as “survival of the fittest”, “elimination of the unfit” and “struggle for existence” created negative feelings in some. Natural selection came to be looked upon as a disruptive force, leading to continual struggle and suffering and death for those not quite fit enough. De Vries’ Theory of Evolution In 1901, De Vries, presented his theory based on mutation. His theory stated that mutation, or suddenly appearing and well-defined inheritable variation, is a force in the evolution of species and origin of new species. This is in opposition to the slight, cumulative changes present in Darwin’s theory. This theory is accepted by most biologists as a fundamental concept in evolution. The gene is responsible for the characteristics of a species and their offspring; changes in the genes would result in changes in the characteristics of the offspring. Current Theories of Evolution The Wallace-Darwin Theory of Evolution, based on the idea of gradual species changes as a result of natural selection over long periods of time, forms a basis for many of the current thoughts. These current thoughts are not the products of any single individual, but reflect the combined ideas of many. Organisms have descended from species which have lived before them. The idea of common descent implies that certain plants, animals and other organisms could be placed into certain groups according to ancestors which they may have shared. The other idea of gradual, but steady, changes is often referred to as Gradualism. If you recall, this actually conforms to one of the principles of uniformitarianism. Environmental conditions and natural selection are still regarded as powerful forces in evolutionary changes. However, natural selection is not necessarily looked upon as the only force resulting in change. Darwin’s Theory promotes the idea of diversity and new strains of cells and organisms developing by means of natural selection. Present day scientists see natural selection as a guiding force which often actually tends to minimize or stabilize the number of changes, rather than maximize them. Selective forces may not permit that many new variations to reproduce themselves. In this way, natural selection really acts in a conservative manner. Present day scientists also try to dispel the idea of natural selection as destructive or as a force which somehow actively destroys the “unfit”. Most often, the success of a variation depends upon an organism’s ability to reproduce or pass genes on to more offspring. Biology 30 15 Lesson 14 It is mainly this difference in individual reproductive abilities, rather than being killed or destroyed at an early age, which determines whether a variation continues into future generations. Unsuccessful variations are those which do not have good reproductive abilities, but they may still live out their lives. Another line of thought among some individuals and groups relates to rate of change. Darwin’s Theory implies evolution is occurring gradually and over long periods of time. There are now some who relate “catastrophism”, or sudden, catastrophic actions, as having major effects in evolution. This idea was first mentioned under the preceding heading of Special Creation. Some groups see catastrophic events occurring periodically under divine influence. Some scientific groups also have a theory of catastrophism. The scientific view maintains that populations tend to remain fairly stable, until some major, natural events upset the normal balance in large areas or even on the entire planet. Major transitions in populations can take place at these times. Large numbers of individuals and even entire species can die out. Survivors could be quite different in traits as they continue new lines of development. To distinguish between the scientific and the divine concepts, the scientific one is sometimes referred to as Punctuated Equilibrium. Punctuated Equilibrium was put forth by American scientists Gould and Eldredge in l972, based on their observations that many species appeared suddenly and disappeared suddenly. Between appearing and disappearing, the species tended to remain relatively unchanged. Looked at another way, one could say that intervals of stability or “equilibrium” were periodically interrupted, or “punctuated”, by brief periods of change. During a change, new species would evolve from pre-existing ones. Fossil, geologic and other evidences which indicate significant changes occurring periodically in the earth’s conditions add credibility to the theory of Punctuated Equilibrium. These significant changes are often marked by mass extinctions. Two of the more notable ones were about 225 million years ago, when an estimated one-half of all marine organisms died, and the other change was between 60 and 70 million years ago. During this last interval, most of the large reptiles or dinosaurs disappeared and mammals made their appearance. The following graphic depicts the earth’s history and some highlights occurring during various time periods. Biology 30 16 Lesson 14 Era Cenozoic Period Quaternary Tertiary Mesozoic Paleozoic Precambrian Millions of Years Ago 1 Main Events Humans 64 “Modern” mammals, angiosperms ... Cretaceous 136 Dinosaurs declining; first flowering plants; small mammals increasing. Jurassic 195 Dinosaurs’ numbers peaking; first mammals, birds. Triassic 225 Reptiles and gymnosperms become prominent. Many marine species disappear. Permian 280 First gymnosperms; many insect orders. Carboniferous 345 Great numbers of ferns, amphibians; first reptiles, winged insects... Devonian 410 Many fish, insects; first trees, amphibians. Silurian 440 First land plants. Ordovician 530 First fish (vertebrates). Cambrian 570 Appearance of many invertebrates: snails, clams, sponges, corals... 4,500 Few fossils. Some monerans, algae, fungi, worms... Questions are continually raised about the reasons for the two major extinctions mentioned and about four or five others which apparently took place. The most recent speculation about the last mass extinction (especially involving dinosaurs), included the possibility of an impact between earth and a large meteor or meteors. If this occurred, a large amount of dust and gases could have entered the atmosphere, beginning significant climatic and other changes. Some scientists suggest that the extinctions were gradual rather than sudden, which would tend to downplay the possibility of an impact with a meteor. Longer term climatic changes could have been tied in with certain geologic events, such as continents slowly rising or being uplifted and inland seas retreating. Climate could have become less “tempered”. That is, the ranges could have become greater, with hot days hotter and cold days colder. Biology 30 17 Lesson 14 Increase in volcanic activities could also have played a part, with the addition of ash and dust into the atmosphere. There is even a possibility that all of the gradual changes could have been occurring when a meteor strike did take place, simply altering the speed of change at some point. Trying to establish the accuracy of various theories regarding origin and evolution is a difficult task. It is likely that there will never be completely acceptable answers to everyone. Perhaps, elements of all the various beliefs and theories have some validity. As time goes on, new methods of research and new findings will probably change some ways of thinking and perhaps even introduce completely new theories. So even as there is physical and biological evolution, there is also an “evolution” in the thinking and theories on the concept. Evidences of Evolution Many techniques have to been utilized to study the changing nature of organisms over periods of time. In some instances, evolution can be seen in laboratory situations – particularly in organisms with short generation times. Strains of viruses and bacteria subjected to certain physical conditions, chemicals or drugs (mutagens) have been found to change in the nature of their genes in relatively short periods of time. Scientists currently regard our planet as being approximately 4.6 billion years old. The oldest fossils of once living cells have been dated as being about 3.4 billion years. Many evolutionary changes among organisms take place over many years. As a result, other techniques must be used as direct human observation, even if carried out over a number of generations, is not sufficient Fossils Paleontology, or the study of plant and animal fossils, has provided a wealth of information, supporting and strengthening the idea of evolution. Fossil evidence not only provides information about previous life, but can indirectly reveal some interesting conditions as they may have existed with respect to the physical forces shaping our planet and the changing climates. Examples are: Finding fossils of tropical plants in Antarctica and a fossilized deciduous forest in an area of the Canadian Arctic has supported the theory of continental plate movements or continental drift. Significant climatic and earth surface changes could also be speculated on when fossils of obviously sea-living creatures are found in deserts or mountains. Biology 30 18 Lesson 14 Fossils are considered to be any signs of previously existing life. The following are some common forms of fossils. Preserved and frozen fossils. Actual cells or bodies of organisms themselves. There can be considered the best fossils. Quicksand, tar pits (a source of saber-toothed tiger fossils), peat bogs, tree sap and permafrost (a source of wooly mammoth fossils) are some conditions or places where remarkably well-preserved remains have been found. These are formed when an organism was quickly trapped in ice or tree sap which hardened into amber. Petrified fossils. Minerals penetrate and replace the hard parts of an organism. The result is exact replications of cell shapes or tissue formations in these petrified fossils which can provide scientists with many details. Trace fossils. The fossils show signs of organisms, although the actual remains are not present. Common trace fossils include body or body part imprints, usually in sedimentary rock layers. Outlines of entire bodies, cell features or even animal (or human) tracks leave information about organisms, even though actual remains have long disappeared. Molds. A mold of an organism forms when it is buried in sediment and then decays, leaving an empty space. Casts. When minerals fill a space left by a decayed organism, a cast of the organism is formed. Imprints. Thin objects, such as leaves, can leave imprints when the sediment that they fall into hardens. Scientists have been able to date many fossils in various ways. Knowing the ages of particular geologic strata or earth layers has been one way. Another method of testing is based on the rate of radioactive decay of certain molecules. Living bodies ingest and incorporate certain radioactive molecules into their tissues. After death, these molecules decay or break down into other forms at very specific rates. A radioactive form of carbon, Carbon 14, has a half-life of approximately 5700 years. That is, it takes about 5700 years for one-half of a sample of Carbon 14 to change to more stable molecules (of nitrogen l4), another 5700 years for onehalf of the remaining half to decay, and so on. By comparing the proportions of radioactive to non-radioactive substances in fossils to those in still-living organisms, scientists can quite accurately date many fossil finds. Carbon dating is fairly accurate for fossils not much older than 50 000 years. Another radioactive substance, Potassium 40 (Calcium-Argon) has a half-life of about 1.3 billion years and is often used for dating fossils much older than 50 000 years. Rather then testing the radioactive matter in a fossil, potassium dating is carried out on the rock matter in which the fossil is found. Proportions of radioactive to non-radioactive matter are then compared with those in relatively recent rock or earth strata. Comparisons of fossils in different earth layers and in different locations have shown changes taking place in species over time. One species which has been commonly used as an example is the horse, which evolved from a much smaller-sized, many-toed form to what it is today. Other examples of organisms that have been studied to observe transitions within a species are clams, Biology 30 19 Lesson 14 rhinoceroses and apes. Some fossils hint at divergence between major organism groups. Such an example could be that of bird-reptiles, with some fossil remains showing birdlike forms with some reptilian traits. The following illustration represents one such bird-reptile – an Archaeopteryx. Despite being found in diverse locations and kinds of places, fossils are really not that common for several reasons. Bodies of organisms, especially soft-bodied ones, tend to be broken down relatively quickly. Unless they are preserved almost immediately, as in amber or resin, low oxygen or low temperature conditions, or in some other manners, they will not remain intact very long. Places where such preservations can happen are not extensive Places were fossils may be found, such as in sedimentary rock, may not be very easy to get at or to search through. For some of these reasons, fossil records can show considerable numbers of gaps (like having pages of books missing). Gaps in fossil records especially bothered Darwin in his studies and observations. Since he felt that changes proceeded slowly and not in leaps, there should have been many long lines of fossil records. These were not readily evident in his time. Since then, many more fossils have been found and many gaps have been filled in. However, there are still many “blank spaces” in trying to trace the histories of various groups of organisms. Comparative Anatomy The study and comparison of body forms of plants and animals has probably been the most common method of trying to establish the degrees of relationships between organisms. Such comparisons also show that changes are taking place within and between groups. Observations of general conformations or external body features have been (and still are) frequently used to place individuals into specific groups. The more alike that organisms looked, the more closely related they were assumed to be. The resemblance of structure and anatomy can be classified as either homologous structures or analogous structures. Homologous structures are body parts of different (species of) organisms which appear to be derived from the same embryonic tissues. That is, they seem to have common origins. However, different environments have resulted in modifications to produce slight differences in structures and often different uses. Biology 30 20 Lesson 14 Homologous organs: these vertebrates have the same basic arrangements of bones, but they have been put to different uses. Homologous structures are interpreted as results of divergent evolution. This is an evolutionary trend where a certain group splits and the resulting sub-groups begin to show greater diversities over time. The many breeds of dogs evolving from a common wolf-like ancestor would be an example of divergence taking place. Opposite to this divergent type of change is one where some unrelated groups begin to show convergent evolution over time. This is shown by analogous structures. Analogous structure are where embryonically unrelated parts have come to function similarly. (Different origins, similar functions.) In the following diagram, the whale paddle and fish fin have similar functions, but have different origins, likewise with the bird wing and the fly wing. Examples of convergent evolution. Biology 30 21 Lesson 14 In addition to the preceding examples of convergence, other examples involve many marsupial species in Australia and placentals in other areas. A wombat in Australia is somewhat similar to the groundhog (or woodchuck) in Saskatchewan, and the Tasmanian devil has shown convergence in numerous features with badgers and wolverines. People using comparative anatomy as a means of trying to determine relationships between organisms must be careful to distinguish structures as either homologous or analogous. Studies of homologous and analogous parts, along with fossil, biochemical and other studies, are used by scientists to assist in setting up phylogenetic trees. These diagrammatic representations indicate how some scientists think certain plant or animal groups developed and their relationships with others. Vestigial Organs Vestigial organs are structures that were functional in the organisms’ ancestors but over time have become much reduced in size and often non-functional. In a human, such vestigial features could include the sometimes troublesome appendix, the remains of another eyelid and reduced facial-head muscles, such as those controlling ear movements. Biology 30 22 Lesson 14 Embryology Studies of embryos of different species can sometimes show the degree of relatedness between them. In many species, early embryonic development is very similar. As developments progress, changes become more noticeable and the less related that species are, the greater the differences. Comparative Biochemistry Scientists are able to study and compare the structures of chlorophylls, hemoglobins, enzymes, hormones and other compounds of different organisms. Proteins and, in particular, their amino acid sequencing can be analyzed in quite a bit of detail. Researchers now have the ability to determine the actual amino acid sequences of most individual proteins. As well, they have the ability to test proteins from different organisms and to determine just how many amino acids may be different. For example, one particular cell protein in humans and chimpanzees has l04 amino acids and all are the same. This similar protein in a certain type of fungus has 44 amino acids which are different. In constructing phylogenetic trees, scientists will sometimes use the number of differences in the amino acids to determine when groups may have split apart. The fewer the Biology 30 23 Lesson 14 differences, the more recent the split and the more closely related groups are. Some individuals claim to be able to use a molecular clock to date particular groups according to how many amino acids are different between them. That is, an assumption is made that amino acid changes occur at fairly regular time intervals in most species. The number of changes or differences in selected proteins between two groups can be multiplied by the estimated time for each change. This would give the estimated time when the two groups split or branched. Chromosome or Genetic Similarity When organisms reproduce, they transmit certain characteristics to their offspring. Transmission of characteristics is accomplished by a substance known as DNA, which makes up chromosomes and genes. The actual structure of DNA and the way in which it passes along an organism’s characteristics have already being examined in other parts of this course. In the preceding section on biochemistry, amino acids and amino acid sequencing were mentioned as important factors in comparing species. It is the DNA which determines the identity of the amino acids used and their sequencing in protein synthesis. Therefore, similarities or differences between organisms are really based on the genetic component. As with previous techniques, the greater the similarity between organisms in their chromosomes, the greater the chance of being more closely related. General chromosome numbers and their structures could be observed and used as a basis for making relatively fast, general comparisons of organisms. If more precise analysis is needed, scientists can study nucleotide sequencing in DNA to determine just how similar two chromosomes from different organisms are. On a general level, determining whether or not certain individuals belong to the same or related species can be done by noting the reproductive success between them. The numbers and structures of chromosomes play an important part in deciding if two individuals can mate or cross to produce fertile offspring. If an egg and a sperm differ in the numbers or structures of their chromosomes, it is unlikely that successful fertilization or reproduction will take place. Since different species do have different chromosome numbers and characteristics, this is one factor keeping them separate. In some situations, crossing between two related species can occur, with offspring being produced. However, in most instances, the offspring of such crosses are sterile or infertile. The most common cross is that between horses and donkeys. Breeding a male donkey and a female horse results in an infertile offspring – the mule. Mules have greater endurance and are stronger than horses, in addition that have the potential to grow taller than either parent. These characteristics are due to hybrid vigour. Biology 30 24 Lesson 14 Actual Evolution in Action Evidences of particular actions or situations are most reliable when actual observations are made. This applies to organic variations or evolution as readily as for other situations. As was mentioned earlier, changes in organisms have been noted – especially in those with relatively short generation times. Changes are commonly noticed in viral or bacterial strains, either in laboratories or outside of them. Other changes, recorded for study purposes, have included a population of peppered moths in England, which changed in colour as environmental conditions changed. In the 1700’s, trees in the countryside were covered by gray-green lichens. Two types of moths could be found, light coloured ones and dark coloured ones. The dark coloured ones were rare because they could be seen on the tree trunks and they were eaten. But as the Industrial Revolution occurred, trees became darkened by soot. The light moths were eaten and the population of moths became predominately dark. Another change is the squirrels in the Grand Canyon in the United States. Originally a single population, it was apparently split into two groups by a change in the course of the Colorado River. As time progressed, changes in the two groups resulted in the formation of two separate species: the Abert squirrel and the Kaibab squirrel. Microevolutions, or small changes occurring in populations, are possible to see in many other groups of animals or plants. Humans often increase the frequencies of these, especially in domesticated organisms, by selective breeding or other practices. At certain points in time, smaller changes eventually accumulate to where a species becomes quite different than it was previously. Macroevolution, or major changes occurring over a short period of time may also occur. This would result in the organism no longer being the same species it was previously. That is, if it were possible to bring them together, crossing and reproduction would not take place between individuals of the “old” and “new” groups. In studying evolution in general it is common to note many pathways coming to an end. Extinctions are common features of phylogenetic trees. Scientists estimate that over 99% of all species that lived at one time are now extinct. It is not difficult to think of examples of organisms that are presently endangered on the prairies. Some of these could include prairie dogs, burrowing owls, swift foxes, whooping cranes and bison. Biology 30 25 Lesson 14 Major Forces or Conditions Affecting the Earth’s Crust Tectonic Plate Movements In l9l5, German meteorologist and geophysicist Alfred Wegener published some of his conclusions on the concept of continental drift. The major idea was that continents at one time were more or less one large land mass. Fragmentation took place and the various land masses drifted apart. Wegener stated that the fragmentation took place about 200 million years ago. Initially greeted with some skepticism, continental drift or tectonic plate movement is now a well-established fact. Extensive studies and mappings of the crusts beneath ocean surfaces (especially with the use of satellite technologies), along with other findings, has added a great deal of information as to what has happened and even as to what is likely to happen. Movements beneath the earth’s surface, owing to its molten interior, have translated into exterior crustal movements. The lighter outer crust of the earth is made up of a series of plates, some of which are part of continents and some are part of ocean floors. These plates “float” on top of more dense, molten matter. The plates move in various ways according to the convectional movements beneath them. They move apart in places, which appears to be the main action in areas of the Atlantic Ocean presently. In other areas, they have collided or come together to add to the process of continent formation or mountain building, as one plate slides beneath the other. In still other places, plates slide past each other, scraping and deforming as they do so. The edges of plates are frequently marked by volcanic activity or earthquakes or both. Scientists now say that the land masses have fragmented, moved, come together and fragmented a number of times. The last time that they were all close together was about 200 million years ago and the name Pangaea was given to this “super” continent. It consisted of a northern continent called Laurasia and a southern land mass called Gondwanaland. The northern mass eventually fragmented into North America and Eurasia and smaller masses. The southern mass eventually produced the continents of South America, Africa, Antarctica and Australia. India, which was part of Gondwanaland, eventually pushed into the Eurasian land mass. The following graphics illustrate how scientists think movements of continental masses occurred after Pangaea fragmented. You may be interested in reading about, or seeing, some of the future projections that some scientists are making about plate movements. If you are, some of the more recent geology texts will likely have these. Biology 30 26 Lesson 14 Pangaea: 200 million years ago 135 million years ago 65 million years ago Present Biology 30 27 Lesson 14 Significance of Tectonic Plate Movements In the short time intervals that humans are familiar with, most movements of the earth’s crust are insignificant in individuals’ life times. The only major exceptions are when individuals are directly affected by some of the consequences related to these movements – such as volcanic actions, earthquakes and related actions. Less obvious are some of the indirect effects which could involve very large land areas and possibly the entire planet. Some of these could include climatic changes due to volcanic ash or dust from major eruptions. These effects are generally not as dramatic and not as obvious as the conditions which actually caused them. Over thousands and millions of years, however, plate movements do have major effects on many physical conditions. Climatic Effects of Plate Movements Plate movements would obviously affect climate. For instance, being close to the equator at one time permitted the present continent of Antarctica to experience much warmer and more moist conditions. This is reflected in some of the fossils found there being that of tropical organisms. Climate is not just determined or affected by latitude. Different positions of continents establish and affect ocean currents. These have major influences on climatic conditions and climatic patterns over adjacent land masses. Climates are also affected by the mountain ranges created by volcanism or usually by plates converging or moving together. Temperature and precipitation or moisture levels are important influences on living organisms. As these conditions changed over millions of years, they affected the developments and changes of species. Changes in climate are related to other conditions which also had effects on life forms. Some of these other conditions could have been the effects or results of climatic changes. The melting of ice caps or the onset of glaciations are included in this, along with accompanying ocean level changes. Other conditions could have themselves caused climatic changes and subsequent results. The ash or dust from volcanoes was already mentioned. Currently, the greenhouse effect and global warming is receiving considerable attention. The burning or combustion of fossil fuels is releasing carbon and other gases into the atmosphere. These have the effect of permitting short wave radiation through to the earth and keeping long wave radiation from escaping. This results in temperatures going up. Biology 30 28 Lesson 14 Another item of consideration is the depletion of some atmospheric ozone over various regions of the planet. This is allowing higher levels of ultraviolet light to reach the earth’s surface. The full significance of this on plant and animal life is not yet known but, for the most part, it does not seem to be favourable. Other Physical Influences While the preceding actions or conditions have been mentioned as playing significant roles in changes occurring on this planet, they are not the only ones. Additional ones could include atmospheric conditions (such as oxygen levels), soil conditions, water salinities and topographies or relief. Summary Signs or evidences of change among living things can take a variety of forms. Where possible, a number of these evidences are used together in attempting to establish patterns of development, changes and relationships among groups. In the process, the attentions of different groups such as paleontologists, geologists, comparative anatomists, geneticists and physiologists, may all be utilized. However, accuracy of interpretation is only as good as the data or collected information. For this reason, while some evolutionary pathways and relationships seem clear, others are not. Many of these doubtful ones will likely remain so, considering the long periods of time involved and the scarcity of information or clues at various points. Evolving or changing is a noticeable action in all that is around us. It is apparent in the non-living and living and from very small areas of our planet out into the greater expanses of our galaxies and universe. Most people accept the concept of these changes or evolutions happening. Differences of opinions do exist on actual mechanisms causing the changes and especially on the reasons for the changes actually taking place. These reasons range from Divine guidance to genetic and environmental influences. To different people, the various reasons bring different degrees of acceptance or non-acceptance. It may very well be that no one reason is totally correct and none are entirely wrong either. Perhaps if there is one reason in the “why” of evolution, it may involve some aspect of all the reasons presented thus far and some that we may not yet be fully aware of. Biology 30 29 Lesson 14