5.1 EVOLUTION
Nature of science: Looking for patterns, trends and discrepancies—there are common features in the bone structure of vertebrate limbs despite their varied use. (3.1)
Understandings:
• Evolution occurs when heritable characteristics of a species change.
• The fossil record provides evidence for evolution.
• Selective breeding of domesticated animals shows that artificial selection can cause evolution.
• Evolution of homologous structures by adaptive radiation explains similarities in structure when there are differences in function.
• Populations of a species can gradually diverge into separate species by evolution.
• Continuous variation across the geographical range of related populations matches the concept of gradual divergence.
Applications and skills:
• Application: Development of melanistic insects in polluted areas.
• Application: Comparison of the pentadactyl limb of mammals, birds, amphibians and reptiles with different methods of locomotion.
• Evolution occurs when heritable characteristics of a species change.
• The fossil record provides evidence for evolution.
• Selective breeding of domesticated animals shows that artificial selection can cause evolution.
• Evolution of homologous structures by adaptive radiation explains similarities in structure when there are differences in function.
• Populations of a species can gradually diverge into separate species by evolution.
• Continuous variation across the geographical range of related populations matches the concept of gradual divergence.
Applications and skills:
• Application: Development of melanistic insects in polluted areas.
• Application: Comparison of the pentadactyl limb of mammals, birds, amphibians and reptiles with different methods of locomotion.
what is evolution
Evolution is defined as the process of cumulative change in the heritable characteristics of a population. The word heritable means that the changes must be passed on genetically from one generation to the next, which implies that evolution does not happen overnight. The word cumulative is in the definition to stress the fact that one change is usually not enough to have a major impact on a species. Finally, the word population is in the definition because the changes do not affect just one individual.
Over time, if enough changes occur in a population, a new species can arise in a process called speciation. The members of the new population will be different enough from the pre-existing population that they came from that they will no longer be able to interbreed. Such a process is rarely observable during a human lifetime. It has been argued that once evolution by natural selection is understood, many of the mysteries of nature are revealed.
Over time, if enough changes occur in a population, a new species can arise in a process called speciation. The members of the new population will be different enough from the pre-existing population that they came from that they will no longer be able to interbreed. Such a process is rarely observable during a human lifetime. It has been argued that once evolution by natural selection is understood, many of the mysteries of nature are revealed.
the fossil record and evolution
Fossils are the petrified remains or traces of animals and plants, and the fossil record is the accumulation of evidence from these remains and traces, such as skeletons and footprints. Palaeontologists have been collecting and classifying fossils in an organized fashion for almost two centuries.
• Overall, the life that existed more than 500 million years ago was vastly different in appearance from life today.
• Although planet Earth has had extensive oceans for most of its existence, fish fossils have only been found in rocks 500 million years old or younger (less than 15% of the 3.5 billion year existence of life on our planet).
• Although most of the top predators today are mammals such as bears, orcas, big cats, and wolves, none of them existed at the time of the dinosaurs or before.
• Apart from organisms such as certain types of sharks, cockroaches, and ferns, the majority of living organisms today have no similar form in the fossil record.
• Overall, the life that existed more than 500 million years ago was vastly different in appearance from life today.
• Although planet Earth has had extensive oceans for most of its existence, fish fossils have only been found in rocks 500 million years old or younger (less than 15% of the 3.5 billion year existence of life on our planet).
• Although most of the top predators today are mammals such as bears, orcas, big cats, and wolves, none of them existed at the time of the dinosaurs or before.
• Apart from organisms such as certain types of sharks, cockroaches, and ferns, the majority of living organisms today have no similar form in the fossil record.
ageing fossils
The age of a rock can be determined by carefully examining differences in the ratios of isotopes. Isotopes are versions of atoms that are heavier or lighter than other versions of the same atom (carbon-14 has more mass than carbon-12). If a fossil of a bone or shell has a high level of carbon-14, for example, it is younger than a bone or shell that has a very low level of carbon-14. This is because carbon-14, also written 14C, is
radioactive but slowly loses its radioactivity; as it gives off its radioactivity, it changes its identity into another atom, nitrogen-14. This process of a radioactive parent isotope changing into a stable daughter isotope is called decay. The speed at which this happens is expressed as an isotope’s half-life. Half-life is defined as the time it takes for half of the parent isotope to decay into a stable daughter isotope.
The half-life of 14C is 5730 years, meaning that, when an animal dies, its bones will have lost half their 14C after 5730 years. After 11 460 years, half of that amount (now 25% of the original amount) will have decayed. Why is this important? Because by looking at the ratio of radioactive 14C to stable 14N, it is possible to determine the age of a fossil. If there is 12.5% of the radioactive isotope and 87.5% of the stable isotope, that means that three half-lives have gone by and the fossil is 17 190 years old. After a certain number of half-lives, there are so few 14C atoms left that it is difficult to determine the age of the fossil with any accuracy. Fortunately, if there is insufficient 14C, there are other radioactive isotopes that have much longer half-lives, such as 40K (potassium-40).
radioactive but slowly loses its radioactivity; as it gives off its radioactivity, it changes its identity into another atom, nitrogen-14. This process of a radioactive parent isotope changing into a stable daughter isotope is called decay. The speed at which this happens is expressed as an isotope’s half-life. Half-life is defined as the time it takes for half of the parent isotope to decay into a stable daughter isotope.
The half-life of 14C is 5730 years, meaning that, when an animal dies, its bones will have lost half their 14C after 5730 years. After 11 460 years, half of that amount (now 25% of the original amount) will have decayed. Why is this important? Because by looking at the ratio of radioactive 14C to stable 14N, it is possible to determine the age of a fossil. If there is 12.5% of the radioactive isotope and 87.5% of the stable isotope, that means that three half-lives have gone by and the fossil is 17 190 years old. After a certain number of half-lives, there are so few 14C atoms left that it is difficult to determine the age of the fossil with any accuracy. Fortunately, if there is insufficient 14C, there are other radioactive isotopes that have much longer half-lives, such as 40K (potassium-40).
artificial selection and evolution
The fossil record is far from complete, but the science of breeding domesticated animals, for example cattle, horses, dogs, sheep, and pigeons, provides a good record of recent changes in heritable characteristics.
By watching which males mate with which females, animal breeders can see which characteristics the offspring will have. Of the offspring produced, not all will be equally valuable in the eyes of a breeder. Some cows produce better milk, other cows produce better meat; one breeder may be interested in better milk, another in better meat. Over the years, breeders have learned to choose the males and females with the most desirable genetic characteristics and breed them together.After practising selective breeding for dozens and sometimes hundreds of generations, farmers and breeders realized that certain varieties of animals now had unique combinations of characteristics that did not exist before.
Although this is evidence that evolution is happening as a result of an accumulation of small changes over time, the driving force is, of course, human choice. The farmers and breeders choose which animals will reproduce and which will not. This is called artificial selection and it should be obvious that it is certainly not the driving force of evolution in natural ecosystems.
By watching which males mate with which females, animal breeders can see which characteristics the offspring will have. Of the offspring produced, not all will be equally valuable in the eyes of a breeder. Some cows produce better milk, other cows produce better meat; one breeder may be interested in better milk, another in better meat. Over the years, breeders have learned to choose the males and females with the most desirable genetic characteristics and breed them together.After practising selective breeding for dozens and sometimes hundreds of generations, farmers and breeders realized that certain varieties of animals now had unique combinations of characteristics that did not exist before.
Although this is evidence that evolution is happening as a result of an accumulation of small changes over time, the driving force is, of course, human choice. The farmers and breeders choose which animals will reproduce and which will not. This is called artificial selection and it should be obvious that it is certainly not the driving force of evolution in natural ecosystems.
evolution of homologous structures by adaptive radiation
Other evidence for evolution comes in the form of homologous anatomical structures, which are similar in form but which are found in seemingly dissimilar species. One of the most striking examples of this is the five-fingered limb found in animals as diverse as humans, whales, and bats. Such limbs are called pentadactyl limbs because ‘penta’ means five and ‘dactyl’ refers to fingers. Although the shape and number of the bones may vary, the general format is the same, despite the fact that the specific functions of the limbs may be very different. Darwin explained that homologous structures were not just a coincidence but evidence that the organisms in question have a common ancestor.
species divergence
The process of an evolving population changing significantly enough so that the production of offspring with the original population becomes impossible is called speciation. In short, two populations of a species have diverged (separated), and a new species has evolved from an old one; both species will then continue on their separate ways.
Theory of knowledge:
• Evolutionary history is an especially challenging area of science because experiments cannot be performed to establish past events or their causes. There are nonetheless scientific methods of establishing beyond reasonable doubt what happened in some cases. How do these methods compare to those used by historians to reconstruct the past?
• Evolutionary history is an especially challenging area of science because experiments cannot be performed to establish past events or their causes. There are nonetheless scientific methods of establishing beyond reasonable doubt what happened in some cases. How do these methods compare to those used by historians to reconstruct the past?
adaptive radiation
Adaptive radiation occurs when many similar but distinct species evolve relatively rapidly from a single species or from a small number of species. This happens as variations within a population allow certain members to exploit a slightly different niche in a more successful way. A niche is a position or role within a community of an ecosystem. By natural selection and the presence of some kind of barrier, a new species can evolve. A barrier separating populations might be a mountain range or a body of water. The reason why there are so many different species of lemur with different specialties is because of adaptive radiation.
continuous variation and the concept of gradual divergence
In the picture bellow the species come from a common ancestor. If any two of the species tried to mate, they would not successfully produce fertile offspring. In some cases, the branches of the phylogenetic tree can become spaced so far apart that the species, although once closely related, do not physically resemble each other anymore. For example, when comparing a bird that has a long, thin beak to another with a short, fat beak, it is difficult to imagine that they are both descendants from the same species. And yet biologists have observed this in many species, notably ones that are spread over a wide geographical area.
trasient polymorphism
Within a population there is often more than one common form. Different versions of a species are referred to as polymorphisms (meaning many shapes) and can be the result of a mutation. One example of such an organism is Biston betularia, the peppered moth, which lives in temperate climates.
This species of moth can have a peppered (grey) form or a melanic (black) form; the melanic form is a rare mutation that usually affects less than 1% of a population. The grey form is well camouflaged against light-coloured surfaces, such as tree branches covered with lichens. One of the reasons why they are much more numerous in the population is that black moths are seen more easily against light-coloured lichens and thus are more frequently preyed upon by birds.
This species of moth can have a peppered (grey) form or a melanic (black) form; the melanic form is a rare mutation that usually affects less than 1% of a population. The grey form is well camouflaged against light-coloured surfaces, such as tree branches covered with lichens. One of the reasons why they are much more numerous in the population is that black moths are seen more easily against light-coloured lichens and thus are more frequently preyed upon by birds.