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Transcript
UNIT 4: Evolution Mr. Tamashiro Biology I Define evolution • Evolution is the cumulative change in the heritable characteristics of a population. • If we accept not only that species can evolve, but also that new species arise by evolution from preexisting ones, then the whole of life can be seen as unified by its common origins. • Variation within our species is the result of different selection pressures operating in different parts of the world, yet this variation is not so vast to justify a construct such as race having a biological or scientific basis. State conditions for Hardy-Weinberg equilibrium in a population • Derived in 1908, • Hardy-Weinberg asserts that the genetic structure of a non-evolving population remains constant over the generations. – If mating in a large population occurs randomly without the influence of: • natural selection, • the migration of genes from neighboring populations, • or the occurrence of mutations, then, the frequency of alleles and of genotypes will remain constant over time. State conditions for Hardy-Weinberg equilibrium in a population and why these conditions are not likely to appear in nature. • Such conditions are so restrictive that they are not likely to occur in nature precisely as predicted, but the Hardy-Weinberg equilibrium equation often gives an excellent approximation for a limited number of generations in sizeable, randomly mating populations. • Even though genetic recombination is taken into account, mutations, gene flow between populations, and environmental changes influencing pressures of selection on a population do not cease to occur in the natural world. State that evolution is the result of genetic changes that occur in constantly changing environments. • Populations tend to produce more offspring than the environment can support. • If the mortality rate remains lower than the natality rate then a population will keep growing. • As more offspring are produced, there will be less resources available to other members of the population. • If there is an over production of offspring this will result in a struggle for survival within the species as the resources become scarce and individuals in the population will start to compete for these. • This results in an increase in mortality rate as the weaker individuals in the population will lose out on these vital resources that are essential for their survival. OUTLINE THE EVIDENCE FOR EVOLUTION PROVIDED BY THE FOSSIL RECORD, SELECTIVE BREEDING OF DOMESTICATED ANIMALS AND HOMOLOGOUS STRUCTURES. Outline the evidence for evolution provided by the fossil record • Fossils, selective breeding and homologous structures have provided scientists with evidence that support the theory of evolution. • As they started to study fossils they realized that these were not identical but had similarities with existing organisms. This suggested that organisms changed over time. Fossil record: • A fossil is the ancient preserved remains of an organism. • The fossil can be dated from the age of the rock formation. • Sequences of fossil can show the gradual change of an organism over geological time. • Continuous fossil records are rare with most containing large time gaps until subsequent discoveries are made. • Evolution and the fossil record This is an excellent site with links to all aspects of this section of the syllabus including the fossil record. Outline the evidence for evolution provided by, selective breeding of domesticated animals • As the domestic breeds have similar characteristics to the wild ones and can still breed with them. • As selected wild individuals with desirable characteristics were bred, over time this resulted in a more desirable species from a human point of view. • This suggests that not only have these animals evolved but also that they can evolve rapidly. An example of this is the taming of wild wolves and their selective breeding in order to produce the domestic dogs we know today. Selective breeding: • man has selectively breed animals and plants for thousands of years. • If an animal posses a characteristic that is considered useful or valuable then this animal is selected for breading. • The hope then is that this characteristic will be present in the next generation and at a higher frequency than before. • In subsequent generations it may even then be possible to select from an even more advantageous characteristic. Outline the evidence for evolution provided by homologous structures. • Finally scientists have found a number of homologous structures within different species. • Many bones in the limbs are common to a number of species and therefore suggests that these have evolved from one common ancestor. Homologous structures: • All of life is connected through evolutionary history and consequently those organisms more closely connected might reasonably be expected to share common structures or homologous. • Group of organisms closely related share a common form or derived trait which has been inherited from the common ancestor. Homologous structures: • This classic example of homologous structures is the pentadactyl limb of the vertebrate. – – – a) Humerus b) Radius c) Ulna • In each example the bones are modified and adapted to the locomotion of the animal. • In homologous structures it is normal to find that parts of the structure will be modified, enlarged or reduced (vestigial). • Divergence: The pentadactyl limb structure shows adaptation and modification from a common limb (ancestor) structure. • Convergence: Two organisms with different ancestors have a limb structure that fulfills the same function but has evolved form different origins. Examples Wing of a bird and the wing of an insect. Explain the use of comparative embryology, DNA or protein sequence comparisons, and other independent sources of data to create a branching diagram (cladogram) that shows probable evolutionary relationships. cladogram • is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. • Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational phylogenetics are now very commonly used in the generation of cladograms. Explain the use of comparative embryology, DNA or protein sequence comparisons, and other independent sources of data to create a branching diagram (cladogram) • Early embryos of animals show surprisingly similar features, revealing a common ancestry. • Protein molecules essentially constitute the bottom line in studying the phenotypes of organisms. • Similarities and differences in the amino acid sequences of the same molecule (e.g. hemoglobin) taken from different species produce a phylogeny. The phylogeny revealed by studying protein structure reflects the same phylogeny as comparative anatomy and embryology, but with a much finer resolution. • The ultimate is the comparison of the base sequences of variable regions of DNA (in particular mitochondrial DNA) taken from different organisms. • The analysis of DNA provides a molecular clock against which the geological clock can be compared. State how several independent molecular clocks, calibrated against each other and combined with evidence from the fossil record, can help to estimate how long ago various groups of organisms diverged evolutionarily from one another. • The molecular clock (based on the molecular clock hypothesis (MCH)) is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. • It is used to estimate the time of occurrence of events called speciation or radiation. • The molecular data used for such calculations is usually nucleotide sequences for DNA or amino acid sequences for proteins. • It is sometimes called a gene clock or evolutionary clock. Identify evidence of change in species using fossils, DNA sequences, anatomical similarities, physiological similarities, and embryology. • • • • • • In embryology, the developing fetus is studied, and similarities with other organisms are observed. For example, annelids and mollusks are very dissimilar as adults. If, however, the embryo of a ragworm and a whelk are studied, one sees that for much of their development they are remarkably similar. Even the larvae of these two species are very much alike. This suggests that they both belong to a common ancestor. It is not, however, true that a developing organism replays its evolutionary stages as an embryo. There are some similarities with the more conserved regions, but embryonic development is subjected to evolutionary pressures as much as other areas of the life cycle. Read more: Evidence of Evolution - Species, Similar, Evolutionary, Organisms, Molecular, and Common http://science.jrank.org/pages/2610/EvolutionEvidence.html#ixzz15Sk01JKN Identify evidence of change in species using fossils, DNA sequences, anatomical similarities, physiological similarities, and embryology. Embryological Suggestions of Common Ancestry Evolution Genetics Embryo's and our Common Ancestor's http://www.youtube.com/watch?v=h3Lkac890c0 State that populations tend to produce more offspring than the environment can support. • This increases the chance of survival of the population as a whole • a single death is less disastrous in a population of 1,000 than it is in a population of 10. Explain that the consequence of the potential overproduction of offspring is a struggle for survival. • Populations of living organisms tend to increase exponentially. – More offspring are produced than the environment can support. There is a struggle for important resources such as food and space. Intraspecific competition. Some individuals survive and others die. – Characteristics in organisms differ from one another. Some have characteristics which make them better suited to survive in their environment. These are the most likely to survive. Identify and illustrate that long-term survival of species is dependent on a resource base that may be limited. • Refer back to populations: • Carrying capacity Analyze the effects of genetic drift on the diversity of organisms in a population. • Genetic Drift • It is the change brought about in the gene frequency of a population by various factors. • It may be involved in the elimination of genes pertaining to certain characters. Explain how reproductive or geographic isolation affects speciation. • The environment may impose an external barrier to reproduction, such as a river or mountain range, between two incipient species but that external barrier alone will not make them separate, full-fledged species. • Allopatry may start the process off, but the evolution of internal (i.e., genetically-based) barriers to gene flow is necessary for speciation to be complete. • If internal barriers to gene flow do not evolve, individuals from the two parts of the population will freely interbreed if they come back into contact. • Whatever genetic differences may have evolved will disappear as their genes mix back together. • Speciation requires that the two incipient species be unable to produce viable offspring together or that they avoid mating with members of the other group. • Here are some of the barriers to gene flow that may contribute to speciation. They result from natural selection, sexual selection, or even genetic drift: The evolution of different mating location, mating time, or mating rituals: • Genetically-based changes to these aspects of mating could complete the process of reproductive isolation and speciation. • For example, bowerbirds (shown to the right) construct elaborate bowers and decorate them with different colors in order to woo females. • If two incipient species evolved differences in this mating ritual, it might permanently isolate them and complete the process of speciation. Different species of bowerbird construct elaborate bowers and decorate them with different colors in order to woo females. The Satin bowerbird (left) builds a channel between upright sticks, and decorates with bright blue objects, while the MacGregor’s Bowerbird (right) builds a tall tower of sticks and decorates with bits of charcoal. Evolutionary changes in mating rituals, such as bower construction, can contribute to speciation. Lack of "fit" between sexual organs: • Hard to imagine for us, but a big issue for insects with variablyshaped genitalia! These damselfly penises illustrate just how complex insect genitalia may be. Offspring inviability or sterility: • All that courting and mating is wasted if the offspring of matings between the two groups do not survive or cannot reproduce. • In our fruit-flies-in-rottenbananas-in-a-hurricane example, allopatry kicked off the speciation process, but different selection pressures on the island caused the island population to diverge genetically from the mainland population. Geographic isolation can instigate a speciation event — but genetic changes are necessary to complete the process. Offspring inviability or sterility: • What might have caused that to happen? Perhaps, different fruits were abundant on the island. The island population was selected to specialize on a particular type of fruit and evolved a different food preference from the mainland flies. • Could this small difference be a barrier to gene flow with the mainland flies? Yes, if the flies find mates by hanging out on preferred foods, then if they return to the mainland, they will not end up mating with mainland flies because of this different food preference. Gene flow would be greatly reduced; and once gene flow between the two species is stopped or reduced, larger genetic differences between the species can accumulate. • Differing selection pressures on the two islands can complete the differentiation of the new species. Geographic patterns: • If allopatric speciation happens, we’d predict that populations of the same species in different geographic locations would be genetically different. • For example, many species exhibit regional "varieties" that are slightly different genetically and in appearance, as in the case of the Northern Spotted Owl and the Mexican Spotted Owl. • Also, ring species are convincing examples of how genetic differences may arise through reduced gene flow and geographic distance. Spotted owl subspecies living in different geographic locations show some genetic and morphological differences. This observation is consistent with the idea that new species form through geographic isolation. Explain how sexual reproduction promotes variation in a species. • Variation is essential for natural selection and therefore for evolution. • Although mutation is the original source of new genes or alleles, sexual reproduction promotes variation by allowing the formation of new combinations of alleles. • Two stages in sexual reproduction promote variation. – Meiosis allows a huge variety of genetically different gametes to be produced by each individual – Fertilization allows alleles from two different individuals to be brought together in one new individual. Heredity • Every organism requires a set of coded instructions for specifying its traits. • For offspring to resemble their parents, there must be a reliable way to transfer information from one generation to the next. • Heredity is the passage of these instructions from one generation to another. The DNA molecule provides the mechanism for transferring these instructions. Compare and contrast the characteristics of asexual and sexual reproduction. Asexual Heredity • In asexually reproducing organisms, all the genes come from a single parent. • As asexually produced offspring are produced by the cell division process of mitosis, all offspring are normally genetically identical to the parent. Sexual Heredity • • • In sexually reproducing organisms, the new individual receives half of the genetic information from its mother through the egg and half from its father from his sperm. Sexually produced offspring resemble, but are not identical to, either of their parents. Some reasons for these variations between sexually reproduced offspring and their parents include crossing over when gametes are formed in each parent and genetic recombination, which is the combining of the genetic instructions of both parents into a new combination in the offspring when fertilization occurs. Explain how natural selection leads to evolution. • Greater survival and reproductive success of individuals with favorable heritable variations can lead to change in the characteristics of a population. • Natural selection is the process by which species adapt to their environment. • Natural selection leads to evolutionary change when individuals with certain characteristics have a greater survival or reproductive rate than other individuals in a population and pass on these inheritable genetic characteristics to their offspring. • The reason that natural selection is important is that it’s the central idea, stemming from Charles Darwin and Alfred Russel Wallace, that explains design in nature. • It is the one process that is responsible for the evolution of adaptations of organisms to their environment. Explain how natural selection leads to evolution. • Simply put, natural selection is a consistent difference in survival and reproduction between different genotypes, or even different genes, in what we could call reproductive success. Charles Darwin studied beak variation of finches on the Galapagos Islands as evidence of natural selection. Illustration from BSCS, Biological Science: Molecules to Man, 1963. Explain why natural selection acts on the phenotype rather than the genotype of an organism. • Since Natural Selection acts only on the phenotype variability is maintained in the population. • When Natural Selection is operating in a population it actually tends to reduce variation by removing individuals (and their genes) which exhibit extreme phenotypes • For example: very tall, very small etc. Explain how natural selection determines the differential survival of groups of organisms. • Natural Selection can cause evolution only when something upsets the dynamic equilibrium in a population and genes which once were not favored help individuals within a population adapt to the new conditions. • In order to understand now this can come about we need to define 3 terms: gene pool, allele frequency, and genetic equilibrium. • • A gene pool is all the genes in all the individuals of a population. • • Allele frequency is the proportion of each allele in the gene pool • • Genetic equilibrium occurs when allele frequencies in a population remain the same from one generation to the next. Explain how natural selection determines the differential survival of groups of organisms. Changes in Genetic Equilibrium (Microevolution) • What are the mechanisms that cause the genetic equilibrium to change? • The mechanisms include: • Mutation -- provides the variations that can be acted upon by natural selection. Even though mutations are: – relatively rare – rarely beneficial – often recessive and hidden Explain how natural selection determines the differential survival of groups of organisms. • • Mutations are the only source of new genetic material added to a gene pool. In a species all other mechanisms of evolution merely shuffle the genetic material that is already present. These include: – Genetic recombination during cross over events in meiosis and the shuffling of genes caused by the random pairing of gametes during fertilization can bring about a change in gene frequencies. – Genetic drift results in changes in allele frequencies because of random (chance) fluctuations possible in small populations. Flip a coin 4 times and it's possible that it could come up heads every time. It is much less possible (actually impossible) that a coin tossed 1000 times would always come up heads. The larger the sample size the more likely the expected ratio (500 heads: 500 tails) – Gene flow occurs when individuals from different populations of the same species migrate from one group to another. Since genes are carried along with the organisms the gene pool must increase or decrease in size. – Gene flow between different species although once thought to be impossible, then rare, is now acknowledged as a major source of change in gene frequency, especially in prokaryotes Explain how natural selection determines the differential survival of groups of organisms. Variation in Natural Selection • When the frequencies of genes change by whatever mechanism Natural Selection can operate on the individuals to establish a new genetic equilibrium in one of the following ways: – Stabilizing Selection – Directional Selection – Disruptive Selection Stabilizing Selection • favors the average individual in a population. • Variation at the extremes is selected against by opposing biotic or abiotic factors in the environment. • An example might be if an organism is too small to compete for a mate or too large to hide from a predator. Only those organisms of average size are least vulnerable. Directional selection. • If one of the opposing forces in the environment dominates selection occurs favoring genes which help the population overcome the new pressure until genetic equilibrium is reestablished. • An example is a pesticide (such as DDT) which kills 99% of a mosquito population. The population of the one percent that survives quickly increases along with the gene or genes which helped them overcome the toxic effects of the pesticide. Disruptive selection. • In some cases the average condition becomes the target of environmental pressure. • Individuals at either extreme are better able to cope with the change and their genes increase in frequency. Explain two examples of evolution in response to environmental change; one must be antibiotic resistance in bacteria. (Other examples could include: the changes in size and shape of the beaks of Galapagos finches; pesticide resistance, industrial melanism or heavymetal tolerance in plants.) • Before Penicillin was invented, bacteria was the leading cause of death. • However, once it began to be used, since it's an antibiotic, some individuals of bacteria may carry the gene Penicillinase, which codes for an enzyme that deactivates Penicillin, making them resistant to an antibiotic such as Penicillin. • Thus, when it is indeed used, they will be the only ones left to reproduce and new bacteria will also be resistant to the antibiotic. • The Peppered Moth is another example of evolution in response to environmental change. • When Britain began industrialising, soot would come from factories and land on trees. • A species of peppered moth with a lighter color vanished and those with a darker color flourished because they could hide themselves easily. Distinguish between the accommodation of an individual organism to its environment and the gradual adaptation of a lineage of organisms through genetic change. • • • • • Living organisms may adapt to changing environments through nongenetic changes in their structure, metabolism, or behavior or through natural selection of favorable combinations of alleles governing any or all of these processes. Genetic and behavioral adaptations are sometimes difficult to identify or to distinguish without studying the organism over a long time. Physical changes are slow to develop in most organisms, requiring careful measurements over many years. Examining fossil ancestors of an organism may help provide clues for detecting adaptation through genetic change. Genetic change can institute behavioral changes, making it all the more complicated to determine whether a change is solely a behavioral accommodation to environmental change. • • • • • Through the use of print and online resources in library-media centers, students can research the effects of encroaching urbanization on undeveloped land and consider the effects on specific species, such as the coyote (not endangered) and the California condor (endangered). Such examples can illustrate how some organisms adapt to their environments through learned changes in behavior, and others are unsuccessful in learning survival skills. Over a long time, organisms can also adapt to changing environments through genetic changes, some of which may include genetically determined changes in behavior. Such changes may be difficult to recognize because a long time must elapse before the changes become evident. Studies of the origins of desert pup fish or blind cave fish may help students understand how gradual genetic changes in an organism lead to adaptations to changes in its habitat.