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Bio - Evolution • Various theories of evolution and life exist: – 300 BC Greeks theorized that an organism’s form is related to its function. – 1769 Charles Bonnet observed that fossils do not resemble modern organisms, and theorized that they are from a previous era. He is the first to use the term ‘evolution’ to describe the development of diverse life-forms. – 1809 Jean-Baptiste Lamarck reasoned that the fossils of extinct animals were the ancestors of those living today. • Organisms constantly strive to improve themselves • The Principle of Use and Disuse – body structures develop or waste away depending on their use or disuse • Principle of Inheritance of Acquired Characteristics – once a structure is modified, that organism will pass it on to its offspring. – Weismann would disprove Lamarck’s theory by showing that although an organism may adapt to its surroundings during its lifetime, these adaptations are not passed on to offspring. 1 Bio - Darwinian Evolution • During Darwin’s voyage on the HMS Beagle, he found evidence that challenged the accepted idea that all critters had been created exactly as they are now seen, and were unchanging. • Charles Lyell had written in Principles of Geology that the surface of the Earth had changed slowly over many centuries. Darwin’s observations of different islands and shorelines presented questions that could be explained only thru a similar process of slow change over time. – Ex: Fossils of armadillos that closely resembled, but were not exactly like the living ones in the same area. 2 Bio - Darwinian Evolution • In the Galapagos Islands, Darwin noted that many of the species of plants and animals were very similar to those on the coast of South America, but were not identical. He would propose that a common ancestor species had migrated to the islands, and its offspring developed differently over time from the offspring of the same species on the mainland. • Darwin named this idea of change over time as descent with modification – evolution. 3 Change Over Time…… 4 Bio - Darwinian Evolution • Darwin looked at the practice of artificial selection and hypothesized that there was a force in nature that acted the same way. – Artificial selection is the purposeful breeding of animals with specific traits, in the hopes that those traits would be passed on to offspring. ….. i.e. dog breeding, etc. 5 http://images.google.com/imgres?imgurl=http://www.botgard.ucla.edu/html/botanytextbooks/economicbotany/images/Capsicum/Capsicu mtypes.jpg&imgrefurl=http://www.botgard.ucla.edu/html/botanytextbooks/economicbotany/Capsicum/a1099tx.html&h=298&w=356&sz= 31&hl=en&start=5&um=1&tbnid=eAaW6NYUc21KlM:&tbnh=101&tbnw=121&prev=/images%3Fq%3Dartificial%2Bselection%26um %3D1%26hl%3Den%26client%3Dfirefox-a%26rls%3Dcom.google:en-US:official%26sa%3DG Bio - Darwinian Evolution • Thomas Malthus in 1798 proposed that human populations are capable of increasing faster than their food supply. – We do not outdistance our food supply because our population growth is slowed by death due to disease, famine, war. 6 Bio - Darwinian Evolution • Darwin applied Malthus’ idea to all species populations – the idea that one organism has the potential to produce many offspring in its lifetime, BUT only a limited number of those offspring are able to survive due to environmental limiting factors. – Only the individuals that survive will be able to mate and reproduce, so……”Individuals that have physical or behavioral traits that better suit their environment are more likely to survive and will reproduce more successfully than those that do not have such traits.” ----- natural selection – In time, the individuals that carry these favorable characteristics increase in a population and the individuals without decrease, so the nature of the population changes. ----- evolution (change over time) – Since different habitats present different challenges, organisms of a species will differ because it has changed in response to its specific environment ----adaptation. • Adaptation – feature that has become common in a population due to the selective advantage it provides. 7 Bio - Darwinian Evolution • Darwin wrote a paper on his theory of evolution in 1844, but would not publish his ideas until 1858, when Alfred Russel Wallace, a naturalist, asked him to help get his essay published – Wallace’s essay also proposed evolution by natural selection, so Darwin presented his manuscript with Wallace’s essay at a public scientific meeting. • Though there was very strong resistance to the ideas, the evidence and arguments made people think and slowly the idea gained acceptance by other biologists. • Later discoveries, especially in the field of genetics, have given more information on how natural selection can bring about evolution of a species, resulting in some modernization and tweaking of the original hypothesis, but NOT fundamentally changing it. 8 Bio - Darwinian Evolution • Darwin’s Theory of evolution by natural selection is supported by four main points: 1. Variation (differences among individuals in a species) exists within the genes of every population or species as a result of random mutations and translation errors. 2. Because of this variation, in a particular environment, some individuals of a population or species are better suited to survive and have more offspring (natural selection). 3. Over time, the traits that make certain individuals of a population able to survive and reproduce, tend to spread thru the population. 4. There is overwhelming evidence from fossils and many other sources that living species have evolved from organisms that are now extinct. 9 • The evolutionary history of a species is called phylogeny. Bio – Darwinian Evolution - Updated 1. Now that we know genes are responsible for inherited traits, we can see that natural selection results in certain forms of a trait becoming more common in a population…..Natural selection causes the frequency of certain alleles to increase or decrease over time. • 10 New variations caused by mutations and recombinations of alleles during sexual reproduction provide endless sources of new variation for natural selection to act on. The Fossil Record Fossils provide a record of Earth’s past life-forms, and present evidence that species have changed over time….fossilized species resemble but are different from living individuals. – – – 11 Paleontologists (scientists who study fossils) use radiometric dating to determine the age of fossils, and are fairly accurate. Once the fossils and/or sediment surrounding them are dated, then fossils of organisms can be arranged from oldest to youngest, and a pattern of evolution can be seen. Darwin predicted that intermediate forms would be found that would bridge the gap between organisms that are extinct and living organisms…..and many have been. • The fossil record is the only evidence we have of many organisms that are now extinct. However, the fossil record is not, and will probably never be complete. – Some critters live in environments that do not form fossils, as fossils are generally formed when the organism or its traces are quickly buried by sediments carried by water, wind or volcanic eruptions • Environments that are conducive to the formation of fossils include wet lowlands, slow-moving streams, lakes, shallow seas, and areas near volcanoes that spew volcanic ash. • Upland forests, mountains, grasslands and deserts generally do not produce a large amount of fossils. – Fossils only form if the animal remains have not been scattered, eaten or decayed before being buried. • Animals with hard exoskeletons like arthropods, are more likely to be fossilized than a soft-bodied organism. • Even incomplete, the fossil record is strong evidence for evolution. 12 Bio – Evidence for Evolution • Evidence of new species evolving, is found by comparing several different aspects of the organisms – living and fossils. – Homologous structures – traits that are similar in different species because they share a common ancestor. • Ex: Forelimbs of vertebrates are similar – arm/hand in humans, front feet of dogs, wings in birds 13 Bio – Evidence for Evolution – Vestigial organs – structures or organs that seem to serve no useful function. • Ex: appendix in humans, hind legs on whales 14 Bio – Evidence for Evolution – Analogous structures – traits that are similar in function, but are not inherited from a common ancestor. 15 Bio – Evidence for Evolution – Biochemistry – Species that share a recent common ancestor, should have fewer differences in the proteins and nucleotide sequences…..a long ago common ancestor would result in more differences in related species’ proteins and nucleotide sequences. • Ex: Amino acid differences from human Hemoglobin Protein: – – – – – 16 Gorilla – 1 Rhesus monkey – 8 Mouse – 27 Frog – 67 Lamprey - 125 Bio – Evidence for Evolution – Embryology – embryos of different species may have almost identical patterns of development, suggesting that they share common genes for embryonic development, and therefore a common ancestor. • At some point during embryonic development, all vertebrates have a tail, buds that become limbs, and pharyngeal pouches (develop into structures in the throat in humans). 17 Bio – Natural Selection • There are four driving points of natural selection for all real populations….. – All populations have genetic variation – The environment presents challenges to successful reproduction – Individuals tend to produce more offspring than the environment can support, so that individuals of a population must compete to survive. – Individuals that are better suited to the challenges survive and reproduce more than individuals who are poorly suited to the challenges. 18 Bio – Natural Selection • The process of forming new species (members of a species can only breed successfully within their own species) is called speciation. – Speciation results from the divergence of a species. • Divergence refers to an accumulation of differences between groups of the same species they adapts to different environments. • Subspecies are the first step of speciation – the organisms of the species have developed differing adaptations, but are still able to interbreed successfully. 19 An experiment demonstrating allopatric speciation in the fruit fly (Drosophila pseudoobscura) conducted by Diane Dodd. A single population of flies was divided into two, with one of the populations fed with starchbased food and the other with maltose-based food. After the populations had diverged over many generations, the groups were again mixed; it was observed that the flies would mate only with others from their adapted population. Mechanisms for Evolution • Individuals do not evolve – populations do. • In any population, a specific gene will occur in a percent of the individuals – this is called allele frequency. – If the frequency of that gene remains fairly constant, then the population is said to be in genetic equilibrium. – These population are not evolving – little to no change is occurring because it is not needed. 20 Mechanisms for Evolution • Many things can alter the percentage of genes in a given gene pool, which will result in evolution. • Natural Selection is the predominant mechanism of evolution for larger populations and established gene pools. **Remember, natural selection acts as a result of genetic variation. – Stabilizing Selection favors average individuals in a population – variation outside the norm makes the individual less likely to survive. – Directional Selection favors an extreme of a trait, eliminating the average and opposite extremes. – Disruptive selection favors individual either extreme, but eliminates the average. 21 Mechanisms for Evolution • In small populations, other factors can act to spur an evolutionary change in that population. • Mutations - disruptions in an individual’s genetic code – Many are lethal and will thus be quickly eliminated – Useful mutations may result in evolution through natural selection • Ex: The recessive gene that causes sickle cell anemia also provided immunity to malaria • Genetic Flow occurs when individuals move in or out of a population, resulting in new genes being introduced to one population and becoming unavailable to another. 22 Mechanisms for Evolution • Genetic Drift – alteration of allelic frequencies by chance events. – Small populations becoming isolated thru belief or religion, or from environmental factors • Amish in Pennsylvania – percentage of a recessive allele that results in short arms and legs, and extra fingers and toes is 1/14 as opposed to 1/1000 in the rest of the US • Blue People of the Troublesome Creek, Kentucky 23 Mechanisms for Evolution – Geographic isolation may separate similar populations from one another, so that they evolve separate from one another, possibly resulting in speciation over time. • Lava flows…..islands breaking from the mainland…. • Eventually the two different populations that have been separated from one another, may become so different, that they can no longer interbreed. 24 Mechanisms for Evolution – Reproductive isolation occurs when formerly interbreeding populations can no longer mate and produce fertile offspring. • Genetic material is so different that fertilization cannot occur • May also result as a difference in behaviors….. – One population mates in the fall, the other in spring…..or one is nocturnal and the other is diurnal. 25 Mechanisms for Evolution – Polyploidism (individual or species with a multiple of the normal set of chromosomes) can also cause speciation….. • Remember, polyploidism is caused by total nondisjunction during meiosis – none of the chromosomes separate, so one egg or sperm gets twice as many as it is supposed to have. • The polyploid individuals will not be able to mate and produce fertile offspring with normal individuals in its population. • But, they may be able to interbreed with other same polyploids, and have fertile offspring, resulting in a totally new species (since members of a species should have the same number of chromosomes, with small exceptions due to genetic mutations, i.e. Trisomy 21, etc) • Common in plants, not so much in animals 26 Bio – Darwinian Evolution - Updated • There are two tempos of evolution….both have evidential support and neither has been shown to be the only correct possibility…… • • 27 Gradualism in which change resulting in a new species occurs gradually over a long period of time. Punctuated equilibrium, where a species may remain unchanged for very long periods of time, only to undergo rapid spurts of change as a result of major environmental changes. Patterns of Evolution • Divergent radiation – species that were once very similar, gradually become less and less alike. – Adaptive radiation: One ancestral species evolves into an array of species, to fit differing habitats…common on islands. • Ex: Darwin’s finches 28 Patterns of Evolution • Convergent evolution – distantly related organisms evolve similar traits. – Share a similar environment, so make some of the same adaptations over time. – They are not related, and do not interbreed, they just become more and more alike in 29 mantispid preying mantis