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EVOLUTION Evolution can be defined as genetic change in a line of descent through successive generations. Evolution is the processes of biological and organic change in organisms by which descendants come to differ from their ancestors. Shared ancestry is one of the core ideas in evolution. Taxonomists are scientists who organize all living (and fossil) organisms to determine Phylogenetic relationships. For example: The Phylogenetic tree above looks at the relationships among invertebrates (animals without backbones). Invertebrates are said to be monophyletic in that they all evolved from a single common ancestor (ancestral colonial choanoflagellates). Chordates are vertebrates (animals with backbones) like us. Our closest relative in the invertebrates are Echinoderms (starfish etc.). You may ask what do we have in common with starfish? We are both deuterostomes meaning our embryos have a common developmental pattern. Taxonomy places organisms in a hierarchy of classification. Every organism has a binomial scientific name. For example: humans are Homo sapiens. The first part of the scientific name (Homo) is the genus (a group of closely related species). The last part of the binomial name is the species. The classification system appears as follows: Kingdom Animalia Heterotrophs (get their food from digesting other organisms) Phylum Chordata Animals with a notochord etc. Class Mammalia Animals that nurse their young Order Primates Animals with nails instead of claws Family Homonidae Animals that walk upright Genus Homo Animals that use tools Species sapiens Modern humans Charles Darwin - 1831 – traveled the world on the HMS Beagle - read and studied other scientist of his day – such as: Thomas Malthus – who discovered that organisms tend to outgrow their resources, so its members must compete for the resources\ remaining Georges Cuvier – the father of paleontology would discovered that fossils are relics of the past. Charles Lyell - geologist who discovered the Earth is really very old. Microevolution – cumulative genetic changes that give rise to new species. Macroevolution – large-scale patterns, trends, and rates of change among groups of species. Evolution occurs in populations not in individuals. A population is a group of individuals occupying a given area. Populations have a great deal of variation. Just look at the differences in the individuals sitting around you. Individual humans have the same morphological traits (2 arms, 2 legs etc.), their bodies function in the same way (for example to digest food), and they have the same behavioral traits. However, the details of the traits vary from individual to individual (for instance, height, hair color, skin color etc.) Gene Pool different alleles (i.e. red hair vs black hair) exist in a population. These genes make up the populations gene pool. If a gene gains new prominence in a population (like almost everybody has red hair) then it is said that microevolution has occurred. Mutations occur at the level of DNA. One change of a nucleotide can result in major changes in the phenotype (what the individual looks like). Microevolution leads to new species as small changes accumulate in the population to produce new species. These small changes over generations are referred to as natural selection. The main points of natural selection are as follows: A. The individuals of a population vary in their body form, functioning, and behavior. B. Many variations can be passed from generation to generation. C. Some traits are more advantageous than others in a particular environment. D. Those traits that allow an individual to survive and pass on that trait to the next generations or more fit. E. A population is evolving when some forms of a trait become more or less common in the population. F. Over time, shifts in the makeup of gene pools have led to the diversity of life forms on Earth. Natural selection can change the gene frequency in a population. This is known as Genetic drift. Genetic drift occurs most rapidly in small, isolated populations. This can occur to several ways including: 1. The Founder effect. A small populations inhabits an area where that species previously didn’t exist (like an island). Purely by chance some alleles are likely to be absent in the new population. 2. The Bottleneck effect. A large population is reduced to a very small population and some alleles disappear. 3. Gene flow can change when individuals enter or leave a population. What is a "Species" - a species is a reproductively isolated group of organisms. In other words, they share a common gene pool. How fast does evolution occur? - evolution can occur gradually (by natural selection) or in bursts. Species tend to remain constant for long periods of time and then quickly evolve. This is referred to as "Punctuated Equilibrium". The driving force behind punctuated equilibrium appears to be major environmental change. Fossils – only the hard parts of animals and plants become fossilized. For example: bones, teeth, shells, and seeds. The organism to be fossilized must be buried in sediments or volcanic ash. With time, minerals replace to biological matter making fossils. Fossilization is a rage process. Many fossils will not be uncovered because the layers they are in are deep in the Earth. In addition, speciation occurs in a small area, making it difficult to find transitional fossils. Biogeography - the study of the world distribution of animals and plants asks the basic question of why animals appear where they do. Species occur where they do because they evolved there from ancestral species. Comparative Morphology – reconstructing evolutionary history through comparison of morphological characteristics. Homologous structures have a common embryological development and evolutionary history but may have different functions. For example: bats and humans have the same bone structure in their arms but bats use their arms to fly and we use ours to swing and grasp tools. In this case we would say bat arms and human arms are homologous. Analogous structures have the same functions but different evolutionary origins. For example: fly wings and bat wings are analogous because they have the same function but they are not homologous because they come from different evolutionary origins. Comparing different developmental patterns The embryos of diverse organisms develop similarly and have nearly identical stages. Differences in adults are often the result of mutations that altered the onset, rate, or time of completion of certain developmental steps. For example: see the figure in your text comparing the development of chimp versus human skulls. The differences in adults of these two species comes about as the result of differences in growth patterns. Comparing Biochemistry – DNA comparisons tell us how we are related to other species. For example: Humans and chimps are better than 98% identical. In fact, humans and chimps are closer to each other than crocodiles are to alligators, or horses are to zebras. Extinction – is a natural occurrence. Will over 90% of all species that have lived have gone extinct. Mass extinction often occurs as the result of a catastrophic event such as an asteroid collision. Mass extinction is followed by the sudden appearance of many new species due to adaptive radiation where existing animals evolve to fill vacant niches. Human Evolution – probably started when our ancestors became bipedal. The ability to do this required changes to the shape of the spine, hips and legs. The structure of the human hand allowed humans to become tool users. Changes in dentition allowed humans to become omnivores and greatly expand their diet. Enlargement of the brain case enabled humans to develop better behaviors for survival. Early Earth - Earth began about 4.5 billion years ago. By four billion years ago the Earth's crust had solidified. Oxygen was in short supply or totally absent. The atmosphere was composed mainly of gaseous hydrogen (H2), nitrogen (N2), carbon monoxide (CO), and carbon dioxide (CO2). The absence of oxygen allowed organic molecules to develop and accumulate. Primitive cells eventually developed that used energy from the sun to produce food, and oxygen was released as a byproduct. Eventually enough oxygen accumulated for eukaryotic cells (like ours, that use oxygen) to evolve.