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Darwin and his theory of evolution Changing ideas Who was Charles Darwin? Artificial selection In order to test his hypothesis, Darwin studied plant and animal breeders The breeders knew that individuals within a species varied, some producing more or less fruit, some producing more or less milk, etc. Darwin noted that these variations could be passed to offspring Artificial selection Farmers and breeders would choose the traits they most favored and breed only those Artificial selection is the process of humans choosing the traits that they find most useful from the natural variations already present Small variations Darwin applied this idea of variation to wild species Sometimes a trait would develop that would allow the organism a better chance of survival Small, hereditary variations (adaptations) are the basis for evolution in a population Adaptation This neck variation is known as an adaptation An adaptation is any inherited characteristic that increases an organisms chance for survival Natural selection Organisms with adaptations favorable to the current environment will be the ones most likely to survive and reproduce Since those that are better adapted for survival leave more offspring, their advantageous traits (adaptations) will increase in frequency in the population. Change Over Time After many generations, the population will be almost entirely made up of individuals with the advantageous trait (all adapted to their environment)! Essentials to Remember … Organisms cannot “try” to adapt, nor does it give them what they need!!! Variation must be present in the population! Either an organism has the trait (adaptation) to help it survive & reproduce or it doesn’t!!! Common descent Darwin stated that species can change over time, passing favorable traits on to the next generation This principle of change over time is called descent with modification With this in mind, Darwin proposed another idea. He suggested that if species change over time and become new species, that there must be a point in history where there was a single, common ancestor, that evolved and changed over time to form all the species of life on earth today Evidence for evolution 1. 2. 3. 4. There are four major pieces of evidence that support Darwin’s theory The fossil record Geographic distribution of living species Homologous structures Similarities in embryology The fossil record Darwin not only collected live samples, but dead ones as well, fossils Some of the fossils Darwin collected resembled organisms of his time, while others did not Darwin saw the fossil record as a detailed record of evolution through time Geographic distribution of living species Darwin noticed how several different species of finch, all living in different areas, shared similar traits Some of these similar traits are known as analogous structures - traits similar in function, but not necessarily appearance, due to a similar environmental influence Example – Two different species, one living on South America, the other living in Africa, live in similar environmental conditions (climate, habitat, niche). Both species have similar traits and behavior because the similar environments cause them to adapt in similar ways. Homologous structures A structure is considered homologous if it comes from the same embryonic tissue, but has a different mature form Example – Bird wings, human arms, and dolphin flippers all serve different functions, but they all formed from the same part of the embryo Darwin noticed these similarities and differences and suggested several things These similarities suggest that all life descended with modification from a single, common ancestor We could group animals according to how recently they shared an ancestor, ie how similar the structures are Embryology Vertebrate organisms at the embryo stage of development seem to have certain similarities These similarities are related to the tissues and cells all growing in similar ways and may also be the reason for homologous structure formation Embryology Evolution in populations Evolution does not occur in individuals, it occurs in populations A gene pool is the entire group of genes that a population shares Relative frequency is the number of times an allele occurs in a population, usually written as a % Relative frequency has nothing to with the Law of Dominance A change in relative frequency implies evolution is occurring Types of natural selection Natural selection can affect the frequency of phenotypes in 3 ways Directional selection Stabilizing selection Disruptive selection Directional Selection Directional selection occurs when natural selection favors a single phenotype and this causes a directional shift in the allele frequency In the graph to the left, a food shortage causes the supply of small seeds to run low, so natural selection favors the birds with large beaks and we see a shift in the allele frequency Stabilizing Selection This type of natural selection occurs when the individuals with a middle or average phenotype have the higher frequency The extreme ends will be favored against An example of this is human birth weight – Babies born in the “normal” range will have a higher chance for survival than those at the 2 extremes Disruptive Selection This happens when individuals at both extremes have a higher frequency than those in the average range An example of this is bird selection of seed size. If the amount of seeds with average size decreases while amounts of seeds with larger and smaller size increase, birds specializing in those 2 sizes will increase in frequency, while those that do not will decrease