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Evolution and Natural Selection Biology Evolution • Idea put forth by others before Charles Darwin, but Darwin is credited with putting all of the prior knowledge together and it make real sense. • A change in the characteristics of a population from one generation to the next; gradual development of organisms from other organisms since the beginning of life. Evolution • Common Ancestry – Shared descent – Do all organisms share a common ancestor? – Research and shared characteristics between all of life seem to agree with this theory Evolution by Natural Selection • Natural Selection – The mechanism of evolution – 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. – These successful characteristics must also be shared within the population. Adaptations • An inherited trait that has become common in a population because the trait provides a selective advantage Population • All of the individuals of a species that live in a specific geographical area and that can interbreed. Change over Time • Gradualism – Gradual change in a population over a long period of time that leads to new species. – Does this type of evolution occur? Change over time • Punctuated Equilibrium – Periods of rapid change in population genetics that are separated by periods of little or no change How did Darwin visualize these changes? • Fossils – Trace or remains of an organism that lived long ago, preserved in sedimentary rock. • Paleontologist – Scientists who study fossils • So was Darwin an early paleontologist? What do we know now that Darwin didn’t? • We now know that earth is about 4.5 billion years old • Organisms have inhabited Earth for most of its history • All organisms living today share common ancestry with earlier, simpler life-forms. Relatedness • Vestigial structures – Structures that are present in only a rudimentary capacity compared to other related organisms. Relatedness Relatedness Relatedness Relatedness • Homologous Structures – Structures that share common ancestry – A similar structure in 2 organisms can be found in the common ancestor of the organisms. – The limb bones of mammals Species Formation • What is a species? – A group of closely related organisms that mate naturally – Examples: • Humans, dogs, cats – Not Examples: • Mule (Infertile offspring of a male horse and female donkey) Homo sapien Speciation • Process by which new species form. • Divergence – Accumulation of differences between groups Speciation Variation Isolation Natural Selection Divergence New Species Population • All organisms within a species that live in a specific area that interbreed. 3 Key Features of Populations 1. Size 2. Density 3. Dispersion Population Size Small vs Large • Small Population – Most likely to become extinct – Disturbances can dramatically affect this population – Inbreeding • Better chance for non-beneficial traits being passed on • Large Population – Many possible mates – More variation – Can better survive disturbance – Less chance of inbreeding Population Density • Number of individuals in a given area • City vs farming community – More people in a city – More diversity of individuals in a city Population Dispersion Checked vs Unchecked Populations • Checked Population growth – Typical of most natural populations – Limiting factors cause populations to stabilize – Carrying Capacity • Unchecked Population Growth – Humans follow this curve – So do bacteria – Limiting Factors play less of a role Population Density Factors • Density-dependent – Disease – Competition for resources – Predation • Density-independent – Climate – Weather – Pollution Growth Rate and Population Size • Birth Rate vs Death Rate for Humans http://www.sonic.net/~evolve/wp/human_ecolog y/worldpopulation_2004a.gif Population Genetics • How many of you remember alleles? – – – – A = dominant, a = recessive AA = homozygous dominant Aa = heterozygous aa = homozygous recessive • The frequency of alleles in a population can be explained by the Hardy-Weinberg Principle – Allele Frequency=how often alleles occur in a population Hardy-Weinberg Principle • The frequency of alleles in a population do NOT change unless evolutionary forces act on the population. • Stable population? • Also, the Hardy-Weinberg Principle holds true if there is no inbreeding p2 + 2pq + q2 = 1 p+q=1 Using Hardy-Weinberg What does p and q mean? p2 + 2pq + q2 = 1 p+q=1 p= dominant alleles q= recessive alleles p2 = frequency of homozygous dominant q2 = frequency of homozygous recessive pq = frequency of heterozygous Hardy-Weinberg Example What information do we • For a disease know? caused by a 2 = 0.25 p recessive allele, 2 p = ? = p .25 .5 calculate the number of carriers in q = ? = 1 p 1 .5 .5 2 2 .5 .25 q =?= a population if 2pq = ? p2 = 0.25. Use Hardy-Weinberg to p + q = 1 solve this problem. Now can we find out 2pq or the frequency of heterozygotes? Hardy-Weinberg Continued p = .5 p2 = .25 q = .5 q2 = .25 Now determine the number of heterozygotes 2pq = 2 *.5 * .5 = .5 What does this mean? p 2 pq q 1 2 2 .25 .5 .25 1 Evolutionary Forces that Control Populations 1. 2. 3. 4. 5. Mutation Gene flow Genetic drift Nonrandom mating Natural selection Mutation • GREAT SOURCE OF VARIATION ANT MAKES EVOLUTION POSSIBLE • Change in genotype which leads to change in phenotype Gene Flow • Migration of individuals into and out of a population • Immigration: individuals move into a population; adds new alleles to a population • Emigration: individuals move out of a population; takes away alleles from a population Nonrandom Mating • Preference to mate with others that live nearby or are attractive in some way to that individual • Examples: HUMANS, birds Genetic Drift • Small populations • The number of alleles is so small that loss of one individual can effect the whole population • Natural disasters, drought Natural Selection • The frequency of certain alleles increases or decreases depending on the effect those alleles have on survival and reproduction. • One of the most powerful agents to changes in the genetics of populations Polygenic Traits • Traits due to interactions of multiple genes on multiple chromosomes • Examples: skin color, height Normal Distribution • Based on the fact that most traits occur on an average basis in nature. In other words, being too different can lead to you being dead in nature. Directional Selection • Natural selection favors a particular phenotype so the frequency shifts in one direction Stabilizing Selection • This type of selection makes the frequency of the most common alleles more common.