How Populations Evolve

Things We Will Know How to summarize Hardy Weinberg principal Describe the five forces that cause genetic change in a population Identify why selection against unfavorable recessive traits is slow Contrast Directional and Stabilizing Selection

Hardy-Weinberg Principle G.H. Hardy and Wilhelm Weinberg researched to understand trait frequencies Found that dominant traits do not overcome recessive traits within populations. Populations typically do not change traits drastically unless acted on by an evolutionary force within the population.

Hardy-Weinberg Principle What are the five forces that can cause populations to evolve? Populations Can evolve through Gene mutation Gene flow Nonrandom mating Natural selection Genetic drift

Gene Mutations Gene mutations are very slow They mutate 1 to 10 times per 100,000 cell divisions Not all gene mutations are result in phenotypic changes, Still, some do cause change. Example: Sickle Cell Anemia Mutation is a source of variation in populations

Gene Flow Migration is the major form of gene flow. Gene flow occurs because new individuals (immigrants) add alleles to a population Gene flow also occurs when individuals depart (emigrants) taking important traits away.

Nonrandom Mating Mating with individuals nearby is nonrandom mating. Inbreeding is a way to keep traits in a population from changing; lower frequency of heterozygous individuals Another example of nonrandom mating is when females choose their mates based on popular or beneficial traits

Genetic Drift Natural disasters can wipe out large numbers within a population Losing large numbers causes loss of strong and beneficial traits. Because Natural Disasters are Random: A random change in traits within a population is called a Genetic Drift

Natural Selection Survival of the FITTEST Traits that are beneficial allow certain individuals to survive. Certain traits are not favorable and the animals containing those traits usually do not get to reproduce For example: Sickle Cell anemia is slowly declining in the U.S. because it is rarely passed on to another generation.

Slowly Removing Unfavorable Traits Tsar of Russia Alexi Nikolayevich had a homozygous recessive trait for hemophilia He died early and never passed one his traits, but all of his sisters were carriers Did the trait die with him? NO, every sister passed on the trait for several generations, but no one knew because it was not a phenotypic trait

Distribution of Traits Polygenic traits are traits that are influenced by dozens of genes. Polygenic traits cause normal distribution or an average curve among populations Usually called a Bell Curve

Contrasting Selection Directional selection is when one an extreme trait is favored and the opposite extreme trait is not. One extreme individual lives to reproduce while the other is lost in history Stabilizing Selection is when neither extreme traits survive, but the average individuals survive to reproduce

Directional Selection Stabilizing Selection