Evolution & Natural Selection
Learning Objectives Know what biological evolution is and understand the driving force behind biological evolution. know the major mechanisms that change allele frequencies and therefor biological evolution. Understand the theory of natural selection and the 3 conditions that must exist in order for natural selection to occur.
What is evolution? Change over time What is biological evolution? Change in the genetic make up of a population over generational time*
What does that mean? Genetic makeup: all the genes and their alleles in a population Genetic change: changes in the frequencies of alleles over time
What causes changes in allele frequencies in a population? (Mechanisms of biological evolution) 1. Mutation 2. Genetic drift 3. Migration 4. Natural selection 5. Artificial selection 6. Sexual selection
1. Mutation The change in nucleotide sequence of DNA base substitution, base deletion, triplet deletion Creates new alleles which??? What happens to mutations in somatic cells? Mutations in somatic cells die with the individual may even cause death of the individual What happens to mutations in gametogenic cells? mutations in the gametes can potentially be passed on to offspring & affect a population s variability
2. Genetic drift A random change in allele frequencies due to chance events earthquakes, fires, floods which traits are passed on to offspring? Surviving allele frequencies do not represent the original population Two types of Genetic Drift 1. bottleneck effect 2. founders effect
2. Genetic Drift Founders Effect Group breaks away from the general population May have taken a disproportionate frequency of an allele from the population reproducing will then increase that allele frequency over the original larger population could lead to allele fixation! Real life example: The Amish and polydactyly
2. Genetic Drift Bottleneck Effect A change led to death of a large random portion of the population New population may have a different frequency of alleles, with the possibility that some alleles are lost Pleistocene extinction event (11,000 ya) Ice age caused a large mammalian extinction small number survived result??? today these mammals are so identical to each other as to be considered clones. who are they? Regardless of the type of genetic drift, the effect always reduces genetic variation within a population.
3. Migration The transfer of alleles from one population to another effect?... Allele frequencies change in both populations
4. Natural selection Nature (the environment) selects for or against individuals with certain traits leading to unequal reproductive success Three conditions are necessary for this to occur 1. Variation for a trait 2. Heritability 3. Unequal reproductive success
4.a Variation for a trait Key concepts to understand about trait variation. Without variation, there is nothing to select! Not limited to physical features! organism may vary in physiological, biochemical & behavioral ways, too
4.b Heritability The offspring must inherit trait from parent through genes BUT.. acquired traits (skills or knowledge gained during an organism s lifetime) cannot be passed on to an organism s offspring
4.c Unequal reproductive success Individuals most suited for reproduction in their environment generally leave more offspring than individuals with other traits some organisms are more likely to overcome the struggle to survive & reproduce, passing on their genes
4.c Measuring reproductive success Fitness is a measure of reproductive success relative to other individuals - of the same? the most fit individuals produce the largest # of viable offspring, passing on their genes to the next generation fitness can change over time & across habitats
4. Natural selection, cont. Over time, the traits that lead organisms to have greater reproductive success (have higher fitness) will increase in frequency in the population while traits that reduce reproductive success will decrease
4. Natural selection, cont. Leads to evolutionary adaptation an adaptation is an inherited trait; the specific feature(s) that make an organism have higher fitness AND adaptation is a process by which organisms become better matched to their environment over generational time Why don t humans just evolve against drug resistant bacteria too?
4. Natural selection is NOT...... a choice. Organisms do not evolve because they want to or need to.
Remember! There are three things required for natural selection to occur 1. Variation for a trait 2. Heritability 3. Unequal reproductive success
Next time more mechanisms of biological evolution