Today: Thinking About Darwinian Evolution Part 1: Darwin s Theory Perplexing Observations Mystery of the Black Death?? What is evolution?? And what is this finch doing?!? We owe much of our understanding of EVOLUTION to CHARLES DARWIN. Darwin s Origin of Species (1859): 1. Descent with Modification (Evolution) 2. Natural Selection as a mode of Evolution Darwin s Natural Selection: Observation #1: Individual Variation Observation #2: Struggle for Existence Inference: Differential Reproductive Success or Natural Selection (Remember: Mendel publishes in 1866 ) Darwin illustrates his mechanism with examples from ARTIFICIAL SELECTION, the breeding of domesticated plants and animals If artificial selection can change so much in a relatively short time, argues Darwin, then natural selection should be capable of considerable modifications. Darwin also proposed that natural selection, over time, could produce new species from ancestral species. This was his explanation for the 14 unique species of finches he observed on the Galapagos Islands. 1
The Historical Context From 1600 s +, Geologists begin mapping multiple distinct layers of rock with fossils in them. Lines of Evidence Lines of Evidence: Comparative Morphology Naturalists begin to study Biogeography: the distribution of organisms Many are unique to isolated places! Lines of Evidence: Comparative Embryology Thus Darwin s views are supported by multiple, independent lines of evidence. Evolutionary patterns of Homology that match patterns in space (Biogeography) and time (the Fossil Record). Who s who??? 2
Examples of Natural Selection? ADAPTATIONS are the result of natural selection. Insecticide-Resistant Insects Antibiotic-Resistant Bacteria Drug-Resistance in HIV Three Important Constraints on Natural Selection: 1.Natural selection occurs only at the POPULATION LEVEL. (Population =a group of interbreeding individuals of the same species, sharing a common geographic area.) 2.Natural selection can only amplify or diminish HERITABLE VARIATION. 3.Natural selection is SITUATIONAL; an adaptation in one situation may be useless or even detrimental in another. Population: localized group of individuals belonging to the same species. Every species distributed over range, but composed of multiple populations. Populations may be more or less isolated. The total aggregate of genes in a population at any one time is its GENE POOL (all alleles at all loci) Generationto-generation change in a population s frequencies of alleles What could cause the frequency of a particular allele to change?? MICROEVOLUTION Four Primary Causes of Microevolution: 1. Mutation 2. Natural Selection 3. Gene Flow 4. Genetic Drift 3
1. Mutations: A change in an organism s DNA can change the gene pool of a population Important as the original source of Genetic Variation! Mutations can be lethal, neutral, or provide an advantage! 2. Natural Selection: can affect the frequency of traits in the POPULATION in three different ways: 1. Directional Selection- favors one extreme 2. Diversifying Selection- favors variants at both opposite extremes 3. Stabilizing Selection- selects against the extremes Types of Selection: Examples?? 3. Gene Flow: Populations may gain or lose alleles due to the migration of fertile individuals or gametes between populations. 4. Genetic Drift: a change in a population s allele frequencies due to CHANCE. Genetic Drift is most effective in small populations! Types of Genetic Drift 1.The Bottleneck Effect- Disasters randomly reduce the population size. The surviving population may not be representative of the original population s gene pool. 4
Types of Genetic Drift 2. The Founder Effect- a few individuals from a larger population colonize an isolated new habitat. Genetic drift occurring in this new colony is known as the founder effect Polydactyly -- extra fingers or sometimes toes -- is one symptom of Ellis-van Creveld syndrome. The syndrome is commonly found among the Old Order Amish of Pennsylvania, a population that experiences the founder effect. Special Types of Selection: Sexual Selection Some traits are advantageous simply because males or females prefer them Special Types of Selection: Balanced Polymorphisms Two or more alleles maintained in a population by balancing selection. Example: Sickle Cell Disease High incidence due to increased malarial resistance! HIV and Evolution Malaria and Sickle Cell Disease A colorized SEM of HIV emerging from a white blood cell. 5
HIV and Evolution A combination of drugs that act on different viral targets can be used to fight HIV. This is known as highly active antiretroviral therapy (HAART). HIV Evolution in a Single Patient T0 T3 T1 You Try: In-Class, Part 1 6