1 Natural Selection and Evolution

2 Std/Obj  Know why natural selection acts on the phenotype rather than the genotype of an organism.  Know how natural selection determines the differential survival of groups of organisms.  Know a great diversity of species increases the chance that at least some organisms survive changes in the environment.  Know new mutations are constantly being generated in a gene pool.  Know why alleles that are lethal in a homozygous person may be carried in a heterozygote and thus maintained in a gene pool.

3 Evidence for Evolution (change over time) 1.Fossil record –Earth is older than previously believed 2.Mechanisms of heredity –modern genetics  molecular evidence –All cells have DNA, RNA, ribosomes, the same 20 amino acids, and use ATP as an energy carrier –DNA similarities ex. Human and Chimp –phylogenetic trees Diff in # aa Diff in # aa Human beta chain 0 Gorilla1 Gibbon2 Rhesus monkey8 Dog15 Horse, cow25 Mouse27 Chicken45 Frog67 Sea slug127 Soybean 124

4 Phylogenetic Tree

5 3. Comparative Anatomy Homologous structures Have same evolutionary origin, with similar structure but different function. Analogous structures Have different structure and similar function, but different evolutionary origin

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8 Contributers  Carolus Linnaeus - developed a classification of plants and animals  Jean-Baptiste Lamarck - first scientist who put his beliefs into a system that attempted to explain evolution.  Thomas Malthus - concerned with relationship of human population growth to food supplies.

9 Charles Darwin ( )  Evolution is descent with modification  Ship’s naturalist on the HMS Beagle ( )  Fossils of Patagonia: similarity of fossil and living forms (e.g., armadillos).  Fauna of Galapagos Islands: each island had similar physical conditions but distinct species of mockingbirds, tortoises. divergence in isolation from a common ancestor

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11 Theory of Evolution by Natural Selection 1.All species produce offspring at a faster rate than food supplies can increase. 2.Except for identical twins, no two individuals are exactly alike. 3.In each generation, more individuals are produced than can survive. 4.Individuals that possess certain traits have an advantage over those that do not. 5.The environment determines whether a trait is beneficial. 6.Traits are inherited and passed on to the next generation. 7.Over time, successful variations accumulate in a population, and a new species may appear. 8.Geographical isolation may lead to the formation of a new species.

12 Fitness  Selection is variation in the average reproductive success among different phenotypes.  Fitness is a quantitative measure of reproductive success.  Selection is variation in average fitness among different phenotypes.  Phenotypes will be differentially affected by selection if they have consistent (average) differences in fitness.

13 Population genetic models of selection  Usually assume that the phenotypes are completely determined by the genotype.  They therefore typically assign fitness values directly to genotypes.  Sophisticated pop. gen. models don’t make this assumption, and neither do quantitative genetic models.

14  Natural selection acts on individuals, but the consequence is a change in the population phenotypic distribution  Natural selection acts on phenotypes, but changes the genotypic distribution (without underlying genetic variation, evolution will not occur)  Each generation is the product of selection on their parents  Natural selection acts on existing traits. New traits arise as a modification of old features and in combination with new mutations -adaptations  Natural selection is non-random, but not progressive

15 Darwin vs Lamarck

16 Agents of Evolutionary Change  Mutation –Mutation rates are generally so low they have little effect on proportions of common alleles.  ultimate source of genetic variation  Gene flow –movement of alleles from one population to another  tend to homogenize allele frequencies -% of any specific allele in the gene pool (all alleles of a population’s genes)  Nonrandom mating –assortative mating - phenotypically similar individuals mate  Genetic drift – statistical accidents –Frequencies of particular alleles may change by chance alone.  important in small populations –founder effect - few individuals found new population (small allelic pool) –bottleneck effect - drastic reduction in population, and gene pool size

17 Genetic Drift - Bottleneck Effect

18  Selection – Only agent that produces adaptive evolutionary change –artificial - breeders exert selection –natural - nature exerts selection  variation must exist among individuals  variation must result in differences in numbers of viable offspring produced  variation must be genetically inherited –natural selection is a process, and evolution is an outcome –Selection pressures :  avoiding predators  matching climatic condition  pesticide resistance

19 Interactions Among Evolutionary Forces  Levels of variation retained in a population may be determined by the relative strength of different evolutionary processes.  Gene flow versus natural selection –Gene flow can be either a constructive or a constraining force.  Allelic frequencies reflect a balance between gene flow and natural selection.

20 Natural Selection Can Maintain Variation  Frequency-dependent selection –Phenotype fitness depends on its frequency within the population.  Negative frequency-dependent selection favors rare phenotypes.  Positive frequency-dependent selection eliminates variation.  Oscillating selection –Selection favors different phenotypes at different times.

21 Heterozygote Advantage  Heterozygote advantage will favor heterozygotes, and maintain both alleles instead of removing less successful alleles from a population. –Sickle cell anemia  Homozygotes exhibit severe anemia, have abnormal blood cells, and usually die before reproductive age.  Heterozygotes are less susceptible to malaria.

22 Sickle Cell and Malaria

23 Forms of Selection  Disruptive selection –Selection eliminates intermediate types –Leads to the evolution of 2 new species  Directional selection –Selection eliminates one extreme from a phenotypic array  Stabilizing selection –Selection acts to eliminate both extremes from an array of phenotypes –Can reduce variation in pop

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25 Divergent evolution-the process of two or more related species becoming more and more dissimilar ex. Adaptive Radiation-an ancestral species evolves into an array of species to fit a number of diverse habitats -a type of birds in Hawaiian Islands have the same body size and shape, but different color and beak shapes Patterns of Evolution

26 Rates of Evolution   Large populations evolve very slowly or almost not at all. – –Advantageous mutations pass very slowly through a pop.   Small populations can evolve very rapidly. – –Advantageous mutations can be passed very quickly through the population.