Processes of Evolution Chapter 16 Overwhelming Evidence for Evolution • A) From Biogeography • B) From Comparative anatomy • C) From Geologic discoveries A) Biogeography - is the science which deals with geographic patterns of species distribution and the processes that result in such patterns. • Size of the known world expanded enormously in the 15th century • Discovery of new organisms in previously unknown places could not be explained by accepted beliefs – How did species get from center of creation to all these places? B) Comparative Morphology • Study of similarities and differences in body plans of major groups • Puzzling patterns: – Animals as different as whales and bats have similar bones in forelimbs – Some parts seem to have no function C) Geological Discoveries • Similar rock layers throughout world • Certain layers contain fossils • Deeper layers contain simpler fossils than shallow layers • Some fossils seem to be related to known species Darwin’s Voyage • At age 22, Charles Darwin began a fiveyear, round-the-world voyage aboard the Beagle • In his role as ship’s naturalist he collected and examined the species that inhabited the regions the ship visited QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Voyage of the Beagle Galapagos Islands Darwin Wolf • Volcanic islands far off coast of Ecuador Pinta Genovesa Marchena • All inhabitants are descended from species that arrived on islands from elsewhere Santiago Bartolomé Fernandia Seymour Baltra Rabida Pinzon Santa Cruz Santa Fe Tortuga San Cristobal Isabela Española Floreana Lyell Provided the foundation for Darwin to draw his conclusions • Lyell’s Principles of Geology • Subtle, repetitive processes of change, had shaped Earth • Challenged the view that Earth was only 6,000 years old, to millions of years Malthus - Struggle to Survive this too shaped Darwins’ views • Thomas Malthus, a clergyman and economist, wrote essay that Darwin read on his return to England • Argued that as population size increases, resources dwindle, the struggle to live intensifies and conflict increases Darwin put together the short term struggle for existence (Malthus) with the great geological timescale (Lyell) Galapagos Finches • Darwin observed finches with a variety of lifestyles and body forms • On his return he learned that there were 13 species • He attempted to correlate variations in their traits with environmental challenges QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Darwin’s Theory Survival of the fittest On the Origin of Species • Darwin’s book • Published in 1859 • Laid out in great detail his evidence in support of the theory of evolution by natural selection Populations Evolve • Biological evolution does not change individuals • It changes a population • Traits (features) in a population vary among individuals • Evolution is change in frequency of traits When does evolution not occur? When 5 conditions are met • No mutation • Random mating • Gene doesn’t affect survival or reproduction • Large population • No immigration/emigration Hardy-Weinberg Rule At genetic equilibrium, proportions of genotypes at a locus with two alleles are given by the equation: p2 AA + 2pq Aa + q2 aa = 1 Frequency of allele A = p Frequency of allele a = q No Change through Generations STARTING POPULATION THE NEXT GENERATION 490 AA butterflies dark-blue wings 490 AA butterflies dark-blue wings 490 AA butterflies dark-blue wings 420 Aa butterflies medium-blue wings 420 Aa butterflies medium-blue wings 420 Aa butterflies Medium-blue wings 90 aa butterflies white wings 90 aa butterflies white wings THE NEXT GENERATION 90 aa butterflies white wings Gene Mutations • Infrequent but inevitable • Every gene is subject to mutation to some degree • Changes the frequency of alleles in a population = EVOLUTION Number of individuals in population Directional Selection • Allele frequencies shift in one direction Number of individuals in population Number of individuals in population Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3 Giraffes • Baby giraffes in Africa • Some have inherited long necks others shorter ones • When food is limited only the ones with longer necks can feed • The others die of starvation • Now the population has individuals with longer neck alleles on average Natural Selection • A difference in the survival and reproductive success of different phenotypes • Acts directly on phenotypes and indirectly on genotypes Number of individuals in population Stabilizing Selection • Intermediate forms are favored and extremes are eliminated Number of individuals in population Number of individuals in population Range of values for wing-color trait at time 1 Range Range of values of values for wing-color for the trait trait at time at time 22 Range of values for wing-color trait at time 3 Stabilizing Selection: Another Example • Weight distribution for 13,370 human newborns (yellow curve) correlated with death rate (white curve) Number of individuals in population Disruptive Selection • Forms at both ends of the range of variation are favored • Intermediate forms are selected against Number of individuals in population Number of individuals in population Range of values for wing-color trait at time 1 Range of values for wing-color trait at time 2 Range of values for wing-color trait at time 3 Balanced Polymorphism • Polymorphism - “having many forms” • Occurs when two or more alleles are maintained at frequencies greater than 1 percent Sickle-Cell Trait: Heterozygote Advantage • Allele causes sickle-cell anemia when heterozygous • Heterozygotes are more resistant to malaria than homozygotes HbS Malaria case Sickle cell trait less than 1 in 1,600 1 in 400-1,600 1 in 180-400 1 in 100-180 1 in 64-100 more than 1 in 64 Genetic Drift • Random change in allele frequencies brought about by chance • Effect is most pronounced in small populations • Sampling error - Fewer times an event occurs, greater the variance in outcome Bottleneck • A severe reduction in population size • Causes pronounced drift • Example – Elephant seal population hunted down to just 20 individuals – Population rebounded to 30,000 – Electrophoresis revealed there is now no allele variation at 24 genes Founder Effect • Effect of drift when a small number of individuals start a new population • By chance, allele frequencies of founders may not be same as those in original population • Effect is pronounced on isolated islands Inbreeding • Nonrandom mating between related individuals • Leads to increased homozygosity • Can lower fitness when deleterious recessive alleles are expressed • Amish, cheetahs Gene Flow • Physical flow of alleles into a population • Tends to keep the gene pools of populations similar • Counters the differences that result from mutation, natural selection, and genetic drift • Prevents the two populations forming new species!!!