Evolution lecture 5 Natural selection AWhat is fitness It s not how long you live but how many offspring you leave behind Fitness is a relative term The most fit genotype is assigned a fitness value of 1 The difference between any two fitness value gives us the selection coefficient B Types of selection assuming some distribution for example a normal distribution for a given phenotype i Stabilizing selection the intermediate phenotype is favored ii Directional selection one of the extreme phenotypes is favored iii Disruptive selection both of the extreme phenotypes are favored iv Sexual selection a type of natural selection acting on traits directly involved in obtaining mates Will tend to enhance sexual dimorphism Intrasexual selection eg male male competition for mates Intersexual selection eg female mate choice Epigamic traits are those selected for by opposite sex As an important aside what are males and females In humans males are XY and females are XX but in birds males are ZZ and females are ZW If it isn t chromosomal what is it Intersexual selection has fascinated Biologists since Darwin s day C Adaptations i Adaptations to the physical environment leading to geographical variation ii Adaptations to the biological environment may lead to coevolution D Why evolution does not result in perfectly adapted organisms i Evolution does not mean necessarily mean improvement it just means change The environment is constantly changing and species are all continuously evolving The Red Queen hypothesis You have to run faster than that to stay in the same place ii Historical constraints the Panda s thumb iii Compromise finding mates vs avoiding predators iv Time and energy are limiting bone strength v Selection only selects from the variation that is present cream of the crop or the lessor of evils Evolution lecture 6 Speciation AWhat is a species The important thing to realize is that this could be a whole lecture i Morphospecies concept using anatomical features problems with variation and with change ii Biological species concept a species is a group of potentially interbreeding natural populations that are reproductively isolated from other groups Problems with asexual organisms extinct organisms geographical separation and ring species B Isolating mechanisms i Premating prezygotic mechanisms that act to prevent zygote formation Habitat isolation species occupy different habitats Temporal isolation reproduction occurs at different times Behavioral isolation eg courtship displays are very different Mechanical isolation physical anatomy doesn t correspond Gametic isolation egg and sperm cannot fuse ii Postmating postzygotic mechanisms that come in to play after fertilization Hybrid inviability hybrids don t survive Hybrid sterility hybrids survive but cannot reproduce Hybrid breakdown hybrids survive and can mate but their offspring are sterile C Modes of speciation i Allopatric speciation pop s are separated by a geographical barrier speciation is NOT inevitable The Dumb bell model divides ancestral population into two large populaions Peripheral isolate model isolation of small population at the periphery of range Important because gene pool may already be different population is small and the environment may be different Repeated isolation of populations may lead to adaptive radiation ii Sympatric speciation new species arise without geographic isolation Autopolyploidy one species doubles its chromosome number self fertilization likely Allopolyploidy 2 different species form an offspring with an increased sets of chromosomes Disruptive selection but what prevents interbreeding John Latto 6 21 07 Evolution lecture 7 Geological time and the fossil record AThe fossil record i Types of fossil unaltered remains altered remains molds and casts trace fossils ii Where fossils occur specific conditions lead to fossil record being incomplete and biased iii What do fossils tell us iv Dating fossils Steno s law of superposition radioactive decay amino acid isomers B Geological time scale i Precambrian era 4 6 billion to 542 million years ago LOTS happens but little is fossilized By the end of the Precambrian there is an abundant fauna of multicellular marine invertebrates ii Paleozoic era 542 to 251 million years ago Cambrian hard bodied marine inverts explosion of diversity Ordovician first vertebrates Silurian first land plants followed by first land animals arthropods Devonian Age of fishes land vertebrates amphibians Carboniferous extensive forests origin of reptiles Permian radiation of reptiles major extinction of marine invertebrates iii Mesozoic era 251 to 65 million years ago Triassic appearance of dinosaurs mammals and birds Jurassic age of dinosaurs Cretaceous appearance of flowering plants major extinction of dinosaurs iv Cenozoic era 65 million years ago to the present day Paleogene appearance of pollinating insects radiation of birds and mammals Neogene appearance of Homo sapiens Pleistocene epoch humans appear ice ages Holocene epoch recorded history C Plate tectonics i Origin of the theory of continental drift ii Pangea forms at the end of the Paleozoic iii Splits into Laurasia and Gondwana in Mesozoic John Latto 6 21 07
View Full Document
Unlocking...