Evolution and Natural Selection Pre Darwin Origin of species Earth relatively young (thousands of years) ; this hypothesis was being replaced

Transcription

1 Evolution and Natural Selection Pre Darwin Origin of species Earth relatively young (thousands of years) ; this hypothesis was being replaced In the 1700 s and early 1800 s, geological evidence suggested that the earth was not young, but quite old, and that it had undergone considerable change over its history. Massive geologic formations, such as the Grand Canyon, were seen as the result of slow geologic processes There were a limited number of fossils found and most did not appear dramatically different from current species; species were thought not to change between generations and the number of species on the earth was constant. Organisms were thought to be perfectly adapted to their environment. Post- Darwin Origin of species Species are related by descent Adaptation to the environment is the result of the interplay of random variation and natural selection The number of species was not constant and species changed over time. Descent with modification Conditions necessary for decent with modification Variation within a population The variation is heritable The reproductive success depends on the available variation.

2 Evolution theory Variation exists among individuals within a species Organisms produce more offspring than the environment can support Competition exists among individuals The organisms whose variation best fit them to the environment are the ones who are most likely to survive, reproduce, and pass those desirable variations to the next generation Gene Variation Macroevolution - Evolutionary change on a grand scale, encompassing novel designs, evolutionary trends and episodic mass extinction. Microevolution - Differential survival and reproduction due to natural selection. Gradually alters population to include more individuals with advantageous characteristics. Gene Variation Darwin s explanation of evolution: Adaptation by natural selection is responsible for evolutionary changes within a species(microevolution), and accumulation of these changes leads to development of new species (macroevolution). Evidences for evolution: fossil record Fossils are preserved traces of once-living organisms created when organisms become buried in sediment and calcium in hard surfaces mineralizes. Often provide evidence of successive evolution. Fossilization process limits available fossils Requires burial, sedimentary rock, deposition of minerals replacing hard parts of an organism. Soft parts preserved in impressions or casts in soft sediment Dating the fossil record isotopic dating

3 Timelines Molecular Record Evolutionary theory allows evolutionary change involves substitution of new versions of old genes. New alleles arise by mutation and come to predominate through favorable selection. Molecular Clocks The longer the time since divergence, the greater the number of differences in nucleotide sequence of cytochrome C. Changes accumulate at constant rate. Molecular Record Phylogenetic Trees Evolutionary history of a gene can be mapped as a phylogenetic tree. Mechanisms of evolution Genetic drift Random fluctuations in the allele frequencies within a population Mutation A random change in the coding of an allele or gene Natural selection (including sexual selection) the differential reproduction of genotypes caused by factors in the environment. Anatomical Record products of evolutionary process Homologous Structures - Structure and function have diverged between body parts of different animals, but are derived from same part present in a common ancestor. Forelimbs of Vertebrates Analogous Structures - Features resemble each other as a result of parallel evolution in separate lineages. Flippers of penguins and dolphins Vestigal Organs - Organs no longer of use. Products of evolution Adaptation organisms are not perfectly adapted ( have to make due with your genetics) Examples: panda thumb Hardy-Weinberg Rule

4 From 1920 s onward, scientists began formulating theory of how alternative gene forms (alleles) behave in a population, and how changes in gene frequencies lead to evolutionary change Hardy and Weinberg pointed out in the absence of forces, in a large population with random mating, allelic frequencies remain constant. Hardy Weinberg equilibrium Hardy-Weinberg - individuals do not evolve Assumptions: Large population size Random mating No mutation No immigration Absence of natural selection Mutation Error in replication of a nucleotide sequence in DNA (Very slow rate). Migration Movement of individuals from one population to another (Dependent on strength of selective forces). Genetic Drift Change in allelic frequencies due to random events. Founder Effect - Population started by few individuals and thus a restricted gene pool (Rare genes may become common). Bottleneck Effect - Gene pool becomes very small, usually due to small population size. Nonrandom Mating Individuals with certain genotypes mate with more or less commonly than expected on a random basis. Inbreeding - Mating with relatives. Increases homozygosity

5 Selection Some individuals leave behind more progeny than others. Artificial - Breeder selects desired characteristics. Natural - Environment determines adapted characteristics. Forms of Selection Disruptive Selection acts to eliminate intermediate phenotypes. Stabilizing Selection acts to eliminate both extremes from an array of phenotypes. Directional Selection acts to eliminate one extreme from an array of phenotypes. Three Forms of Selection Sickle-Cell Anemnia Hereditary disease affecting hemoglobin molecules. Sickle-Cell homozygosity frequently leads to a reduced life span. Heterozygosity causes enough hemoglobin to be produced to keep red blood cells healthy. Very common in Africa. Stabilizing selection as heterozygosity infers less susceptibility to malaria. One of leading causes of death in Africa. Stabilizing Selection in Sickle-Cell Peppered Moths and Industrial Melanism Until the mid nineteenth century, Peppered Moths,Biston betularia, had predominately light-colored wings. Subsequently, dark individuals became predominant. Industrial smog helped turn lichens on tree trunks dark. Contrasting colors between trunk color and moth color led to differential predation by birds. Peppered Moths and Industrial Melanism Second half of the twentieth century saw widespread implementation of pollution controls, thus trends reversed and light colored moths again dominated. But, caution must be taken, as the selective agent could be some factor other than wing coloration.

6 Industrial Melanism Example of directional selection Species Concept A species is generally defined as a group of organisms unlike other such groups and does not integrate extensively with other groups in nature. Species Formation Local populations adapt to the specific circumstances each faces. When they become different enough, the populations become ecological races. Natural selection reinforces differences through isolating mechanisms. Two races become incapable of interbreeding and are considered two separate species. Prezygotic Isolating Mechanisms Prevent formation of Zygote: Geographic Isolation Ecological Isolation Behavioral Isolation Temporal Isolation Mechanical Isolation Prevention of Gamete Fusion Geographical and Ecological Isolation In Oaks Postzygotic Isolating Mechanisms Prevent zygotes from developing into normal, functional offspring. Improper development Reduced fertility or sterility.