Evolution Sexual reproduction and speciation Natural selection Natural selection and evolution Modern examples of Evolution Darwin His book On the Origin of Species by means of Natural Selection Nov. 24, 1859 – This book presented the first convincing case for evolution – Covered many aspects of living organisms including diversity, origins and relationships, similarities and differences, geographical distribution, and adaptations to their environment Darwin made two major points in his book – Species evolved from ancestral species and were not specially created – Natural Selection is a mechanism that could result in this evolutionary change Five Agents of Evolutionary Change Mutation Gene Flow Nonrandom Mating Genetic Drift Selection Mutation Mutation rates are generally slow A single gene may mutate 1 to 10 times per 100,000 cell divisions (although some genes may mutate more frequently) Nonetheless, mutation is the ultimate source of genetic variation Gene Flow Movement of alleles from one population to another = movement and reproduction – Powerful mechanism of change because members of two different populations may exchange genetic material – Occur if, for example, one organism moves from one place to another. If the characteristics of the newcomer differ from the native organisms and it adapts well and mate successfully, the genetic composition of the population will be altered Nonrandom Mating – Does not change the frequency of the alleles , but the proportion of homozygous individuals because relatives are likely to be genetically similar and thus produce offspring with two copies of the same allele Genetic Drift In small populations, frequencies of particular alleles may change drastically by chance alone. These changes occur randomly, as if the genes were drifting If the gametes of only a few individuals form the next generation, the alleles they carry may by chance not be representative of the parent population from which they were drawn (Bottleneck effect) The Bottleneck Effect See the following site – http://www.brooklyn.cuny.edu/bc/ahp/LAD/C2 1/C21_Bottleneck.html – Bottleneck effect (BIOdotEDU) Selection As Darwin pointed out, some individuals leave behind more progeny than others, and the rate at which they do so is affected by phenotype and behavior – Artificial Selection –breeder selects – Natural Selection -in natural selection, environmental conditions determine which individuals in a population produce the most offspring For Natural Selection to occur and produce evolutionary changes three conditions must be met: 1. Variation must exist among individuals in a population. Natural selection favors individuals with some traits over individuals with alternative traits 2. Variation among individuals results in differences in number of offspring surviving in the next generation. Because of their phenotype or behavior, some individuals are more successful than others 3. Variation must be genetically inherited. For natural selection to result in evolutionary change, the selected differences must be inherited. Darwin, Wallace and Evolution by Natural Selection Evolution – – Accumulation of changes in the heritable characteristics of a population – Processes that have transformed life on Earth – Charles Darwin developed the theory that evolution occurs as a result of natural selection (explained in The Origin of Species, 1859). What was Wallace’s contribution?