Natural Selection PDF

Differences between individuals in adaptation, survival and reproduction as the basis for natural selection  Traits must be heritable for evolutionary change Evolution to...

Differences between individuals in adaptation, survival and reproduction as the basis for natural selection  Traits must be heritable for evolutionary change Evolution to occur Evolution is the change in the heritable characteristics of a population over time Populations are groups of organisms of the same species living in an area e Pop u a l s hav n ula ti id io ove ons ev Indiv le variat a b r tim olv herit e e Natural Selection Observations by Charles Darwin led him to propose natural selection as a mechanism by which evolutionary change can occur. Darwin based the idea of natural selection on his and others’ observations and inferences about the natural world. Ecological Observations: Inference 1: 1. Populations have the potential to Not all offspring that Inference 3: NATURAL SELECTION: increase exponentially are produced will Differences in survival and reproduction 2. Populations generally remain stable survive and reproduce among individuals are not random; traits once they reach a certain size because of a struggle that help and organism survive and 3. Natural resources are limited for resources reproduce are passed on at a higher rate and will increase in proportion in the Heredity Observations: Inference 2: population from one generation to the 4. Individuals in a population vary in Some individuals are next. Over time, the heritable many characteristics more likely to survive characteristics of the population 5. Many characteristics are heritable; and reproduce than change; the population has evolved. passed on from parent to offspring others because of their heritable traits The basis of natural selection as presented by Darwin (1868), based on the summary by Mayr (1982) Biological “Fitness” Fitness is a measure of an organism's ability to survive to reproductive age, find a mate, and produce offspring. Individuals are more or less “fit” which means they are more or less likely to survive and reproduce as a result of a selective pressure and their heritable traits. Less fit individuals (green beetles) have a variation selected against by the selection pressure (the bird predator) More fit individuals (brown beetles) have a variation selected for by the selection pressure Differential Survival and Reproduction Variations that promote survival and/or reproduction in the face of the selection pressure will be passed on to the next generation during reproduction at a higher frequency than variations that reduce survival and/or reproduction. The brown phenotype had a survival advantage compared to the green phenotype beetle. As a result, more brown beetles are able to survival and reproduce, passing the alleles for the brown phenotype to the next generation. Differential Survival and Reproduction Over generations, the frequency of the selected variation will increase in frequency in the population. The heritable features of the population will have changed over time, so the population will have evolved. Pace of Evolution due to Natural Selection The pace of change in a population due to natural selection can vary Rapid change with strong depending on the strength selection pressure of the selection pressure. Rapid evolution will occur if the selection pressure is higher Grad weak ual chan selec ge Gradual evolution tion p with ressu re will occur if the selection pressure is lower Changes in allele frequency in the gene pool as a consequence of natural selection Natural Selection Results in Adaptation Adaptations are forms that correspond to function. The differential survival and reproduction results in a population that is adapted to its environment because individuals have adaptations that allow them to survive and reproduce. Adaptations are the result of natural selection in the population. Adaptations are phenotypes that are maintained through natural “The penguins are adapted to their environment.” selection. “White fur is an adaptation of the polar bear.”