Evolution Acts on Populations

Evolution acts on populations 12.1 - Decent with modification - Evolution is change in allele frequency in a population over time - The only way an allele frequency can change is from generation to generation Evolution thought has evolved for centuries 12.2 Aristotle 350 BCE- individuals in a species are basically identical and species are unchanging Buffon 1749 AD- species change as they spread from their original location - Closely related species arose from a common ancestor Lamarck 1809- new species come from existing species through environmental forces - Reseaoned that organisms that used one part of their body repeatedly would increase their abilities Lyell 1830- all changes in nature are gradual - Natural process are slow and steady and that earth is much older than previously thought Darwin and Wallace 1859- individuals in a population are different; species arise through the process of natural selection - Decent with modification to describe gradual change from ancestral type - Individuals better able to obtain resources were more likely to survice and reporidcue The concept of natural selection - This preservation of fourable variations and rejections of injurious variations - Occurs when individuals with certain genotypes those that are best suited to teh environment have grgeater reproductive success than other indicudals Evolutionary theory continues to expand - Modern evolutionary synthesis which suggests that genetic mutation create heritable variation and that this variation is the raw material upon which natural selection acts Natural selection molds evolution 2.3 Adaptations enhance reproductive success - Some members of any population will not survive to reproduce - A struggle for existence is therefore inevitable - The variation inherent in each species means that some individuals in each population are better than othera at obtaining nutrients and water, avoiding predators, tolerating temperature changes, attracting mates or reproducing - Adaptations in teh evolutionary sense include only those structures, behaviors, or physiological processes tat are heritablle and that contribute to reproductive success Natural selection eliminated poorly adapted phenotypes - Selective forces “weed out” individuals with poorly adapted phenotypes, indirectly changing allele frequencies - Environmental conditions constantly change so evolution never really stops Natural selection does not have a goal - Many people believe that natural selection leads to ever more “perfect” organisms or that evolution works toward long term goal - Several factors combine to prevent natural selection from producing all of the traits that a species might find useful - Every genome has limited potential imposed by its evolutionary history - No population contains every allele needed to confront every possible change in the environment - Disaster such as floods and volcanic eruptions can indiscriminatley eliminate the best allele combinations, simply by chance - Some harmful genetic traits are out of natural selections reach - Artificail selection DOES have a goal: humans select specific diaries traits What does survivial of the fittest mean - Successful reproduction is the only way for an organism to perpetuate its genes, fitness depends on the ability to survive just long enough to reproduce - Plants that grow, flower, produce seeds, and die within just a few weeks may have fitness equal to redwood trees that live for centuries - Fitness includes not only the total number of offspring produced but also the proportion that reaches reproductive age Evolution is inevitable in real population 12.4 At hardy-weinberg equilibrium, allele frequencies do not change - Highly unlikely situation in which allele frequencies and genotypes do not change from one generation to the next - Natural selection does not occur - Mutations do not occur so no new alleles arise - The population is indefinitely large or at least large enough to eliminate random changes in allele frwuencies - Individuals mate at random - Individuals do not migrate into or out of the population In reality allele frequencies always change - A population at hardy weinberg equilibrium does not evolve - A real population biolates some of teh assumptions of hardy weinberg - Allele frequencies change when natural selection, mutation, genetic drift, nonrandom mating, or migration occurs - Evolution is inevitable Natural selection can shape population in 3 main ways 12.5 - Natural selection changes the genetic makeup of a population by favoring the alleles that contribute to reproductive success and selecting against those that do not - Individuals with the “best” phenotypes are more likely to pass their alleles to the next generation; those with poorly suited phenotypes are less likely to survive long enough to reproduce Three modes of natural selection 1. Directional 2. Disruptive 3. Stabalization Directional selection - One extreme phenotype is fittest and the environment selects against the others Disruptive selection - Two or more extreme phenotypes are fitter than the intermediate phenotype Stabalizing selection - Occurs when extreme phenotype are less fit than the optimal intermediate phenotype Sexual selection directly influences reproductive success Sexual dimorphism - Is a difference in appearance between males and females - One sex may be much larger or more colorful than the other or one sex may have distictive structues such as horns or antlers