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Transcript
The Evolution of Populations What is Evolution? A change over time in the genetic composition of a population Human evolution… The gene pool Is the total aggregate of genes for a particular trait in a population Consists of all genes for that traits in all individuals of the population e.g. population: 500, two alleles: Red (R) and White (r). Total: 1000 genes for flower color in the population Example: alleles frequencies over time Each allele has a given frequency (proportion) in the population. R Æ 320*2 + 160 = 800 = 0.8 r Æ 20*2 + 160 = 200 = 0.2 Genotypes 20 rr 320 RR 160 Rr Alleles frequencies 50 YEARS FROM NOW R Æ = 0.6 r Æ = 0.4 Alleles frequencies TODAY Voila! EVOLUTION! The Evolution of Populations What is a Population? A group of individuals of the same species living in a given environment peacocks Species: Organisms that share a more or less distinctive form and are capable of breeding and having fertile offspring female Charles Darwin… (still evolving) Evolution through natural selection theory: 1859 male What is the time scale we Many and Many generations in general, million of years Microorganisms and viruses can have a rate of evolution must consider to see visible in a few generations evolutionary changes? Why? short generation spans and sometimes also a high mutation rate 1 Before Darwin’s ideas 1) The idea of fixed species 2) Lamarck and adaptive evolution Animals develop the characteristics according to the environmental needs (neck elongates with the appearance of taller trees) These characteristics can be passed to the next generation WRONG! Evolution •Change in gene (allele) frequencies in a population A+a = 0.65+0.35= 1 That lead to change in the expressed characteristics we observe. These changes happen due mainly to… Natural Selection Individuals whose genetic combination is favored have a better chance to survive, reproduce, and pass their genes on to the next generation. The environment is the selective agent Fitness Apache pocket mouse (lives on lava) Rock pocket mouse (lives on white sand) Reproductive success of each individual within the population. Individuals with those genes will be less predated and will reproduce more than the others The fitness is determined by how well the characteristics of the individual match the demands of the environment in which it lives Genes for white and dark fur were favored in each population of mice and this led to changes in the allele frequencies 2 Theory of natural selection (Darwin): 1) Genetic variation: Individuals within a species differ from each other 2) Inheritance: Offspring are similar to their parents 3) Excess of reproduction: More offspring are generally produced than those to survive to maturity. Factors like predation, disease and competition take place 4) Population size remains fairly constant: if environmental conditions do the same Important! Natural selection do not affect the genes directly but indirectly by selecting those phenotypes that better match the environmental requirements for the species Natural selection operates at the individual level, but the evolutionary changes that produce operates at the population level. Factors that lead to Evolution Changes in the genetic composition of a population 1- Small population size: Genetic drift Bottleneck effect The population is severely reduced in number due to chance Æ a natural disaster, etc. Genetic diversity is also reduced. Mating among the remaining individuals will not replace the lost genes Parent population 50 % 50 % Bottleneck: drastic reduction caused for example by a change in the environmental conditions First generation after bottleneck 25 % 75 % Survival individuals 3 A4 migration Original population Founder effect: A5 •Some individuals migrate and become the founders of a new, isolated population. •Only part of the original gene pool is present in the new population New population Founder effect in the case of Flores man? Flores island An adult pygmy and an adult Australian Founder effect in the case of the Lake Erie water snake? The Lake Erie water snake is limited to the islands in the western section of the Lake Erie Northern water snake Lake Erie water snake The difference in color patterns is related to different allele frequencies for color pattern 4 3- Mutations Point mutations (only one base is changed in the sequence) Chromosomal aberrations (Major change in the DNA at the level of the chromosomes) For only one individual mutations may not appear relevant But the cumulative impact of mutations when considering the entire population can be significant! 4- Natural Selection CANADA MAP AREA Beaufort Sea Porcupine herd range N TE OR RR T H IT WE O S RI T ES • Fairbanks Fortymile herd range ALASKA YUKON Genetic exchange due to migration of fertile individuals or gametes between populations ALASKA 2- Gene flow = Migration • Whitehorse The two caribou populations share the same area in part of the year, but they rarely breed with members of the other population Individuals whose genetic combination is favored for the environment a better chance to survive and pass their genes # of individuals Modes of Natural Selection Evolved Original population population (a) Directional selection Shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators. Original population Which one may be related to… Reduces variation? Important in speciation? Environment changes? Phenotypes (fur color) (c) Stabilizing selection (b) Disruptive selection Favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage. Removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. 5