Zoology101: Animal BiologyLast Lecture Outline Lecture 34 1. Why are there so many kinds of organisms?2. Process of evolution by natural selectionCurrent lecture 1. MicroevolutionMicroevolution• Natural selection can provide a mechanism to create evolutionary change and help explain the existence of adaptations • Microevolution: variation in heritable traits within a population and differential survival and or reproduction◦ change in traits in a population over time • Population: a localized group of individuals capable of interbreeding and producing fertile offspring • Heritable traits: traits with genetic basis • genotype: genetic composition of an individual • phenotype: observable characteristics • Variations in genotype give rise to variation in phenotype: selection acts on phenotype • Alleles: alternative versions of a gene that produce a different phenotypic event • Microevolution◦ change in allele frequencies in a population from one generation to the next◦ Natural selection on phenotype by evolutionary change ultimately implies change in genotypes in a population • Why is genetic variation important?◦ Variation is the raw material for evolution◦ Environment (and thus selection pressure) is unlikely to remain constant • Gene pool: all of the alleles in a population • Polymorphism: where there are 2 or more versions of a trait within a population • an allele becomes fixed when an entire population is homozygous at that locus • Creating genetic variation◦ mutation (structure in gene or chromosome) ◦ Sex: combining existing alleles in new ways ▪ meiosis: gamete formation▪ fertilization • Four basic processes causing change ◦ mutations: occurrence of new mutation directly changes allele frequency, relatively low mutation rates in most organisms (generally neutral or have a small harmful effect)◦ genetic drift: changes in allele frequencies between generations due to random events▪ tends to reduce genetic variation through the loss of alleles ▪ can have a positive, negative, or no effect on population level of adaptation ▪ always present in some degree (especially in a small population)▪ some can lead to large amounts of drift • founder effect• bottleneck effect: may be particularly relevant for understanding how humans affect the viability of plant and animal population ◦ gene flow: change in allele frequency between generations due to input of individuals or gametes (e.g pollen) from other populations ▪ tends to reduce genetic variation among populations ▪ migration may have a positive, negative, or no effect on degree of adaptation on focal population ◦ Natural selection: only one that shows a predictable high level of adaptation▪ stabilizing, disruptive/diversifying, directional ▪ Stabilizing: favors individuals in the middle of phenotypic gradient▪ Disruptive/diversifying: favors individuals at opposite ends of the phenotypic gradient▪ directional: favors individuals at one end of phenotypic
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