1. Natural Selection Can Only Occur If There Is Variation Among Members of the Same Species

1. Natural selection can only occur if there is variation among members of the same species. WHY?

Variation in a population results from mutation and the recombination of alleles during meiosis and fertilization. Mutation

• New alleles arise from the mutation of existing alleles, usually by changing one or a few base pairs Meiosis: Independent assortment as Crossing over of homologous homologous chromosomes randomly chromosomes during prophase I results orient at metaphase I causing a in a recombination of maternal and randomized inheritance of maternal paternal alleles within chromosomes and paternal chromosomes within gametes Fertilization:

• New combinations of alleles appear during fertilization • As the unique set of haploid alleles in the egg • Combine with the unique set of haploid alleles in the sperm 2. Species tend to produce more offspring than the environment can support. · observation 1: All species have such great potential fertility that their population size would increase exponentially if all individuals that are born reproduced successfully.

· observation 2: Populations tend to remain stable in size except for seasonal fluctuations. 3. Individuals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die or produce fewer offspring.

• observation 3: Environmental resources are limited.

· inference 1: Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of offspring surviving each generation (= struggle for existence). 4. Individuals that reproduce pass on characteristics to their offspring.

• Survival in the struggle for existence is not random, but depends in part on the hereditary constitution of the surviving individuals. Those individuals whose inherited characteristics best fit them to their environment are likely to leave more offspring than less-fit individuals (= natural selection). • this unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with the inheritance of favorable characteristics accumulating over the generations. In other words: Natural selection is differential success in reproduction ( unequal ability of individuals to survive and reproduce). 5. Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species.

• evolution is the cumulative change in the heritable characteristics of a population, or • the changes in allelic frequencies in the gene pool of a population over time, as a result of natural selection, genetic drift, gene flow, and mutation pressure. 6. Adaptations are characteristics that make an individual suited to its environment and way of life.

• Natural selection occurs through an interaction between the environment and the variability inherent among the individual organisms making up a population. • The product of natural selection is the adaptation of populations of organisms to their environment. 7. Application: Changes in beaks of finches on Daphne Major. • yearly variation in rainfall in the Galapagos Islands can be extreme due to the weather cycles known as El Nino • in wetter years, all sizes of seeds are available in large numbers • smaller birds with smaller beak sizes eat more efficiently and have higher rates of surviving and reproducing • smaller beaked birds pass on the trait of smaller beaks to their offspring • as a result, in the following year the average beak size decreases • that is, the frequency of the smaller beak alleles increases over time • in dryer years, smaller seeds are quickly consumed, leaving only larger, harder seeds • larger birds with larger beak sizes eat more efficiently and have higher rates of surviving and reproducing • larger beaked birds pass on the trait of larger beaks to their offspring • as a result, in the following year the average beak size increases • that is, the frequency of the larger beak alleles increases over time • As predicted by evolutionary theory, natural selection produces gradual changes in traits in response to changes in the environment 8. Application: Evolution of antibiotic resistance in bacteria.

• shortly after development of antibiotics (e.g. penicillin) nearly all bacteria were killed during an application of antibiotics • some variants of bacteria had a heritable trait that gives them resistance to antibiotics • the resistant bacteria have higher rates of surviving and reproducing • resistant bacteria pass on the trait of antibiotic resistance to their offspring • in each following generation the percentage of antibiotic resistant bacteria increases • that is, the frequency of the antibiotic resistance alleles increases over time • As predicted by evolutionary theory, natural selection produces gradual changes in traits in response to changes in the environment