Evolutionary Biology BIOL 3350 Dr Lisa Rapaport Lecture 5 Part 1 Notes Selection I What is Selection A Selection a mechanism of evolution that favors individuals that are better adapted to their environment the fittest organisms survive while the unfit or weak organisms die Individuals that are better adapted to their environment have higher rates of survival and higher rates of reproduction higher chance of passing their traits onto offspring of the next generation Individuals that are poorly adapted to their environment have lower rates of survival and lower rates of reproduction lesser chance of passing their traits onto offspring of the next generation B Stabilizing selection Keeps the allele frequencies and the phenotypes stable in the population Selection acts against extremes thus causing less variation overtime in the phenotypes represented in the population C Example of stabilizing selection for coat color Mice in the light environment tend to have light colored fur because that allows them to be more camouflaged and less frequently detected by predators thus enabling them to live longer and reproduce more than mice with dark fur in this light environment because the mice with dark fur are more easily detected by predators Light fur becomes more prominent in light environments because the light colored mice are the ones that are surviving and reproducing at higher rates while the dark colored mice are the ones that predators are spotting and consuming thus the dark colored mice are the ones that are dying off more quickly and have less chances to reproduce Mice in the dark environment tend to have dark colored fur because that allows them to be more camouflaged and less frequently detected by predators therefore enabling them to live longer and reproduce more than mice with light fur in this dark environment because the mice with the light fur are more easily detected by predators Dark fur becomes more prominent in dark environments because the dark colored mice are the ones that are surviving and reproducing at higher rates while the light colored mice are the ones that predators are spotting and consuming thus the light colored mice are the ones that are dying off more quickly and have less chances to reproduce Selection stabilizes coat colors so that they match their environment 1 The mechanism of selection predation acts against those individuals that do not match their environment D Directional Selection Directional selection a mode of natural selection in which an extreme phenotype is favored over other phenotypes causing the allele frequency to shift over time in the direction of that phenotype Selection is acting against one extreme and is changing allele frequencies in the population The environment drive change in phenotypes E Losos et al 2004 Anolis hindlimb length No change in the control population Control population half above and half below In the experimental population they were all above the mean controlled values which suggests that selection has occurred This shows directional selection F Finch Beak Depth Before the drought variation in beak depth After the drought the beak depth shifted to a higher value beak depth shifted to a higher value due to the change in the environment The selection acted against the thinner beaks due to the change in the environment Change in the mean value of the traits shows that selection has occurred Force of selection is driving towards larger beaks 2 Variation Among Individuals Allopatric speciation when 2 populations of the originally same species are separated geographically and therefore evolve into different species Pea coat color is not governed by polygenic traits Cladogenesis a speciation event The change in guppy morphology and ability to escape a predator directional selection I Genetic Mutations A How do new alleles come about DNA repair mistakes DNA replication mistakes Creates genetic variability B Point Mutations 1 Point mutation single base pair substitution Sometimes repair mechanisms do not work so a brand new allele is produced Point mutations can cause a different amino acid to be coded for and this alters the protein can make the protein non functional Point mutations can be subtle and cause little change if any at all or they can be drastic and cause many problems These kinds of point mutations can be 1 of 2 types transitions or transversions Transition changes a purine nucleotide to another purine A G or a pyrimidine nucleotide to another pyrimidine C T about 2 out of 3 single nucleotide polymorphisms SNPs are transitions Thymine and Cytosine Pyrimidines Adenine and Guanine Purines Cytosine can be transformed into a thymine for example Transversion changes a purine nucleotide to a pyrimidine or a pyrimidine nucleotide to a purine Transversions are more complicated chemical transformations than transitions Transitions have fewer chemical steps Transitions are more common than transversions 2 Insertions and Deletions Insertions and deletions of bases can occur and these create new alleles Insertion when a new base is added to a sequence Deletion when a base is removed from a sequence If insertions or deletions occur in a non coding region no dramatic change occurs to the organism s phenotype If insertions or deletions occur in a coding region major consequences occur and you can tell in the organism s phenotype 3 C Consequences of point mutations 1 Silent mutations Silent mutation DNA mutation that does not significantly alter the phenotype of the organism in which it occurs can occur in non coding regions outside of genes within introns or they can occur within exons Another name for a silent mutation is a synonomous mutation When a silent mutation occurs a different base is substituted in for another base but this substitutions does not change the amino acid that is produced no change in the product These mutations are very common 2 Missense mutations Missense mutation when the change of a single base pair causes the substitution of a different amino acid in the resulting protein this amino acid substitution may have no effect or it may render the protein nonfunctional If change an amino acid in the protein can result in altering the protein and causing the protein to not be able to do its job Occasional creates a beneficial allele and that is how we get new adaptations Advantageous mutations are passed onto offspring because they allow for longer survival and or faster reproduction rates 3