BCOR 12 1st Edition Lecture 5 Outline of Last Lecture I The Scientific Process II How to Solve an Expected vs Observed Hardy Weinberg Problem a Used to determine if a population is evolving III Mechanisms that affect Allele Frequencies Outline of Current Lecture I Types of Natural Selection II Biological Species Concept III Speciation IV Reproductive Barriers Current Lecture Natural selection is the ONLY mechanism in evolution that causes shifts in the population s traits Figure obtained from http www apsnet org edcenter advanced topics PopGenetics Pages NaturalSelection aspx To explain these curves we ll use the example of the pocket mouse which has a variation of colors in it s coat ranging from dark to very light 1 Directional Selection causes a shift in the mouse s coat color from a medium brown more towards a darker color Probably due to predation on the lighter colored mice These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute 2 Disruptive Selection mice of a medium color are preyed on more frequently than light or dark colored mice so it causes a dip in the middle of the phenotypes 3 Stabilizing Selection mice of very light and dark colors are being preyed on so the medium color becomes the most prominent 4 Is not relevant for this course Disruptive selection can lead to speciation Natural selection is based on behavior genotype and phenotype of individuals Chapter 24 Speciation Biological Species Concept a species is a group of populations whose members have the potential to interbreed in nature and produce viable fertile offspring Speciation the evolutionary process in which one species splits into two or more species o Can be slow or quick depending on what is changing Gene flow is the ONLY mechanism that holds a gene pool of a species together If there isn t any gene flow between members that used to be of the same population then speciation has occurred There are several different species concepts but our class will be focusing on the biological species concept But this concept has its limitations Limitations of the biological species concept Difficult to test this species concepts ex Hard to test with fossil evidence Many species reproduce asexually this concept doesn t apply to these individuals It is based only on gene flow Remember that species must remain reproductively isolated from one another Prezygotic Reproductive Barriers barriers that restrict mating or hinder fertilization if mating does occur Types of prezygotic reproductive barriers Habitat isolation Behavioral isolation Temporal reproductive timing isolation Mechanical isolation the sex parts don t align between two species Gametic isolation gametes don t fuse Postzygotic Reproductive Barriers a reproductive barrier that prevents hybrid zygotes produced by two different species from developing into viable fertile adults Types of postzygotic reproductive barriers Reduced viability Reduced hybrid fertility Hybrid breakdown Example A mule is a hybrid between a male donkey and a female horse however this hybrid is sterile due to the odd number of chromosomes This is a postzygotic barrier for the hybrid between the two species cannot breed Example The American Toad and the Fowler s toad do not interbreed in the wild but they can in captivity What keeps them from interbreeding in the wild is the time of the year in which they breed The American Toad breeds early in the summer while the Fowler s toad breeds late in the summer This is a prezygotic barrier of a temporal isolation sorts