Lecture 23 Evolutionary Processes I Natural Selection Learning Objectives Describe components of modern synthesis and requirements for evolution by natural selection Understand and calculate relative fitness Explain how evolution of antibiotic resistance illustrate natural selection in action as well as the role of mutation in evolution Compare and contrast directional stabilizing balancing and disruptive selection with respect to how they impact levels of genetic variation and phenotypic mean Explain how the repeated loss of complex structures counters misconceptions about how natural selection works Evolutionary Biology Why We Care Evolutionary processes have affected all organisms on Earth including us Evolutionary processes continue to impact populations and species Understanding how and why species change over one or more generations is fundamental to Human activities are accelerating rates of evolution in certain cases HOW Understanding evolution also has practical applications such as fighting infectious diseases and understanding biology saving endangered species The modern evolutionary synthesis represents the combination of Darwin Wallace s theory of the evolution of species by natural selection Gregor Mendel s theory of genetics as the basis of biological inheritance and mathematical population genetics Genetic variation in populations arises spontaneously through random mutation and is constantly reshuffled into new alleic combinations by meiosis random fertilization Evolution consists primarily of changes in the frequencies of alleles between one generation and another as a result of genetic drift gene flow natural selection Speciation is usually due to the gradual accumulation of small genetic changes and most easily occurs when populations are reproductively isolatedgeographic barriers Darwin s BIG idea A Paradigm Shift Did not publish ideas on NS for 21 years until Wallace s independent work spurred Knew his ideas would rock the victorian world Victorian worldview changed earth was ancient species could and di change over generations new species evolved from ancestral species and some species went extinct human evolution part of same natural process Among most important and enduring contributions withstood experimental tests EVOLUTION Most biologists think that small evolutionary changes accumulate over long time periods to generate large evolutionary changes like speciation Natural selection works fast enough that we can often observe rapid phenotypic genotypic change in poplautions in nature lab Evolution by natural selection does not push organisms forwards toward some ideal form it is not progressive NS reduces the representation of those traits that are less in current environment As environmental conditions change the traits that lead to increased reproductive success in environment can also change Natural selection can move populations away from Hardy Weinberg Equilibrium Of the processes that can change allele frequencies natural and sexual selection are the only processes that produces adaptations heritable traits that increase survival and or reproductive success Biologists often compare relative fitness fitness of individual relative to others Timescale for evolution by natural selection depends upon generation time of organ Evolution of antibiotic resistance in the bacterium that causes TB Evolution o A chance mutation occurs o Drug therapy kills most bacteria w out mutation o Mutant cells proliferate o Drug therapy is ineffective against mutant cells Types of Selection Directional Selection one tail of phenotypic distribution is favored If directional selection continues long enough the favored alleles eventually reach a frequency of 1 0 and are said to be fixed o Over time genetic diversity is reduced o Selection can act on discrete traits and polygenic traits skin color o Directional artificial selection drives phenotypic divergence in dogs o Human activities change enviroments causing unintended directional Antibiotic resistance disease causing bacteria resistant to antibiotic Pesticide resistance insects mites resistant to pesticides Herbicide resistance weedy plants resistant to herbicides NOT artificial selection not trying to produce resistant things Balancing Selection Heterozygote advantage o Biologists reserve this term in cases where 1 gene influences trait discrete o Ex carriers of sickle cell Hh more resistant to malaria than HH o Maintains recessive allele in populations Stabilizing Selection individuals w intermediate phenotypes are favored o Population mean stays the same o Over time reduces both tails of the phenotypic distribution o Tends to reduce genetic variation in the trait narrower distribution o Ex very small and very large babies r most likely to die Disruptive Selection o Intermediate phenotypes are selected against and extreme phenotypes are favored o Increases genetic variation but does not change the mean value of a trait o Opposite that of stabilizing o Ex juvenile black bellied seed crackers that had very long or very short beaks survived long enough to breed Reductive Evolution People assume that selection always drives organisms toward greater complexity o NOT TRUE this is incorrect If reduced complexity yields higher survival and reproduction organisms evolve in that direction Loss of complex features is termed reductive evolution Ex Eye loss in cave populations of many different species Kentucky cave shrimp if having reduced morphological complexity leads to higher fitness then the frequency of the alleles associated with that phenotype will increase in pop Ex many parasitic worms have very simple morphologies some lost digestive system Lecture 24 Sexual Selection Objectives Compare and Contrast intrasexual and intersexual selection and provide examples Explain Fisherian runaway model and provide examples Describe direct vs indirect benefits of female mate choice Sexual Selection Process by which individuals that posses certain heritable traits are more successful at attracting obtaining mates thus reproduce at higher rate relative to population Directed by female choice or male male competition results in traits that lead to greater mating success Males compete more intensely for mates than do females Females are choosier about their mates than are males Sexual Dimorphism differences between the sexes in traits related to attracting obtaining mates Sexual selection is generally stronger on males thus sexually