Chapter 23: The Evolution of Populations - Campbell Biology

Chapter 23 The Evolution of Populations Lecture Presentations by Nicole Tunbridge and © 2021 Pearson Education, Inc. Kathleen Fitzpatri...

Chapter 23 The Evolution of Populations Lecture Presentations by Nicole Tunbridge and © 2021 Pearson Education, Inc. Kathleen Fitzpatrick  Objectives To show that natural selection that causes adaptive evolution Show the importance of genetic variation Illustrate the use of Hardy Weinberg Equation Understand the effects of genetic drift and migration on a population © 2021 Pearson Education, I nc. Figure 23.1b © 2021 Pearson Education, Inc. The Smallest Unit of Evolution  A common misconception is that organisms evolve during their lifetimes  Natural selection acts on individuals, but only populations evolve  Consider, for example, a population of medium ground finches on Daphne Major Island  During a drought, large-beaked birds were more likely to crack large seeds and survive  The finch population evolved by natural selection © 2014 Pearson Education, Inc.  Early criticism of Natural Selection Required long spans of time Could not be tested experimentally Only described selection among pre-existing variations Provided no source of variation Did not explain Mendel most certainly read Origin of Species, but Darwin likely did not know of Mendel’s work studying inheritance. The connection wasn’t made until the 20th century. We now know that new alleles arise by mutation and that sexual reproduction creates new combinations of those alleles. The Evolution of Populations refers to evolutionary change above the species level Microevolution consists of adaptations that evolve within a population, confined to one gene pool The “Modern Synthesis” defines evolution as the change in the genetic make-up of a population over time. Given the shorter time spans, experiments to test natural selection are possible. Kettlewell: peppered moth Carroll & Boyd: Soapberry bugs Antibiotic Resistance Experiments of Rosemary and Peter Grant Literally hundreds of others Trees prior to Trees after industrial industrial pollution pollution Most moths are Most moths are light variety dark variety  Antibiotic Resistance Selection works on individuals, but populations evolve Genetic variation ultimately is the result of mutation which creates new alleles A gene pool consists of all the alleles for all the loci in all individuals in an interbreeding population  How do we measure the change in the genetic make- up of a polulation? The Hardy-Weinberg equation p2 + 2pq + q2 = 1 p = frequency of one allele (dominant) q = frequency of other allele (recessive) p2 = frequency of homozygous dominant In a flower population, there are two alleles of a color gene, CR and CW. These two alleles are partially dominant. CR CW CR CR CW CW Gene Pool Mendel always got a 1:2:1 ratio in his F2 crosses because his allelic frequencies were always 50%. The Hardy-Weinberg equation allows allelic frequencies to be any number. p+q=1 p2 + 2pq + q2 = 1 p = frequency of red allele q = frequency of white If the population is in Hardy- Weinberg equilibrium, the allelic frequencies and genotype frequencies do NOT change from one generation to the next. The Factors that can affect the gene pool of a population, resulting in evolutionary change Mutation – creates new alleles Non-random mating Gene flow – migration of individuals in or out of the population Genetic drift – chance events Natural selection – non-random reproductive success Gene Flow: The transfer of alleles into or out of a population by migration Genetic Drift: Random events are likely to affect small populations more than large ones Genetic Drift: Random events are likely to affect small populations more than large ones Genetic Drift: Random events are likely to affect small populations more than large ones Bottlene ck Effect When the population size crashes, it can dramatically affect the gene pool Founder The same thing Effect can happen if a small group splits from the population and These colonizes a new individuals location don’t die, rather…. ….these folks  Just to review…. p2 + 2pq + q2 = 1 A population in Hardy-Weinberg equilibrium is a population that is not evolving. The frequencies of alleles remain constant from one generation to the next. The Hardy-Weinberg equation allows us to measure the amount of change due to various factors such as natural selection Depending on which phenotypes are beneficial, natural selection can alter the frequency distribution of inherited traits in three basic ways Natural Selection leads to adaptation Sexual Selection: Some traits are adaptive only in terms of attracting mates Genetic variation is preserved in populations in various ways Neutral Variation Diploidy Heterozygote Advantage (balancing Selection) Frequency Dependent Selection (Balancing Selection) Diploidy  Diploidy maintains genetic variation in the form of hidden recessive alleles  Heterozygotes can carry recessive alleles that are hidden from the effects of selection  Alleles that lower an individual’s fitness can be maintained in a population if they are recessive © 2014 Pearson Education, Inc. Balancing Selection  Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population  Balancing selection includes  Heterozygote advantage  Frequency-dependent selection © 2014 Pearson Education, Inc.  Frequency-dependent selection occurs when the fitness of a phenotype declines if it becomes too common in the population  Selection can favor whichever phenotype is less common in a population  For example, frequency-dependent selection selects for approximately equal numbers of “right-mouthed” and “left-mouthed” scale-eating fish © 2014 Pearson Education, Inc.  Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes  Natural selection will tend to maintain two or more alleles at that locus  For example, the sickle-cell allele causes deleterious mutations in hemoglobin but also confers malaria resistance © 2014 Pearson Education, Inc. The Sickle Cell Allele  Sickle-cell disease is a genetic disorder that strikes individuals with two copies of the sickle-cell allele  This allele affects the structure and function of hemoglobin, reducing the oxygen carrying capacity of red blood cells  Though sickle-cell disease is lethal, frequency of the allele is as high as 15–20% in some regions © 2014 Pearson Education, Inc. Events at the Molecular Level  A point mutation in the sickle-cell allele changes one amino acid, causing improper protein folding, and binding of the proteins into chains forming a fiber © 2014 Pearson Education, Inc. The difference between normal and sickle cell hemoglobin is one base substitution leading to one amino acid difference in the protein This change of one amino acid results in the tendency of the molecules to stick together, distorting the shape of the red blood cell Having one copy of the sickle cell allele is protective against malaria Being homozygous for sickle cell lowers your fitness Being heterozygous for sickle cell increases your fitness in areas where malaria is Imagine that a population exists in which no mutations ever occur. Could such a population evolve? A.Yes B.No C.Maybe  Formation of New Alleles New alleles arise by mutation, change in the nucleotide sequence of DNA Mutations can be caused by replication errors or exposure to certain types of radiation or chemicals Even a point mutation, change in a single nucleotide, can have significant impact on phenotype Why Natural Selection Cannot Fashion Perfect Organisms 1. Selection can act only on existing variations 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Chance, natural selection, and the environment interact © 2014 Pearson Education, Inc.