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TOK Natural Selection is a theory. How much evidence is required to support a theory and what sort of counter evidence is required to refute it? 1 Chapter 23 The Evolution of Populations “Evolution is indeed a matter of chance, but in the random lottery of mutations, some numbers and combinations better meet the imperatives of ecological necessity, and they arise and are selected for repeatedly.” --Sean B. Carroll Essential Idea The diversity of life has evolved and continues to evolve by natural selection. 3 A Common Misconception Many people think individuals evolve. This is not true. Populations evolve as a result of natural selection acting on each individual within a given population. Those individuals better fit to survive are more likely to reproduce and pass on genes that will benefit future generations. 4 Microevolution Evolution on a small scale. The change in the genetic make up of a population from generation to generation. 5 Darwin’s “Problem” Darwin’s problem was that there were many limitations of science in his time. He did not have a good explanation for how such heritable variations that were required for natural selection appeared in a population. Nor did he have an explanation for how they were transmitted from organisms to their offspring. 6 Recall the Blending Hypothesis: At the time, the blending hypothesis was what people used to explain why offspring look like both parents. Darwin and others realized this was wrong because it would eliminate variation within a population. Ironically, shortly after Darwin published Origin, Gregor Mendel published his paper on genetics. 7 Mendel’s Pea Experiments Mendel’s paper went unnoticed for nearly 50 years. In the early 20th century, as scientists uncovered the work of Mendel, it became apparent that its implications and relatedness to Darwin’s idea were profound. 8 Population Genetics Scientists began drawing parallels between Darwin and Mendel, and melded them into what is known as population genetics--the study of how populations change over time. 9 The Modern Synthesis As more was learned about Darwin and Mendel, scientists developed the Modern Synthesis--a comprehensive theory of evolution that incorporates many fields of science. 10 Populations Populations are groups of individuals that can breed with one another and are localized in certain regions. Some populations are isolated from others. Still others can easily mix with like members of a population. 11 Populations Within a population, all of the genes are called the gene pool and it consists of all alleles at a given locus. If only one allele exists in a population, it is said to be fixed and all individuals are homozygous. If more than one allele exists, then individuals are either homozygous or heterozygous. 12 Allele Frequency Consider a population of 500 with 2 alleles, CR and CW CRCR gives Red CWCW gives White CRCW gives Pink 13 Allele Frequency Our Population Breakdown: 320 red, CRCR 160 Pink, CRCW 20 White, CWCW These numbers suggest a blending hypothesis Why can’t we use blending? 14 The Hardy-Weinberg Theorem This theorem is a way to examine how allele frequencies change over time when only segregation and independent assortment are working on the alleles. The properties of a non-evolving gene pool--in the absence of natural selection. The theorem states that the frequencies of the alleles will remain constant in a population when it is not evolving. 15 The Hardy-Weinberg Theorem In other words, sexual reproduction alone will not change the frequency of the alleles in a non-evolving population. 16 The Hardy-Weinberg Theorem The theorem describes Mendelian inheritance in non-evolving populations. It also helps us to understand long-term evolutionary change--that is, the preservation of genetic variation gives the opportunity for natural selection to occur. 17 Hardy-Weinberg Frequency In our population, there are 1000 copies of genes (500 individuals, 2 copies). 800 of them are CR 200 of them are CW When we have 2 alleles, by convention we represent them as p and q. p = CR = 800/1000 = 0.8 or 80% q = CW = 200/1000 = 0.2 or 20% 18 The Hardy-Weinberg Theorem To determine the probabilities in our wild flower example: The chance of CRCR is: p • p = p2 = 0.8 • 0.8 = 0.64 64% The chance of CRCW is: p • q = 2pq = 0.8 • 0.2 = 0.32 32% The chance of CWCW is: q • q = q2 = 0.2 • 0.2 = 0.04 4% 19 The Hardy-Weinberg Equation The Hardy-Weinberg Equation becomes: p2 + 2pq +q2 = 1 Again, with a non-evolving gene pool, the frequencies of alleles will remain constant if mating is random. You can think of it like a deck of cards, no matter how many times you shuffle them, the types of cards and their frequencies remain the same. 20 5 Reasons Hardy-Weinberg Doesn’t Hold True: Departure from these 5 conditions results in evolution. 1. 2. 3. 4. 5. Extremely large population size. No gene flow. No mutations. Random mating. No natural selection. 21 Mutation and Sexual Recombination These provide variety within gene pools. Mutations are changes in the nucleotide sequences that give rise to new genes and new alleles. Sometimes they’re good, usually they are not. Most mutations occur in somatic cells and are never passed on. Only a small percentage of gametes ever get into the populations, so any mutation occurring in the gametes likely won’t get passed on. 22 Mutation and Sexual Recombination Mutation rates in general are low. The larger the organism the less likely a mutation will occur and vice-versa. For example: Plants and animals with long generation times are relatively large and have a much lower frequency of mutations than do microorganism and viruses. 23 Mutation and Sexual Recombination Sexual recombination is the best way to produce variation within a population on a generation to generation time scale. Movie 24 Mutation and Sexual Recombination There are 3 main understandings which play a major role in evolution by altering allele frequencies: 1.Natural selection 2.Genetic drift 3.Gene flow 25 1. Natural Selection There are a lot of things that need to be understood about natural selection and the role it plays in evolution. 26 1. Natural Selection As you know, when organisms have traits/adaptations that make them better suited for survival, they are said to be more fit to survive. Adaptations are characteristics that make an individual suited to its environment and way of life. 27 1. Natural Selection This survival often changes the allele frequency within a population. Natural selection can only occur if there is variation among members of the same species. 28 1. Natural Selection Species tend to produce more offspring than the environment can support. Those individuals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die and produce fewer offspring over time. 29 1. Natural Selection Those individuals that have reproductive success pass the characteristics on to their offspring. 30 1. Natural Selection Thus natural selection will increase the frequency of those characteristics that make individuals better adapted and decrease the frequency of other characteristics leading to changes within the species. 31 2. Genetic Drift Genetic drift is an unexpected fluctuation in allele frequency from one generation to the next. This is often due to a chance event where a large proportion of the population is wiped out. 32 2. Genetic Drift There are two situations which increase the likelihood of genetic drift that have a large impact on a population: A. The bottle neck effect B. The founder effect 33 A. The Bottle Neck Effect A sudden change in the environment which drastically changes a population can have a profound impact on the genetic makeup of the population. It may change the population in such a way that the survivors no longer represent the original population. The survivors are said to have gone through a “bottleneck.” 34 B. The Founder Effect When a few organisms become isolated from a large population and establish a new population whose gene pool is not reflective of the new population, we say the “founder effect” has occurred. These founders pass through an isolation bottleneck and represent a gene pool with altered allele frequencies. 35 3. Gene Flow Gene flow occurs when populations gain or lose alleles as organisms come and go within a population. Gene flow tends to reduce differences between populations. 36 Variation Variations are heritable differences within a population and comprise the raw material for diversity and natural selection. Only the genetic component of variation can have evolutionary consequences as a result of natural selection. 37 Variation Variation within a population comes from either discrete characters or quantitative characters: Discrete-an either or basis determined from a single locus. Quantitative-comes from 2 or more loci that determine the phenotype. 38 Variation Variation can arise from mutation, meiosis, and the process of sexual reproduction between individuals in a species. This variation is what natural selection operates on. 39 Fitness The adaptive advantage of an organism which allows it to make a genetic contribution to the gene pool of the next generation. In other words, the reproductive success of an organism. 40 Modes of Selection Natural selection alters the frequency distribution of heritable traits in three ways: 1. Directional selection. 2. Disruptive selection. 3. Stabilizing selection. 41 Directional Selection This is most common when a population’s environment changes or when members of a population migrate to a new habitat with different environmental conditions. QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. 42 Disruptive Selection This occurs when conditions favor individuals in both extremes over those of normal average phenotypes. It can be important in the early stages of speciation. QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. QuickT ime™ and a TIF F (Uncompressed) decompressor are needed to see this picture. Quick Time™ and a TIFF (U nc ompress ed) decompres sor are needed to s ee this pic ture. 43 Stabilizing Selection Acts against extreme phenotypes, it favors the intermediates. It reduces variation and maintains the status quo of a given phenotype. QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. QuickT ime™ and a TIF F (Uncompressed) decompressor are needed to see this picture. Quick Time™ and a TIFF (U nc ompress ed) decompres sor are needed to s ee this pic ture. 44 Selection, In General Regardless of the mode of selection, selection works to favor certain heritable traits through differential success. Disruptive and stabilizing selection tend to reduce variation, but there are methods nature uses to preserve it. 45 Methods Nature Uses: 1. Diploidy 2. Balancing Selection A. Heterozygous advantage B. Frequency dependent selection 3. Neutral Variation 46 Diploidy Many eukaryotes are diploid and this hides a lot of variation from selection. Recessiveness can be transferred from generation to generation even if they are harmful because they only cause harm when inherited from both parents when the zygote is formed. 47 Balancing Selection Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population. Heterozygous advantage--acts in a way that is favored by natural selection over either homozygous form. Frequency dependent selection--the fitness of any one morph declines if it becomes too common in a population. 48 Malarial Parasite, Human QuickTime™ and a H.264 decompressor are needed to see this picture. 49 Malarial Parasite, Mosquito QuickTime™ and a H.264 decompressor are needed to see this picture. 50 Plasmodium falciparum http://www.nature.com/nrg/journal/v8/n7/fig_tab/nrg2126_F1.html 51 Plasmodium falciparum Inside the red blood cells, Plasmodium feeds on the hemoglobin, and the amino acids that make it up to produce more of itself. The Plasmodium transforms the cell into a machine that will help it reproduce and avoid transport to the spleen. 52 QuickTime™ and a decompressor are needed to see this picture. Sickle Cell Anemia A single base-pair substitution causes the cellular machinery to insert the wrong amino acid to the beta globin. The defective hemoglobin collapses onto itself taking on a needle shape. 54 http://t1.gstatic.com/images?q=tpn:ANd9GcRX7NQlvgnBnolAaG-2HxbUcBtuHROXt1yfk8ViEBlhslsZPKTE&t=1 Sickle Cell Anemia If this disease is fatal, why is the allele for it maintained in the population? 55 Sickle Cell Anemia Homozygotes (aa) are unlikely to survive to reproductive age. Heterozygotes, however, have a good deal of resistance to malaria. When the Plasmodium invades the body and into a heterozygote’s RBC, the cell sickles and loses its ability to pump potassium. 56 Sickle Cell Anemia The Plasmodium depends on the potassium for survival and cannot survive and reproduce without it. These individual’s immune systems quickly recognize the sickled cells as foreign and remove them along with the parasite. Thus the individual’s bout with malaria is greatly reduced. The mechanism for protection is not completely understood. 57 Other Blood Disorders Other blood disorders are prevalent in areas where malaria is found. Ovalocytosis. Thalassemia. 58 QuickTime™ and a decompressor are needed to see this picture. Heterozygous Advantage 60 Neutral Variation Some of the genetic variation has little or no effect on reproductive success. Much of the difference we see is found in untranslated parts of the genome. Confers no advantage--are called pseudogenes. Genetic drift can increase or decrease the frequency of pseudogenes. Difficult to measure--very debatable. 61 Sexual Selection Sexual selection is natural selection for mating success. It can result in sexual dimorphism--differences between the sexes in secondary characteristics. There are two types of sexual selection: Intrasexual selection. Intersexual selection. Males are usually the showier sex. 62 Intrasexual Selection In this, we have direct competition of one sex for mates of the opposite sex. A male often patrols a group of females and prevents other males from mating with her. He is often the psychological winner via a ritual that discourages competitors. This prevents harm to him and increases his own fitness. 63 Intersexual Selection Individuals of one sex are choosy in selecting mates from the other sex. In most cases, a female’s choice depends on the showiness of a male. Example: Peacocks display sexual dimorphism and both inter- and intrasexual selection. 64 66 An Interesting Aside Regarding showiness, the most intriguing thing is that it is often a hindrance to their survival. The benefits, however, seem to outweigh the costs. When a female chooses a showier male, she is often choosing the healthiest mate with the best genes. This allows the male to pass his genes on to his offspring. 67 Selective Breeding and Evolution Other forms of selective breeding of domesticated animals shows that artificial selection can cause evolution. Horses, cats, pigeons, dogs, etc. Here is an example: 68 QuickTime™ and a H.264 decompressor are needed to see this picture. Selective Breeding and Evolution Here is another example involving plants. 70 QuickTime™ and a H.264 decompressor are needed to see this picture. Natural Selection It doesn’t fashion perfect organisms: 1. Evolution is limited by historical constraints. 2. Adaptations are often compromises. 3. Change and natural selection interact. 4. Selection can edit only existing variation. 72 1. Evolution is Limited By Historical Constraints. Each species comes from a long line of ancestral forms. Ancestral anatomy isn’t scrapped by a new form, it’s a slow change. This helps to explain why you don’t see an example of every species that has ever lived preserved in the fossil record. 73 QuickTime™ and a xvid decompressor are needed to see this picture. 2. Adaptations are Often Compromises What makes us better in some ways, hinders us in others. Take the tail of the peacock as an example. It makes the bird better in that it allows it to get a mate, it hinders it in its ability to evade predators. 75 3. Chance and Natural Selection Interact Chance events can alter a gene pool such as when a storm blows birds or insects over an ocean and to a new environment. The genes which arrive may not be the best in the former population. The organisms pass through a “bottleneck.” 76 4. Selection Can Only Edit Existing Variation Natural selection favors the fittest phenotype in any population, and new variations can’t arise on demand. 77