Lecture 11 Mechanisms of Evolution & Sexual Selection PDF

BIO1202: Genetics and Evolution MECHANISMS AND PROCESSES OF EVOLUTION & SEXUAL SELECTION LECTURE 11 Dr. Arianne-Elise Harris But first….a shoutout to the ‘founding fathers’ of Evolution ERASMUS DARWIN (1731 – 1802): https://earlyevolution.oeb.harvard.edu/erasmus-darwin JEAN BAPTISTE-LAMARCK (1744- 1...

BIO1202: Genetics and Evolution MECHANISMS AND PROCESSES OF EVOLUTION & SEXUAL SELECTION LECTURE 11 Dr. Arianne-Elise Harris But first….a shoutout to the ‘founding fathers’ of Evolution ERASMUS DARWIN (1731 – 1802): https://earlyevolution.oeb.harvard.edu/erasmus-darwin JEAN BAPTISTE-LAMARCK (1744- 1829): https://early-evolution.oeb.harvard.edu/jeanbaptiste-lamarck THOMAS MALTHUS (1766- 1834): https://ucmp.berkeley.edu/history/malthus.html CHARLES DARWIN (1809-1882): https://earlyevolution.oeb.harvard.edu/charles-darwin Mechanisms: the processes of evolution POPULATION Localized group belonging to the same species SPECIES Naturally occurring breeding group of organisms that produce fertile offspring GENE POOL Total aggregate of genes in a population at any one time 1.Most species are not evenly distributed over a geographic range. 2.Individuals are more likely to breed with others from their population center. FUNFACT: Haplotype (A haplotype is a group of genes in an organism that are inherited together from a single parent) Mechanisms: the processes of evolution Fundamental to the process is genetic variation upon which selective forces can act in order for evolution to occur. Evolution only occurs when there is a change in gene frequency within a population over time. (A reminder: A gene is a portion of DNA that determines a certain trait. An allele is a specific form of a gene. Genes are responsible for the expression of traits. Alleles are responsible for the variations in which a given trait can be expressed). These genetic differences are heritable and can be passed on to the next generation — which is what matters in evolution: long term change. Mechanisms: the processes of evolution 1. Mutation 2. Gene flow (Migration) 3. Natural selection as mechanisms of change 4. Genetic drift Mechanisms: the processes of evolution Mutations are changes in the DNA. Mechanisms: the processes of evolution Mechanisms: the processes of evolution A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations. Although mutation is the original source of all genetic variation, mutation rate for most organisms is pretty low. So, the impact of brand-new mutations on allele frequencies from one generation to the next is usually not large. However, natural selection acting on the results of a mutation can be a powerful mechanism of evolution. Mechanisms: the processes of evolution Gene flow involves the movement of genes into or out of a population, due to either the movement of individual organisms or their gametes (eggs and sperm, e.g., through pollen dispersal by a plant). Organisms and gametes that enter a population may have new alleles, or may bring in existing alleles but in different proportions than those already in the population. Gene flow can be a strong agent of evolution. Sex can introduce new gene combinations into a population. This genetic shuffling is another important source of genetic variation. Gene flow Mechanisms: the processes of evolution Genetic drift is one of the basic mechanisms of evolution. In each generation, some individuals may, just by chance, leave behind a few more descendants than other individuals. The genes of the next generation will be the genes of the "lucky" individuals, not necessarily the healthier or "better" individuals. Genetic drift example We have a very small rabbit population that's made up of 8 brown individuals (genotype BB or Bb) and 2 white individuals (genotype bb). Initially, the frequencies of the B and b alleles are equal. What if, purely by chance, only the 5 circled individuals in the rabbit population reproduce? (Maybe the other rabbits died for reasons unrelated to their coat color, e.g., they happened to get caught by a hunter.) In the surviving group, the frequency of the B allele is 0.70, and the frequency of the b allele is 0.30. In our example, the allele frequencies of the five lucky rabbits are perfectly represented in the second generation. Because the 5-rabbit "sample" in the previous generation had different allele frequencies than the population as a whole, frequencies of B and b in the population have shifted to 0.70 and 0.30 respectively. From this second generation, what if only two of the BB offspring survive and reproduce to yield the third generation? In this series of events, by the third generation, the b allele is completely lost from the population. Population size and Genetic Drift When alleles are lost for a population and other allele frequency reach 100% it is referred to as fixation. This is occurs over a short time period for small populations. Larger populations are unlikely to change this quickly as a result of genetic drift. Genetic drift, unlike natural selection, does not take into account an allele’s benefit (or harm) to the individual that carries it. That is, a beneficial allele may be lost, or a slightly harmful allele may become fixed, purely by chance. https://www.khanacademy.org/science/biology/her/heredity-and-genetics/a/genetic-drift-founder-bottleneck The bottleneck effect The bottleneck effect is an extreme example of genetic drift that happens when the size of a population is severely reduced. Events like natural disasters (earthquakes, floods, fires) can decimate a population, killing most individuals and leaving behind a small, random assortment of survivors. The allele frequencies in this group may be very different from those of the population prior to the event, and some alleles may be missing entirely, thus genetic diversity is reduced. The smaller population will also be more susceptible to the effects of genetic drift for generations (until its numbers return to normal), potentially causing even more alleles to be lost. https://www.khanacademy.org/science/biology/her/heredity-and-genetics/a/genetic-drift-founder-bottleneck The founder effect The founder effect is another extreme example of drift, one that occurs when a small group of individuals breaks off from a larger population to establish a colony. The new colony is isolated from the original population, and the founding individuals may not represent the full genetic diversity of the original population. That is, alleles in the founding population may be present at different frequencies than in the original population, and some alleles may be missing altogether. The founder effect is similar in concept to the bottleneck effect, but it occurs via a different mechanism (colonization rather than catastrophe). https://www.khanacademy.org/science/biology/her/heredity-and-genetics/a/genetic-drift-founder-bottleneck Mechanisms: the processes of evolution Natural selection, occurs when one allele (or combination of alleles of different genes) makes an organism more or less fit, that is, able to survive and reproduce in a given environment. If an allele reduces fitness, its frequency will tend to drop from one generation to the next. Video on Natural Selection https://www.youtube.com/watch?v=2mTVtToioLg&ab_channel=Primer https://www.youtube.com/watch?v=goePYJ74Ydg&ab_channel=Primer https://www.youtube.com/watch?v=0ZGbIKd0XrM&ab_channel=PrimerPrimer Mechanisms: the processes of evolution Natural selection 1. There is variation in traits. For example, some beetles are green and some are brown. 2. There is differential reproduction. The environment can't support unlimited population growth, not all individuals get to reproduce. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do. 3. There is heredity. The surviving brown beetles have brown baby beetles because this trait has a genetic basis. Mechanisms: the processes of evolution Natural Selection Mechanisms: the processes of evolution The population will not only evolve (change in its genetic makeup and inherited traits), but will evolve in such a way that it becomes adapted, or better-suited, to its environment. Natural selection acts on an organism’s phenotype, or observable features. When a phenotype produced by certain alleles helps organisms survive and reproduce better than their peers, natural selection can increase the frequency of the helpful alleles from one generation to the next – that is, it can cause microevolution. Microevolution is defined as changes in the frequency of a gene in a population. These are subtle changes that can occur in very short periods of time, and may not be visible to a casual observer. ** Microevolution happens on a small scale (within a single population), while macroevolution happens on a scale that transcends the boundaries of a single species. Mechanisms: the processes of evolution Natural Selection End result: The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown. Mechanisms: the processes of evolution Natural Selection Mechanisms: the processes of evolution Natural Selection The phenotypes and genotypes favored by natural selection aren't necessarily just the ones that survive best. Instead, they're the ones with the highest overall fitness. Fitness is a measure of how well organisms survive and reproduce, with emphasis on "reproduce." Mechanisms: the processes of evolution Natural Selection There are three basic ways that natural selection can influence distribution of phenotypes for polygenic traits in a population. 1. Stabilizing selection 2. Directional selection 3. Disruptive selection Stabilizing selection, intermediate phenotypes are more fit than extreme ones. Directional selection, one extreme phenotype is more fit than all the other phenotypes. Disruptive selection, both extreme phenotypes are more fit than those in the middle. Mechanisms: the processes of evolution Hardy-Weinberg equilibrium If a population is in a state called Hardy-Weinberg equilibrium, the frequencies of alleles, or gene versions, and genotypes, or sets of alleles, or genetic variation, in that population will stay the same over generations (and will also satisfy the Hardy-Weinberg equation). Hardy-Weinberg equilibrium Hardy-Weinberg equilibrium In order for a population to be in Hardy-Weinberg equilibrium, or a non-evolving state, it must meet five major assumptions: 1. No mutation. No new alleles are generated by mutation, nor are genes duplicated or deleted. 2. Random mating. Organisms mate randomly with each other, with no preference for particular genotypes. 3. No gene flow. Neither individuals nor their gametes (e.g., windborne pollen) enter or exit the population. 4. Very large population size. The population should be effectively infinite in size. 5. No natural selection. All alleles confer equal fitness (make organisms equally likely to survive and reproduce). Hardy-Weinberg equilibrium – explanation Imagine we have a large population of beetles. Let's say this population is infinitely large. The beetles of our infinitely large population come in two colors, dark gray and light gray, and their color is determined by the A gene. AA and Aa beetles are dark gray, and aa beetles are light gray. In our population, let's say that the A allele has a frequency of 0.30, while the a allele has a frequency of 0.70. If a population is in Hardy-Weinberg equilibrium, allele frequencies will be related to genotype frequencies by a specific mathematical relationship, the Hardy-Weinberg equation. So, we can predict the genotype frequencies we'd expect to see (if the population is in Hardy-Weinberg equilibrium) by plugging in allele frequencies as shown below: Hardy-Weinberg equilibrium – explanation Let's imagine that these are, in fact, the genotype frequencies we see in our beetle population (9%, percent AA, 42% Aa, 49% aa). With this ratio our beetles appear to be in Hardy-Weinberg equilibrium. Now, let's imagine that the beetles reproduce to make a next generation. What will the allele and genotype frequencies will be in that generation? To predict this, we need to make a few assumptions: First, let's assume that none of the genotypes is any better than the others at surviving or getting mates. If this is the case, the frequency of A and a alleles in the pool of gametes (sperm and eggs) that meet to make the next generation will be the same as the overall frequency of each allele in the present generation. https://www.khanacademy.org/science/biology/her/heredity-and-genetics/a/hardy-weinberg-mechanisms-of-evolution Hardy-Weinberg equilibrium – explanation Second, let's assume that the beetles mate randomly (as opposed to, say, blue beetles preferring other blue beetles). If this is the case, we can think of reproduction as the result of two random events: the selection of sperm from the population's gene pool and the selection of an egg from the same gene pool. The probability of getting any offspring genotype is just the probability of getting the egg and sperm combo(s) that produce that genotype. We can use a modified Punnett square to represent the likelihood of getting different offspring genotypes. Here, we multiply the frequencies of the gametes on the axes to get the probability of the fertilization events in the squares: https://www.khanacademy.org/science/biology/her/heredity-and-genetics/a/hardy-weinberg-mechanisms-of-evolution Hardy-Weinberg equilibrium – explanation As shown above, we'd predict an offspring generation with the exact same genotype frequencies as the parent generation: 9% AA, 42% Aa, and 49% aa. If genotype frequencies have not changed, we also must have the same allele frequencies as in the parent generation: 0.30 for A and 0.70 for a. What we've just seen is the essence of Hardy-Weinberg equilibrium. If alleles in the gamete pool exactly mirror those in the parent generation, and if they meet up randomly (in an infinitely large number of events), there is no reason—in fact, no way—for allele and genotype frequencies to change from one generation to the next. In the absence of other factors, you can imagine this process repeating over and over, generation after generation, keeping allele and genotype frequencies the same. Since evolution is a change in allele frequencies in a population over generations, a population in Hardy-Weinberg equilibrium is, by definition, not evolving. Hardy-Weinberg Equation Videos https://youtu.be/oc9fJCAIRJs https://www.youtube.com/watch?v=D5NVlAaT-OA LET’S WORK A LIVE EXAMPLE: Tiger stripes In a given population of tigers, 5% have zig-zag stripes represented as the recessive alleles: zz. Homozygous dominant tigers have wavy stripes, represented by ZZ. Using the principles of the Hardy-Weinberg equilibrium, determine the allele frequencies within the population and the probability of tigers being born heterozygous dominant. HW equilibrium = P^2 + 2Pq + q^2 = 1 Population = 100% allele frequency q^2 = 5% or 0.05 q = SQRT(0.05) or SQRT(5) = 0.2 OR 20% 20% of our tiger population is homozygous recessive. Thus, (100-20) 80% of tiger pop. is homozygous DOMINANT. Thus, the % of tigers born heterozygous dominant = 0.32 OR 32%. NEXT WEEK: Final lecture for BIO1202: Laboratory evolution, species formation and evolutionary applications LAST TUTORIAL (tutorial 10) WILL BE NEXT WEEK AS WELL. FINAL EXAM: In-person. Date: 22nd May, 2024, 1-3pm. Topics: All lectures & materials after test 2