1.3 Microevolution (PDF)

1.3 Microevolution These changes are driven by mechanisms that directly impact genetic variation and are observable within a few generations. ...

1.3 Microevolution These changes are driven by mechanisms that directly impact genetic variation and are observable within a few generations.  1.3 Microevolution Natural selection acts on individuals, but only populations, not individuals, evolve – For example, medium ground finches evolved in response to seed shortage on Daphne Major – Larger-beaked birds that could eat the more plentiful large seeds survived at a higher rate – Offspring of the survivors tended to have large beaks; average beak depth increased in the next generation – The population, not its individual members, evolved I can explain the role of genetic variation in the process of evolution and how genetic diversity of a We species affects its ability to withstand environmental pressures. do!  1.3 1.3Microevolution Microevolution W 1.3 Microevolution  1.3 Microevolution Alleles show a different forms of a gene 12345 You are responsible for taking your own notes You will need to use your notes and textbook to study 1.3 Microevolution  1.3 Microevolution Population (in ecology). 1. A group of individuals of the same species living together (live in the same area)and can reproduce with each other (interbreed)for continuation of the species  1.3 Microevolution A gene pool refers to the combination of all the genes (including alleles) present in a reproducing population or species (genetic makeup).which consists of all copies of every type of allele at every locus in all members of the population. A large gene pool has extensive genomic diversity and is better able to withstand environmental challenges..  1.3 Microevolution An allele frequency is calculated by dividing the number of times the allele of interest is observed in a population by the total number of copies of all the alleles at that particular genetic locus in the population. Allele frequencies can be represented as a decimal, a percentage, or a fraction. Figure 23.UN01 These alleles show incomplete dominance For example, imagine a population of 500 wildflower plants with two alleles, C R and CW, for a locus that codes for flower pigment. These alleles show incomplete dominance; thus, each genotype has a distinct phenotype. Plants homozygous for the C R allele (C RC R) produce red pigment and have red flowers; plants homozygous for the CW allele (CWCW) produce no red pigment and have white flowers; and heterozygotes (C RCW) produce some red pigment and have pink flowers Figure 23.UN01 genotypes for red, white and pink flowers  1.3 Microevolution Consider a population of 500 wildflowers with 320 red flowers R R R W W (C C ), 160 pink flowers (C C ) and 20 white flowers (C C )W 1. Calculate the number of copies of each allele 2. Calculate the frequency of each allele. I can define Population, gene pool and allele frequency and will be able to calculate allele frequency and genotype frequency in a population  1.3 Microevolution Consider a population of 500 wildflowers with 320 red R R R W flowers (C C ), 160 pink flowers (C C ) and 20 white W W flowers (C C ) 1. Calculate the number of copies of each allele – CR = (320 × 2) + 160 = 800 – CW = (20 × 2) + 160 = 200 2. Calculate the frequency of each allele, divide the number of copies of each allele by the total number of alleles in the population. Frequency of CR = 800/(800 + 200) = 0.8 (80%) Frequency of CW= 200/(800 + 200) = 0.2 % (20%) The sum of alleles is always 1 (100%)  1.3 Microevolution Allele frequencies can also be calculated for a population – For diploid organisms, the total number of alleles at a locus is the total number of individuals times two. – Count two dominant alleles for each homozygous dominant individual and one for each heterozygote – The same logic applies for recessive alleles Consider the population of 500 wildflowers with 320 red flowers (CRCR), 160 pink flowers (CRCW) and 20 white flowers (CWCW) Calculate the number of copies of each allele – CR = (320 × 2) + 160 = 800 – C = (20 × 2) + 160 = 200 W  1.3 Microevolution By convention, if there are two alleles at a locus, p and q are used to represent their frequencies q is recessive and p for dominant. The frequency of all alleles in a population will add up to 1 – That is, p + q = 1 To calculate the frequency of each allele, divide the number of copies of each allele by the total number of alleles in the population – p = frequency of C R= 800/(800 + 200) = 0.8 (80%) – q = 1 − p = 0.2 (20%) The sum of alleles is always 1 (100%) – 0.8 + 0.2 = 1  1.3 Microevolution The Hardy-Weinberg Equilibrium The Hardy-Weinberg equation describes the expected genetic makeup for a population that is not evolving at a particular locus. In a population that is not evolving, allele and genotype frequencies will remain constant from generation to generation,  1.3 Microevolution The frequency of genotypes can be calculated R R 2 – C C = p × p = p = 0.8 × 0.8 = 0.64 R W – C C = pq + qp = 2pq = 2 × 0.8 × 0.2 = 0.32 W W 2 – C C = q × q = q = 0.2 × 0.2 = 0.04 The frequency of genotypes can be confirmed using a Punnett square  1.3 Microevolution The Hardy-Weinberg Equilibrium The Hardy-Weinberg equation describes the expected genetic makeup for a population that is not evolving at a particular locus. In a population that is not evolving, allele and genotype frequencies will remain constant from generation to generation,  The Hardy-Weinberg equilibrium is a principle stating that the genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors.  The Hardy-Weinberg Equation The Hardy-Weinberg equilibrium is a principle stating that the genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors. If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then 2 2 – p + 2pq + q = 1 – where p2 represents the frequency of homozygous dominant gentotype and 2 q represent the frequencies of the homozygous recessive genotypes, and 2pq represents the frequency of the heterozygous genotype  1.3 Microevolution The Hardy-Weinberg equation can be used to test whether a population is evolving Genetic variation is required for a population to evolve, but does not guarantee that it will One or more factors that cause evolution must be at work for a population to evolve  1.3 Microevolution Hardy-Weinberg Equilibrium If a population is not evolving, genotype and allele frequencies will be constant from generation to generation Such a population is in Hardy-Weinberg equilibrium.  Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg approach describes a population that is not evolving In real populations, allele and genotype frequencies often do change over time Such changes occur when one or more of the conditions for Hardy-Weinberg equilibrium are not met Table 23.1 Sexual Selection: When individuals select mates based on specific traits that are not randomly distributed within the population, it can lead to an increase in frequency of alleles associated with desirable traits. This selective mating can significantly alter allele frequencies over time, diverging from predictions made by the Hardy-Weinberg equilibrium.  1.3 Microevolution Genetic equilibrium describes the condition of an allele or genotype in a gene pool (such as a population) where the frequency does not change from generation to generation. It is also called as Hardy-Weinberg equilibrium. Any factor which can disturb the alleles in a population is likely to affect the Hardy-Weinberg equilibrium. The factors affecting Hardy-Weinberg equilibrium are: No Mutation No Genetic drift No Random mating. No Gene flow No Natural selection Large population Hardy Weinberg principle of genetic equilibrium  Hardy–Weinberg Principle: five conditions can disrupt genetic equilibrium and cause evolution to occur Nonrandom mating Small population size Gene flow from immigration or emigration Mutations Natural selection 1.3 Microevolution 1.3 Microevolution  1.3 1.3Microevolution Microevolution W  1.3 Microevolution Microevolution, the change in allele frequencies in a population over generations, is evolution at its smallest scale Three mechanisms cause allele frequency change: – Natural selection (adaptation to the environment) – Genetic drift (chance events alter allele frequencies) – Gene flow (transfer of alleles between populations) Animation: Causes of Evolutionary Change  1.3 Microevolution Natural selection acts on individuals, but only populations, not individuals, evolve – For example, medium ground finches evolved in response to seed shortage on Daphne Major – Larger-beaked birds that could eat the more plentiful large seeds survived at a higher rate – Offspring of the survivors tended to have large beaks; average beak depth increased in the next generation – The population, not its individual members, evolved I can explain the role of genetic variation in the process of evolution and how genetic diversity of a We species affects its ability to withstand environmental pressures. do! 1.3 Microevolution 1.3 Microevolution  1.3 Microevolution * The smaller the sample, the greater the chance of random deviation from a predicted result * Genetic drift is a process in which chance events cause allele frequencies to fluctuate unpredictably from one generation to the next * Genetic drift tends to reduce genetic variation through the random loss of alleles I do! 2. I can compare the effects of natural selection, genetic drift, and gene flow on allele frequencies.  1.3 Microevolution * Genetic drift is significant in small populations * Genetic drift can cause allele frequencies to change at random * Genetic drift can lead to a loss of genetic variation within populations * Genetic drift can cause harmful alleles to become fixed I do! 2. I can compare the effects of natural selection, genetic drift, and gene flow on allele frequencies. 1.3 Microevolution  1.3 Microevolution * The founder effect occurs when a few individuals become isolated from a larger population * Allele frequencies in the smaller founder population are different from those in the parent population For example, genetic drift could occur if a few individuals are indiscriminately blown to a new island by a storm I do! 2. I can compare the effects of natural selection, genetic drift, and gene flow on allele frequencies.  1.3 Microevolution * The bottleneck effect occurs when there is a drastic reduction in population size due to a sudden change in the environment * The resulting gene pool may no longer be reflective of the original population’s gene pool * If the population remains small, it may be further affected by genetic drift I do! 2. I can compare the effects of natural selection, genetic drift, and gene flow on allele frequencies. 1.3Microevolution Figure 23.1a The medium ground finch (Geospiza fortis) is a seed-eating bird that inhabits the Galápagos Islands. In 1977, the G. fortis population on the island of Daphne Major was decimated by a long period of drought: Of some 1,200 birds, only 180 survived. The surviving birds had larger, deeper beaks, indicating that this population of finches had evolved I do!  1.3 Microevolution-– Application & Literacy Skills Student Answer: Average beak depth (mm) After the drought of 1977 on the 10 Finches mostly fed on large, hard island of Daphne Major, of the seeds that were plentiful. Birds 1200 finches on the island, only with larger, deeper beaks were 180 survived. Researchers Peter 9 better able to crack and eat these and Rosemary Grant observed larger seeds and they survived at that during the drought, small soft a higher rate than finches with seeds were in shorty supply. Hard 8 seeds were more abundant. Refer smaller beaks. The average beak to the graph and explain what depth in the next generation was occurred to the finch population 1976 1978 greater than it had been in the 0 pre-drought population. as a result. (similar to the (after prior 3 years) drought) You do!  1.3Microevolution 3.1.10 Explain, using examples, how natural selection affects allele frequency 3.1.11 Define the term adaptive evolution Natural selection also affects allele frequency. If an allele confers a phenotype that enables an individual to better survive or have more offspring, the frequency of that allele will increase. Natural selection decreases the frequency in a population of genes that decrease fitness and increases the frequency of genes that increase fitness. Adaptive evolution pertains to evolutionary changes in an organism that make it suitable to its habitat. The changes result in an increased chance of survival and reproduction. The changes enable the particular organism to fit to an environment.  1.3 Microevolution * Balancing selection preserves variation at by maintaining stable frequencies of two or more phenotypes * Balancing selection includes: - Frequency-dependent selection - Heterozygote advantage I do! 3. I can describe the effect of sexual selection and balancing selection with examples.  1.3 Microevolution In frequency-dependent selection, the fitness of a phenotype depends on how common it is. Frequency-dependent selection is a type of natural selection where the fitness of a phenotype depends on its frequency relative to other phenotypes in a given population. This selection can be either positive or negative: - For example, frequency-dependent selection results in approximately equal numbers of “right-mouthed” and “left-mouthed” scale-eating fish - Prey maintain both phenotypes by altering their behavior to defend against whichever is most common I do! 3. I can describe the effect of sexual selection and balancing selection with examples.  1.3 Microevolution Balancing Selection: This term refers to a type of natural selection where genetic diversity is maintained by selective pressures that favor different phenotypes in different contexts or at different times. In this scenario, if one type of mouth orientation becomes too common, it becomes less advantageous as prey adapt. This dynamic causes the population frequencies of left- and right-mouthed fish to oscillate and tend to balance out around 50% over time, maintaining a balance of both phenotypes in the population.  1.3 Microevolution * Heterozygote advantage occurs when heterozygotes have a higher fitness than both kinds of homozygotes * Natural selection will tend to maintain two or more alleles at that locus - For example, the deleterious sickle-cell allele is maintained at relatively high frequencies in some regions due to heterozygote advantage I do! 3. I can describe the effect of sexual selection and balancing selection with examples.  1.3 Microevolution * 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 I do! 3. I can describe the effect of sexual selection and balancing selection with examples.  1.3 Microevolution Effects on Individual Organisms Individuals homozygous for the sickle-cell allele have sickle-cell disease,they have distort the red blood cell into a sickle shape in low-oxygen conditions. In heterozygotes, some sickling occurs, but not enough to cause the disease Evolution in Populations Individuals that are homozygous for the sickle-cell allele are strongly selected against Heterozygotes experience few harmful effects, but are more likely to survive malaria than homozygotes Where malaria is common, heterozygote advantage increases the frequency of sickle-cell alleles I do! 3. I can describe the effect of sexual selection and balancing selection with examples.  1.3 Microevolution Heterozygote Advantage I do! 3. I can describe the effect of sexual selection and balancing selection with examples.  1.3 Microevolution Explain balancing selection through frequency-dependent selection and heterozygote advantage "Heterozygote protection" refers to the phenomenon where individuals who are heterozygous (possessing two different alleles for a particular gene) have a selective advantage over individuals who are homozygous (possessing two identical alleles for a particular gene) in certain conditions. This selective advantage can help maintain a pool of alleles that could be beneficial if the environment changes. One example of heterozygote protection is sickle cell anemia. Sickle cell anemia is a genetic disorder caused by a mutation in the hemoglobin gene, which results in the production of abnormal hemoglobin molecules that can cause red blood cells to become sickle-shaped and less efficient at carrying oxygen. Individuals who are homozygous for the sickle cell allele (having two copies of the sickle cell gene) have a severe form of the disease and may die at a young age. However, individuals who are heterozygous (having one normal and one sickle cell allele) have a selective advantage in areas where malaria is common, as the sickle cell trait provides some protection against the disease. This selective advantage can help maintain the sickle cell allele in the population, as individuals who are heterozygous for the sickle cell allele are more likely to survive without severe health conditions ,and pass on their genes to the next generation,while still gaining the protective benefits against malaria, as their red blood cells are somewhat abnormal in shape and structure which impairs the parasite's ability to invade or reproduce effectively. Heterozygote protection helps maintain a pool of alleles that could be beneficial if the environment changes. While homozygosity can be advantageous in stable environments, heterozygosity can provide a selective advantage in changing environments, helping to maintain genetic diversity in the population. 1.3 Microevolution  Hardy-Weinberg Equilibrium Heterozygote Presence: Because recessive alleles only express their traits in homozygous individuals (those who have two copies of the allele), they can be "carried" in heterozygous individuals without showing any harmful effects. This means that the allele can be passed on to the next generation without being subjected to strong negative selection.  Hardy-Weinberg Equilibrium.In the absence of random events (an infinitely large population), are the allele frequencies of the original population expected to change from generation to generation? In a theoretical scenario with an infinitely large population and no random events (like mutation, migration, or selection), the allele frequencies would not be expected to change from generation to generation. This is because, in such a population, the effects of genetic drift are minimized, and the system would reach a state of genetic equilibrium. Under these conditions, the population would maintain constant allele frequencies over generations. This situation aligns with the assumptions of the Hardy-Weinberg equilibrium, where allele frequencies remain stable unless influenced by external factors.  Hardy-Weinberg Equilibrium In a small population with random gamete selection, the allele frequencies can change significantly from generation to generation due to genetic drift. Genetic Drift: Random sampling errors can cause certain alleles to be overrepresented or underrepresented in the next generation. This effect is stronger in smaller populations, Bottleneck Effect: If a small population experiences a sudden reduction in size (e.g., due to environmental events), the allele frequencies can shift dramatically based on which individuals survive, potentially leading to reduced genetic diversity. Founder Effect: If a new population is established by a small number of individuals, the allele frequencies of that new population may differ significantly from the original population, simply due to the random selection of the founding members.  Recap / Summary Summary of important points: (Either the teacher can recap the lesson or students can be asked to recap the important points) Genetic variation plays a very important role in the process of evolution Genetic diversity of a species affects its ability to withstand environmental pressures. Natural selection acts on individuals, but only populations, not individuals, evolve Formation of New Alleles (Mutations), Altering Gene Number or Position, Rapid reproduction and sexual reproduction are all sources of genetic variation. A population is a group of individuals of the same species that live in the same area and interbreed The gene pool consists of all copies of every allele at every locus in all members of the population Allele frequencies: To calculate the frequency of each allele, divide the number of copies of each allele by the total number of alleles in the population p + q = 1 (p = frequency of dominant allele, q = frequency of recessive allele) The Hardy-Weinberg equation can be used to test whether evolution is occurring in a population. It is also used to determine the percentage of a population carrying a specific allele I You 123 do! do!