Scale & Forces of Evolution - Microevolution PDF

Scale and Forces of Evolution The Scale of Evolution Microevolution - occurs over a short period of time in a population or species Macroevolution - occurs over geologic time above the level of species ...

Scale and Forces of Evolution The Scale of Evolution Microevolution - occurs over a short period of time in a population or species Macroevolution - occurs over geologic time above the level of species  Genes in Populations Individuals do not evolve. So if individuals cannot evolve, then how does evolution happen? Populations of individuals in the same species have a collective gene pool in which all future offspring will draw their genes from. - This allows natural selection to work on the population and determine which individuals are more “fit” for their environment 1. Why natural selection cannot work on a single individual?  Genes in Populations Gene pool Genotype Alleles Allele frequency - version of a - the total set of - genetic makeup gene, - how frequently gene copies for all of an organism - a heritable unit a particular allele genes in a - alleles, or variant that controls a appears in a population forms of a gene particular feature popuation of an organism  Genetic variation - the genes of organisms within a population change DNA mutation Gene flow Sexual reproduction - alteration in the DNA - transfer of genetic - mating (genetic sequence (error or material from one recombination) environmental factors) population to another Evolution occurs in a population when allele frequencies change What causes allele over time. frequencies to change?  Hardy and Weinberg and Microevolution 1 3 5 7 The population is There is no Mutation at a DNA All mating is totally infinitely large. emigration or level is not occurring random. immigration occurring. 2 4 6 Natural Selection is All members of the All individuals population are able to produce the same not occurring. breed and do breed. number of offspring.  Hardy-Weinberg Equilibrium 𝑝+𝑞=1 𝑝² + 2𝑝𝑞 + 𝑞² = 1 p q 𝑝² the frequency of the dominant allele. the frequency of the recessive allele the frequency of individuals with the homozygous dominant genotype 2pq 𝑞² is the frequency of individuals with the the frequency of individuals with the heterozygous genotype homozygous recessive genotype. It is an ideal state that provides a baseline against which Note scientists measure gene evolution in a given population.  Hardy-Weinberg Equilibrium Example A population of cats can be either black or white; the black allele (B) has complete dominance over the white allele (b). Given a population of 1,000 cats, 840 black and 160 white, determine the allele frequency, the frequency of individuals per genotype, and number of individuals per genotype.  Hardy-Weinberg Equilibrium Step 1: Find the frequency of white cats, the homozygous recessive genotype, as they have only one genotype, bb. Black cats can have either the genotype Bb or the genotype BB, and therefore, the frequency cannot be directly determined. Frequency of individuals = Individuals/Total population = 160/1,000 = 0.16 Frequency of white cats = 0.16; therefore, 𝑞² = 0.16  Hardy-Weinberg Equilibrium Step 2: Find 𝑞𝑞 by taking the square root of 𝑞². √(𝑞²) = √(0.16) q = 0.4  Hardy-Weinberg Equilibrium Step 3: Use the first Hardy-Weinberg equation (𝑝 + 𝑞 = 1) to solve for p. 𝑝+𝑞=1 𝑝=1−q 𝑝 = 1 − (0.4) 𝑝 = 0.6  Hardy-Weinberg Equilibrium Step 4: Square 𝑝 to find 𝑝². 𝑝 = 0.6 𝑝²= (0.6)² 𝑝²= 0.36  Hardy-Weinberg Equilibrium Step 5: Multiply 2 × 𝑝 × 𝑞 to get 2𝑝𝑞. 2pq = 2 (0.6) (0.4) 2pq = 0.48 Therefore: 𝑝+𝑞=1 The frequency of the dominant alleles: 𝑝 = 0.6 0.6 + 0.4 = 1 The frequency of the recessive alleles: 𝑞 = 0.4 The frequency of individuals with the dominant genotype: 𝑝² = 0.36 𝑝² + 2𝑝𝑞 + 𝑞² = 1 The frequency of individuals with the heterozygous genotype: 2𝑝𝑞 = 0.48 The frequency of individuals with the recessive genotype: 𝑞𝑞² = 0.16 0.36 + 0.48 + 0.16 = 1 Frequencies can be checked by substituting the values above back into the Note Hardy-Weinberg equations.  Hardy-Weinberg Equilibrium Step 6: Multiply the frequency of individuals (𝑝², 2𝑝𝑞, and 𝑞²) by the total population to get the number of individuals with that given genotype. 𝑝² × total population = 0.36 × 1,000 = 360 black cats, BB genotype. 2pq × total population = 0.48 ×1000 = 480 black cats, Bb genotype. q² × total population = 0.16 ×1000 = 160 white cats, bb genotype. Hardy-Weinberg Equilibrium Comparing Generations To know if a population is in Hardy-Weinberg Equilibrium scientists have to observe at least two generations. If the allele frequencies are the same for both generations then the population is in Hardy- Weinberg Equilibrium.  Hardy-Weinberg Equilibrium Step 1: Recall: Solve for q2. The previous generation had allele frequencies of 𝑝 Individuals with the recessive genotype/total population = 128/800=0.16 = 0.6 and 𝑞 = 0.4. The next generation of cats has a total population of 800 cats, 672 black and 128 white. Step 2: Use 𝑞² to solve for 𝑞𝑞. There is no need to solve the entire equation, Is the population in Hardy-Weinberg Equilibrium? because if 𝑞 has changed, then 𝑝 has also changed. If 𝑞 remains the same, then 𝑝 will remain the same. q2 = 0.16 √(q)2 = √(0.16) q = 0.4 Hardy-Weinberg Equilibrium Is the population in Hardy-Weinberg Equilibrium? Because the recessive allele frequency (q) has remained the same, the population is in a state of Hardy-Weinberg Equilibrium.  Forces of Evolution Natural selection 1 Migration (Gene flow) 2 Mating 3 Mutations 4 Genetic Drift 5 Natural Selection - main mechanism for microevolution - occurs when there are differences in fitness among members of a population e.g Sickle-cell anemia Genotype Phenotype Fitness Somewhat reduced fitness because of no AA 1 OO% normal hemoglobin resistance to malaria Enough normal hemoglobin to AS highest fitness because of resistance to malaria prevent sickle-cell anemia 1 OO% abnormal hemoglobin, Greatly resuced fitness because of sickle-cell SS causing sickle-cell anemia anemia  How natural selection can keep a harmful allele in a gene pool? ▪The allele (S) for sickle-cell anemia is a harmful autosomal recessive. It is caused by a mutation in the normal allele (A) for hemoglobin (a protein on red blood cells). ▪Malaria is a deadly tropical disease. It is common in many African populations. ▪Heterozygotes (AS) with the sickle-cell allele are resistant to malaria. Therefore, they are more likely to survive and reproduce. This keeps the S allele in the gene pool. CREDITS: This presentation template was created by Slidesgo, including icons byCREDITS: Flaticon,This andpresentation infographicstemplate & imageswas by Freepik. created by Slidesgo, including icons by Flaticon, and infographics & images by Freepik. Why natural selection keep a harmful allele in a gene pool?  Types of Selection Artificial Selection - not a type of natural selection - mimics natural selection in that certain traits are chosen to be passed down to the next generation - instead of nature or the environment in which the species lives being the deciding factor for which traits are favorable and which are not, it is humans that do the selecting of traits during artificial selection  Types of Selection Directional Selection - occurs when one of two extreme phenotypes is selected for e.g The beak length of the Galapagos finches changed over time due to available food sources.  Types of Selection Disruptive Selection - occurs when phenotypes in the middle of the range are selected against - can be influenced by human interaction e.g London’s peppered moths  Types of Selection Stabilizing Selection - occurs when phenotypes at both extremes of the phenotypic distribution are selected against - works mostly on traits that more than one gene controls the phenotype (polygenic) e.g Number of offsprings Stabilizing selection results in an average between too many (when there is a danger of malnourishment0) and too few (when the chance of no survivor is highest) Migration (Gene Flow) - movement of individuals into or out of a population - Gene flow occurs when individuals move into or out of a population e.g American servicemen had children with Vietnamese women *American servicemen changed the allele frequencies in the Vietnamese gene pool  3. Was the gene pool of the American population also affected? Why or why not? Mating - some species choose any available individual that is available as a partner with no regard for which characteristics they show keeps the alleles that are being passed down from generation to generation random. - however, many animal species are selective when finding a mate. These individuals look for particular traits in a mate that will translate to an advantage for their offspring makes the gene pool shrink and fewer traits available for the next generation, causing microevolution. Mutations - creates new genetic variation in a gene pool. mutations that matter for evolution are those that occur in gametes - mutations provide the genetic variation needed for other forces of evolution to act Genetic Drift - a random change in allele frequencies that occurs in a small population When a small number of parents produce just a few offspring, allele frequencies in the offspring may differ, by chance, from allele frequencies in the parents.  Special conditions Bottleneck effect - occurs when a population suddenly gets much smaller - might happen because of a natural disaster such as a forest fire - by chance, allele frequencies of the survivors may be different from those of the original population.  Special conditions Founder effect - occurs when a few individuals start, or found, a new population - by chance, allele frequencies of the founders may be different from allele frequencies of the population they left Founder Effect in the Amish Population. The Amish population in the U.S. and Canada had a small number of founders. How has this affected the Amish gene pool? NATURAL SELECTION Natural selection can lead to speciation, where one species gives rise to a new and distinctly different species.  HOW SPECIATION OCCURS? In speciation, an ancestral species splits into two or more descendant species that are genetically different from one another and can no longer interbreed.  SPECIATION Allopatric speciation - involves geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation - involves speciation occurring within a parent species remaining in one location  Summary The Hardy-Weinberg theorem There are five forces of The population is the unit of Microevolution occurs over a states that, if a population meets evolution: natural selection, evolution. A population’s gene certain conditions, it will be in migration (gene flow),mating, short period of time in a pool consists of all the genes of equilibrium. In an equilibrium mutation, and genetic drift. population or species. all the members of the population, allele and genotype Natural selection for a polygenic Macroevolution occurs population. For a given gene, frequencies do not change over trait changes the distribution of the population is characterized time. The conditions that must be over geologic time above phenotypes. It may have a by the frequency of different met are no mutation, no the level of the species. stabilizing, directional, or alleles in the gene pool. migration, very large population size, random mating, and no disruptive effect on the natural selection. phenotype distribution. Thank you for listening!