Population Genetics and Speciation 1. 2. 3. 4. 5. Population Genetics Microevolution Gene Pool Allele Frequency Phenotype Frequency A. B. C. D. E. Total genetic information in a population Portion of gene copies of a given allele Study of the frequency and interaction of alleles and genes in populations Change in the collective genetic material of a population Ratio of individuals with a given phenotype to the total population A. B. C. D. E. Population Genetics Microevolution Gene Pool Allele Frequency Phenotype Frequency C D A B E Population Genetics is the study of evolution from a genetic point of view (it is the study of microevolution) Microevolution—a change in the collective genetic material of a population • Population–members of the same species that can interbreed. It is the smallest unit in which evolution occurs. http://www.abc.net.au/science/news/enviro/EnviroRepublish_1417697.htm Populations show natural variety within a species. Many quantitative traits (height and weight etc.) follow a bell shaped curve. http://www.bulbnrose.org/Heredity/Mather/poly1.jpg Environmental factors—amount of food, quality of food, etc. Genetic factors • Mutations—random changes in genes • Recombination—reshuffling of genes • Random pairing of gametes Gene pool — total genetic information available in a population http://www.cartoonstock.com/lowres/rde3053l.jpg Allele frequency—expressed as a percent: it is determined by dividing the number of a certain allele by the total number of alleles of all types in the population Phenotypic frequency—expressed as a percent: it is the number of individual with a particular phenotype divided by the total number of individuals in the population. Developed by Wilhelm Weinberg (German physician) and Godfrey Hardy (British mathematician) Godfrey Hardy Wilhelm Weinberg http://anthro.palomar.edu/synthetic/synth_2.htm States that genetic frequencies in a population tend to remain the same from generation to generation unless acted on by outside influences. It is based on a “hypothetical population” that is not evolving. http://www.cartoonstock.com/directory/h/human_evolution.asp http://mariewinnnaturenews.blogspot.com/2007_11_25_archive.html In Genetic Equilibrium: • No net mutations occur • Population size remains constant • The population is infinitely large • Individuals mate randomly • Selection does not occur This flock of mallards probably violates some or all of the conditions necessary for the Hardy-Weinberg genetic equilibrium It is highly unlikely that all five of the conditions in the Hardy-Weinberg Model will happen in the real world. Therefore, Genetic Equilibrium is impossible in nature. It is a theoretical state that allows us to consider what forces could disrupt such balance (equilibrium). 1. 2. 3. 4. 5. 6. 7. 8. Immigration Emigration Gene Flow Genetic Drift Sexual Selection Stabilizing Selection Disruptive Selection Directional Selection A. B. C. D. E. F. G. H. Individuals move out Individuals move in Choice of mates based on favorable traits Genes move from one population to another Average trait is selected One extreme trait is selected Two extreme traits are selected Allele frequencies change randomly 1. 2. 3. 4. 5. 6. 7. 8. Immigration Emigration Gene Flow Genetic Drift Sexual Selection Stabilizing Selection Disruptive Selection Directional Selection B A D H C E G F Disruptions to the Hardy-Weinberg equilibrium can result in evolution. The five requirements for genetic equilibrium can be disrupted by the following outside forces: • • • • • Mutation Gene Flow Genetic Drift Nonrandom Mating Natural Selection Mutations occur constantly at very low rates under normal conditions. Exposure to mutagens (mutationcausing agents, i.e. radiation and chemicals) can increase mutations rates. Mutations produce new alleles for a trait They can be harmful, harmless or helpful Helpful mutations are a vital part of evolution. Individuals enter and leave populations constantly. Their “genes” move with them. This is called Gene Flow. Factors influencing gene flow include: • Immigration—movement of individuals into a population • Emigration—movement of individuals out of a population • Migration and dispersal patterns can also influence the movement of individuals into new populations • Birth and Death Rates also remove or add genes from individuals to a population. In nature, population sizes are restricted rather than infinitely large. Genetic Drift can occur in small populations of organisms • Genetic Drift—the random change in allele frequency in a population • Significant changes can happen in small populations if even a single organism either fails to reproduce or reproduces too much. http://en.wikipedia.org/wiki/File:Northern_Elephant_Seal,_San_Simeon2.jpg If the frequency of an allele reaches zero in a population, then (assuming you started with two alleles), there is only one left. All individuals will be homozygous for that trait---creating no variations. This weakens a species. • Ex: Northern Elephant Seal Homozygous for every gene tested Genetic Drift can lead to a bottleneck effect in which variations are reduced overtime. http://biology.unm.edu/ccouncil/Biology_203/Summaries/PopGen.htm Organisms do not mate randomly in nature. Mate selection is influence by: • Geographic proximity— choose mates nearby: can result in kinship mating • Assortative Mating— choose mates with similar traits: reduces variation • Sexual Selection—choose mates based on favorable traits http://upload.wikimedia.org/wikipedia/commons/5/51/Oregon_zoo_peacock_male.jpg Natural Selection—organisms with favorable traits are more likely to survive and reproduce, passing on their favorable genes to the next generation. It is an ongoing process in nature and an important disruption to equilibrium. Three patterns of Natural Selection: Stabilizing Selection: individuals with the average form of a trait have the highest fitness. Ex: Lizard body size http://upload.wikimedia.org/wikipedia/commons/c/c6/Selection_Chart.PNG Disruptive Selection: individuals with either extreme variation of a trait have the highest fitness. Ex: Shell Color of Limpets http://upload.wikimedia.org/wikipedia/commons/c/c6/Selection_Chart.PNG Directional Selection: individuals with one extreme of a trait have the highest fitness Ex: Nose and tongue lengths of anteaters http://upload.wikimedia.org/wikipedia/commons/c/c6/Selection_Chart.PNG 1. 2. 3. 4. 5. 6. 7. Speciation Geographic Isolation Allopatric Speciation Reproductive Isolation Sympatric Speciation Gradualism Punctuated Equilibrium A. B. C. D. E. F. G. A slow change in a species over millions of years Bursts of rapid change Formation of a new species Physical separation of populations Inability to mate or produce offspring Speciation resulting from geographic isolation Speciation resulting from reproductive isolation 1. 2. 3. 4. 5. 6. 7. Speciation Geographic Isolation Allopatric Speciation Reproductive Isolation Sympatric Speciation Gradualism Punctuated Equilibrium C D F E G A B Speciation—formation of a new species • Species: a single kind of organism whose members are morphologically similar and can interbreed to produce fully fertile offspring. • Two types of speciation: Allopatric Speciation and Sympatric Speciation Allopatric Speciation: species arise as a result of geographic isolation. (Allopatric = different homelands) Geographic Isolation—physical separation of members of a population • Gene flow between the new subpopulations stops and the two begin to diverge • Eventually, they become incompatible for mating, creating new species. • Debate exists as to whether or not allopatric species are different enough to be considered new species. Examples of Geographic Isolation http://cas.bellarmine.edu/tietjen/images/geographic_isolation.jpg http://evolution.berkeley.edu/evosite/history/images/geog_isolation.gif Sympatric Speciation —occurs when two subpopulations become reproductively isolated within the same geographic area. Reproductive Isolation—the inability of members of the same species to mate • Can be caused by disruptive selection • Two types: prezygotic isolation and postzygotic isolation Prezygotic (premating) isolation: (different mating seasons, different mating calls, etc.) Behavioral Isolation Habitat Isolation Other Prezygotic Isolation Mechanisms include Mechanical Isolation Temporal Isolation Gametic Isolation Postzygotic (postmating) isolation: offspring do not fully develop, die, or are infertile. Hybrid is weak and likely to die Hybrid is sterile http://www.geo.arizona.edu/Antevs/nats104/SymptrcSpctnSml.jpg Two Theories: • Gradualism —slow change over millions of years • Punctuated Equilibrium—short bursts of rapid change http://bioap.wikispaces.com/file/view/gradualism.gif/94608242/gradualism.gif Evidence exists that suggests that both have taken place over time. http://silvertonconsulting.com/blog/wp-content/uploads/2010/06/c7-1-23-finches.jpg http://www.doctortee.com/dsu/tiftickjian/cse-img/biology/evolution/horse-evolution-2.jpg Hardy and Weinberg went on to develop an equation that can be used to discover the probable genotype frequencies in a population and to track their changes from one generation to another. The equation is: p2+2pq+q2 = 1 p= frequency of the dominant allele q = frequency of the recessive allele See handout Punnett Squares allow geneticists to predict the probability of offspring genotypes for particular traits based on the known genotypes of their two parents The Hardy-Weinberg equation essentially allowed geneticists to do the same thing for entire populations. Before Hardy and Weinberg, it was thought that dominant alleles must, over time, wipe out recessive alleles (genophagy = “gene eating”) According to this wrong idea, dominant alleles always increase in frequency from generation to generation. Hardy and Weinberg demonstrated that dominant alleles can just as easily decrease in frequency.