Selection Lecture nd Wed. June 2 . Recap from last time: There are three essential mechanisms underlying evolution. 1. Variation ( mutation, gene migration, genetic recombination) 2. Heritability or those traits that are inherited. 3. Natural Selection or the differential capacity for survival. пЃ® Different things produce selection пЃ® Internal environment пЃ® The external environment SELECTION ??? How do measure such a concept Based on non-random breeding and on вЂњfitnessвЂќ Fitness = W = ability of some genotypes to leave more offspring to the next generation When W = 1 indicates best fit genotype (all offspring ) When W = 0 lethal genotype ( 0 offspring) When W = 0.5 genotype leaves ВЅ offspring for next generation What is Fitness? вЂў Fitness = species' fitness lies at the heart of Darwin's original theory. The genetic contribution of an individual to succeeding generations. The ability to produce healthy offspring. Source: Webster's Revised Unabridged Dictionary, В© 1996, 1998 MICRA, Inc. What is relative Fitness? вЂў The fitness of an individual relative to other individuals in a population. Calculation of fitness W = Reproductive rate of unfavored alleles reproductive rate of favored alleles Consider the condition that for вЂњnormalsвЂќ W = 1 SELECTION COEFFICIENT(S) вЂў вЂў вЂў вЂў Measure of the intensity of selection S = 1-W S = 0 is most fit S = 1 is least fit Think of the lab example вЂў Fish A and Fish D вЂў Fish A had W= 1 so most fit Fish D had W= 0.6 вЂў For Fish DвЂ™s Fitness W= Reproductive rate of unfavored types (D) Reproductive rate of favored (A) The opposite could hold true вЂў It will depend on the internal and external environments of the fish. Thus, Fish D may have W= 1 in a different pond so now W= Reproductive rate of unfavored types (A) Reproductive rate of favored (D) вЂў S will be 1- W or equal to 0 A. Selection against dominant allele Given the genotypes AA Aa aa вЂўDominant genes can be good (favorable), bad (unfavorable) or neutral вЂўMost disadvantageous: dominant lethal time of lethal events effects W вЂўEnvironmental event may change fitness of dominant genesвЂ”could disappear in single generation вЂўPartial selection against dominants Ex. Dwarfism- Achondroplasia вЂў Achondroplasia is a genetic disorder of bone growth that is evident at birth. A major type of dwarfism. вЂў It affects nearly one in every 25,000 births вЂў Non specific to races, creeds or sexes. вЂў Achondroplastic dwarfism is characterized by an average-size trunk, short arms and legs, and a slightly enlarged head and prominent forehead. A case of Dwarfism Achondroplastic dwarfs produce 19.6% offspring as normal population (no differences in survival) Dwarfism is a dominant allele W = 19.6/100 = 0.196 fitness value Since S = 1-W S = 0.804 selection coefficient against dwarfs i.e. 80.4% of the expected offspring are removed solely by selection In this dominant case вЂў Achondroplasia is not favored for even though has a dominant A allele вЂў Non dominant alleles are now favored for normal births to occur вЂў How does this work in terms of evolution? Note that A is gradually lost over time %A In population W = 0.2 W = 0.1 W=0 Time This level of selection leads to elimination of dominant and fixation of the recessive. Ignores mutation rate. B. Selection against recessive allele Genotypes AA Aa aa Aa not affected in complete dominance Therefore, elimination of recessive is very slow. If there is co-dominance or incomplete dominance, elimination of recessive can be faster I.e. Aa is disadvantageous Ex. Is Cystic Fibrosis вЂў CF is a genetic disorder that affects the respiratory, digestive and reproductive systems. There are approximately 30,000 people in the United States with CF вЂў There is an inflammation of the mucous membranes causing excess mucous to form and clump in the lungs. The presence of two mutant genes (g) is needed for CF to appear. Each parent carries one defective gene (g) and one normal gene (G). The single normal gene is sufficient for normal function of the mucus glands, and the parents are therefore CF-free. Each child has a 25 percent risk of inheriting two defective genes and getting CF, a 25 percent chance of inheriting two normal genes, and a 50 percent chance of being an unaffected carrier like the parents. C. Selection favoring heterozygotes Genotypes AA Aa aa Co-dominance or incomplete dominance must be involved if Aa is favored Reminder: co-dominance= Situation in which two different alleles for a genetic trait are both expressed autosomal dominant, recessive gene incomplete dominance=A condition where a heterozygous off- spring has a phenotype that is distinctly different from, and intermediate to, the parental phenotypes Specific Terms вЂў Sickel Cell anemia- abnormal blood cells with irregular sickle cell shape. вЂў When these hard and pointed red cells go through the small blood tube, they clog the flow and break apart. This can cause pain, damage and a low blood count, or anemia. What makes the red cell sickle? вЂў Hemoglobin . This protein carries oxygen inside the cell. Any changes in this protein causes the hemoglobin to form long rods in the red cell when it gives away oxygen. вЂў The hemoglobin allele S, is what is responsible for the disease. Why has this disease not been depleted if it is caustic? вЂў The heterozygotes for the S allele are resistant to malaria. http://www.scinfo.org/sicklept.htm The Statistics phenotype normal s Mild anemia Severe anemia genotypes HbN, HbN Fitness 1.0 (W) In USA In Africa 0.85 HbN, HbS HbS, HbS 0.9 0.1 1 0.14 What sort of Selection will favor heterozygotes like the sickle cell anemia case? The choices вЂў Disruptive вЂў Stabilizing вЂў Directional The Answer: Stabilizing Selection вЂў Selection eliminates the extremes. вЂў It prevents the changes of the middle range вЂў DoesnвЂ™t change the more common phenotypic traits shown in the population D. Selection for polygenic traits 1. Stabilizing selection = elimination of extremes from the population Before selection % pop After selection вЂў In nature, natural selection is most commonly stabilizing. The average members of the population, with intermediate body sizes, have higher fitness than the extremes. Natural selection now acts against change in form, and keeps the population constant through time. вЂў The phenotypic distribution before selection is a relatively broad bellcurve. вЂў The Variance is also reduced вЂў Text material В© 2002 by Steven M. Carr Stabilizing Selection ex. вЂў Human infants with an average/intermediate birth weight will have the higher survival rate. Disruptive Selection вЂў The middle of the range of phenotypes are selected against вЂў A bimodal distribution results 2. Disruptive selection = elimination of intermediate individuals from population i.e. increases population variability Character displacement The implication of the distribution As a result, the population will be monomorphic for one of the homozygous genotypes spanning one of the two peaks in the population distribution. Which homozygous genotype comes to predominate, however, depends on the initial allele frequencies in the population. http://darwin.eeb.uconn.edu/eeb348/lecture-notes/selection/node10.html Most common Disruptive Selection Example вЂў Diverse beak sizes in finch populations. Has high amounts of large and small beaks but few middle sized beaks. вЂў This suggests considerable variation in body and beak size (large beaks are better for large seeds but can also eat small seeds favored by finches with small beaks). Directional Selection вЂў Tries to eliminate one of the two extremes вЂў Distributions are either right or left skewed 3. Directional selection X X X Height =elimination of individuals from the population causing progressive shift in an average trait through time The implications вЂў Often, shifts in environmental conditions, such as climate change or the presence of a new disease or predator, can push a population toward one extreme for a trait. вЂў In periods of prolonged cold temperatures, natural selection may favor larger animals because they are better able to withstand extreme temperatures. вЂў http://www.bioproject.info/GENERAL_BIOLOGY/Evolution/Natural_selection_in_populations/Directional_selection.html Directional Examples вЂў Components of fitness, such as survival after food depletion in Cliff Swallows вЂў Insecticide resistance in insects вЂў heavy metal tolerance in plants Summary вЂў selection coefficient (S) вЂў Relative Fitness coefficient (W) вЂў There are three kinds of natural selection 1. Stabilizing- peak will get narrower 2. Disruptive- 2 peaks form 3. Directional- peak shifts to one side specifically вЂў One would think most populations are in a normal distribution. вЂў However, we can now suggest that вЂњThe selection on the traits affected by many genes ( or environmental pressures) can favor both extremes, the intermediate values, or only one extreme.вЂќ вЂў Stabilizing Selection and Directional Selection are fairly common in various populations. вЂў Disruptive Selection-this is uncommon, but of theoretical interest because it suggests a mechanism for species formation without geographic isolation Final Thought вЂў Selection is the agent of evolution that solely produces adaptive evolutionary changes.