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.
Different things produce selection • Internal environment • The external environment
SELECTION ??? Fitness = W = ability of some genotypes to leave more offspring to the next generation How do measure such a concept Based on non-random breeding and on “fitness” When W = 1 indicates best fit genotype (all offspring ) When W = 0 lethal genotype ( 0 offspring) When W = 0.5 genotype leaves ½ offspring for next generation
What is Fitness? • 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, © 1996, 1998 MICRA, Inc.
What is relative Fitness? • 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 “normals” W = 1
SELECTION COEFFICIENT(S) • Measure of the intensity of selection • S = 1-W • S = 0 is most fit • S = 1 is least fit
Think of the lab example • Fish A and Fish D • Fish A had W= 1 so most fit Fish D had W= 0.6 • For Fish D’s Fitness W= Reproductive rate of unfavored types (D) Reproductive rate of favored (A)
The opposite could hold true • 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) • S will be 1- W or equal to 0
A. Selection against dominant allele Given the genotypes AA Aa aa • Dominant genes can be good (favorable), bad (unfavorable) or neutral • Most disadvantageous: dominant lethal time of lethal events effects W • Environmental event may change fitness of dominant genes—could disappear in single generation • Partial selection against dominants
Ex. Dwarfism- Achondroplasia • Achondroplasia is a genetic disorder of bone growth that is evident at birth. A major type of dwarfism. • It affects nearly one in every 25,000 births • Non specific to races, creeds or sexes. • 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 • Achondroplasia is not favored for even though has a dominant A allele • Non dominant alleles are now favored for normal births to occur
How does this work in terms of evolution?
W = 0.2 W = 0.1 Time Note that A is gradually lost over time % A In population W = 0 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 • 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 • 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 • Sickel Cell anemia- abnormal blood cells with irregular sickle cell shape. • 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? • 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. • The hemoglobin allele S, is what is responsible for the disease. Why has this disease not been depleted if it is caustic? • The heterozygotes for the S allele are resistant to malaria. http://www.scinfo.org/sicklept.htm
What sort of Selection will favor heterozygotes like the sickle cell anemia case?
The choices • Disruptive • Stabilizing • Directional
The Answer:Stabilizing Selection • Selection eliminates the extremes. • It prevents the changes of the middle range • Doesn’t change the more common phenotypic traits shown in the population
1. Stabilizing selection = elimination of extremes from the population Selection for polygenic traits Before selection % pop After selection
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.
The phenotypic distribution before selection is a relatively broad bell-curve. • The Variance is also reduced • Text material © 2002 by Steven M. Carr
Stabilizing Selection ex. • Human infants with an average/intermediate birth weight will have the higher survival rate.
Disruptive Selection • The middle of the range of phenotypes are selected against • 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 • Diverse beak sizes in finch populations. Has high amounts of large and small beaks but few middle sized beaks. • 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 • Tries to eliminate one of the two extremes • Distributions are either right or left skewed
X X X 3. Directional selection Height =elimination of individuals from the population causing progressive shift in an average trait through time
The implications • 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. • In periods of prolonged cold temperatures, natural selection may favor larger animals because they are better able to withstand extreme temperatures. • http://www.bioproject.info/GENERAL_BIOLOGY/Evolution/Natural_selection_in_populations/Directional_selection.html
Directional Examples • Components of fitness, such as survival after food depletion in Cliff Swallows • Insecticide resistance in insects • heavy metal tolerance in plants
Summary • selection coefficient (S) • Relative Fitness coefficient (W) • 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
One would think most populations are in a normal distribution. • However, we can now suggest that “The selection on the traits affected by many genes ( or environmental pressures) can favor both extremes, the intermediate values, or only one extreme.” • Stabilizing Selection and Directional Selection are fairly common in various populations. • Disruptive Selection-this is uncommon, but of theoretical interest because it suggests a mechanism for species formation without geographic isolation
Final Thought • Selection is the agent of evolution that solely produces adaptive evolutionary changes.