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Lecture 13

Biology 1001A- Lecture 13.docx

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Department
Biology
Course
Biology 1001A
Professor
Tom Haffie
Semester
Fall

Description
Biology 1001A Lecture 13: Variation and Evolutionary Forces November 10, 2011 Next Time: Evolutionary Forces and Sex Recall Last Lecture:  At a locus one allele may be dominant over another allele. The dominance status of an allele describes its effect on the phenotype of the heterozyotes… NOT whether the allele is helpful, harmful or neutral.  CLICKER QUESTION: o Most genetic disorders are associated with recessive alleles. Why?  A. In general, recessive alleles tend to be rare  B. By definition, recessive alleles reduce an individuals alleles  C: Selection eventually weeds out all copies of a harmful dominant allele from the population, but a harmful recessive allele can “hide” from selection in heterozygous genotypes.  Dominance and recessive status of an allele aren’t defined based on the fitness effects, but the dominance status of an allele is going to be important in determining how thoroughly selection can change the frequencies of a particular allele.  In other words, the dominant status of a particular allele turns out to be important. It turns out to be a potential CONSTRAINT on the degree to which selection can favor the spread of a dominant allele. How effectively selection can weed out a deleterious allele. The dominant status of an allele can affect the speed of evolution as well as the possible outcome of evolution.  For example, a new favorable allele, beneficial to the population. A new favorable allele starts in a population starts off with low frequency. This allele is recessive. If it is recessive, that is the other deleterious allele is dominant, then eventually selection is going remove every single of the dominant alleles, harmful dominant alleles. The beneficial allele should reach fixation and replace all of the deleterious dominant alleles.  HOWEVER, if a new beneficial mutation arises, that is DOMINANT to the other alleles at that locus, some small proportion of those harmful recessive alleles can be sheltered or hidden from selection, because there is a diploid population. Those deleterious recessive alleles can be carried by heterozygous individuals. These individuals are still going to be healthy because they will carry the dominant allele, but selection is isn’t going to see and weed out every copy of the recessive allele.  The dominant allele spreads faster in the population compared to recessive. o There is a greater expression of dominant alleles. Recessive alleles can be only found in heterozygous individuals. Therefore, takes longer time for recessive allele to grow in the population. Selection is constraint by the dominant status alleles and by diploids.  Although powerful enough to cause adaptive evolution, there are many constraints on selection: 1. Selection can be constraint by genetic correlations among traits.  i.e. Deeper bird beaks selectively favoured during dry environment  Cannot select beak depth, without changing other components (correlation) such as body size. Genetic correlation between traits might affect not only beak depth, but body size, beak shape and etc. This is due to genes being pleiotropic – one gene change affects another gene that may or may not be related. 2. Selection can be constrained by time.  Environment may be changing more rapidly than evolution can occur.  A population may continue to adapt to past conditions, but if environment changes fast, those conditions will not apply anymore. 3. Selection can be constrained by available genetic variation.  Very little genetic difference between cheetahs  Can affect the population if a disease can affect each individual.  Natural selection requires genetic variation Why is Genetic Variation so important?  Genetic Variation affects the population’s evolutionary potential. The ability it will have sufficient genetic variation for selection to have something to work on.  Shaping individual fitness and population fitness because it affects evolutionary potential Quantifying Genetic Variation  Of an individual o Proportion of HETROZYGOUS loci  The HIGHER the proportion your heterozygous at, the HIGHER your diversity o Inbreeding coefficient  Constructing pedigrees  Of a population o Proportion of POLYMORPHIC loci  What % of population exhibits MORE than ONE allele o Alleles per locus  How many alleles are there at each loci  More alleles means more diverse population Directional Selection  Directional selection refers to selection AGAINST on one extreme end of the graph o Ex. Yellow side of the graph  Selection is AGAINST short tail lengths  Selection FAVOURS birds with longer tails  If directional selection sustained for a long time, eventually, we will see a decrease in Genetic Variation  We see Genetic Variation because of the environments o Every year, environment can change (i.e. water cycle) o Selection causes animals to adapt that o Therefore, Genetic Variation occurs Spatial Variation in Selection  Selection does not always result in loss of variation  When environments vary, spatially, certain alleles may be favoured in a particular environment  Population maintains HIGH amount of genetic variation Stabilizing Selection  In stabilizing selection, selection works AGAINST BOTH extreme ends of the graph.  MIDDLE part of graph is FAVOURED  We see a reduction in Genetic Variation o Ex. Bird Tail
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