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Unformatted text preview: Evolu&on Phylogeny and Specia&on Feb 7th ­ 11th Ques&ons Hardy ­Weinberg!! Modes of Selec6on Gene Flow and Gene6c Dri; Random vs. non ­random (dri;?/ma6ng?/assor66ve ma6ng?) •  Darwinian Fitness/Selec6on Coefficient •  •  •  •  Will go over during and a;er class ac6vity: •  Phylogeny, taxonomy, and some key terms Why use Hardy ­Weinberg?  ­  Null hypothesis to prove evolu6on is not occurring  ­  Popula6on Gene6cs “Popula6on gene6cs is concerned with gene and genotype frequencies, the factors that tend to keep them constant, and the factors that tend to change them in popula6ons.” “It is also largely concerned with the study of polymorphisms. It directly impacts counseling, forensic medicine, and gene6c screening.” For many human autosomal recessive traits the heterozygote cannot be dis6nguished from the normal homozygote. Calcula6ng the amount of heterozygotes (carriers of recessive allele) in a popula6on is very important. Hardy ­Weinberg Assump&ons •  •  •  •  •  •  Extremely Large Popula6on Random Ma6ng No Muta6on No Gene Flow No Gene6c Dri; No Natural Selec6on These assump6ons can hold true when we are looking at individual alleles, such as in the Cys6c Fibrosis example. Thus we can use H ­W equa6on to calculate the allele frequency of such genes in human popula6ons. Hardy ­Weinberg Formula p + q = 1 p2 + 2pq + q2 = 1 p = frequency of the dominant allele in the popula6on= “A” q = frequency of the recessive allele in the popula6on= “a” p2 = percentage of homozygous dominant individuals= “AA” q2 = percentage of homozygous recessive individuals= “aa” 2pq = percentage of heterozygous individuals= “Aa” •  Know this terminology! Different problems will use different terms but, in the end all you need to know is what the above 5 terms stand for. Tips for solving H ­W problems It is very likely that you’ll be given the amount/percentage of homozygote recessive in the word problem itself, as it is much easier to know who is “aa” due to the phenotype difference. 1.  Calculate q2. Count the individuals that are homozygous recessive Calculate the percent of the total popula6on they represent (some6mes the percentage is given). This is q2. 2. Find q. Take the square root of q2 to obtain q, the frequency of the recessive allele. 3. Find p. The sum of the frequency of both alleles, p+q = 1. (100%) You know q, so p= 1 ­q 4. Find p2 by squaring p. 5. Find 2pq by plugging in the values for p and q. The frequency of the heterozygotes is represented by 2pq. PROBLEMS+ANSWERS You have sampled a popula6on in which you know that the percentage of the homozygous recessive genotype is 36%. Using that 36%, calculate the following: a) The frequency of the "aa" genotype. b) The frequency of the "a" allele. c) The frequency of the "A" allele. d) The frequencies of "AA" and "Aa.” •  The frequency of the "aa" genotype. Answer: 36%, as given in the problem itself. •  The frequency of the "a" allele. Answer: The frequency of aa is 36%, which means that q2 = 0.36, by defini6on. If q2 = 0.36, then q = 0.6, again by defini6on. Since q equals the frequency of the a allele, then the frequency is 60%. •  The frequency of the "A" allele. Answer: Since q = 0.6, and p + q = 1, then p = 0.4; the frequency of A is by defini6on equal to p, so the answer is 40%. •  The frequencies of the genotypes "AA" and "Aa." Answer: The frequency of AA is equal to p2, and the frequency of Aa is equal to 2pq. So, using the informa6on above, the frequency of AA is 16% (i.e. p2 is 0.4 x 0.4 = 0.16) and Aa is 48% (2pq = 2 x 0.4 x 0.6 = 0.48). PROBLEMS+ANSWERS Within a popula6on of bulerflies, the color brown (B) is dominant over the color white (b). And, 40% of all bulerflies are white. Calculate the following: a) The percentage of bulerflies in the popula6on that are heterozygous. b) The frequency of homozygous dominant individuals. Answers The first thing you'll need to do is obtain p and q. So, since white is recessive (i.e. bb), and 40% of the bulerflies are white, then bb = q2 = 0.4. To determine q, which is the frequency of the recessive allele in the popula6on, simply take the square root of q2 which works out to be 0.632 (i.e. 0.632 x 0.632 = 0.4). So, q = 0.63. Since p + q = 1, then p must be 1  ­ 0.63 = 0.37. Now then, to answer our ques6ons. First, what is the percentage of bulerflies in the popula6on that are heterozygous? Well, that would be 2pq so the answer is 2 (0.37) (0.63) = 0.47. Second, what is the frequency of homozygous dominant individuals? That would be p2 or (0.37)2 = 0.14. PROBLEMS+ANSWER Cys6c fibrosis is a recessive condi6on that affects about 1 in 2,500 babies in the Caucasian popula6on of the United States. Please calculate the following. a)  b)  The frequency of the recessive allele in the popula6on. The frequency of the dominant allele in the popula6on. •  The percentage of heterozygous individuals (carriers) in the popula6on. The frequency of the recessive allele in the popula6on. Answer: We know from the above that q2 is 1/2,500 or 0.0004. Therefore, q is the square root, or 0.02. That is the answer to our first ques6on: the frequency of the cys6c fibrosis (recessive) allele in the popula6on is 0.02 (or 2%). •  The frequency of the dominant allele in the popula6on. Answer: The frequency of the dominant (normal) allele in the popula6on (p) is simply 1  ­ 0.02 = 0.98 (or 98%). •  The percentage of heterozygous individuals (carriers) in the popula6on. Answer: Since 2pq equals the frequency of heterozygotes or carriers, then the equa6on will be as follows: 2pq = (2)(.98)(.02) = 0.04 or 1 in 25 are carriers. Fitness “The quality of being suitable” Manifested through an organism’s phenotype and its interac6on with the environment. Natural Selec&on Differen6al reproduc6ve success of different phenotypes resul6ng from interac6on with the environment and leading to a change in the gene6c composi6on of a popula6on. Selec&on Modes Direc&onal Selec&on  ­ One trait is favored over others  ­ Results in direc6onal changes Examples: ar6ficial selec6on, an6bio6c resistance and industrial melanism Stabilizing Selec&on  ­Promotes phenotypic uniformity by favoring the average phenotype  ­ Examples: number of offspring per birth in humans and amount of eggs birds lay Disrup&ve Selec&on •  Promotes phenotypic differences, favoring the presence of mul&ple alleles •  Gene pool may become split into two dis6nct gene pools: specia6on Balancing Selec&on • Maintained by heterosis: when hybrid progeny have higher fitness than either of the parental organisms •  Promotes gene6c variability in popula6ons • Prevents loss of deleterious alleles •  Sickle Cell Anemia •  People homozygous for the trait (ss) are severely anemic •  Normal people (SS) are suscep6ble to malaria •  Heterozygous people are slightly anemic but have the advantage of malaria resistance Gene&c DriX “The change in allele frequency from one genera6on to the next that occurs because of chance alone.” Chance plays a role in determining whether a given individual will survive and reproduce. • Random change in gene frequencies • Leads to fixa6on of some alleles and loss of others at random to environmental condi6ons • Does not lead to adapta6ons • In small popula6ons, it can override some effects of natural selec6on • In large popula6ons neutral alleles may be lost or fixed due to dri;. It just takes a longer 6me. Gene Flow “Gene flow is the exchange of genes between popula6ons, and between species” Migra6on into or out of a popula6on can change allele frequencies, as well as introduce gene6c varia6on in a popula6on. Assor&&ve Ma&ng A dic6onary defini6on of “Assor66ve” ­ characterized by assor6ng ­ to associate, consort •  Posi6ve Assor6ng ­ to ‘associate’ (mate) with someone similar –  Inbreeding and selfing leads to loss of gene6c variability (heterozygosity) and expression of recessive phenotypes •  Nega6ve Assor6ng ­ to mate with someone different –  outbreeding leads to an increase in heterozygosity. Darwinian Fitness • Rela6ve measure compared to other genotypes in a popula6on • Value may vary over6me and place • Most successful variant assigned value of w=1 • Fitness is enhanced by survival, physiological efficiency, ma6ng success • The bolom line is reproduc6ve success Selec&on Coefficient •  s = 1 ­w • The larger S, the stronger selec6on on an allele or trait and the lower its fitness • s=1 is a lethal allele Taxonomy, Phylogeny & Systema&cs Terms •  Taxonomy ­ iden6fying and classifying organisms •  Phylogeny ­ study of evolu6onary history •  Systema&cs ­ study of biodiversity in an evolu6onary context •  Taxon ­ the named taxonomic unit at any level –  ( canis = taxon at genus level; mamallia at class level) •  Characters ­ analogous to “allele” in a popula6on •  Character states ­ alternate values for each character –  e.g. if character is hair color, then the character states could be brown, black, blonde, red etc. Shared Characters •  Shared derived character  ­ synapomorphy A shared trait found among two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. •  Shared primi&ve character ­ sympleisiomorphy A shared trait found among two or more taxa, but which is also found in taxa with an earlier common ancestor. e.g five toes seen on the hind legs of rats and apes. This character ­state originated very early in Tetrapoda and occurs in other tetrapod groups, e.g. in lizards. Cladogram From: ...
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This note was uploaded on 09/23/2011 for the course SCI 240 taught by Professor Staff during the Spring '11 term at S.F. State.

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