Lesson: Measuring Microevolution

Transcription

1 Lesson: Measuring Microevolution Recall that a GENE is a unit of inheritance. Different forms of the same gene are called LLELES (uh-leelz ) lleles arise from an original gene via the process of MUTTION. Mutations can happen in any cell as the DN replicates before the cell divides. Usually, the mistake is repaired. Sometimes it is not. Mutations that happen in a SOMTIC CELL (body cell) can t be passed on. Mutations that happen in a GERMLINE CELL can be passed on to offspring. In females, germline cells undergo meiosis to produce eggs (ova) In males, germline cells undergo meiosis to produce sperm. Like traits, a mutation can be beneficial helps survival and reproduction harmful (= deleterious) interferes survival and reproducrtion neutral does not affect survival and reproduction Probability The probability of an event is the precise chance that it will happen. For example, if you roll a die (singular of dice), there is a one in six probability (chance) that you will roll a one because a die has six sides, and only one side bears a one. There s also a one in six probability that you will roll a six or any other number. You also can calculate the probability that two events will happen together. This is called combined probability. If two events are independent of one another, (For example, if you roll a die twice, then the result the second roll does not depend on the result of the first roll.), then the probability of both of them happening is the product of their independent probabilities. This is called The Product Rule. For example, if you roll a die twice, then the probability that you will roll a one and a six is the product of each of those results probability. The chance of rolling a one is one in six (1/6) The chance of rolling a six is one in six (1/6) The chance of rolling a one and a six in two rolls is equal to: 1/6 x 1/6 = 1/36 That means that, statistically, you should have to roll a die 36 times to roll a one and a six, back-toback. You might get a one and a six on the first try or the hundredth. But the most probable result is 36 rolls. Statistics is about predictions. Once something happens, it no longer has a probability. It s already happened. What does this have to do with genetics? Everything! Because every time an organism sends its eggs or sperm on the road to create a member of the next generation, the likelihood that any given allele of a gene will end up in that egg or sperm is like a roll of a die. If a gene has two alleles, and an individual has one of each (Xx), then the probability that X will end up in the sperm is 1/2 (50%, or 0.5). The probability that x will end up in the gamete is also 1/2 (50%, or 05.) If there are two events possible, then each event has a 50:50 chance of happening.

2 Probability and Inheritance Every diploid organism has two copies of every gene (one from mom, one from dad). Let s consider a gene called. It codes for an important enzyme, enzyme. dominant allele () codes for normal, functional enzyme. recessive allele (a) codes for non-functional enzyme. Genotypes and Phenotypes n organism with genotype has phenotype of all normal, wild type enzyme. n organism with genotype a has phenotype of half the amount of normal, wild type enzyme X. n organism with genotype aa has phenotype of no enzyme a, and it is not viable. In this case, half the amount is enough to function normally. (This makes the allele dominant.) n organism with genotype can make sperm or eggs carrying ONLY the allele. n organism with genotype aa can make sperm or eggs carrying ONLY the a allele. n organism with genotype a can make sperm or eggs carrying either the or a allele. You can predict the expected genotypes of offspring of any given mating with a Punnett Square. Let s say you are breeding two flies, one with genotype (female), and one with genotype a (male). What will be the genotypes and phenotypes of their babies? 1. The homozygous female produces eggs with only alleles. Put an beside each square. 2. The heterozygous male produces sperm with either or a. Put one of those above each square. a The inside of each box represents the possible genotype of 1/4 of the offspring possible from this mating. Fill the squares by entering the letter ( or a) that borders each of its sides. Like so: a a a In this mating, 50% of the offspring will be genotype, and 50% will be genotype a. ll are viable. What if two of the a flies from the offspring above were to mate together? Fill in the square! a a If you did this correctly, 25% will be, 50% will be a (carriers) and 25% will be aa. You can do a Punnett Square for any possible genotype combination and predict the expected ratio of offspring genotypes (and phenotypes).

3 To Share Genes, or Not. If a recessive allele is harmful (as in the case of the mutant enzyme a), there can be problems. Inbreeding (mating between close relatives) increases the probability that two copies of a particular allele will be identical because they were inherited from a common ancestor. If that allele happens to be harmful (= deleterious), then it is clear that inbreeding increases the chance of harmful recessive traits being expressed. This is why a hybrid organism (one whose parents are not closely related) tends to be healthier and more vigorous than an inbred one. Heterozygosity at many gene loci means that even if an organism carries a harmful recessive allele, it will not express (show) it, because it is masked by the dominant allele. Organisms heterozygous at many gene loci are usually healthier and more robust than those who are homozygous at many gene loci. They exhibit HYBRID VIGOR. Outbreeding (matings between unrelated individuals) increases heterozygosity at many gene loci, and thus increases hybrid vigor. Send in the Clones Some organisms can reproduce exact copies of themselves without mixing their genes with those of a mate. Their offspring are all genetically identical to the parent and to each other. population of genetically identical organisms is called a CLONE. In a stable environment, the clone may persist. (If it ain t broke, don t fix it.) But most environments are not stable. They change. nd if the clone individuals are not well-suited to a change, they may LL die. Sex ensures a variety of genetically different offspring, some of which might survive in a changing environment. Sex is life s way of not putting all of its eggs in one basket. So sex is good in a hostile, changing world. ~>~>~>~>~>~>~>~>~>~>~>~>~>~>~> Changing Environments, Changing Genotypes Remember that mutation is the raw material of evolution. Without multiple forms of genes (alleles), there can be no evolution. MICROEVOLUTION is a change in a population's genes without speciation (reproductive isolation). MCROEVOLUTION is the origin of two new species from an ancestral one. lso remember that there are five different processes that can make a population evolve. 1. MUTTION: gene in the population changes 2. SMLL POPULTION SIZE: small gene pool leads to quicker evolution. 3. MIGRTION: Individuals immigrate into or emigrate out of the population 4. SSORTTIVE MTING: Individuals pair off according to phenotype. a. positive assortative mating similar individuals prefer each other as mates b. negative assortative mating dissimilar individuals prefer each other as mates 5. NTURL SELECTION: Individuals of a particular genotype have a better chance of reproducing than individuals of other genotypes.

4 Measuring Changes in Gene Frequencies Most natural populations are not clones. Genetic diversity (heterozygosity) is a good insurance policy that helps prevent a population from going extinct. What is frequency? The frequency of anything is how often it occurs compared to all the chances it has to occur. For example, if a population of 1000 beetles has 800 red members and 200 black members, then you can calculate their frequencies by dividing the number of each color of beetle by the total number of beetles: Red beetles: 800/1000 = 0.8 (80%) Black beetles: 200/1000 = 0.2 (20%) We can do the same thing with genes in a population. REMEMBER THT EVERY MEMBER OF THE POPULTION HS TWO COPIES OF EVERY GENE. SO THE NUMBER OF GENES (and alleles) WILL LWYS BE DOUBLE THE NUMBER OF INDIVIDULS IN THE POPULTION. In our beetles, a single gene codes the enzymes responsible for the color of the wing covers (elytra). ** Dominant allele (R) codes for red elytra (RR or Rr beetles will be red) ** Recessive allele (r) codes for black elytra (rr beetles will be black) How do we determine the frequency of R and r in our population of 1000 beetles? In population genetics shorthand the frequency of the dominant allele is represented as p the frequency of the recessive allele is represented as q because there are only two alleles at this locus, p + q = 100% of the alleles; p + q = 1.0 Back in 1908, two scientists, George Hardy (a British mathematician) and Wilhelm Weinberg (a German physician) independently reported that the relative frequencies of genotypes (at ONE gene locus with TWO alleles) could be represented by this equation: p 2 + 2pq + q 2 = 1.0 in which p 2 = the proportion (frequency) of RR homozygotes 2pq = the proportion (frequency) of Rr heterozygotes q 2 = the proportion (frequency) of rr homozygotes If you know the values of p and q, you can plug them into the Hardy-Weinberg equation and predict how many of the individuals in the next generation should be RR, Rr, and rr IF THE POPULTION IS NOT EVOLVING. ( population that is not evolving is said to be in Hardy- Weinberg equilibrium.) In our example, p = 0.8 and q = 0.2. Plug in! (0.8) 2 + 2(0.8)(0.2) + (0.2) 2 = In plain English, this means that in the next generation 64% of the beetles should be RR 32% of the beetles should be Rr 4% of the beetles should be rr

5 If you don t know the values of p and q, you can estimate them by counting the number of individuals expressing the recessive trait, because you KNOW that all those individuals are rr. Let s say in a neighboring population of 1000 beetles there are 500 red beetles and 500 black beetles. Unpack the problem: 1. The black beetles (rr) make up 50% (0.5) of the population. 2. The frequency of rr individuals is equal to q To find the value of q, take the square root of q The square root of 0.5 = 0.7, which means that q = You can now solve for p. 6. If p + q = 1.0, then 1.0 q = p = 0.3, which means that p = 0.3 You can now plug the values into the Hardy-Weinberg equation: (0.3) 2 + 2(0.3)(0.7) + (0.7) 2 = which means that the next generation of beetles in this population should be 9% (0.9) RR, 42% (0.42) Rr, and 50% ().5) rr if the population is NOT EVOLVING (is in Hardy-Weinberg equilibrium). For the population to not evolve, the following criteria must be met: 1. NO MUTTION: The gene in question does not change. 2. NO MIGRTION: Individuals do not immigrate into or emigrate out of the population 3. LRGE POPULTION: The population consists of very great number (theoretically infinite!) of individuals. 4. NO SSORTTIVE MTING: Every individual in the population has an equal chance to mate 5. NO NTURL SELECTION: Neither allele confers an advantage over the other, resulting in differential reproduction IF YOUR POPULTION IS IN HRDY-WEINBERG EQUILIBRIUM, THEN THE GENOTYPE PROPORTIONS ND LLELE FREQUENCIES SHOULD NOT CHNGE FROM GENERTION TO GENERTION.