Biology SN2 Lecture 3: Microevolution PDF

Biology SN2 Lecture 3 Microevolution Hardy-Weinberg equilibrium (absence of evolution in a population) & Mechanisms of evolution Chapter 23 Genetic Basis of Evolution Evolution occurs in populations not individuals. Individuals do not evolve in their lifetime. Populations...

Biology SN2 Lecture 3 Microevolution Hardy-Weinberg equilibrium (absence of evolution in a population) & Mechanisms of evolution Chapter 23 Genetic Basis of Evolution Evolution occurs in populations not individuals. Individuals do not evolve in their lifetime. Populations (definition): all the individuals of the same species that live in a particular place at the same time. Variations in traits Some variation due to environment, and some due to heredity Evolution is the change in the frequency of genes within a population over several generations. Changes in the gene frequency directly influence the traits and characteristics of a species  Genetic Variation Genes Without genetic variation, key mechanisms of evolutionary change like natural selection cannot operate. Genes are the units of heredity which are made up of DNA and are located on chromosomes; Gene: A section of DNA that contains the instructions for a trait. A segment of the DNA that usually codes for a specific protein is called a gene.  Important definitions Alleles & Gene pool Alleles: Genes can have two or more forms called alleles. Many species have two alleles for every gene, one copy from each parent. The gene pool, consists of the total diversity of genes and alleles in a population or species Alleles  Genetic Variation New alleles arise from mutations Genetic variation, exists because the genes—the functional units of hereditary information that provide the blueprint of an organism—in different individuals are made up of slightly different segments (forms) of DNA. Different forms of a gene, which arise through mutations that change DNA sequences, are known as alleles. Mutation: A change in an organism’s DNA. New alleles can arise by mutation.  Most animals are Diploid Two sets of chromosomes Humans have 23 pairs of chromosomes from a Example: for two alleles - P and p; human somatic cell Genotype: PP, Pp, or pp 1 complete set of 23 chromosomes from mom 1 complete set of 23 chromosomes from dad  Genotype & Phenotype The genotype is the combination of alleles that an organism contains. For any particular trait they can be heterozygous (different) or homozygous (same). The phenotype is the physical trait that occurs because of the alleles. The genotype is the genetic makeup, and is represented by letters, one for each allele. A phenotype is the outward expression of the genotype. proteins A A Review Genotype & Phenotype Genotypic diversity Ex: AA, aa, BB, Bb Phenotypic diversity Ex: Orange head, head with 2 black spots, striped all the way down the back…  Let’s review Phenotypes: How many and what? A) Flowers B) Moths Genotypes? How many and what? A) Flowers B) Moths Alleles? How many and what? A) Flowers B) Moths  Genetic Variation makes Evolution Possible Individuals of a population generally have the same number and kind of genes Genes come in different allelic forms, and this leads to variations in their traits Diploid organisms have 2 alleles at each genetic locus Each individual only has a small fraction of the alleles present in the population’s gene pool The gene pool, consists of the total diversity of genes and alleles in a population or species. In general, the greater the genetic diversity in a population, especially the greater number of alleles present, the more capable a species will be to adapt to changing circumstances in their environment.  Evolution: genetic change over time or a process of descent with modification. Coltman et al. (2003) found that over a 30-year period, when about 10% of the males were removed by hunting each year, the average size of males and the average size of their horns decreased. Horn size in bighorn sheep is a heritable trait. Because trophy hunting selectively eliminates rams with large horns, it favors rams with genes for small horns. Université de Sherbrooke, QC  Evolution of Populations: the study of Microevolution Microevolution: Is change in the genetic makeup of a population from generation to generation (evolutionary change below the species level) Example: Ground Finch 1977 severe drought during drought, small soft seeds in short supply, large hard seeds more plentiful large-beaked birds were more likely to crack large seeds and survive The finch population evolved by natural selection This is microevolution since the change didn’t result in a new species… but the genetic makeup of the population has changed  Genotype and Allele Frequency Geno- Numb Genotype Genotype frequency: type er frequency The proportion of a particular AA 490 0.49 genotype in the population Aa 420 0.42 Ex: population of 1000 individuals aa 90 0.09 total 1000 1 Allele frequency: (A or a) Allele frequency: proportion of a Ex: 1000 individuals (diploid) X 2 = 2000 alleles specific allele in a pop. 490 AA = 980 A alleles Allele Number Allele of each frequency 420 Aa = 420 A alleles and 420 a alleles allele 90 aa = 180 a alleles A 1400 0.7 a 600 0.3 Total = 1400 AA 600 aa total 2000 1 Allele freq = 0.7 (1400/2000) 0.3 (600/2000)  Genetic Equilibrium Frequencies of alleles do not change from generation to generation unless influenced by outside factors. A population whose allele frequency does not change from one generation to the next are at equilibrium. At equilibrium: not evolving with respect to the gene / trait being studied. If allele frequency changes over generations ➔ evolution is occurring. Genetic Equilibrium MICROEVOLUTION Generation 1 Generation 1 No change in There is a allelic change allelic frequency. frequency. Generation 2 Generation 2 Population Population is is at NOT at equilibrium. equilibrium. Generation 3 Generation 3 Population is NOT EVOLVING. Population is EVOLVING.  Hardy-Weinberg (W-H) Principle In 1908 Hardy and Weinberg showed that allele and genotype frequencies in a population will remain constant from generation to generation unless acted upon by forces that cause change. In a large population where all male-female mating are equally likely and occur randomly, allele frequency in the population will not change over generations: the population’s genetic structure is said to be in H-W equilibrium / genetic equilibrium H-W principle of genetic equilibrium tells us what to expect when a sexually reproducing population is not evolving Can use phenotype frequencies to calculate the expected genotype & allele frequencies  Hardy-Weinberg Principle How measure the absence of evolution in a population Example: For trait (flower color) with only 2 alleles: CR or CW Allele freq. of either allele (CR or CW) is represented by C R CR a number that ranges from 0 (totally absent from a population) to 1 (all the alleles of a given locus are the same in the population). Sum of allele freq of CR & CW = 1 C W CW Let p = freq. of CR allele, q = freq. CW allele Therefore p + q =1 C R CW 320 CRCR = 640 CR alleles Population 160 CRCW= 160 CR alleles and 160 CW alleles of 500 20 CWCW = 40 C W alleles flowers Total = 800 CR 200 CW Allele freq = 0.8 0.2 =p+q=1  Hardy-Weinberg Principle 320 CRCR = 640 CR alleles 160 CRCW = 160 CR alleles and 160 CW alleles 20 CWCW = 20 C W alleles Total = 800 CR 200 CW Allele freq = 0.8 0.2 = p+q=1 When we know value of p or q, we can calculate the value of the other, p = 1 - q and q = 1 – p Squaring both sides p + q = 1 → (p+q)2 = 1 this equation used to describe relationship of GENOTYPE frequencies: p2 + 2 pq + q2 = 1 (all individuals in population) (freq CRCR) (freq CRCW) (freq CWCW)  HW & wildflower population With allele frequency can calculate genotype frequency With Allele frequency: p = 0.8, q = 0.2 CR C R (64%) Can determine Genotype Frequency: p2 + 2 pq + q2 = 1 CRCR= p2 = 0.8 * 0.8 =.64 C W CW (4%) CRCW=2pq= 2*0.8*0.2 = 0.32 CWCW =q2 = 0.22 = 0.04 CR C W (32%)  Hardy-Weinberg equation Calculating genotype and allele frequencies Calculating ALLELE Frequencies: For a gene with only 2 alleles: p+q=1 – p = frequency of dominant allele – q = frequency of the recessive allele Calculating GENOTYPE frequencies of a population: p2 + 2pq + q2 = 1 p2 frequency of homozygous dominant genotype 2pq frequency of heterozygous genotype q2 frequency of homozygous recessive genotype  Example Problem: In a human population of 1000: 840 are tongue rollers : TT or Tt 160 are not tongue rollers (tt) What is the frequency of the dominant allele (T) in the population ? p2 + 2pq + q2 = 1 Freq TT Freq Tt Freq tt  Solution p2 + 2pq + q2 = 1 Freq TT Freq Tt Freq tt ALLELE freq’s: q2 = 160/1000 = 0.160, so q= √.16 = 0.4 p=1-q p=1-0.4 = 0.6 So…. – p=0.6 p2= 0.36, q = 0.4 q2 =0.16 Frequency of: – homozygous dominant (TT) = 0.36 (360/1000 people) – Heterozygote (Tt) = 2 x 0.6 x 0.4 = 0.48 (480/1000 people) – Homozygote recessive (tt) = 0.16 (160/1000 people)  HW & Genetic equilibrium Any population in which the distribution of alleles conforms to p2+2pq+q2 = 1 is at genetic equilibrium HW principle of genetic equilibrium tells us what to expect when a sexually reproducing population is NOT EVOLVING Genetic equilibrium occurs only if certain conditions are met: Departure from these assumptions results in evolutionary change (microevolution)  Variation alone is not enough to cause evolution **MECHANISMS OF EVOLUTION** Evolution is a departure from the HW principle Degree of departure between observed allele frequency and those expected by the HW principle indicates the amount of evolutionary change. Hardy Weinberg equilibrium states that the frequencies of alleles and genotypes in a population will remain constant unless acted upon by outside agents or forces Main MECHANISMS that can alter allele frequencies in populations & cause microevolution: 1. Natural selection Natural selection is the only evolutionary mechanism that 2. Genetic drift consistently causes adaptive evolution. 3. Gene flow / migration  New genes and new alleles originate by mutation *Mutation rates are slow Mutation: Produces Genetic Variation New genes and new alleles originate only by mutation. A mutation is a change in the nucleotide sequence of an organism’s DNA. Most mutations occur in somatic cells and are lost when the individual dies. Only mutations in gametes can be passed on to offspring, and only a small fraction of these spread through populations and become fixed. Mutation Rates: *Tend to be low in animals and plants; more rapid in microorganisms Plants & animals: Average: one in every 100,000 genes per generation  Deviation from Hardy-Weinberg Natural Selection Differential success in reproduction Results in certain alleles (those that increase fitness) being passed to the next generation in greater proportions  Deviation from Hardy-Weinberg Natural Selection = ADAPTIVE Evolution Of all the factors that can change a gene pool, only natural selection leads to adaptation of an organism to its environment. From the range of variations available in a population: Natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations. CW allele would decline frequency of the CR allele would increase  3 ways Natural Selection can Selective pressures in an alter population’s env. tend to ‘select’ for freq. distribution specific phenotypes in a pop.: –Favors individuals –Favors individuals at –Favors intermediate at one extreme of the both extremes of the variants and acts against phenotypic range phenotypic range extreme phenotypes  Deviation from Hardy-Weinberg Genetic Drift: Random Shift chance / random events determine which alleles are passed on to the next generation Describes how allele frequencies can fluctuate unpredictably from one generation to the next. Alleles may be eliminated from population purely by chance regardless if beneficial or harmful. Statistically, the smaller a sample, the greater the chance of deviation from a predicted result {ex. coin toss}, therefore GD is especially important in small populations Decreases genetic variation within a population Increase genetic differences among different populations  Deviation from Hardy-Weinberg Genetic Drift – Describes how allele frequencies can fluctuate unpredictably from one generation to the next – Tends to reduce genetic variation  Deviation from Hardy-Weinberg Genetic Drift Genetic Drift: Bottleneck The bottleneck effect occurs when the numbers of individuals in a population are drastically reduced Events like natural disasters (earthquakes, floods, fires) can decimate a population, killing most individuals and leaving behind a small, random assortment of survivors. Allele frequencies in this group may be very different from those of the population prior to the event, and some alleles may be missing entirely. By chance, some alleles may be over- represented and others underrepresented or even eliminated among the survivors.  Genetic Drift: Founder Effect Occurs when a few individuals become isolated from a larger population Small fraction of a population establishes a new population, they Ex 2. Dwarfism bring with them only a small Amish population stems from a small number of fraction of the genetic variability about 200 immigrants, one must have carried a gene defect for the disorder. in the original population Allele freq of newly found population different than those of the parent population.  Effects of Genetic Drift: Summary 1. Genetic drift is significant in small populations 2. Genetic drift causes allele frequencies to change at random 3. Genetic drift can lead to a loss of genetic variation within populations 4. Genetic drift can cause harmful alleles to become fixed  Deviation from Hardy-Weinberg Gene flow Gene flow: Gain or loss of alleles Results from the movement of fertile individuals - Migration of breeding individuals between populations cause a movement of alleles May cause a population to lose alleles (loosing genetic variability), and other population to gain some (increasing genetic variability) Mixing of individuals between populations tends to reduce differences between populations over time. Can counter genetic drift and natural selection  Nonrandom mating Sexual Selection Sexual selection: selection for mating success. Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics Sexual selection can result in selection for sexual characteristics that can give individuals advantages in mating. Females can choose males based on favoured characteristics Males compete for females and can have specialized adaptations for fighting  Sexual Selection INTRAsexual selection: direct competition among individuals of one sex for mates of the opposite sex (within the same sex) INTERsexual selection: Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex (aka mate choice) Intrasexual selection http://www.youtube.com/watch?v=L54bxmZy_NE&featu re=related http://www.youtube.com/watch?v=E1zmfTr2d4c Intersexual selection Why are certain birds sexually dimorphic? Not all birds are i.e. Starlings *Experiments show that birds that are sexually dimorphic tend to have high parasite loads. So those that are bright, beautiful, have long colourful tails is a sign that they are healthy (low parasite load) Better for female to choose these males. *AMER. ZOOL., 30:287-298 (1990) Parasites and Sexual Selection in Birds of Paradise  Learning Objectives: Terms to know: gene, allele, gene pool, genotype, phenotype, genetic equilibrium, allele frequency, genotype / allele frequency, gene pool, microevolution, mutation, Understand the Hardy Weinberg principle – population at genetic equilibrium (no evolution) – p2 + 2pq + q2 = 1, p+q=1 – Know this formula and how to use it. Genetic equilibrium occurs if certain 5 conditions are met. What are they? Evolution is a departure from the HW principle Name and describe all factors that lead to microevolution. (Natural Selection, Gene flow , Genetic drift) Be able to give examples of each Natural selection leads to adaptive evolution. – Selection favors certain phenotypes (directional, disruptive, stabilizing). What is Sexual Selection? – Know the 2 types of sexual selection and give examples. Review  Review: HW equation: p2 + 2pq + q2 = 1 Freq AA Freq Aa Freq aa If you have the genotype frequencies can calculate: allele frequencies If you have the allele frequencies can calculate: genotype frequencies Always begin with homozygous recessive (q2) Example problem: 1. If red is dominant over white for roses and 64% of the rose population is red… i) what is the frequency of the allele for white in the gene pool? ii) In the same population, what percentage is heterozygous? ANS: i) 64% DOMINANT: 100-64 = 36 recessive →.36 = q2 → q = 0.6 ii) p+q =1, 1 - 0.6 = 0.4 = p Hetero = 2pq= 2(0.4)(0.6)= 0.48 → 48%  Example problems: 2. Brown eyes are dominant over blue eyes. If 49% of a population is blue-eyed, what percentage will be homozygous dominant? Step 1: q2 =.49 → q = 0.7 1 - 0.7 = 0.3 = p Step 2: % homo dominant = p2 = (0.3)2 = 0.09 = 9%