Topic 15: Microevolution PDF

Topic 15 Microevolu tion 101-NYA-05 General Biology Topic 15: Microevolution Microevolutio n 1.Diversity 2.Defining Microvolution 3.Hardy-Weinberg 4.Deviating from Hardy- Weinberg Topic 15: Microevolution Diversity Kinds of Characters Quantitative characters: Vary along a continuum / sp...

Topic 15 Microevolu tion 101-NYA-05 General Biology Topic 15: Microevolution Microevolutio n 1.Diversity 2.Defining Microvolution 3.Hardy-Weinberg 4.Deviating from Hardy- Weinberg Topic 15: Microevolution Diversity Kinds of Characters Quantitative characters: Vary along a continuum / spectrum within a population Usually due to polygenic inheritance (additive effects of 2 or more genes influence single phenotypic character) Discrete characters: Generally either-or Usually determined by a single locus with different alleles with distinct impacts on the phenotype Topic 15: Microevolution Diversity Polymorphism When 2 or more discrete characters are present & noticeable within a population Applies only to discrete characters, not quantitative characters Contrasting forms called “morphs” e.g. Red-flowered & white-flowered morphs in a wildflower population, or butterflies Human populations are polymorphic for a variety of physical (e.g. freckles) & biochemical (e.g. blood types) characters Topic 15: Microevolution Defining Microevolution Genetic Basis of Evolution Evolution occurs in populations not individuals Individuals do not evolve in their lifetime. Populations: 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 Evolutionary change: inherited Topic 15: Microevolution Defining Microevolution ‘Types’ of Evolution Micro-evolution Generation-to-generation change in allele frequency in a population Generally subtle changes Macro-evolution Large-scale changes in organisms (obvious differences in traits) These changes are observable only after many generations Something of a distinction without a difference Examination of the same process at different scales Both are due to non-random selection of random variation Topic 15: Microevolution Defining Microevolution Evolution of Populations 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. Gene Pool: all the alleles for all the loci present in the population Diploid organisms have 2 alleles at each genetic locus and each individual only has a small fraction of the alleles present in the population’s gene pool. Topic 15: Microevolution Defining Microevolution Evolution of Populations Genotype Genotype # Genotypic frequency frequency AA 490 0.49 The proportion of a particular Aa 420 0.42 genotype in the population aa 90 0.09 e.g. population of 1000 individuals Total 1000 1 Allele frequency Proportion of a specific allele in a Allele Allele # population frequency 490 AA = 980 A alleles A 1400 0.7 420 Aa = 420 A alleles & 420 a alleles a 600 0.3 90 aa = 180 a alleles Total 2000 1 1400 A 600 a Topic 15: Microevolution Hardy-Weinberg 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 locus being studied. If allele frequency changes over generations → evolution is occurring: Microevolution Hardy-Weinberg Principle: Allele and genotype frequencies do not change from generation to generation (no evolution is occurring) in a population at genetic Topic 15: Microevolution Green = GG or Gg Orange = Hardy-Weinberg gg Generation 1 Genetic Equilibrium No change in genotypic or allelic frequency. Generation 2 Population is at equilibrium. Population is not evolving. Generation 3 Topic 15: Microevolution Hardy-Weinberg Genetic Equilibrium Evolution of a population can be explained as changes in allelic frequencies over time G.H. Hardy & W. Weinberg considered this change in allelic frequencies → developed mathematical model for situation where genetic makeup of a population does not change Hardy-Weinberg Equilibrium If certain conditions are met, allelic (genotypic) frequencies will not change Genetic frequencies will stabilise to certain proportion: p2 + 2pq + q2 = 1 Topic 15: Microevolution Hardy-Weinberg Genetic Equilibrium Relies on several (minor) assumptions / conditions: Large population No mutation (no new alleles) No migration (isolated) No natural selection No sexual selection (random mating) Sounds reasonable, right? Topic 15: Microevolution Hardy-Weinberg Genetic Equilibrium If all of these conditions are met, then we arrive at HW equilibrium: Genotypic frequencies are determined entirely by allelic frequencies Note: a population can be in HW equilibrium for 1 locus (i.e. 1 gene), but not others Consider 1 gene with 2 alleles: A (dominant) with proportion/frequency ‘p’ a (recessive) with proportion/frequency ‘q’ Such that p + q = 1 (i.e. together they make up 100% of the alleles for that gene in Topic 15: Microevolution Hardy-Weinberg Genetic Equilibrium But let’s consider the actual genotypes: A a A AA = p2 Aa = pq a Aa = pq aa = q2 p2 +2pq + q2= 1 p2 = frequency of homozygous dominant genotype (AA) q2 = frequency of homozygous recessive genotype (aa) Topic 15: Microevolution Hardy-Weinberg Topic 15: Microevolution Hardy-Weinberg But how, you may ask, might we calculate these frequencies? Well, it depends on the frequency of what Note: you can substitute AA for Aa or aa if heterozygous dominance f( isn’t your jam f(a A) ) N = total number of individuals in the sample n = number of individuals with that particular genotype Topic 15: Microevolution Hardy-Weinberg Genetic Equilibrium What about “Fitness”? What if 1 allele is better for you than another? Recall, fitness = reproductive success of a genotype; generally we think of relative fitness (how well a certain genotype does compared to other genotypes in the same population) Most prolific genotype means the one that produces the most offspring Most prolific genotype is considered to have fitness (W) of Topic 15: Microevolution Hardy-Weinberg Example 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 Topic 15: Microevolution Hardy-Weinberg Example p2 + 2pq + q2 = 1 freq TT freq Tt q2 = 160/1000 =freq 0.160,tt so q = √.16 = 0.4 p = 1 - q therefore 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) Heterozygous (Tt) = 2 x 0.6 x 0.4 = 0.48 (480/1000 people) Homozygous recessive (tt) = 0.16 (160/1000 people) Topic 15: Microevolution Deviating from Hardy-Weinberg Genetic Equilibrium What happens when these conditions are not met? Failing to meet these conditions means that our population will begin to change We can use HWE as a baseline when observing changes in allelic / genotypic frequencies in an evolving population Topic 15: Microevolution Deviating from Hardy-Weinberg Allele Frequencies (Microevolution) Allele frequencies are determined by 5 factors: 1. Mutation 2. Genetic Drift 3. Gene Flow 4. Sexual Selection 5. Natural Selection Topic 15: Microevolution Deviating from Hardy-Weinberg 1. Mutation 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 (average: one in every 100,000 genes per generation) Topic 15: Microevolution Deviating from Hardy-Weinberg 2. Genetic Drift Change in gene pool due to chance; result of random events / changes that cause allele frequencies to change Can end up getting rid of beneficial, harmful, or neutral alleles (because it’s chance) Typically occurs in small, unrepresentative populations Can lead to decreased genetic variation within a population (but increases differences between populations) 2 important mechanisms: Bottleneck effect Topic 15: Microevolution Deviating from Hardy-Weinberg 2. Genetic Drift: Bottleneck Effect Occurs when an event causes only a small fraction of a population to survive Since population is small, it’s possible / likely that there’s imbalanced representation of alleles from the original population e.g. insect survival Less-frequent through alleles the may no winter, longer at-risk exist (if/ endangered no carriers species Topic 15: Microevolution Deviating from Hardy-Weinberg 2. Genetic Drift: Founder Effect Occurs when very few individuals from larger population establish a new colony Bring only small sample of original genetic diversity New population has only the alleles brought by the founders (less e.g. diverse than Prevalence oforiginal Huntington’s disease amongst Afrikaners (South population) Africa) due to presence of disease Topic 15: Microevolution Deviating from Hardy-Weinberg 3. Gene Flow Results from the movement of fertile individuals - migration of breeding e.g. Spread of individuals between populations cause insecticide a movement of alleles resistance amongst various mosquito May cause a population to lose alleles populations or (loosing genetic variability), and other immigration of population to gain some (increasing beetles with genetic variability) different colouration Mixing of individuals between populations tends to reduce differences between populations Topic 15: Microevolution Deviating from Hardy-Weinberg 4. Sexual Selection Results in sexual dimorphism (differences between sexes) e.g. secondary sexual characteristics not directly tied to reproduction Differ in size, colouration, Topic 15: Microevolution Deviating from Hardy-Weinberg 4. Sexual Selection Mate choice & competition Mate choice: (generally selection by females): may be at odds with other selective pressures – e.g. large colourful feathers on birds of paradise make you easier to spot by predators & mean you might have difficulty getting away) Intrasexual selection: competition amongst males for mating opportunities / dominance (e.g. size, ritual displays, combat) Topic 15: Microevolution Deviating from Hardy-Weinberg 5. Natural Selection Of all the factors that can change a gene pool, only natural selection leads to adaptation of an organism to its environment. Adaptive Evolution From the range of variations available in a population: Natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations. Topic 15: Microevolution Deviating from Hardy-Weinberg 5. Natural Selection Way of discriminating in favour alleles that make an individual more fit, given limited resources 2 major types of natural selection: Positive selection: when a specific trait allele makes an organism more fit → allele is favoured & will increase in frequency Negative selection: when a trait allele causes reduced fitness → allele is disfavoured & will decrease in frequency Fixation: when a specific allele is the only one present in a population A ‘fixed’ allele has a frequency of 1 (100%) Topic 15: Microevolution Deviating from Hardy-Weinberg Original population 5. Natural Selection: Stabilizing Frequency of individuals Selection Select against both phenotypic extremes Trend towards homogeneity (reduced variation) Phenotypes (fur colour) Generally occurs in a stable environment Disadvantage: if there is a sudden change & the stabilized phenotype becomes unfit, there is a greater risk of extinction e.g. Infant birth weight: relatively narrow range of Topic 15: Microevolution Deviating from Hardy-Weinberg Original population 5. Natural Selection: Directional Frequency of individuals Selection Select for one phenotypic extreme Shifts allele frequency curve in 1 direction (move average) Phenotypes (fur colour) Allows for adaptation to new conditions Often caused by exposure to a changing or novel environment Artificial selection (breeding organisms to produce preferred phenotypes) e.g. Bacterial resistance to antibiotics: if there is a range of resistance within a population & it becomes exposed Topic 15: Microevolution Deviating from Hardy-Weinberg 5. Natural Selection: Directional Selection Peter and Rosemary Grant from Princeton have spent more than 30 years studying the Galapagos finches. During a prolonged drought, the birds that survived were predominantly those with large beaks that could crack large Topic 15: Microevolution Deviating from Hardy-Weinberg Original population 5. Natural Selection: Disruptive Frequency of individuals Selection Favour 2 extreme phenotypes Select against the majority of the population (intermediate phenotypes) Phenotypes (fur colour) May be due to new barrier (disperse population) or change in conditions (average organisms perish) Often results in 2 sub-populations (may eventually become distinct species) Can occur with a geographic barrier e.g. Drought causing most common food source of Topic 15: Microevolution Deviating from Hardy-Weinberg 5. Natural Selection: Disruptive Selection Good example of how disruptive selection operates is found in the three-spine sticklebacks. Dolph Schluter’s group at UBC, showed that two morphs of this species exist in some coastal freshwater lakes. The small morph lives in the open water of the lake and feeds on small plankton. The large morph lives on the bottom of the lake and feeds on insects and crustaceans. Topic 15: Microevolution Deviating from Hardy-Weinberg 5. Natural Selection: Overview