BIOL 1111 Chpt 17 Microevolution PDF

CHAPTER 17- Microevolution: changes within populations -​ Most species of a population look alike but are not identical -​ The difference in features is called phenotypic variation -​ Phenotypic variation within popn can be caused by genetic differences between individuals or by environme...

CHAPTER 17- Microevolution: changes within populations -​ Most species of a population look alike but are not identical -​ The difference in features is called phenotypic variation -​ Phenotypic variation within popn can be caused by genetic differences between individuals or by environmental factors that individuals experience -​ Can determine the cause experimentally -​ Ex: acidity in soil may influences flower color in hydrangeas Microevolution involves looking at changes in the genetic makeup the genotype of populations of species over time Quantitative variation when individuals differ in small incremental ways like the length of their toes, the number of hairs on their heads or their height -​ A broad low curve indicates a lot of variation among individuals -​ A high narrow curve indicates little variation among individuals -​ Qualitative variation is the variation that exists in two or more discrete states with intermediate forms often being absent (x: blood types, snow geese have either blue or white feathers) -​ -​ Polymorphism(poly:many, morphos:form) is the existence of discrete variants of a character -​ Phenotypic polymorphism is described quantitatively by calculating the frequency of each trait. -​ Genetic research focus on the structure of genes their function and how they are regulated -​ Population genetics focuses on the genetic variation that exists within a popn and how this changes over time as a result of evolution -​ Evolution can be caused by 4 distinct processes  ​ Mutation ​ Gene flow ​ Genetic drift ​ Natural selection -​ Genetic variation happens because individuals possess different versions of the same genes -​ A gene can have several different alleles -​ evolution : a change in allele frequencies from gen to gen -​ Gene pool: the sum of all alleles at all gene loci in all individuals in a popn/the frequency of the alleles of one locus within a popn -​ -​ Biologists began to use the technique of electrophoresis to detect the presence and size of various proteins -​ Differences in amino acid sequences of a protein reflect changes in the gene sequence (polymorphism) -​ Both protein coding and non coding regions of DNA harbour extensive genetic variation -​ Genetic variation is the raw material of evolutionary change it has 2 potential sources/causes 1.​ The production of new alleles 2.​ The rearrangement of existing alleles into new combinations -​ A simple system that has been used to study microevolutionary processes is the diploid plant snapdragon(Antirrhinum spp) -​ Flower color in snapdragons is controlled by a single gene 17.2d The Hardy–Weinberg Principle Is a Null Hypothesis That Defines a Population That Is Not Evolving -​ Null hypothesis : a prediction of what researchers would see if that particular factor had no effect -​ Hardy weinberg principle: mathematical model that specifies the conditions that are necessary so that allele frequencies and genotype frequencies do not change from one generation to the next -​ According to HW genetic equilibrium is possible if all the following conditions are met:  1.​ No mutation occurring 2.​ Population closed to migration 3.​ Population infinite in size 4.​ All genotypes free of selection (no NS or sexual selection) 5.​ Random mating with respect to genotype -​ Research method using the HW principle Allele Frequencies ★​ Two alleles: Cᴿ (red) and Cᵂ (white) ★​ p (Cᴿ) = 0.7, q (Cᵂ) = 0.3 ★​ Genotype Frequencies in Parents ★​ CᴿCᴿ = 0.45 (45%) ★​ CᴿCᵂ = 0.50 (50%) ★​ CᵂCᵂ = 0.05 (5%) ★​ Gamete Formation ★​ Each parent produces gametes with allele frequencies: ○​ Cᴿ = 0.7 (70%) ○​ Cᵂ = 0.3 (30%) ★​ Genotype Frequencies in Offspring (Hardy-Weinberg Equation: p² + 2pq + q² = 1) ★​ CᴿCᴿ = p² = 0.49 (49%) ★​ CᴿCᵂ = 2pq = 0.42 (42%) ★​ CᵂCᵂ = q² = 0.09 (9%) ★​ Random Mating ★​ Gametes combine randomly, ensuring allele frequencies stay the same across generations. -​ The allele and genotype frequencies of a popn can change due to migration into and out of of the population -​ Gene flow refers to organisms or their gametes moving from one population to another and may introduce new alleles into a popn shifting its allele and genotype frequencies -​ Gene flow violates HW because HW requires that popn’s are closed to migration -​ -​ Genetic drift is when allele frequencies in a popn change from one gen to another by chance -​ It causes allele frequencies to move up and down in unpredictable ways -​ Usually leads to reduced genetic diversity in small popn -​ Smaller popn typically experience greater effects of genetic drift which is associated with a decrease in genetic polymorphism -​ Clearly violating HW model(requiring infinitely large popn size) -​ Genetic drift is driven by the founder effect and popn bottlenecks -​ Founder effect: due to a few individuals starting new popn -​ Popn Bottleneck : reduction in allele frequency due to popn reduction -​ Factors such as disease starvation and hunting may kill a large proportion of individuals in a popn. -​ This decreases the size of the gene pool and the genetic diversity of the popn and rare alleles can be totally lost from the popn -​ A mutation is a change to the double stranded sequence of DNA. -​ Common factors that cause mutations are ​ Radiation ​ Errors in copying DNA ​ Movement of transposable elements from one place in the genome to another -​ Genetic recomb between homologous chromosomes during meiosis is a source of genetic variation in a popn -​ But recombination does not result in mutation -​ For mutations to alter allele frequencies within a population, the mutation must occur in the DNA of germ-line cells (one’s that produce gametes) -​ Mutations are random and spontaneous events -​ Mutations have no effect on fitness(neutral) or they will be harmful(deleterious) to an organism. -​ Three modes of NS 1.​ Directional selection: individuals near one end of the phenotypic spectrum have the highest relative fitness 2.​ Stabilizing selection: individuals expressing intermediate phenotypes have the highest relative fitness  3.​ Disruptive selection: when extreme phenotypes have higher relative fitness than intermediate phenotypes -​ Disruptive selection is less common than directional and stabilizing selection -​ Genetic drift, gene flow, mutations and NS can all change allele frequencies of a population and drive evolutionary change -​ NS is constantly improving the ability of a popn to grow and reproduce in a particular environment that tis to adapt -​ Mating is non random -​ Snow geese usually mate select mates of their own colour and a tall woman is more likely to marry a tall man -​ Non random mating does not result in a change in allele frequencies therefore if it not considered a microevolutionary process -​ The two types of non random mating are inbreeding and sexual selection -​ Inbreeding is mating between individuals that are genetically related because of this parents will share many of the same alleles -​ It is an issue in small popn -​ Its mostly found in plant and invertebrate mamals -​ In inbreeding there is an increase in the proportion of both homozygous genotypes resulting in the lowering of the overall fitness of the popn known as inbreeding depression over successive generations and a decrease in the proportion that are heterozygous. -​ It does not cause evolution because there is no change in allele frequencies over time. -​ Inbreeding depression is a decline in the average fitness of inbred individuals in a popn -​ Recessive alleles tend to be deleterious and yet they perpetuate in a typical popn because they are carried in heterozygotes where they are effectively masked. -​ Deleterious alleles are versions of gene that reduce the fitness of an organism -​ The result of inbreeding depression is simply outbreeding that is introduce introduce individuals from other popn’s which come with new alleles -​ The other type of non random mating is sexual selection which favours individuals with specific traits that enhance their ability to mate with individuals of the other sex. -​ It is established by male competition for access to females and by the females choice of mates. -​ Intersexual selection is the selection based on interactions between males and females -​ Intrasexual selection is the selection based on the interactions between members of the same sex. -​ Sexual selection pushes phenotypes toward one extreme. -​ Heterozygous individuals protect the presence of recessive alleles from being selected out of a popn -​ The masking of recessive alleles in heterozygotes makes it almost impossible to eliminate them fully from a popn -​ In small popn a combination of natural selection and genetic drift can eliminate harmful recessive alleles -​ Balancing selection is a type of NS in which more than one allele is actively maintained in a popn -​ NS preserves balancing selection when 1.​ heterozygotes have higher relative fitness 2.​ When different alleles are favoured in a different environments 3.​ When the rarity of a phenotype provides a selective advantage -​ Heterozygote advantage: an evolutionary circumstance in which individuals that are heterozygous at a particular locus have higher relative fitness than either homozygote -​ Being heterozygous at many gene loci provides some advantage by allowing organisms to respond effectively to environmental variation. -​ Genetic variability can also be maintained within a popn when different alleles are favoured in different places or at different times -​ Genetic variability is maintained in a popn simply because rare phenotypes have higher relative fitness than more common phenotypes -​ Frequency dependent selection: a form of NS in which rare phenotypes increase its frequency in the popn -​ The agents of evolution cause microevolutionary changes in the gene pools of popn’s