BI111 chapter 17 - Lecture notes PDF

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Chapter 17- Microevolution: changes within populations Evolution: change in allele frequencies from one generation to the next ! Microevolution: change in the genetic makeup of a population from one generation to the next ! Quantitative Variation: individuals differ in small, incremental ways!

- ex. Variation of weights of people in bio class !

Qualitative Variation: characters exist in 2 or more discrete states and intermediate forms are absent ! - ex. Snow geese have either blue OR white feathers ! Polymorphism (poly= many; morphos= form): the existence of discrete variants of a character !

- ex. Human blood A, B, AB, O!

Phenotypic Variation: variability of phenotypes in a population ! - Ex, height, weight, body shape! - Can be caused by genetic differences or environmental factors, or combination of both! - Sometimes organisms with same genotype exhibit different phenotypes ! - Ex. Soil acidity for same flower can make them different colours Genetic Variation • Raw material of evolutionary change! • 2 potential sources:! - Production of new alleles! - Rearrangement of existing alleles into new combinations ! Hardy-Weinberg Principle • Mathematical model that predicts genotype frequencies: ! - used as a null/control against which observed data can be compared! • Specifies the conditions necessary for allele and genotype frequencies do not change from one generation to the next (genetic equilibrium)! • Genetic equilibrium only possible if conditions are met:! - No migration from other populations! - Population is infinite in size! - No mutation occurring! - All genotypes survive and reproduce equally well! - Individuals mate randomly in population with respect to genotype ! • “what would the genetic makeup of a population be at a particular locus if the population was not evolving?”! p^2 + 2pq + q^2 =1

•

317 total x 2 alleles each =634 (denominator)

f(b)=p=

139 + 139 + 49 634

= 327 = 0.52 634

bb= 139 Bb= 49 BB= 129 f(B)=q=

129 + 129 + 49 634

= 307 = 0.48 634

p+q= 1 (allele frequencies)

Gene Flow: organisms/gametes move from one population to another and may introduce novel alleles into population, shifting allele and genotype frequencies away from values predicted by hardy-weinberg model!

Genetic Drift: allele frequencies in a population change from one generation to the next simply by chance !

- Leads to reduced diversity since rare alleles are lost ! - Driven by 2 circumstances:! • Founder effects! • Population bottlenecks! Founder Effect: • when a few individuals colonize a distant locality and start a new population, they only carry small sample of parent population’s genetic variation ! • By chance, some alleles may be totally missing from new population whereas other alleles that were rare in original population might be common ! • This change in gene pool is founder effect! Population Bottlenecks • Factors like disease, starvation, hunting may kill large proportion of population, resulting in population bottleneck (amount of survivors in bottleneck)! • Large reduction in population size is associated with a decrease in size of gene pool and therefore genetic diversity ! • Alleles that were rare in original population may be totally lost!

• Both result in populations with very few individuals, very little genetic variation! Mutations: change to the double-strand sequence of DNA!

- Can introduce new alleles and change allele frequencies ! - Most stations occur as a result of normal cellular processes ! • Point mutation (substitution): single nucleotide (base) is changed ! • Insertion: one or more nucleotide base pairs are introduced into a DNA sequence ! • Deletion: one or more nuceliotide base pairs are removed from a DNA sequence ! • Inversion: a segment of DNA breaks and is inserted back into its original position in reverse orientation! • Duplication: DNA is copied twice!

- Mutations can occur in genomes of any cells (ex. Human skin cells)! - But for mutations to alter allele frequencies within a population, the mutation must occur in the DNA of germ-line cells, those that go on to produce gametes (ex. Sperm and egg)!

- Does not tend to lead to increased fitness (either neutral or harmful effect)! - Importance of mutation for evolution: it is the only microevolutionary process that gives rise to genetic novelty ! Natural Selection: key mechanism of evolution, the change in the heritable traits characteristic of a population over generations!

- Favours some combinations of traits over others, resulting in differential survivorship and -

reproduction! Violates Hardy-Weinberg equilibrium since not all genotypes survive and reproduce equally! 3 types of natural selection:!

• Directional Selection: when individuals near one end of the phenotypic spectrum have the highest relative fitness! - Shifts a trait away from the existing mean and toward the favoured extreme! • Stabilizing Selection: when individuals expressing intermediate phenotypes have the highest relative fitness! - Reduces genetic and phenotypic variation!

- Increases frequency of intermediate phenotypes!

• Disruptive Selection: when extreme phenotypes have higher relative fitness than intermediate phenotypes ! - Alleles producing extreme phenotypes become more common!

- Much less common!

Non-random Mating: mates are selected because they have a particular phenotype and thus often a particular underlying genotype !

- Hardy-Weinberg equilibrium is not maintained but by itself, non-random mating does not -

result in a change in allele frequencies …so therefore it is not considered a microevolutionary process! 2 types:!

• Inbreeding: mating between individuals that are genetically related ! - Issue in small populations !

- Major results: an increase in the proportion of both homozygous genotypes in the population over successive generations and a decrease in proportion of individuals that are heterozygous ! Does not cause evolution because allele frequencies do not change overtime!

- Inbreeding depression: decline in average fitness of inbred individuals in population! • Sexual Selection: When individuals consider specific aspects of the other sex before deciding to mate with them! - Favours those individuals with specific traits that enhance their ability to mate with individuals of the other sex! - Usually actors on male species who often possess a range of ornaments (ex. Brightly coloured feathers, long tails, etc.)! - Pushes phenotypes toward one extreme!

- 2 related processes:! • Intersexual Selection: selection based on the interactions between males and females! • Intersexual Selection: selection based on the interactions between members of the same sex! Balancing Selection: type of natural selection where more than one allele is actively maintained in a population! - Natural selection preserves balanced selection when:! • Heterozygotes have higher relative fitness (heterozygote advantage)!

• When different alleles are favoured in different environments ! • When the rarity of a phenotype provides a selective advantage !

Hardy Weinberg- S.I. Session! (definitions on phone)...