Random Events in Population Genetics PDF

2024/08/03 RANDOM EVENTS IN POPULATION GENETICS 1 CHANGE WITHIN AND...

2024/08/03 RANDOM EVENTS IN POPULATION GENETICS 1 CHANGE WITHIN AND BETWEEN POPULATIONS… Non-random processes: Natural selection Random processes: Genetic drift Founder events Gene (allele) substitution …Both random and non-random events can act on a population simultaneously 2 GENETIC/RANDOM DRIFT Genetic drift: Random changes in allele frequency between generations NOT a function of fitness Due to random reproductive success Random sampling of gamete pool Random mortality of adults / juveniles e.g. a freak storm that dislodge mussels from rocks – they cannot contribute offspring to the population anymore. 3  2024/08/03 GENETIC DRIFT (See also Box 6.1., p. 139) 4 GENETIC/RANDOM DRIFT Genetic drift: Random changes in allele frequency between generations NOT a function of fitness Due to random reproductive success Random sampling of gamete pool Random deaths of adults / juveniles e.g. a freak storm that dislodge mussels from rocks – they cannot contribute offspring to the population anymore. Small populations = more likely to experience chance changes in gene frequency faster rate of change larger variability 5 https://www.youtube.com/watch?v=W0TM4LQmoZY 6  2024/08/03 GENETIC DRIFT 1,0 0,9 Pops = 3, N = 10 Pops = 3, N = 100 Pops = 3, N = 1 000 0,8 0,7 Frequency 0,6 0,5 0,4 0,3 0,2 0,1 0,0 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 Generation 7 FOUNDER EFFECT Founder effects The loss of genetic variation when a new colony is formed by a very small number of individuals from a large population Founder event: Establishment of a new population by a small number of founders Gene frequency of the founders are different from that of the original population Population ‘bottleneck’ When only a few individuals (and genetic variation) survive, e.g. due to severe environmental conditions, and later expand again when conditions become favourable 8 FOUNDER EFFECT If a small population colonizes a new area, it could contain the same alleles as those present in the ancestral population, but in different (allele) frequencies. Examples: Several human populations have rare genes in high frequencies, because of the founder effect Northern elephant seals 20 individuals to 30000 9  2024/08/03 One allele can be substituted for another by random drift… The frequency of a gene is as likely to increase as it is likely to decrease, between generations, by random drift. GENE This could result in fixation: SUBSTITUTION When an allele has achieved a frequency of 100% in the population (i.e. the allele is “fixed”; the only allele left in population for that gene). So one allele substituted by another 10 Neutral alleles (neutral mutation) Confer no selective advantage or disadvantage on the individual (no difference in fitness). CHANGE CAN HWE (theoretical): OCCUR BY On average the frequencies of neutral alleles RANDOM remain unchanged from one generation to the next. DRIFT In practice, however: Random drift in frequency occurs, especially when population is small. 11 CHANGE CAN OCCUR BY RANDOM DRIFT In every generation the frequency of a neutral allele has a chance of : increasing, decreasing, or staying constant. If it increases in one generation it again has a chance of: increasing, decreasing, or staying constant. And the next generation…. 12  2024/08/03 A neutral allele thus has a small chance of increasing for two generations in a row. CHANGE CAN It also has an even smaller change of increasing in three successive generations OCCUR BY RANDOM For any one allele, chance of fixation by random DRIFT drift is very small, but not impossible. Rare allele is even less likely to be fixed…but it can happen 13 RANDOM DRIFT VERSUS ADAPTATION 14 GENETIC DRIFT Change within and between populations happen as: Non-random process: Natural selection Random processes: Genetic drift Random changes in allele frequency between Founder events generations NOT because of increase fitness benefits. Gene (allele) substitution Neutral alleles (neutral mutation) Confer no selective advantage or disadvantage on the individual (no difference in fitness). Change in frequencies of neutral alleles cannot happen due to natural selection – only random drift. 15  2024/08/03 Neutral alleles (neutral mutation)… e.g. synonymous substitutions and non-coding DNA… Synonymous sites in DNA: substitutions / mutations that do not change amino acids coded for DNA non-coding (NOT “junk” DNA) does not for proteins (have another function), so cannot be selected – change only through random processes Amino Acid Codon (RNA) Amino Acid Codon (RNA) Leucine (Leu) UUA Arginine (Arg) CGU UUG CGC CUU CGA CUC CGG CUA AGA CUG AGG Lysine (Lys) AAA Aspartic acid (Asp) GAU AAG GAC 16 Neutral change should happen at a constant rate Rate at which neutral alleles lost balanced by rate of alleles added by neutral mutations Kimura’s (1968): Neutral theory of molecular evolution GENETIC DRIFT Rates of molecular change relatively constant per unit time Rate of change appears clock-like (“Molecular clock”) Used to construct phylogenies (more on this later) 17 DRIFT VS. SELECTION Example: Differences between α– and β–haemoglobin Port Jackson shark Species pairs Number of AA differences living fossil, almost unchanged >300 Ma. Human α vs human β 147 Humans Carp α vs human β 149 Fish-like, amphibian, reptilian, mammalian Shark α vs shark β 150 stages α- and β– haemoglobin molecules: have been accumulating changes independently, at roughly constant rates Regardless of external selective circumstances Thus, most changes must be neutral shifts Equivalent forms – equal adaptive utility 18  2024/08/03 GENETIC DRIFT 19 DRIFT VS. SELECTION Molecular change: mostly by genetic drift relatively constant rate between different lineages Morphological change driven mostly by natural selection varies greatly among different lineages e.g. amino acid differences between α– and β–haemoglobin Duplicated into α and β forms before radiation of chordates 20 DRIFT VS. SELECTION Current knowledge DNA (molecular) level: mostly random drift Morphological level / adaptation: mostly selection Amino acid change in proteins Non-synonymous mutations (nucleotide mutation alter amino acid sequence) => codes for different amino acids Influence form and function Relative importance of drift and selection still uncertain 21  2024/08/03 GENETIC DRIFT Tribolium castaneum (flour beetle) bb + b+b+ crossed, and the offspring divided into several populations of two population sizes: N = 10 and N = 100. 22 GENETIC DRIFT Tribolium castaneum (flour beetle) bb + b+b+ crossed, and the offspring divided into several populations of two population sizes: N = 10 and N = 100. 23