1 VI The Causes of Evolution Process based mechanistic evolutionary biology 1 Major Processes Involved in Evolution a Mutation i A relatively rare and sudden heritable change in genetic material DNA b Recombination also destroys genetic variance i Process that gives rise to new combinations of genetic determinants such as during re assortment of parental genes and crossing over during meiosis c Natural Selection variants d Genetic Drift of genetic drift e Gene Flow Process that 1 Generate genetic variation i The non random and differential reproduction of different genotypes acting to preserve favorable variants and to eliminate less favorable i The occurrence of random changes in the gene frequencies of finite populations the smaller the population size the larger the potential impact i The exchange of genetic factors between populations by interbreeding 2 Change the relative proportions of those genetic variants are both required for evolution 2 to take place Evolutionary Processes in Depth 1 Variation and Mutation Chapter 8 9 a Mutation the primary source of variation Classification of Mutations a Macromutation Changes in more than one locus i Changes in the amount of DNA 1 Loss of part or all of chromosome 2 Polyploidy additional sets of chromosomes 3 Aneuploidy one extra or one less chromosome 4 Duplications duplicate multiple genes at one time the duplicate can possibly assume a different function or fade away in next cycle ii Rearrangements of Genes 1 Inversions two breaks on a chromosome that will flip 180 degrees 2 Translocation reciprocal two pieces of chromosome are swapped 3 Fusion causes a decrease due to two forming one 4 Fission causes an increase due to one splitting into two b Point Mutations Changes at a single locus 3 1 Substitutions equal swaps of nucleotides may or may not effect coding for codons for an Amino Acid a Example A being replaced by G can change the codon 2 Deletions removal of a nucleotide can cause a frameshift 3 Insertions insertion of an extra nucleotide can also cause 4 Frameshifts the loss or gain of a nucleotide can cause the codon sequence to shift changing what the strip of DNA will be translated mutation frameshift into to 5 Intragenic Recombination recombination within a specific gene Given that genetic variation is generally abundant what are the agents mechanisms that alter the frequencies of genetic variants and thus cause evolution First determining what factors do not cause changes in gene frequencies would be This allows for a thorough evaluation of a smaller number of potentially significant Hardy Weinberg Castle Equilibrium Principle See pages 223 228 helpful evolutionary mechanisms What is it Three Major Points of HWCEP 4 1 Allele frequencies will tend to remain constant from generation to generation a Different forms of a gene occupying the same locus 2 Genotypes will reach an equilibrium frequency in one generation of random mating and will remain at that frequency to the entire genome a The genetic constitution of an individual ranging from reference to a single locus 3 from 1 2 meiosis and recombination do not alter gene frequencies over time General Examples imaginary population of diploid creatures concerned with a single genetic locus with 2 alleles A1 A2 What are the three possible genotypes at that locus A1 A1 A1 A2 A2 A2 the frequency of A1 in the population can be labeled p the frequency of A2 in the population can be labeled q p q 1 Therefore probability of drawing an A1 containing gamete is p Probability of drawing an A2 containing gamete is q Now the frequencies of the three possible offspring genotypes can be calculated when we draw and pair gametes at random from the population o A1 A1 the probability of drawing an A1 egg is p and an A1 sperm is also p p x p p2 5 o A1 A2 the probability of drawing an A1 egg is p and an A2 sperm is also pq the probability of drawing an A2 egg is p and an A1 sperm is also pq o A2 A2 the probability of drawing an A2 egg is q and an A2 sperm is also q 2pq heterozygotes q x q q2 knowing the allele frequencies in the first generation s gene pool allowed us to calculate the allele frequencies in the second generation s gene pool Now the allele frequencies of the second generation s gene pool can be calculated Calculating Frequency of A1 allele in 2nd generation We know fr A1 A1 fr A1 A2 fr A2 A2 1 1 p2 2pq q2 1 2 so fr A1 p2 pq 3 sub 1 p for q 4 p2 p 1 p 1 p2 2pq q2 1 2 so fr A2 pq q2 3 sub 1 q for p 4 q 1 q q2 5 p frequency of A1 in the 2nd generation is same as 1st generation We know fr A1 A1 fr A1 A2 fr A2 A2 1 5 q frequency of A1 in the 2nd generation is same as 1st generation This carries on through subsequent generations Therefore given certain assumptions 6 1 Allele Frequencies do not change from generation to generation in populations with Mendelian genetic systems those systems which obey Mendel s Laws 2 If allele frequency in a population are given as p and q then genotype frequency will be given by p2 2pq and q2 If evolution is defined as a change in gene frequencies in a population over time and if HWCEP holds in a given population then evolution has not taken place Assumptions of HWCEP 1 Large effective population size Ne a Only mating individuals matter among other 2 No differential migration of genes 3 Random mating of phenotypes 4 No differential population b Ne not all population contribute to gene pool due to male dominance factors 5 Random assortment of homologous chromosomes during meiosis Mendel s 2nd law a k a independent assortment 6 No differential survivorship of reproduction The HWCE finding establishes a null expectation for what will happen to allele and genotype frequencies in populations Given the above simple assumptions populations will not evolve However when HWCE is falsififed it is a demonstration of evolution but not necessarily via natural selection o HWCE is typically falsified o Very seldom are all HWCE assumptions valid What are the possible agents of genotype frequency change evolution 7 1 Genetic drift 2 Gene flow 3 Non random mating a Inbreeding or outbreeding average population average population b Assortative mating A M i Inbreeding mating with a mate that is more genetically related than ii Outbreeding mating with a mate that is less genetically related than the i Positive A M pairing individuals which are more closely alike than average with respect to one or more traits ii Negative A M pairing individuals which are less closely alike