Chapter 24 Evolutionary Genetics © John Wiley & Sons, Inc.

Chapter Outline Genetic Variation in Natural Populations Molecular and Evolution Speciation © John Wiley & Sons, Inc.

Genetic Variation in Natural Populations © John Wiley & Sons, Inc. --Variation in alleles of genes --There are three primary sources of genetic variation Mutations are changes in the DNA structure Gene flow in genes’ movement [Sex determination (non somatic cells)] --It provides the raw material for natural selectionnatural selection Genetic Variation~~phenotype

© John Wiley & Sons, Inc. Phenotypic Variation Phenotypic differences or variations= polymorphisms

Phenotypic Variation Polymorphisms: two or more clearly different phenotypes (forms) co-exist in the same populationphenotypes © John Wiley & Sons, Inc.  In Drosophila, many mutant alleles have been characterized  for phenotypes such as eye color.  In Human, the Duffy Blood Group system is characterized  by a different allele in Chromosome 1 (Duffy allele, Fy a and Fy b ).

A Human Polymorphism © John Wiley & Sons, Inc. Polymorphism and ethnic groups

Variation in Chromosome Structure (genetic variability).  Drosophila polytene chromosomes afford researchers an opportunity to look for variation in chromosome structure in natural populations.  There have been identified many rearrangements of the banding patterns in the polytene chromosomes © John Wiley & Sons, Inc.

Puffs

Variation in Protein Structure  Amino acid differences in proteins can be detected using gel electrophoresis.  Proteins differing in size and charge can be separated by moving them through a starch or polyacrylamide gel (matrix). © John Wiley & Sons, Inc.

Isocitrate Dehydrogenase Variability in Trillium pusillium  Isocitrate dehydrogenase exists as a dimer.  Electrophoresis can distinguish dimers containing two fast-moving subunits, two slow-moving subunits, and a “hybrid” enzyme with one fast and one slow enzyme subunit.   The fast- and slow-moving subunits are allozymes (variant forms) encoded by different alleles of the same locus. © John Wiley & Sons, Inc.

Protein Polymorphisms  Proteins that exhibit electrophoretic variation are polymorphic if at least two of the variants have frequencies greater than 1% in the population.  Proteins that do not exhibit electrophoretic variation are monomorphic. © John Wiley & Sons, Inc.

Variation in Nucleotide Sequences  DNA sequencing can be used to study genetic variation. © John Wiley & Sons, Inc.

Polymorphisms in the Alcohol Dehydrogenase (Ddh) Gene of Drosophila © John Wiley & Sons, Inc. 11 Ddh genes

 Most of the polymorphisms in the Alcohol dehydrogenase (Adh) gene are in noncoding regions (introns and 3' and 5' untranslated regions).  Most of the polymorphisms in the coding region are silent- (it does not alter a protein function and mobility). © John Wiley & Sons, Inc.

Techniques for Detecting Nucleotide Polymorphisms  PCR followed by DNA sequencing  Gene chip technology for identification of SNPs ( single-nucleotide polymorphisms) ~1 to 2kd. © John Wiley & Sons, Inc. Anti-sense RNA

Molecular and Evolution Analysis of DNA (and protein) sequences provide information on the phylogenetic relationships among different organisms, and on their evolutionary history. © John Wiley & Sons, Inc.

Molecular Evolution  Heredity depends on the sequence of nucleotides in DNA and the transmission of DNA molecules from parents to offspring.  When mutations occur, modified DNA molecules are transmitted to the offspring. Over time, mutations accumulate and the DNA sequence is changed; chromosomal rearrangements may also occur. © John Wiley & Sons, Inc. genotype DNA RNA Protein phenotype

DNA and Protein Sequence Analysis vs. Traditional Methods to Study Evolution  DNA and protein sequences follow simple rules of heredity.  Molecular sequence data are easy to obtain and are amenable to quantitative analyses framed in evolutionary genetics theory.  Molecular sequence data allow analysis of evolutionary relationships among organisms that are phenotypically very dissimilar. © John Wiley & Sons, Inc.

DNA and Protein Sequence Analysis vs. Traditional Methods to Study Evolution  It is difficult to obtain DNA or protein sequence data from extinct organisms.  It is not always clear how molecular sequence data relate to questions about evolution at the phenotypic level. © John Wiley & Sons, Inc.

Molecular Phylogenies  Evolutionary relationships among organisms (at the DNA level) are summarized in phylogenetic trees, or phylogenies (branching diagram).  All organisms on earth have descended from a common ancestor.  A phylogeny that shows only the relationships among organisms is an unrooted tree.  A phylogeny that superimposes the relationships on a time scale to show how organisms evolved (variation) is a rooted tree.

Phylogenetic Trees  Lineages bifurcate (nodes) to produce branches.  The terminal branches lead to the organism being studied.  Each bifurcation represents a common ancestor. © John Wiley & Sons, Inc.

Homologous Sequences  The descendants of an (common) ancestral DNA or protein sequence are homologous.  Sequences that resemble each other but are derived from different ancestral sequences are analogous.  Construction of phylogenetic trees should be based on homologous sequences. © John Wiley & Sons, Inc.

Steps Involved in Constructing Phylogenetic Trees  Aligning the sequences to allow comparisons among them  Ascertaining the amount of similarity or difference between any two sequences  Grouping the sequences on the basis of similarity  Placing the sequences at the tips of trees © John Wiley & Sons, Inc.

Parsimony  The principle of parsimony or SIMPLICITY is used to judge the merit of a tree.  The best tree is the one that requires the fewest mutational changes. © John Wiley & Sons, Inc.

Phylogenetic Trees of Hominids Based on Mitochondrial (mt) DNA © John Wiley & Sons, Inc. mt DNA: circular with ~16.6 Kb Sequence ~896 bp Analyzed ~ 283 bp ---number of mutations 145, 147, 148

Speciation © John Wiley & Sons, Inc.

What Is a Species?  A species is a group of organisms that share characteristics.  species is a group of interbreeding, or potentially interbreeding, producing fertile organisms  species is a group reproductively isolated from other species.  Species have been defined in different ways. –Traditionally, species have been defined based on phenotypic characteristics. –Evolutionary genetics, species have a “defined” a “shared genetic pool” © John Wiley & Sons, Inc.

Speciation Species arise when a population of organisms splits into genetically distinct groups that can no longer interbreed with each other. © John Wiley & Sons, Inc. Consequence?

Reproductive Isolation  Prezygotic isolating mechanisms prevent members of different groups from producing hybrid offspring.  Postzygotic isolating mechanisms prevent hybrid offspring from passing on their genes to subsequent generations. © John Wiley & Sons, Inc. Is a feature or mechanism to prevent breeding between species

Prezygotic Isolating Mechanisms  Prezygotic isolating mechanisms prevent mating between individuals form different populations of organisms or by preventing the gametes of these individuals to form zygotes.  These mechanisms include –Ecological or Geographical isolation based on habitat preference (different habitats in the same geographical area) –Temporal or behavioral factors (e.g., different times sexual maturity or different courtship rituals) –Mechanical Anatomical or chemical incompatibilities in reproductive organs or gametes (e.g., failure to mate successfully or to form zygotes) © John Wiley & Sons, Inc.

Postzygotic Isolating Mechanisms  Postzygotic isolating mechanisms operate after hybrid zygotes have been formed.  Mechanisms include –Reduction of hybrid viability (e.g., failure to survive or to reach sexual maturity) –Impaired hybrid fertility (failure to produce functional gametes) © John Wiley & Sons, Inc.

Modes of speciation Allopatric Sympatic © John Wiley & Sons, Inc.

122) Which of the following accurately describes the Hardy-Weinberg genotype frequency expression? a) P 2 + 2p +q2 b) P 2 + 2pq + q3 c) P 2 +2pq+q2 d) P 2 + 3pq+q3 e) (p+q)3 243) What effect does consanguineous mating and assortative mating have on genotypic frequencies in populations? 1. Reduce the frequency of homozyotes 2. Increase the frequency of homozygotes 3. Reduce the frequency of heterozygotes a) 1 b) 2 c) 3 d) 1 and 3 e) 2 and 3 38) What is the ultimate source of all genetic variability? a) Natural selection b) Artificial selection c) Mutation d) Natural selection and artificial selection e) None of these Answer: c Answer: e Answer: c