chapter 24 evolutionary genetics © john wiley & sons, inc

Chapter 24Evolutionary Genetics

© John Wiley & Sons, Inc.

Chapter Outline

Genetic Variation in Natural PopulationsMolecular and EvolutionSpeciation


Genetic Variation in Natural Populations


--Variation in alleles of genes

--There are three primary sources of genetic variation

Mutations are changes in the DNA structureGene flow in genes’ movement[Sex determination (non somatic cells)]

--It provides the raw material for natural selection

Genetic Variation~~phenotype

http://en.wikipedia.org/wiki/Natural_selection


Phenotypic Variation

Phenotypic differences or variations= polymorphisms

Phenotypic Variation

Polymorphisms: two or more clearly different phenotypes (forms) co-exist in the same population


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, Fya and Fyb).

http://en.wikipedia.org/wiki/Phenotypes

A Human Polymorphism


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


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).


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.


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.


Variation in Nucleotide Sequences

DNA sequencing can be used to study genetic variation.


Polymorphisms in the Alcohol Dehydrogenase (Ddh) Gene of

Drosophila


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).


Techniques for Detecting Nucleotide Polymorphisms

PCR followed by DNA sequencingGene chip technology for identification of

SNPs ( single-nucleotide polymorphisms) ~1 to 2kd.


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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.


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.


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.


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.


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.


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.


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


ParsimonyThe 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.


Phylogenetic Trees of Hominids

Based on Mitochondrial (mt)

DNA


mt DNA: circular with ~16.6 KbSequence ~896 bpAnalyzed ~ 283 bp---number of mutations145, 147, 148

Speciation


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”


Speciation

Species arise when a population of organisms splits into genetically distinct groups that can no longer

interbreed with each other.


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.


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)


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)


Modes of speciationAllopatric Sympatic


122) Which of the following accurately describes the Hardy-Weinberg genotype frequency expression?

a) P2 + 2p +q2b) P2 + 2pq + q3c) P2 +2pq+q2d) P2 + 3pq+q3e) (p+q)3

243) What effect does consanguineous mating and assortative mating have on genotypic frequencies in populations?1. Reduce the frequency of homozyotes2. Increase the frequency of homozygotes3. Reduce the frequency of heterozygotes

a) 1b) 2c) 3d) 1 and 3e) 2 and 3

38) What is the ultimate source of all genetic variability?

a) Natural selectionb) Artificial selectionc) Mutationd) Natural selection and artificial selectione) None of these

Answer: c

Answer: e

Answer: c

chapter 24 evolutionary genetics © john wiley & sons, inc

Documents

polymorphisms john wiley

population john wiley

phenotype john wiley

puffs john wiley sons

ddh genes john wiley

ethnic groups john wiley

electrophoretic variation

alleles of genes