Chapter 20Evolution of Genes and

Traits

Darwinian evolution, mutations and molecular evolution, a case study in natural selection, morphological evolution; the origin of new genes and protein functions

Major principles of Darwinian evolution

• Principle of variation—among individuals in a population, variation exists

• Principle of heredity—offspring resemble parents more than unrelated individuals

• Principle of selection—some forms more successful at survival and reproduction in a given environment (natural selection)

• heritable variationheritable differences between populations over time

Phyletic evolution – continuous change over time in a single line of descent

Diversification – the origin of new species over time

Darwin’s Finches and Adaptive Radiation

A phylogenetic tree is a visual representation in tree form of how we think evolution has occurred, often based on fossils.

Famous example:Galapagos finches, with numerous feeding adaptations

Neo-Darwinian evolution—Darwin’s principles w/genetics, population

biology

Different models of selection

• Directional selection- pushes population toward homozygosity and phenotypes toward one extreme

• Balancing selection- favors heterozygotes but maintains all phenotypes

• Disruptive selection- favors both homozygotes, eliminates heterozygotes and increases extremes of phenotypes

Mutations and Molecular Evolution

• 3 different effects on fitness by mutations:– deleterious

– increase efficiency or performance

– no effect (“neutral”)

• can be a little more specific:– effectively neutral mutationsselection intensity so

low that mutation is retained

– effectively selected mutationsselection high enough that mutation is weeded out

Synonymous changes refer to a mutation which substitutes the same amino acid

Deleterious mutations removed by purifying selection-shown by lower rate of nonsynonymous mutations

Mutation rate of synonymous sites higher than nonsynonymous

MOLECULAR CLOCKS: mutations/amino acid differences can be used to estimate evolutionary

divergence times

Natural Selection in Action:An exemplary case

• Genotypes could be measured• The genetic and molecular basis of variation

was identified• The physiological role of the gene/protein

was well understood• The environmental (natural) selection

process was understood• We are talking about the connection

between sickle cell anemia and malaria

Malaria Life Cycle

Red blood cells in someone with sickle-cell trait

Malarial parasites live within red blood cells

Electrophoresis of hemoglobin variants

The hemoglobin molecule

The first seven N-terminal amino acids in normal and sickle cell hemoglobin

polypeptides

GAG

GUG

GLU = Glutamic acid is acidicVAL = Valine is neutral non-polar

Gene frequency for HbS allele high in malaria (mosquitoes-rich) zones

HbAS heterozygous are more resistant to malaria

Survival analysis of sickle-cell genotypes

Morphological evolution-melanism in rock pocket mouse (adaptive changes)

Morphological evolution—melanism in rock pocket mouse

• Pinacate region of SE Arizona has blackish lava rock areas interspersed with pale brown rock areas

• rock pocket mouse (Chaetodipus intermedius) has 2 melanic forms growing in 2 different substrates– dark form inhabits blackish areas– pale ancestral form lives in sand-colored areas

• Nachman et al. found 4 mutations in melanocortin 1 receptor (MC1R) gene of dark mice, causing protein to be constitutively active and lay down pigment constantly

Morphological evolution—melanism in MC1R protein of

organisms

Peppered Moths in Great Britain

Morphological evolution-evolution of albinism in blind

cave fishes(gene inactivation)

Morphological evolution—albinism in blind cave fishes

• albinism common in cave organisms (incl. fishes, crustaceans), often accompanied by eye loss

• genetic studies of Mexican blind cave fish (Astyonax mexicanus) in 2 different populations, Pachón and Molino, revealed different mutations in Oca2 gene—gene inactivation

• Pachón fishes are homozygous for deletion of intron and most of exon in Oca2 gene

Morphological evolution-wing spots on fruit flies (regulatory sequence evolution)

Drosophila melanogaster

Drosophila biarmipes

Origin of New Genes

• New genes and proteins necessary for wholly new functions and processes

• Sources of new genes/DNA:– Polyploidy-duplicate genomes can diverge– Gene duplications-duplicated genes can diverge– Transposition (transposable elements)– Retrotransposition (retrotransposons)– Imported DNA from organelles or horizontal

gene transfer

The alternative fates of duplicated genes

Final Thoughts• The one process now going on that will take millions

of years to correct is the loss of genetic and species diversity by the destruction of natural habitats. This is the folly our descendants are least likely to forgive us. --E. O. Wilson

• It's an important point to realize that the genetic programming of our lives is not fully deterministic. It is statistical - it is in any animal merely statistical - not deterministic. --Richard Dawkins

• If you liked genetics (PBIO 3300/5300), consider taking biotechnology and genetic engineering (PBIO 4500/5500) in the fall. –Allan Showalter