Transcript Chapter 20 - Evolution of genes and traits

Chapter 20
Evolution 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 variationheritable 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 mutationsselection intensity so
low that mutation is retained
– effectively selected mutationsselection high enough
that mutation is weeded out
Mutation rate of synonymous sites higher than nonsynonymous
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
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 acidic
VAL = 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:
–
–
–
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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