Download Natural selection

Evolution
Chapters 13 & 14
DARWINISM
AND DESCENT WITH MODIFICATION
Chapter 13
Pre-Darwinian Thought
• Aristotle
– All members of a species identical
– All species are the work of a Creator
– Species never went extinct or changed
• New species were additions by creator
• George Cuvier
– Studied strata and fossil record
– Catastrophism caused most geological
formations
• Fossils were the extinct organisms that
resulted
• Repopulation from migration of nearby
species
Jean-Baptiste de Lamark
• Proposed 1st mechanism for
evolution
– Remembered for being wrong
• Principles
– Use and disuse
– Inheritance of acquired
characteristics
• Genetics now can refute
– Phenotypes not affecting genotype
aren’t inherited
• Bonsai trees exemplify
Charles Darwin
• Round the world voyage on the H.M.S
Beagle
– Observed similarities and differences b/w
organisms
• Fossils and current species were similar
• Same environments had different organisms
• Different parts of world had similar organisms
– Now known as convergent evolution
– Wrote his works upon returning, but waited to
publish
• Father of evolution
– Genetic change in an organism over time
• Adaptation to better fit a habitat
– He termed as descent with modification
• Decades of backlash and denial
Darwin’s Conclusions
• Organisms have variations
– Certain inherited traits provided for more offspring
– Similarities and differences in organisms
• Organisms struggle to exist
– More individuals produced then can be supported
• Influenced by work of Thomas Malthus  resources limited
population size
• Organisms differ in fitness
– Inherited characteristics allow unequal reproduction
• Those favorable traits accumulate
• Organisms become adapted
– Enhanced ability to survive and reproduce
Darwin’s Published Work
• On the Origin of the Species by Means of Natural Selection
– Descent with modification explains life’s unity & diversity
– Natural selection matches organisms to their environment
• Heritable traits make it more likely an organism will survive to
reproduce
• Exclusions
– Avoided reference to humans
– Term ‘evolution’ not mentioned
• Relied on familiar examples of artificial selection
• Explains succession of current organisms from ancestors
• Alfred Russel Wallace published similar idea first though
Artificial Selection at Work
• All species Canis familiaris, but
different breeds
– Exemplary specimens of bree
– Poor genetic specimens
• Inbreeding causes small gene pools 
increase harmful alleles
• Basset hound
– Ear infections, arthritis, and back problems
• Great Danes
– Heart weakness and joint problems
• Common vegetables from wild
mustard
Natural Selection at Work
• Peter and Rosemary Grant’s
finch work
– Beak size changed with climate
and seed abundance
• Insects and pesticides
– Initially kill 99%
– Each successive application
decreases in effectiveness
• Resistant individuals survive to
reproduce and pass on resistant
allele
• Peppered moth (first lecture)
Key Points of Natural Selection
• Individuals do not evolve
• Only heritable traits are
diminished or amplified
– Not encoded by genes = acquired
characteristics
• Not goal directed
– Is not a perfect process
– Often trade-off of needs (mating
colors/calls)
• Editing, not a creative,
mechanism
Evolutionary Evidence
• Artificial and natural selection
• Fossils
– Organisms trapped in sediment so can provide a fossil record based
on layer of strata (rock) that they appear
• Biogeography is the study of plant and animal distribution
– Closely related species in same geographic region
– Similar appearances in different parts of world due to ecological
niche
Evolutionary Evidence (cont.)
• Comparative anatomy
– Homologous structures have different functions, but are
structurally similar due to common descent
• Post-anal tail and pharyngeal slits in chordates, forearm bones in
mammals
– Vestigial organs such as appendix in humans, femur in some
snakes
• Molecular Biology
– Homologous genes matching closely suggest recent relation
– Universality of DNA, RNA, and genetic code
Chapter 13
POPULATIONS
Populations
• Group of individuals, in the same place at same time
• Can be isolated which prevents interbreeding and
limits gene exchange
– Uncommon now in humans, but mates generally chosen
locally
• Gene pool is total collection of genes in a population
at one time
– Includes all alleles in all individuals
• Often more than 2 alleles per gene
– Relative frequencies (proportion) change over
generations = microevolution
• Hardy-Weinberg equation to verify it isn’t
Genetic Variation
• Darwin and Mendel’s contribution
– Contemporaries, ideas unified in 30’s and 40’s =
broad acceptance
• Alleles are key
– Polygenic (height) or single (blood type)
– Mutation produces new alleles
– Sexual reproduction
• Independent assortment, random fertilization, and
crossing over
• Necessary for evolution, but doesn’t guarantee
– Allelic frequency constant w/o outside factors
Evolutionary Change
• Natural selection
– Certain variations leave more offspring then
others
– Improves match between individual and
environment
• Phenotype exposed to environment  changes
genotype
• Genetic drift
– Chance events that alter allele frequencies
• Gene flow (migration)
– Transfer of alleles between populations
 Reduces differences between populations
Modes of Natural Selection
• Stabilizing: favors intermediate (birth weight); stable environment reduces
phenotypic variation
• Directional: acts against one extreme phenotype (peppered moths); with
environmental change or migration
• Disruptive: favors either extreme (less common); with varied environment
Genetic Drift
– Reduces variation through random loss
• Founder effect
– Isolated individuals form a new
population
• Bottleneck effect
– Sudden event drastically reduces the
population
– Recovery can’t replace genetic
diversity loss
– E.g. Cheetahs
• Two isolated populations remain
– Skin grafts readily accepted b/w
unrelated animals
– Low sperm count
• Recovery shows low variation levels
– Can’t flourish, difficult to breed in
captivity
– Single environmental change could
cause extinction
Fitness and the Environment
• Natural selection blends chance (random
assortment) with sorting (allele preferences)
– Enhances fit between organisms and environment
• Environment changes faster than evolution can
occur
• Fitness is not ‘survival of the fittest’
– Survival doesn’t guarantee reproductive success
– Organism’s contribution to next generation’s gene pool
• Offspring must also survive to reproduce
• Lead to trade offs (praying mantis)
• Selection favors individuals with phenotypic
traits that provide higher reproductive success
than others
Sexual Selection
• Individudals with certain inherited characteristics are
more likely than others to obtain mate
• Creates sexual dimorphism, marked differences
between the 2 sexes that don’t have direct effect on
survival or reproduction
– Include size, color, ornamentation, and behavior
• Intrasexual selection (within same sex)
– Males defends status through force or displays
• Intersexual selection (between sexes)
– Female choice depends on showiness of male
• Not always beneficial, pose risks by making more visible = tradeoff
– Females want mates with ‘good genes
• Midshipman Fish
– Males become singers or
sneakers
– Singing induces females to
lay eggs in male’s nest
– Female leaves and male
resumes singing
• Attract more mates
– Sneakers hangout by
singers
• Sneak in to fertilize eggs
• Resemble females
Natural Selection Isn’t Perfect
• Can only act on existing variations
– Aren’t always ideal
• Evolution limited by ancestry
– Doesn’t scrap existing structure
– Adapts/modifies to something new
• Adaptations often compromises
• Interaction of chance, natural selection, and the
environment
– Change not necessarily best fit environment initially
• Species evolve to be ‘better than’
Chapter 14
ORIGINATION OF SPECIES
Speciation
• Emergence of a new species
– Biological Species Concept
• Species are organisms that can potentially
breed and produce fertile offspring
• Reproductive isolation maintains
species separation
– Two groups can’t interbreed which
prevents gene flow
Reproductive Barriers
• Biological features of an organism that can prevent interbreeding
and maintain a species
• Prezygotic barriers (prevents mating or fertilization between
species)
• Postzygotic barriers (affects after hybrid zygote forms)