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

POPULATIONSChapter 13

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 populationsReduces 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’

ORIGINATION OF SPECIESChapter 14

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

• 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)

Reproductive Barriers