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Evolution
Darwin’s proposal and modern updates
Chapters 16 - 19
Created by Kevin BleierMilton High School
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On the Origin of Species published 1859
1) Species change (they are not fixed)
2) Selection as a mechanism for change
3) “Descent with modification” implies all species related, diverged over evolutionary time (Earth is very old)
Darwin’s revolutions
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Darwin’s natural selection1) Competitive force
2) Genetic variety
3) Selection
4) Generations of continued selection
chapter 16.2
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Darwin’s natural selection Requirement 1: competitive force
something is making it hard for individuals to survive
1) Competition within a species for resources (too many organisms)
2) Competition between species (predation / parasitism, etc)
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Darwin’s natural selection Requirement 2: genetic variety
(everyone has different traits)
Must be genetic (so traits potentially passed on to offspring)
We have spent time here in fall – how do organisms generate genetic variety?
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Darwin’s natural selection Result 1: selection
Individuals are selected for if they are able to survive longer to reproduce more offspring than other individuals (they have traits that benefit them)
Individuals selected against if they do not survive as well – meaning they do NOT produce as many offspring
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Often summarized as “survival of the fittest”
Key idea there is not survival, but fittest
Evolutionary fitness = how many offspring individual can produce
(NOT strength, size, speed, etc)
Darwin’s natural selection
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Darwin’s natural selection Result 2: selection over generations
Those with beneficial trait must continue to out-reproduce the others
Eventually, overall population starts to change (= evolution)
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Natural selection example Reduction in fish body size over 17 years off
coast of South Africa
Can human fishing (predation) drive evolution of fish populations over time?
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Simple natural selection modelBig fish
Small fish
1) competitive force
2) genetic variety3) selection
70% 30%54% 46%
4) selection overmany generations
42% 58%31% 69%
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Darwin’s natural selection1) Competitive force
2) Genetic variety
3) Selection
4) Generations of continued selection
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Darwin had many influences to help develop his idea for evolution
Other scientists also believed that species changed over time – alternative hypotheses for evolution from Lamarck
History of Darwin’s idea
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Darwin vs. LamarckSupported by evidence
Genetic variation
Individuals with beneficial variations reproduce more
Population evolves, NOT individuals
Not supported Individuals change
by use and disuse
Individuals pass acquired changes in life to offspring
Individuals evolve because they “want” change
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Darwin read other subjects for fun
Lyell and geology – studies proposing that Earth is much older than previously thought (now enough time for evolution)
Malthus and economics – growing human population might lead to mass starvation and competition for food(leads to ideas about competitive force)
History of Darwin’s idea
chapter 16.1
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Evidence for 3 ideas?1) Species change (they are not fixed)
2) Species changed from common ancestry over evolutionary time (implying a much older Earth)
3) Selection as a mechanism for change
chapter 16.2
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Darwin observes organisms with slight differences
Ex: Galápagosfinches withdifferent beaks
Adapted to eatdifferent food sources
Species change (Darwin)
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Darwin also finds fossils of organisms unlike any that live today
Ex: giant sloth in Argentina
(modern armadillos andsloths related, but MUCHsmaller)
Species change (Darwin)
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From “AIDS: Evolution of an Epidemic”
http://www.hhmi.org/biointeractive/hl/
Species change (viral resistance)
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Resistance to antibiotic medicines in some species of pathogenic bacteria
We have only seen this recently (first wide use of antibiotic penicillin in 1940s)
Open books to p. 484
Species change (modern pathogens)
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Evidence for 3 ideas?1) Species change (they are not fixed)
2) Species changed from common ancestry over evolutionary time (implying a much older Earth)
3) Selection as a mechanism for change
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Homo = _______
Same evolutionary history (same bone structures)
Different functions in different environments
Homologous structures
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Genes and proteins with important cell functions have been largely unchanged in evolutionary history
Ex: Proteininvolved in celldivision(cytokinesis)
Homologous DNA / protein sequences
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Homologous development Embryo stage of development shows
similarities in many animals (then divergence)
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Respiration / photosynthesis pathways
Mitosis pathways (eukaryotes)
…and more!
Proteins involved are very similar, unchanged for billions of years of life’s history
Homologous cell processes
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Vestigial structures
“use it or lose it”
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Vestigial structures
“use it or lose it”
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A species does not always have to add something new, it can evolve by LOSING traits as well
Great example: tapeworm
A reminder
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Evidence for 3 ideas?1) Species change (they are not fixed)
2) Species changed from common ancestry over evolutionary time (implying a much older Earth)
3) Selection as a mechanism for change
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Ideas for selection started with interviewing pigeon breeders
= artificialselection
(domesticating plants)
(domesticating silver foxes)
How does change happen?
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Sure
Antibiotic resistance only occurs in era of antibiotics
Also example of bedbug resistance to insecticide chemicals
Does natural selection occur?
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Evidence for 3 ideas?1) Species change (they are not fixed)
2) Species changed from common ancestry over evolutionary time (implying a much older Earth)
3) Selection as a mechanism for change
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Evolution does not “finish” at “perfection”
Ex: Eye setup and blind spots
Imperfections in humans
incoming light
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Populations are not isolated, and often evolve in response to each other
Coevolution – two species are competing to “one up” each other with adaptations
◦Ex: predators and prey, plants and herbivores
Patterns of evolution
chapter 16.3
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Divergent Convergentevolution evolution
True homology
Fake homology
common ancestor
new species
different ancestries
some similarities begin to develop in same environment
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Adaptive radiation
Special case of divergent evolution - when many niches available
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Convergent evolution example
Australian mole (marsupial)
European mole (mammal)
similar adaptations to live underground but very different
ancestries
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Gradualism vs. punctuated equilibrium
Patterns of evolution
Slow, even changethroughout history
Long periods of no change with bursts of rapid change
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Radiometric dating – half-life of radioactive atoms is very reliable, like a clock ticking
Half-life: how long it takes for half of any size sample to become stable (some take millions of years, some thousands of years)
Relative dating – using commonly found fossils to estimate age of new fossil
How long ago did an event occur?
chapter 19.2
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One area lacking in Darwin’s analysis 1) Competitive force makes it hard to
survive
2) Variety of heritable traits
3) Selection of organisms to out-reproduce others
4) Generations of out-reproduction to foster population change
How is this variety generated? How does inheritance work?
chapter 17.2
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Modern update to Darwin Variety of traits caused by genes (with
different alleles)
New evolution definition: When allele frequencies change in a population’s gene pool over generations
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Gene pool ideaR = black fur r = sandy fur
RR
Rr
RrRr
rr
rrrrrr
rr
rrrr
rrrr
rr
rr
rr
rrRr
Rr
rr
20 total organisms
14 / 20 = rr = 70%
Gene pool7 / 40 R = 17.5%
1 / 20 = RR = 5%5 / 20 = Rr = 25%
33 / 40 r = 82.5%
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Natural selection causes evolution Evolution = change in gene pool allele %s
Ex: Lava flow creates black rock environment
Genotypes # organisms initially
# organisms 25 years later
RR 1 12Rr 5 8rr 14 0
R = black fur r = sandy fur
black fur phenotype
sandy fur phenotype
initially: R = 17.5% r = 82.5%25 yrs: R = 80.0% r = 20.0%
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Evolutionary change Natural selection chose organisms with
phenotypes to survive longer and reproduce more offspring
Over generations, this causes allele frequencies to shift
Modern evolutionary theory identifies five total evolutionary forces (four others besides natural selection)
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Five evolutionary forces1. Natural selection2. Sexual selection3. Mutation4. Genetic drift5. Gene flow
3 types:
a) Stabilizing selection
b) Directional selectionc) Disruptive
selection
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Types of natural selectiona) Stabilizing selection
Original population (green) shifts to favor intermediate phenotype, away from both extremes (blue)
Ex: Lizards have become medium size
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Types of natural selectionb) Directional selection
Original population (green) shifts toward one direction of a phenotype (blue)
Ex: anteater populations have evolved longer tongues to reach ants
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Types of natural selectionc) Disruptive selection
Original population (green) shifts to EITHER extreme, away from intermediate (blue)
Ex: white or black limpet shells camouflage, NOT tan shells
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Five evolutionary forces1. Natural selection2. Sexual selection3. Mutation4. Genetic drift5. Gene flow
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Evolution cause #3: mutation Raw material for any change, but causes
very little change by itself
Example: new recessive mutation at another gene causes albino coloration in just one mouse
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Five evolutionary forces1. Natural selection2. Sexual selection3. Mutation4. Genetic drift5. Gene flow
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Evolution cause #2: sexual selection
Mate choice for particular characteristics makes certain traits more prominent
Mates “selecting” other mates because their phenotypes are “sexy”
Increased reproductionmakes trait moreprominent in futuregenerations
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Evolution cause #2: sexual selection
Initially: R = 74.4% r = 25.6%
Genotypes # peacocks(initially)
# peacocks(25 generations
later)GG 25 4Gg 17 8gg 3 28
G = boring coloration g = sexy, bright coloration
25 generations: R = 20.0% r = 80.0% later
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Five evolutionary forces1. Natural selection2. Sexual selection3. Mutation4. Genetic drift5. Gene flow
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Evolution cause #4: Genetic drift Any random change that shifts allele %s
Evolution does NOT have to be caused by selection
Especially affects small populations – may even eliminate alleles from gene pool
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Evolution cause #4: Genetic drift Example: flood kills many of the mice in a
population randomly
Genotypes # mice before disaster
# mice afterdisaster
RR 8 0Rr 14 2rr 6 4
R = black fur r = sandy fur
black fur phenotype
sandy fur phenotype
Before disaster: R = 53.6% r = 46.4%After disaster: R = 16.7% r = 83.3%
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Bottleneck Effect –
Occurs after an event greatly reduces the size of the population
Results in a loss of genetic diversity
Example: northern elephant seal
Evolution cause #4: Genetic Drift
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Founder Effect
Occurs after a small number of individuals colonize a new area
Leads to loss of genetic diversity in the population
Evolution cause #4: Genetic Drift
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Five evolutionary forces1. Natural selection2. Sexual selection3. Mutation4. Genetic drift5. Gene flow
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Evolution cause #5: Gene flow Entry of new individuals or exit of current
members (their genes are flowing in or out)
Genotypes # mice in desert
# mice + 10 new sandy fur
miceRR 25 25Rr 17 17rr 3 13
Initially: R = 74.4% r = 25.6%After migration: R = 60.9% r = 39.1%
R = black fur r = sandy fur
black fur phenotype
sandy fur phenotype
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Evolution at two scales Microevolution – “small” changes within a
population – chapter 17.2
Macroevolution – “large” scale change involving new species, broader groups of organisms – chapter 17.3
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How do new species form? When do populations become so different
that they are new species?
Biological species concept – when male and female can make fertile offspring together
If they are so different that they do not mate (or cannot mate), then they are different species
chapter 17.3
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Speciation at work?
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Why does speciation happen? Populations separated by a landform
(ocean, mountain) (geographic isolation), begin to become different in genetics
Eventually, they are so different that they do not mate (reproductive isolation)
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Artificial speciation
Diane Dodd’s fruit fly lab, 1989
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Two approaches to building Darwin’s tree of life 1) Linnean system of groups
2) Cladistics
chapter 18.1 / 18.2
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Linneaus and taxa Taxa – group names below to classify:
(least specific)domain kingdom phylum class order family genus species (most specific)
Early classifying based on structural analysis
Example: illustration p. 425
chapter 18.1
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Binomial nomenclature rules Organism’s scientific name: Genus epithet
Capitalize genus name, NOT species name
Italicize if typing, underline if handwriting
Humans: Homo sapiens
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Problems with Linnean system1) Misleading similarities result in mis-classifying
(often due to convergent evolution)
Ex: birds AND mammals have 4-chamber hearts (but little else in common)
2) Transitional organisms strain the definitions of groups
Ex: Platypus – the egg-laying mammal (or should that be mammary-gland containing reptile?)
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Modern classification - cladistics Creates a system based on traits, not on
group names
Organizes into clades – every organism and their common ancestor who has a certain trait
chapter 18.2
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common ancestor with a jaw
Cladograms
common ancestor to all these animals
lamprey salamandershark tuna turtle dog
connection to other organisms on tree
evolution of jaws
the jaw clade
evolution of air sac (swim bladder)
the lung / lung derivative clade
common ancestor with an air sacevolution of tetrapody (4 legs)
the tetrapod clade
common ancestor with 4 legs
evolution of amniotic egg
the amniote clade
common amnioteancestor
evolution of mammary glands
the mammal clade
no jawsno airsac
no 4 legsno swim
bladder
Goal: to understand the phylogeny = evolutionary historyof species
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One last misconception “Humans evolved from chimpanzees”
Both are modern species … BOTH evolved to their modern forms from a common ancestor
other mammals, vertebrates, animals, eukaryotes, life
chimpanzees humans
common ancestor