Evolution

Darwin’s proposal and modern updates

Chapters 16 - 19

Created by Kevin BleierMilton High School

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

Darwin’s natural selection1) Competitive force

2) Genetic variety

3) Selection

4) Generations of continued selection

chapter 16.2

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)

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?

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

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

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)

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?

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%

Darwin’s natural selection1) Competitive force

2) Genetic variety

3) Selection

4) Generations of continued selection

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

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

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

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

Darwin observes organisms with slight differences

Ex: Galápagosfinches withdifferent beaks

Adapted to eatdifferent food sources

Species change (Darwin)

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)

From “AIDS: Evolution of an Epidemic”

http://www.hhmi.org/biointeractive/hl/

Species change (viral resistance)

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)

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

Homo = _______

Same evolutionary history (same bone structures)

Different functions in different environments

Homologous structures

Genes and proteins with important cell functions have been largely unchanged in evolutionary history

Ex: Proteininvolved in celldivision(cytokinesis)

Homologous DNA / protein sequences

Homologous development Embryo stage of development shows

similarities in many animals (then divergence)

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

Vestigial structures

“use it or lose it”

Vestigial structures

“use it or lose it”

A species does not always have to add something new, it can evolve by LOSING traits as well

Great example: tapeworm

A reminder

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

Ideas for selection started with interviewing pigeon breeders

= artificialselection

(domesticating plants)

(domesticating silver foxes)

How does change happen?

Sure

Antibiotic resistance only occurs in era of antibiotics

Also example of bedbug resistance to insecticide chemicals

Does natural selection occur?

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

Evolution does not “finish” at “perfection”

Ex: Eye setup and blind spots

Imperfections in humans

incoming light

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

Divergent Convergentevolution evolution

True homology

Fake homology

common ancestor

new species

different ancestries

some similarities begin to develop in same environment

Adaptive radiation

Special case of divergent evolution - when many niches available

Convergent evolution example

Australian mole (marsupial)

European mole (mammal)

similar adaptations to live underground but very different

ancestries

Gradualism vs. punctuated equilibrium

Patterns of evolution

Slow, even changethroughout history

Long periods of no change with bursts of rapid change

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

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

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

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%

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%

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)

Five evolutionary forces1. Natural selection2. Sexual selection3. Mutation4. Genetic drift5. Gene flow

3 types:

a) Stabilizing selection

b) Directional selectionc) Disruptive

selection

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

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

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

Five evolutionary forces1. Natural selection2. Sexual selection3. Mutation4. Genetic drift5. Gene flow

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

Five evolutionary forces1. Natural selection2. Sexual selection3. Mutation4. Genetic drift5. Gene flow

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

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

Five evolutionary forces1. Natural selection2. Sexual selection3. Mutation4. Genetic drift5. Gene flow

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

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%

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

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

Five evolutionary forces1. Natural selection2. Sexual selection3. Mutation4. Genetic drift5. Gene flow

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

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

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

Speciation at work?

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)

Artificial speciation

Diane Dodd’s fruit fly lab, 1989

Two approaches to building Darwin’s tree of life 1) Linnean system of groups

2) Cladistics

chapter 18.1 / 18.2

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

Binomial nomenclature rules Organism’s scientific name: Genus epithet

Capitalize genus name, NOT species name

Italicize if typing, underline if handwriting

Humans: Homo sapiens

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?)

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

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

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