lecture 2 - natural selection

8/4/2019 Lecture 2 - Natural Selection

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Evolution and natural selection


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Clock analogy for

some key events

in evolutionary

history


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Cellular tree of life


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Pre-Darwinian theories

Ancient philosophers believed that Naturerandomly produces a huge variety of creatures

only creatures that manage to provide for

themselves and reproduce successfully survive

Until the early 19th century, the prevailing view wasthat differences between individuals of a species

were insignificant departures from their Platonicideal of created kinds


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Theory of uniformitarianism

Simple, weak forces could act continuouslyover long periods of time to produce radicalchanges in the Earth's landscape

Raised awareness of the vast scale ofgeological time

tiny, virtually imperceptible changes in successivegenerations could produce consequences on thescale of differences between species


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Catastrophism

Geologically only catastrophic events had changed

the geological structure of the earth

changes seen within fossilized bones were a result

of a previous catastrophic change where an entireformer and less perfect species was wiped out in

order to give rise to a new species.

Accounted for the discoveries of many "funny

looking bones" found throughout Europe and other

parts of the world


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Lamarcks theory

Of use and disuse


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Malthus's theory of population


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Charles Darwin

1809-1882

Charles Darwin

Sailed as the naturalist of the

ship H.M.S. Beagle (1831-1836)

to survey the south seas (mainly

South America and theGalapagos Islands)

Observed huge diversity in species

in Galapagos island


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Voyage of the HMS Beagle, 1831 1836


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Charles Darwin

Wrote On the Origin of Species by Means of

Natural Selection in 1859

Two main points:

1. Species were not created in their present

form, but evolved from ancestral species.

2. Proposed a mechanism for evolution:

NATURAL SELECTION


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Specialized feeding niches in birds


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Darwin observed beak

variation in the finches of

the Galpagos Islands

13 closely related species that

differ most markedly in the

shape of their beaks

The beak of each species is

suited to its preferred food,

suggesting that beak shapesevolved by natural

selection.


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Natural selection leads to adaptive radiation and

speciation


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Darwinian natural selection

3 conditions necessary for evolution by

natural selection to occur:

1. Natural variability for a trait in a population

2. Trait must be heritable

3. Trait must lead to differential reproduction

A heritable trait that enables organisms to

survive AND reproduce is called an adaptation


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Natural selection

favorable heritable traits become more common insuccessive generations of a population ofreproducing organisms

Acts on observable characteristics of an organism

Individuals with favorable traits more likely tosurvive, reproduce than those with less favorabletraits

Over time, can result in adaptations that specializeorganisms for particular ecological niches

May eventually result in the emergence of newspecies


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Natural selection

1. There is variation in traits.

For example, some beetles are green

and some are brown.

2. There is differential reproduction.

Since the environment can't support

unlimited population growth, not all

individuals reproduce maximally

green beetles tend to get eaten by

birds and have less chances to

reproduce than brown beetles


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Natural selection

3. There is heredity. The surviving brown beetles have

brown baby beetles because this trait

has a genetic basis.

4. End result:

The more advantageous trait, brown

coloration, becomes more common

in the population. If this process continues, eventually,

all individuals in the population will

be brown.


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Fitness

Ability of a particular species to leave offspring in thenext generation compared to other species/variants

e.g., if brown beetles consistently leave more offspring

than green beetles because of their color, the brown

beetles has a higher fitness.


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A species' fitness depends on the environment inwhich the organism lives e.g., the fittest variant of a species during an ice age is

probably not the fittest variant once the ice age is over.

The fittest individual is not necessarily the strongest,fastest, or biggest.

A species' fitness includes its ability to survive, find amate, produce offspring and ultimately leave itsgenes in the next generation

Fitness


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Sexual selection

When natural selection acts on mate-finding and reproductivebehavior

acts on an organism's ability to obtain or successfully

copulate with a mate

often powerful enough to produce features that are harmfulto the individual's survival


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1. Male competition

Males compete for access to females, the amountof time spent mating with females, and evenwhose sperm gets to fertilize her eggs

e.g., male damselflies scrub rival sperm out of thefemale reproductive tract when mating.

2. Female choice

Females choose which males to mate with, howlong to mate, and even whose sperm will fertilizeher eggs.

e.g., some females can eject sperm from an

undesirable mate.

Sexual selection


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no offspring means no genes in the next generation all those genes for living to a ripe old age don't get passed on

to anyone!

That individual's fitness is zero.

Why sexual selection?


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Adaptation

feature that provides some improved

function in an organism

can take many forms; e.g.,

1. behavior that allows better evasion of predators

2. a protein that functions better at body

temperature

3. an anatomical feature that allows the organismto access a valuable new resource


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Katydids have camouflage to look like leaves

Non-poisonous king snakes mimic poisonous

coral snakes

Orchids fool wasps into "mating" with them


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The creosote bush is a desert-dwelling

plant that produces toxins that preventother plants from growing nearby, thus

reducing competition for nutrients and

water

Echolocation in bats is an

adaptation for catching insects


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1. Directional

Only one side of the distribution reproduce Population looks different over time

2. Stabilizing Favors individuals with an average geneticmakeup

Only the middle reproduce; population looks moresimilar over time

3. Disruptive (Diversifying)

Environmental conditions favor individuals at both ends ofthe genetic spectrum

Population split into two groups

3 types of natural selection


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3 types of natural selection


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Industrial revolution

Pollution darkened

tree trunks

Camouflage of moths

increases survival from

predators

Directional selection

caused a shift away from

light-gray towards dark-

gray moths

The case of the

peppered moths


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What are selection pressures in an

organisms environment?

Examples ofbiotic

factors: predators

competitors

mutualists

Examples ofabiotic

factors: resource availability

physical conditions

chemical conditions


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Antibiotic resistance: modern example of

natural selection

Widespread use and misuse of antibiotics

has resulted in increased microbial

resistance to antibiotics Can be considered an evolutionary arms

race:

1. bacteria continue to develop strains that are lesssusceptible to antibiotics

2. medical researchers continue to develop new

antibiotics that can kill them


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How antibiotic resistance is

enhanced by natural selection.

A. population of bacteria

before exposure to an

antibiotic

B. population directly after

exposure, the phase in which

selection took place

C. the distribution of resistance

in a new generation ofbacteria.

A

C

B
http://en.wikipedia.org/wiki/Image:Antibiotic_resistance.svg


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Evidence by example:

Artificial selection

People havedomesticated plants andanimals for thousands ofyears

These activities haveamounted to large, long-term, practicalexperiments that

demonstrate that speciescan change dramaticallythrough selectivebreeding


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Artificial selection

These common vegetables were cultivated from forms of wild mustard. This is

evolution through artificial selection.


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Experiments show that populations can evolve

Female guppies prefercolorful males

Predatory fish also"prefer" colorful males Easier to spot

Fewer predators = morecolorful males

More predators = lesscolorful males

predatory fish inregions with brightlycolored males = lesscolorful males

Mi ti b t t l


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a process, rather than aguiding hand.

mindless and

mechanistichas no goals

not striving to produce"progress" or a balanced

ecosystem

Misconceptions about natural

selection

Evolution does not work this way!


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Natural selection just selects among whatever

variations exist in the population.

The result is evolution.

However, process is not random:Genetic variants that aid survival and reproduction

are much more likely to become common than variants

that don't.


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Microevolution

Evolution on a small scale within a single population A change in gene frequency over time means that the

population has evolved.


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These happy face spiders lookdifferent, but since they can

interbreed, they are considered the same species: Theridion

grallator.

SPECIES - a population or group of populations whose

members have the potential to interbreed with one anotherand produce viable offspring, but who cannot produce

viable offspring with other species.


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Speciation


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Reintroduction

will no longerresult in

interbreeding


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Causes of speciation

1. Geographic isolation

rivers change course, mountains rise, continents drift,organisms migrate, and what was once a continuous

population is divided into two or more smaller

populations.


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Spotted owl subspecies living in different geographic locations

show some genetic and morphological differences. This

observation is consistent with the idea that new species form

through geographic isolation


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2. Reduction of gene flow

Imagine a situation in which a population extends over a broadgeographic range. Individuals in the far west would have zero

chance of mating with individuals in the far eastern end of the range

reduced gene flow, but not total isolation

may or may not be sufficient to cause speciation.

R d i i l i


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Reproductive isolation

R d ti B i


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Habitat isolation

Geographic isolation

Reproductive Barriers

North America

Central America

Madagascar


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Pre zygotic barriers to gene flow that

contribute to speciation

Evolution of different mating location, mating time,

or mating rituals


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Lack of "fit" between sexual organs

Temporal isolation2 species breed at different times of the day or during

different seasons.

These damselfly penises illustrate just how complex insect genitalia may be.

Post zygotic barriers that


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Post zygotic barriers that

contribute to speciation

Reduced hybrid viability Aborted development of hybrid at some embryonic

stage

Reduced hybrid fertility Meiosis doesnt produce fertile gametes in vigorous

hybrids. e.g., donkey + horse = mule (sterile hybrid)

Hybrid breakdown 1st - generation hybrids are fertile, but they cannot

produce fertile offspring in the next generation e.g., different species of cotton


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x =

sterile

Reproductive barriers


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Individualsof differentspecies

Mating

attempt

Habitatisolation

Temporalisolation

Behavioralisolation

Mechanicalisolation

HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION




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Viable

fertile

offspring

Reducehybrid

viability

Reducehybrid

fertility

Hybridbreakdown

Fertilization

Gameticisolation

GAMETIC ISOLATION REDUCED HYBRIDVIABILITY

REDUCED HYBRID FERTILITY HYBRID BREAKDOWN



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Speciation - a lineage-splitting event that produces 2

or more separate species

M d f


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Modes of

speciation

1. Allopatric speciation

speciation that takes place

in populations with

geographically separateranges

Gene flow is interrupted

and new species evolve


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Allopatric speciation of squirrels in the Grand Canyon.

Animals like birds do not show speciation like those animals

that are barred from breeding by the canyon.


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2. Sympatric

speciation

speciation that takesplace ingeographicallyoverlappingpopulations

Chromosomalchanges andnonrandom matingreduce gene flow

Sympatric speciation


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y p pDivergence occurs despite lack of geographic isolation

I i i ti


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Issues in speciation

1. Opportunity knocks

Factors in the environment that might encourage

diversification:

The environment may have offered opportunities forspecialization.

A fragmented environment might make reproductive

isolation likely.

The environment may have provided a release fromcompetition with other insects.


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Being in the right place at the right time is a reason

that one clade might be more diverse than another.


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2. Adaptive Radiation

an event in which a lineage rapidly diversifies, with the

newly formed lineages evolving different adaptations

Adaptive radiation


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Adaptive radiation

One species gives rise to many others in a geologically short

period of time

e.g., Darwins finches

Islands are good

places to find

adaptive radiations

3. Historical changes in diversity:


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3. Historical changes in diversity:

a. Explosion

e.g., about 530 million years ago, a huge variety of marine

animals suddenly burst onto the evolutionary scene- New animals had a variety of new body forms that evolution

has been using to produce "spin-offs" ever since


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Cambrian

explosion

b Extinction


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b. Extinction

About 225 million years ago, >90% of the species alive at thetime went extinct in fewer than 10 million years

Some groups that were dominant before the extinction neverrecovered

Set stage for massive diversification of taxa that filled the emptyniches.
http://www.dodoisland.com/


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Dodos extinction

thought to be due to:

Inability to fly

Inability to adapt topredators

Inability to adapt to

changing climates

The Dodo an extinct bird

Factors affecting extinction rates
http://www.dodoisland.com/


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Factors affecting extinction rates

Natural Extinctions

Climate change Cataclysmic event (volcano, earthquake)

Human Activities

Habitat Loss/Fragmentation Introduction of exotic/invasive species

Pollution

Commercial harvesting

Accidental killing (tuna nets)

Harassing Pet Trade

Urbanization

Damming/Flooding

Agricultural conversion


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Human activities and extinction

Earth took millions of years to recover from previousmass extinctions

Humans major force in the premature extinction ofspecies.

Extinction rate increased by 100-1000x the naturalbackground rate

As population grows, we are expected to take over more ofthe earths surface

may cause the premature extinction of up to a 1/4 of the

earths current species and constitute a 6th massextinction

Speciation Extinction = Biodiversity!

lecture 2 - natural selection

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