EvolutionCh 22-26

AP Biology 2012

Life’s Natural History is a record of Successions & Extinctions

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Ediacaran

Precambrian,

Proterozoic,

&

Archarozoic

An

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Inse

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Rep

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Din

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aurs

Mam

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Bir

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Lan

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lan

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See

d P

lan

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Pla

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Art

hro

po

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Ch

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Jaw

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Tel

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Am

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Ph

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Gre

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Mu

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An

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Mo

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1.5

4500

700

63

135

180

225

280

350

400

430

500

570

Flo

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mya

LaMarck Organisms adapted to

their environments by acquiring traits change in their life timechange in their life time

DisuseDisuseorganisms lost parts because they did not use them — organisms lost parts because they did not use them — like the missing eyes & digestive system of the tapewormlike the missing eyes & digestive system of the tapeworm

Perfection with Use & NeedPerfection with Use & Needthe constant use of an organ leads that organ to increase the constant use of an organ leads that organ to increase in size — like the muscles of a blacksmith or the large in size — like the muscles of a blacksmith or the large ears of a night-flying batears of a night-flying bat

transmit acquired characteristics to next generation

Charles Darwin 1809-1882 British naturalist Proposed the idea of

evolution by natural selection

Collected data on voyage of Beagle, plus compared to more than 500 other studies & experiments

Robert FitzroyRobert Fitzroy

Voyage of the HMS Beagle Invited to travel around the world

1831-1836 (22 years old!) makes many observations of nature

main mission of the Beagle was to chart South American coastline

Voyage of the HMS Beagle Stopped in Galapagos Islands

500 miles off coast of Ecuador

Armadillos are native to the Armadillos are native to the Americas, with most species Americas, with most species found in South America.found in South America.

Glyptodont fossils are also Glyptodont fossils are also unique to South America.unique to South America.

Succession of types

Why should extinct armadillo-like species& living armadillos be

found on the samecontinent?

Mylodon Mylodon (left)(left) Giant Giant ground sloth ground sloth (extinct)(extinct)

“This wonderful relationship in the same continent between

the dead and the living will…throw more light on the appearance of organic beings on our earth,

and their disappearance from it, than any other class of facts.”

Unique species

So Many Questions! Why…

…does a 2 mile forested island tend to have more unique species than a 2 mile inland forest?

…are fish on the east coast of Panama more closely related to Bahamian fish, than to fish on the west coast of Panama?

…do desert animals on different continents tend to have similar body features, yet are more closely related to nearby non-desert animals than to each other?

…do fossil species closer to the surface tend to be more similar to current species than deeper fossils?

…do species tend to have features that match their environment?

Yet all species have ill-fitting body structures, none are “perfect?”

Evolution Biological evolution is the change in a

population’s genetic makeup over generations. Occurs by more than one mechanism The first (accurate) proposed

mechanism was Darwinian natural selection

Natural Selection When several prerequisites or conditions are in place:

Variation Inheritance Competition

…the inevitable result is differential reproductive success = some segments of the population have more offspring on average than others.

Variation Variation - Simply put, everyone isn’t identical!

Specifically, this is referring to phenotypic variation - variation in expressed traits.

In a given environment at a given time, a variation may be helpful to the individual that bears it, harmful, or neither.

If the environment changes, the relative helpfulness of the variation may as well.

Inheritance Inheritance means that the variation/s in question must

have a genotypic basis. Genetics are the source of variation Genetic processes can increase the amount of variation in a

population: Mutation (#1 source of variation) Recombination (crossing-over, independent assortment of

chromosomes in meiosis) Sexual reproduction (fertilization in eukaryotes, gene transfer in

prokaryotes)

Inheritance Which variations are present in a population = random

Because the genetic processes that produce variation are random

“Random, rare, and regular” Not all variations are possible due to genetic/historical

constraints Variations are not directed by the environment, t.e. they DO

NOT ARISE IN RESPONSE TO “NEED” - common mistake!

Competition Resources are limited. All organisms

reproduce as much as possible. Competition is the inevitable result. Competition can be interspecific

(between different populations/species) or intraspecific (between members of the same population). Try to think intraspecific - it’s most

common, most relevant to N.S., and most overlooked

Differential Reproductive Success So everyone’s different…

…because there are so many differences, some of them are probabilistically bound to happen to be helpful or harmful…

…you pass your variations on to your kids… …and not everyone can have the max number of kids that all

survive.

Inevitable result: Someone’s going to have more kids, because they happened to have a variation that gave them an advantage. (D.R.S.)

And because those kids also get that advantage, and now out-represent other kids in the next generation, that variation becomes increasingly common. (N.S.)

Interdisciplinary! The development of this concept was

revolutionary… not just in Biology, but it gave rise to whole other fields as well! Political science Economics Computer science and engineering

(genetic algorithms!)

Random?

So, is natural selection random? Nope. Placing a non-random rule

(which variation is best suited to the present environment) on a random source (mutation, recombination) mathematically produces a non-random result.

Card game example

Discussion: How NOT To Explain Evolution by N.S. The problem: “A population of giraffes lives on the Serengeti

in East Africa. The best leaves that provide the most nutrition are at the tops of the trees. In one generation, the average giraffe’s neck is 1.4 meters long. Fifty generations later, the average giraffe’s neck is 1.55 meters long. How did this happen?”

Explain why each of the following explanations is wrong, wrong, wrong!

Discussion #1: “The more the giraffe stretches its neck to

get to the tops of the trees, the longer its neck becomes. It passes this longer neck on to its babies. So over time, necks get longer and longer.”

Discussion #2: “A longer neck is dominant, so over time,

the dominant longer neck allele becomes more and more common.”

Discussion #3: “Longer necks are necessary for the

species to endure, because without them, giraffes are at risk of starvation. If the giraffe species doesn’t get a longer neck, then it will eventually go extinct.”

Discussion #4: “Giraffes adapted themselves to their

environment. In their case, because of the tree height, this meant evolving longer necks.”

Evolutionary ForcesWhat changes populations?

Forces of evolutionary change Natural selection

traits that improve survival and/or reproduction will accumulate in the population adaptive change

Genetic drift frequency of traits can change

in a population due to chance events random change

Natural Selection Anything that can reduce reproductive success is termed a

selective force or selective pressure. But there is no actual “selection” in the typical English

language sense of the word “Natural selection” - actually kind of a bad term, Darwin later

wished he’d picked another one! He used this one because his first chapter was on what was

already known as “artificial selection” and he was setting up a linguistic contrast

Natural Selection Selection acts on any trait that affects

survival or reproduction predation selection physiological selection sexual selection

Predation Selection Predation selection

act on both predator & prey behaviors camouflage & mimicry speed defenses (physical & chemical)

Physiological Selection Acting on body functions

disease resistance physiology efficiency (using oxygen, food, water) biochemical versatility protection from injury

HOT STUFF!Some fish had the

variation of producinganti-freeze protein

5.5 mya5.5 myaThe Antarctic Ocean The Antarctic Ocean freezes overfreezes over

Effects of Selection Changes in the average trait of a population

DIRECTIONALSELECTION

STABILIZINGSELECTION

DISRUPTIVESELECTION

giraffe neckhorse size human birth weight rock pocket mice

Discussion It is a fairly common misconception

(bolstered by the narration in countless nature documentaries!) that natural selection fashions organisms that are “perfect,” but this is not the case. Why not?

Imperfection Selection can only act on existing variations.

Which variations are present = chance Evolution is limited by historical constraints.

And all organisms bear the scars of their evolutionary history - more on these in a future week

Adaptations are often compromises. The environment is in constant flux, and natural

selection is a change mechanism, not something that can predict the future and plan for it!

Not to mention… what is a “perfect” organism, anyways?

Evolution is not goal-orientedAn evolutionary trend does not mean that evolution is goal-oriented.

Evolution can’t “think ahead!”

Sexual Selection Natural selection purely acting on reproductive success, no

involvement of survival attractiveness to potential mate fertility of gametes successful rearing of offspring

Sexual selection

The lion’s mane…

Females are attracted to males with larger, dark manes

Correlation with higher testosterone levels

better nutrition & health more muscle & aggression better sperm count / fertility longer life

But imposes a cost to male HOT! Is it worth it??

Sexual selection Acts in all sexually

reproducing species the traits that get you mates

sexual dimorphism influences both morphology & behavior can act in opposition to traditional

natural selection

Discussion Consider these facts:

The difference between female and male gametes is that female gametes are whole cells, male gametes contain only genetic material

Sexual selection tends to drive females towards choice behaviors, and males towards display behaviors

What’s going on? In which kinds of species/environments would you expect

to see choosy males and displaying females?

Coevolution Two or more species reciprocally

affect each other’s evolution predator-prey

disease & host competitive species mutualism

pollinators & flowers

Discussion Coevolution is sometimes compared to a passage

from Lewis Carroll’s Through The Looking Glass, in which Alice and the Red Queen are trying to run across a giant chessboard. But because of the incredible properties of the world through the looking glass, running as fast as they can only leaves them in the same spot.

Why/how do scientists compare this to the relationship between pathogen and host, or predator and poisonous prey?

Genetic Drift Chance events changing frequency of

traits in a population not adaptation to environmental conditions

not selection founder effect

small group splinters off & starts a new colony bottleneck

some factor (disaster) reduces population to small number & then population recovers & expands again but from a limited gene pool

Founder effect When a new population is started

by only a small group of individuals just by chance some rare alleles may

be at high frequency; others may be missing

skew the gene pool of new population human populations that

started from small group of colonists

example: colonization of New World

Distribution of blood types Distribution of the O type blood allele in native

populations of the world reflects original settlement

Distribution of blood types Distribution of the B type blood allele in native populations of the

world reflects original migration

Out of AfricaLikely migration paths of humans out of AfricaLikely migration paths of humans out of Africa

Many patterns of human traits reflect this migrationMany patterns of human traits reflect this migration

50,000ya

10-20,000ya

10-20,000ya

Bottleneck effect When large population is drastically

reduced by a disaster famine, natural disaster, loss of habitat… loss of variation by chance event

alleles lost from gene pool not due to fitness

narrows the gene pool

Cheetahs All cheetahs share a small number of alleles

less than 1% intraspecies

diversity as if all cheetahs are

identical twins

2 bottlenecks 10,000 years ago

Ice Age last 100 years

poaching & loss of habitat

MeasuringEvolution of Populations

Populations & gene pools Concepts

a population is a localized group of interbreeding individuals. By definition, all members = same species.

gene pool is the collection of alleles in the population remember difference between alleles & genes!

allele frequency is how common is that allele in the population how many A vs. a in whole population

Evolution of populations Evolution = change in allele frequencies

in a population hypothetical: what conditions would

cause allele frequencies to not change? non-evolving population

REMOVE all agents of evolutionary change§ very large population size (no genetic drift)§ no migration (no gene flow in or out)§ no mutation (no genetic change)§ random mating (no sexual selection)§ no natural selection (everyone is equally fit)

Hardy-Weinberg equilibrium Hypothetical, non-evolving population

preserves allele frequencies

Serves as a model (null hypothesis) natural populations rarely in H-W equilibrium useful model to measure if forces are acting on a

population measuring evolutionary change

W. Weinbergphysician

G.H. Hardymathematician

Hardy-Weinberg theorem Allele frequencies

assume 2 alleles = B, b frequency of dominant allele (B) = p frequency of recessive allele (b) = q

frequencies must add to 1 (100%), so:

p + q = 1

bbBbBB

Hardy-Weinberg theorem Counting Individuals

frequency of homozygous dominant: p x p = p2 frequency of homozygous recessive: q x q = q2 frequency of heterozygotes: (p x q) + (q x p) = 2pq

frequencies of all individuals must add to 1 (100%), so:

p2 + 2pq + q2 = 1

bbBbBB

H-W formulas Alleles: p + q = 1

Individuals: p2 + 2pq + q2 = 1

bbBbBB

BB

B b

Bb bb

What are the genotype frequencies?What are the genotype frequencies?

Discussion

q2 (bb): 16/100 = .16

q (b): √.16 = 0.40.4

p (B): 1 - 0.4 = 0.60.6

q2 (bb): 16/100 = .16

q (b): √.16 = 0.40.4

p (B): 1 - 0.4 = 0.60.6

population: 100 cats84 black, 16 whiteHow many of each genotype?

population: 100 cats84 black, 16 whiteHow many of each genotype?

bbBbBB

p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16

Must assume population is in H-W equilibrium!Must assume population is in H-W equilibrium!

Using Hardy-Weinberg equation

bbBbBB

p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16

Assuming H-W equilibriumAssuming H-W equilibrium

Sampled data Sampled data bbBbBB

p2=.74p2=.74 2pq=.102pq=.10 q2=.16q2=.16

How do you explain the data? How do you explain the data?

p2=.20p2=.20 2pq=.642pq=.64 q2=.16q2=.16

How do you explain the data? How do you explain the data?

Null hypothesis Null hypothesis

Discussion

Populations can be in Hardy-Weinberg equilibrium at one locus (gene), but out of equilibrium at another.

What could cause that to be the case?

Discussion Practice problems, everybody’s

favorite!

Discussion An example AP-style question: “PKU is a rare autosomal recessive disorder. Babies

born with PKU do not make the liver enzyme necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine. 1 in every 10,000 babies (a frequency of 0.0001) is born with the disease, and when they grow older, should avoid some foods.

Assume that the mutation rate in the PKU gene is so low as to be negligible, and that mates are not chosen on the basis of the manufacture of this liver enzyme. Further assume that diet modifications are available to all humans at no particular cost, rendering selective force on this locus negligible.

What percent of the population would you expect to be carriers of the PKU disease allele?”

Application of H-W principle Sickle cell anemia

inherit a mutation in gene coding for hemoglobin oxygen-carrying blood protein recessive allele = HsHs

normal allele = Hb

low oxygen levels causes RBC to sickle breakdown of RBC clogging small blood vessels damage to organs

often lethal

Sickle cell frequency High frequency of heterozygotes

1 in 5 in Central Africans = HbHs

unusual for allele with severe detrimental effects in homozygotes 1 in 100 = HsHs

usually die before reproductive age

Why is the Hs allele maintained at such high levels in African populations?Why is the Hs allele maintained at such high levels in African populations?

Suggests some selective advantage of being heterozygous…Suggests some selective advantage of being heterozygous…

Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells

Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells

1

2

3

Heterozygote Advantage In tropical Africa, where malaria is common:

homozygous dominant die or reduced reproduction from malaria: HbHb

homozygous recessive die or reduced reproduction from sickle cell anemia: HsHs

heterozygote carriers are relatively free of both: HbHs

survive & reproduce more, more common in population

Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.

Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. Frequency of sickle cell allele &

distribution of malaria

Discussion How does a new species arise?

First…what is a species? Biological species concept

defined by Ernst Mayr population whose members can interbreed & produce

viable, fertile offspring reproductively compatible

Western MeadowlarkEastern Meadowlark

Distinct species:songs & behaviors are different enough to prevent interbreeding

Distinct species:songs & behaviors are different enough to prevent interbreeding

But that doesn’t capture every situation

Consider Ensatina salamanders. How many species? Which ones are different species?

Species Definitions Other definitions include:

Morphological or typological - They conform to the same body plan.

Phylogenetic or evolutionary - Share a common ancestor and a unique evolutionary history.

Ecological - Share a specific niche, unique to them and them alone.

“Species” is a human language box. Never forget that nature exists on a continuum!

Discussion Which definitions work or don’t work to

determine whether or not you’re examining different species if you’re studying… Bacteria in a lab petri dish? Hooved mammals in the modern-day

arctic? Dinosaurs? Ancient algae?

How and why do new species originate? Species are created by a series of evolutionary

processes populations become isolated - no gene flow between

them geographically isolated and/or reproductively isolated

isolated populations evolve independently

Isolation allopatric

geographic separation sympatric

still live in same area

An obstacle to mating or fertilization

PRE-zygotic barriers

behavioral isolation

geographic isolation ecological isolation temporal isolation

mechanical isolation gametic isolation

Geographic isolation Species occur in different areas

physical barrier allopatric speciation

“other country”

Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel

Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel

Ammospermophilus spp

Ecological isolation Species occur in same region, but occupy different

habitats so rarely encounter each other reproductively isolated

2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial

2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial

lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest

lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest

Temporal isolation Species that breed during different times of day,

different seasons, or different years cannot mix gametes reproductive isolation sympatric speciation

“same country”

Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

Behavioral isolation Unique behavioral patterns & rituals isolate species

identifies members of species attract mates of same species �

courtship rituals, mating calls reproductive isolation

Blue footed boobies mate only after a courtship display unique to their species

Blue footed boobies mate only after a courtship display unique to their species

sympatric speciation?

firefly courtship displaysfirefly courtship displays

courtship display of Gray-Crowned Cranes, Kenya

courtship display of Gray-Crowned Cranes, Kenya

courtship songs of sympatricspecies of lacewings

courtship songs of sympatricspecies of lacewings

Recognizing your own species

Mechanical isolation Morphological differences can prevent successful

mating reproductive isolation

Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.

Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.

Plants

sympatric speciation?

Mechanical isolation For many insects, male &

female sex organs of closely related species do not fit together, preventing sperm transfer

lack of “fit” between sexual organs: hard to imagine for us… but a big issue for insects with different shaped genitals!

Damsel fly penises

Animals

Gametic isolation Sperm of one species may not be able to fertilize eggs of

another species mechanisms

biochemical barrier so sperm cannot penetrate egg receptor recognition: lock & key between egg & sperm

chemical incompatibility sperm cannot survive in female reproductive tract

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

sympatric speciation?

POST-zygotic barriers Prevent hybrid offspring from

developing into a viable, fertile adult reduced hybrid viability reduced hybrid fertility hybrid breakdown

zebroid

Reduced hybrid viability Genes of different parent species may

interact & impair the hybrid’s development

Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.

Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.

sympatric speciation?

Mules are vigorous, but sterile

Reduced hybrid fertility Even if hybrids are vigorous

they may be sterile chromosomes of parents may differ in number

or structure & meiosis in hybrids may fail to produce normal gametes

Donkeys have 62 chromosomes(31 pairs)

Horses have 64 chromosomes(32 pairs) Mules have 63 chromosomes!

Hybrid breakdown Hybrids may be fertile & viable in first

generation, but when they mate offspring are feeble or sterile

In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile.On path to separate species.

In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile.On path to separate species.

sympatric speciation?

Niles EldredgeCurator

American Museum of Natural History

Rate of Speciation When considering speciation events over

geological time: Does speciation happen gradually or rapidly, uniformly or unevenly? Gradualism

Charles Darwin Charles Lyell

Punctuated equilibrium Stephen Jay Gould Niles Eldredge

Gradualism Gradual divergence over

long spans of time assume that big changes

occur as the accumulation of many small ones

events can increase or decrease speciations worldwide, but overall speciation proceeds fairly regularly

Punctuated Equilibrium Rate of speciation is not

constant Organisms are in

“stasis” for much of their history, with little or no change

When speciation occurs, it tends to be in a rapid burst

Species undergo rapid change when they 1st bud from parent population

Time

Discussion Based upon what you know of

evolutionary history, where do you fall in the debate?

Speciation Rates Regardless of whether punctuated

equilibrium or gradualism holds, speciation rates vary by species and circumstance Speciation can occur over a scale of

millions of years, or much more rapidly! Polyploidy in plants increases

speciation rate to, in some cases, only a few years

Polyploidy and Hybrid Speciation Unlike in animals, in plants, duplicating

the genome (polyploidy) isn’t fatal. The pollination reproduction method

means plants hybridize more often and more readily than animals on average

Sometimes, a diploid hybrid is sterile, but a triploid or tetraploid

hybrid isn’t.So if two plants hybridize, and the

sterile offspring undergoes polyploidy, it can produce fertile,

speciated offspring!

Polyploidy and Hybrid Speciation This has been observed in species like

the Evening Primrose, Raphanobrassica, Hemp Nettle, and the Maidenhair Fern.

Speciation Rates In all species, when a new habitat or

new niche becomes available, speciation rates tend to increase

Adaptive radiation - ecological & phenotypic diversity in a rapidly multiplying lineage

Discussion Scientists generally break it down into

two main reasons why this causes a burst in speciation events. What do you think they could be?

Adaptive Radiation Innovation - The evolution of one trait can make the

evolution of others possible Ex: A harder shell on eggs not only made it possible to

lay them out of water, it opened up many new avenues for expansion, which drove further diversification

Opportunity - Unoccupied niches, such as on new land or after a mass extinction, which new occupiers adapt to

Speciation Rates Ex: Darwin’s finches

Ex: An explosion in bivalve species diversity after the loss of brachiopods in the “Great Dying,” or Permian extinction 250 mya

Extinction But, of course, extinction rates also

fluctuate Higher in times of environmental stress

Million years ago

% of familiesextinct

Discussion A population’s ability to respond to

environmental changes is dictated, in part, by its level of genetic diversity.

Which do you think is most resistant to extinction and why: high-diversity or low-diversity?