Evolution Ch 22-26 AP Biology 2012 Life’s Natural History is a record of Successions & Extinctions Quaternary Tertiary Cretaceous Jurassic Triassic Permian.
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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
aero
bic
Bac
teri
a
Inse
cts
Rep
tile
s
Din
os
aurs
Mam
ma
ls
Bir
ds
Lan
d P
lan
ts
See
d P
lan
ts
Pla
nts
Art
hro
po
ds
Ch
ord
ates
Jaw
les
s F
ish
Tel
eo
st F
ish
Am
ph
ibia
ns
Ph
oto
syn
thet
ic B
acte
ria
Gre
en A
lga
e
Mu
ltic
ellu
lar
An
ima
ls
Mo
llu
scs
1.5
4500
700
63
135
180
225
280
350
400
430
500
570
Flo
wer
ing
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?
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