15-3 evolution
TRANSCRIPT
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15-3Shaping Evolutionary Theory
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Hardy Weinberg Principle
• The Hardy-Weinberg principle is like aPunnett Square for populations, instead of individuals. A Punnett square can predict
the probability of offspring's genotypebased on parents' genotype or theoffspring's' genotype can be used to revealthe parents' genotype. Likewise, the
Hardy-Weinberg principle can be used tocalculate the frequency of particular allelesin a population
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Hardy Weinberg Principle
• The dominant allele is denoted A and the
recessive a. Their frequencies are p and q;
freq(A)= p and freq(a)=q
• The final three possible genotypic
frequencies in the offspring become:
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Hardy Weinberg Principle
•
• Find: Frequencies of A and a.and the genotypic frequencies
of AA, Aa and aa. • Solution:
• f( A) = 12/30 = 0.4 = 40%
• f(a) = 18/30 = 0.6 = 60%
• Then, p + q = 0.4 + 0.6 = 1
• and p2 + 2 pq + q2 = AA + Aa +aa
• = .16 + .48 + .36 = 1
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Assumptions For Genetic
Equilibrium
• No gene mutations
• Large population size
• Limited-to-no immigration or emigration• Gene of interest has no effect on survival
or reproduction
• Mating is random• (see table 15.3 on page 432 for more info)
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• Evolution involves changes in the gene
pool. A population in Hardy-Weinberg
equilibrium shows no change. What the
law tells us is that populations are able to
maintain a reservoir of variability so that
if future conditions require it, the gene
pool can change. If recessive alleleswere continually tending to disappear,
the population would soon become
homozygous. Under Hardy-Weinbergconditions, genes that have no present
selective value will nonetheless be
retained.
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Genetic Drift
• In each generation, someindividuals may, just bychance, leave behind afew more descendents(and genes, of course!)
than other individuals.The genes of the nextgeneration will be thegenes of the ―lucky‖individuals, not
necessarily the healthier or ―better‖ individuals.That, in a nutshell, isgenetic drift. It happens to
ALL populations—there’sno avoiding chance.
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Founder Effect
• Founder effects A founder effect occurs when a newcolony is started by a few members of the
original population. This small populationsize means that the colony may have:
• reduced genetic variation from the originalpopulation.
• a non-random sample of the genes in theoriginal population.
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Founder Effect
• Founder effects . For example, the Afrikaner population of Dutch settlers in South Africa is
descended mainly from a few colonists.Today, the Afrikaner population has anunusually high frequency of the gene thatcauses Huntington’s disease, because
those original Dutch colonists justhappened to carry that gene withunusually high frequency.
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BottleneckAn example of a bottleneck:
Northern elephant seals havereduced genetic variation
because of a population
bottleneck . Hunting reduced
their population size to as few as
20 individuals at the end of the19th century. Their population
has since rebounded to over
30,000—but their genes still
carry the marks of this
bottleneck: they have much less
genetic variation than a
population of southern elephant
seals that was not so intensely
hunted.
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Gene Flow
• Within a population: It can introduce or reintroduce genes to apopulation, increasing the genetic variation of that population.
• Across populations: By moving genes around, it can make distantpopulations genetically similar to one another.The less gene flow between two populations, themore likely that two populations will evolve intotwo species. (like the finches)
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Nonrandom Mating• Proximity usually has a great
deal to do with mate selection – this promotes inbreedingand can lead to a change inallelic proportions.
• .
• Random mating is unlikely tooccur for a variety of reasons.One is that it is simply easier to mate with a nearbyindividual, as opposed to one
that is farther away. Also,especially in animals,individuals compete for matesand active selection of matingpartners occurs. This goesdirectly against the concept of randomness.
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Mutation
• Mutations are the raw
materials of evolution.
• Evolution absolutely
depends on mutationsbecause this is the
only way that new
alleles are created.
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Mutation
• MUTATIONSLack of mutations in apopulation limits genetic change
• The frequency of all allelesremains the same.
• Once a mutation occurs, theallele frequency is changed.
• Mutations add to the geneticvariability of populations over time and are thus the ultimatesource of variation for evolution.
• Mutations increase theopportunity for evolution of adaptations different fromcharacteristics of the ancestralpopulation.
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NATURAL SELECTIONThree kinds of selection cause
changes in the normaldistribution of phenotypes in a
population. Stabilizing
selection eliminates those
phenotypes most different
from the norm, thus reducing
the frequency of phenotypic
extremes. Directional
selection eliminates one
extreme and moves thepopulation toward the other.
Disruptive selection
eliminates average
phenotypes and encouragesthe extremes.
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Natural Selection – Stabilizing
Selection• See the diagram on page 434
• Stabilizing selection favors the norm, the common,average traits in a population. Look at the SiberianHusky, a dog bred for working in the snow. The SiberianHusky is a medium dog, males weighing 16-27kg (35-60lbs). These dogs have strong pectoral and legmuscles, allowing it to move through dense snow. TheSiberian Husky is well designed for working in the snow.If the Siberian Husky had heavier muscles, it would sinkdeeper into the snow, so they would move slower or
would sink and get stuck in the snow. Yet if the SiberianHusky had lighter muscles, it would not be strongenough to pull sleds and equipment, so the dog wouldhave little value as a working dog. So stabilizingselection has chosen a norm for the the size of theSiberian Husky.
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Natural Selection – Directional
Selection
• See the diagram on page 434• Directional selection favors those individuals who
have extreme variations in traits within a population. Auseful example can be found in the breeding of the
greyhound dog. Early breeders were interested in dogwith the greatest speed. They carefully selected from agroup of hounds those who ran the fastest. From their offspring, the greyhound breeders again selected thosedogs who ran the fastest. By continuing this selection for those dogs who ran faster than most of the hound dogpopulation, they gradually produced a dog who could runup to 40mph.
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Natural Selection – Disruptive
Selection
• See the diagram on page 434• Disruptive selection, like directional selection,
favours the extremes traits in a population.Disruptive selection differs in that suddenchanges in the environment creates a suddenforces favouring that extreme. Imagine a snakewhich has a brown and grey colouration andlives on the rocks and in grasslands. Snakes
which are mostly grey hide well in the rocks,snakes which are mostly brown hide well in thegrass but the intermediate snakes (with brownand grey) get noticed in both environments andeaten by predators.
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Sexual Selection
• Sexual selection acts on
an organism's ability to
obtain a mate. Selection
makes many organisms
go to extreme lengths for sex: peacocks maintain
elaborate tails, elephant
seals fight over territories,
fruit flies perform dances,and some species deliver
gifts.
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Sexual Selection• In one kind of sexual
selection, members of onesex create a reproductivedifferential amongthemselves by competing for opportunities to mate. Thewinners out-reproduce the
others, and natural selectionoccurs if the characteristicsthat determine winning are, atleast in part, inherited. In theother kind of sexual selection,
members of one sex create areproductive differential in theother sex by preferring someindividuals as mates.
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Sexual Selection• In one kind of sexual
selection, members of onesex create a reproductivedifferential amongthemselves by competing for opportunities to mate. Thewinners out-reproduce the
others, and natural selectionoccurs if the characteristicsthat determine winning are, atleast in part, inherited. In theother kind of sexual selection,
members of one sex create areproductive differential in theother sex by preferring someindividuals as mates.
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Sexual Selection
• Another type of sexualselection involves theevolution of secondary sexualcharacteristics which
determine the relative"attractiveness" of membersof one sex to the other sex. Such items as courtshipdisplays and male plumage inbirds (e. g., the male
peacock) are obviousexamples
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Reproductive Isolation- Prezygotic
Isolation• Prezygotic Isolation
• Geographic isolation: physical barriers (rivers, oceans,mountains) prevent mixing of populations
• Ecological isolation: species occur in the same are but inhabitdifferent habitats so they don’t encounter each other twospecies whose ranges overlap live in different habitats. As aresult, potential mates from the two species do not encounter one another. During the breeding season in eastern NorthAmerica, five species of small birds known as flycatchers arefound in different habitats in the same area. One speciesprefers open woods and farmland; one frequents beech trees;one is found in alders, one in conifer woods, and one in willowy
thickets.
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Temporal isolation
• Temporal isolation is aprezygotic barrier inwhich the two speciesreproduce at different
times of the day, season,or year. Wood andleopard frogs are anexample of two similar species whose ranges
overlap.• Temporal isolation:
species reproduce indifferent seasons or atdifferent times of the
day
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Prezygotic Isolation
• Behavioral isolation: species differ in their
mating rituals (e.g. differing bird songs,
mating colors, dances, pheromones)
reproduction between similar species isprevented because each group possesses its
own characteristic courtship behaviors.
Wood and leopard frogs exhibit behavioral
isolation because the males of each specieshave vocalizations that only attract females
of their species.
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Prezygotic Isolation
• Mechanical isolation: bodystructure prevents mating
• Even if members of twospecies court and attemptcopulation, mating is notsuccessful. In plants,
mechanical isolation oftenoccurs in flowering plantspollinated by insects. Theflowers of black sage andwhite sage are structurallydifferent and are pollinated
by different species of insects. In this example,each insect speciespollinates flowers of onlyone of the sage species.Therefore, interbreedingdoes not occur.
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Reproductive Isolation- Postzygotic
Isolation• Postzygotic Isolation
• Hybrid zygotes between two species mightoccur, but...
• Embryological arrest: hybrid embryos oftendo not develop properly; no viable offspringis created
• Postzygotic behaviors—hybrid inviability,hybrid sterility, and hybrid breakdown—
prevent gene flow in the unlikely event thatfertilization occurs between two closelyrelated species.
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Reproductive Isolation- Postzygotic
Isolation• Infertility: hybrid offspring might
grow to viable adults but these areinfertile and cannot produce further offspring ? cannot create a newpopulation (e.g. mules, =donkey +horse) The sterility isattributed to the differing
number of chromosomes of the two species: donkeyshave 62 chromosomes,whereas horses have 64. (example in book a LIGER (crossbetween a lion and a tiger)
• Natural selection: If hybrids are less
adapted (weaker, smaller, etc.), theyare removed by natural selection
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Reproductive Isolation-
Postzygotic Isolation• The Mule is a cross between a
donkey stallion (called a jack) and
a horse mare. Hinnies are just
the opposite - a stallion horse
crossed to a female donkey Bothmale and female mules have all
the correct "parts" but they are
sterile and cannot reproduce. A
VERY few mare mules have had
foals, but these are VERY, very
rare. No male mule has ever
sired a foal.
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• Ligers are crossbreeds
between a male lion
while Tigons arecrossbreds between a
male tiger and a female
lion. Ligers are theworld’s largest
cats..Female Tigons and
Ligers can reproduce
where the males cannotso they can never create
a new species by mating
together.
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Camas
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Speciation
• Allopatric speciation isspeciation by geographicisolation. In this mode of speciation, something
prevents two or moregroups from mating witheach other regularly,eventually causing thatlineage to speciate.
Isolation might occur because of great distanceor a physical barrier, suchas a desert or river.
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Speciation
• Unlike the previous
mode, sympatricspeciation does notrequire large-scalegeographic distance toreduce gene flowbetween parts of apopulation. Organismsexploiting a new nichemay automatically reduce
gene flow with individualsexploiting the other niche.This may occasionallyhappen when, for example, herbivorous
insects try out a new hostplant. .
Gene flow has been reduced between flies
that feed on different food varieties, even
though they both live in the same geographic
area.
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Patterns of Evolution
• Adaptive Radiation• This is where species all deriving
from a common ancestor haveover time successfully adapted totheir environment via natural
selection.• Previously, the finches occupied
the South American mainland, butsomehow managed to occupy theGalapagos islands, over 600
miles away. They occupied anecological niche with littlecompetition.
• Watch video Evolution Primer #4:How Does Evolution Really Work?
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Patterns of Evolution
• Co-Evolution• The term coevolution is used
to describe cases where two(or more) species reciprocally
affect each other’s evolution.So for example, anevolutionary change in themorphology of a plant, mightaffect the morphology of an
herbivore that eats the plant,which in turn might affect theevolution of the plant, whichmight affect the evolution of the herbivore...and so on.
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Patterns of Evolution
• Co-Evolution
• Coevolution is likely to
happen when different
species have close ecological
interactions with one another.These ecological
relationships include:
• Predator/prey and
parasite/host• Competitive species
• Mutualistic species
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Patterns of Evolution
• Convergent evolution
• Animals in different parts of the world maylook similar, but it's not because they're
close relatives. Instead, they've evolvedsimilar adaptations because they occupysimilar niches -- dining on ants, hunting inthe high grass, or swimming in the dark --although their evolutionary origins arequite different.
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Convergent evolution
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Convergent evolution
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Gradualism• Gradualism is selection
and variation thathappens gradually. Over a short period of time it ishard to notice. Smallvariations that fit anorganism slightly better toits environment areselected for: a few moreindividuals with more of the helpful trait survive,and a few more with less
of the helpful trait die.Very gradually, over along time, the populationchanges. Change is slow,constant, and consistent.
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Punctuated Equilibrium
• In punctuated equilibrium,change comes in spurts.There is a period of verylittle change, and thenone or a few huge
changes occur, oftenthrough mutations in thegenes of a fewindividuals. Thoughmutations are oftenharmful, the mutationsthat result in punctuatedequilibrium are veryhelpful to the individualsin their environments.
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Punctuated Equilibrium
• Because these mutations are so different
and so helpful to the survival of those that
have them, the proportion of individuals in
the population who have the mutation/traitand those who don't changes a lot over a
very short period of time. The species
changes very rapidly over a fewgenerations, then settles down again to a
period of little change. Watch final video
on evolution
End of 15-2
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End of 15 2 Assessment Questions
• 1. Genetics supported
evolution as well as
genetic drift and
punctuatedequilibrium
End of 15-2
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End of 15 2 Assessment Questions
• 2. Hardy Weinburg – you
should have 3 of these:
• No gene mutations
• Large population size
• Limited-to-no immigration
or emigration
• Gene of interest has no
effect on survival or
reproduction
• Mating is random
End of 15-2
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End of 15 2 Assessment Questions
• 3. An isolating
mechanism such as a
physical barrier (example:
mountains) or in
sympatric speciation aspecies can evolve into a
new species without a
physical barrier possibly
to take advantage of anew food supply.
End of 15-2
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End of 15 2 Assessment Questions
• 4. Adaptive radiation
or divergent evolution