15-3 eVOLUTION

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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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• 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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•

• 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?

http://uk.youtube.com/watch?v=xkwRTIKXaxg&feature=related

http://uk.youtube.com/watch?v=xkwRTIKXaxg&feature=related

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• 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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• 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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• 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

http://www.youtube.com/watch?v=x5NG8SYQSzE&feature=related




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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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• 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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• 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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• 4. Adaptive radiation

or divergent evolution

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