Lecture13 Evol Mechanisms Sv.ppt

8/17/2019 Lecture13 Evol Mechanisms Sv.ppt

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Evolutionary Mechanisms

Microevolution: Evolutionary Forces

• Evolutionary forces can significantly alter the genetic

structure of a population

1. Mutation

2. Genetic drift

3. Gene flow

4. Non-random mating

5. Selection

Mutations

- change in the base sequence of DNA

- genetic diversity……

- neutral

- deleterious

- beneficial


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Mutations

•

mutation rates are low in plantsand animals: 1 mutation in every

100,000 genes per generation

• low in prokaryotes, as well, but

can be a significant evolutionary

force because they have short

generation times

• Random change in gene frequency in small

populations over time: can occur in 3 situations:

• Chance events

• Founder effect

• Bottleneck effect

Genetic Drift

Effect of Genetic Drift in Small

Populations

(c) Chance events in small populations: The carrier of a rare allele does not reproduce.

Frequency of red allele is low inoriginal population.

The only lizard with red allele happensto fall victim to an eagle and dies.

Red allele is lost.


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• Chance events can cause allele frequencies to fluctuate

unpredictably from one generation to the next in small populations, n = 10 plants

Effect of Genetic Drift in Small Populations

C W allele has

disappeared in

generation 3

Genetic Drift Is More Pronounced in Small

Populations

A1 drifted to fixation

A1 was lost

Computer simulation for 100 generations

- change in allele frequency due to chance: striking difference between effects

of drift in small populations versus large populations

Effect of Genetic Drift in Small Populations

in 70/90 populations, genetic drift was found to decrease

genetic variation

gene for leg-bristle morphology

has two alleles, one codes for

straight leg bristles and the other

forked (bent) bristles


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Genetic Drift: Greater Prairie Chicken

Genetic Drift: Greater Prairie Chicken

Genetic drift:

- had reduced the genetic variation

of the small population, n < 50

- may have increased the

frequency of harmful alleles that

resulted in low hatching success

- addition of 271 birds (new

alleles) to Illinois population

increased hatching success from

< 50% to > 90%

Effects of Genetic Drift: A Summary

1. Genetic drift is significant in small populations:chance events can occur in all populations, but they tend to

alter allele frequencies in small populations

2. Genetic drift can cause allele frequencies to change atrandom:the change in allele frequencies from year to year is

unpredictable (unlike natural selection that favors certain

alleles over others)


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Effects of Genetic Drift: A Summary

3. Genetic drift can lead to a loss of genetic variationwithin populations:

Genetic drift can eliminate alleles from a population, which

can affect the population’s ability to adapt to changing

environments

4. Genetic drift can cause harmful alleles to be fixed :

In a very small population, harmful alleles can become fixed,

and the population’s survival can be threatened

Founder Effect

(a) Founder effect: A small sample of a large population establishes a new population.


Several of the travelers happen tocarry the red allele.

Frequency of red allele is muchhigher in new population.

How Do Founder Effects Cause Drift?

• if the founding group is small,its allele frequencies will likelydiffer from those of the source population

•

this new population's allelefrequencies is called a foundereffect

• founder effects are common incolonization of isolatedhabitats: islands, mountains,caves, and ponds

Fruit flies in Europe and N and S Americas


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Human Example of the Founder

Effect

Ellis-van Creveld syndrome

• disproportionate dwarfism

• short arms and legs

• extra fingers, toes

• deformed/missing nails

• dental abnormalities

• small chest

• heart defects

Bottleneck Effect

(b) Population bottleneck: A dramatic but short-lived reduction occurs in population size.


Many survivors of tidal wave happen tocarry red allele.

Frequency of red allele is much higherin new population.

Bottleneck Effect

causes:

* bottlenecks lead to genetic bottlenecks— a sudden

reduction in the number of ….

Northern elephant seal (~30 individuals in 1890)


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BOTTLENECK EFFECT

Cheetah ( Acinonyx jubatus)

Small population size may lead to inbreeding

• high juvenile mortality

• higher susceptibility to disease

low genetic variation: likely resulted

from several historical bottlenecks

Scientific Amer. 254(5)84-92

Human Example of Genetic

Bottleneck

Achromatopsia

•

recessive allele that results in a

vision defect

• on S. Pacific Island of Pingelap Atoll, 20 people out of several

thousand survived a typhoon andfamine (1775)

• likely one survivor carried the loss

of function allele

• 1/20 people on S. Pacific Island

of Pingelap Atoll is afflicted with

the vision disorder

Frequency of mutant allele increased as a result of drift


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•

the transfer of ….. into orout of a population

• a result of migration or

emigration:

Gene Flow

Gene Flow

• equalizes gene frequencies between the source and

recipient populations

• movement of alleles between populations tends to

………… genetic differences between them

• is random with respect to fitness


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Gene Flow and Local Adaptation

Data suggest that females born

in the eastern population are

better adapted to the island than

females in the central

population.

Birds migrate from the mainlandto the island (gene flow)

Ques. Why is one population

better adapted to life on the

island than the other

population?


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Gene Flow and Local Adaptation

Data reveals:

- unequal gene flow from the

mainland

In any year, 43% of first-time

breeders in central population are

immigrants from the mainland,

while only 13% in the eastern

population are immigrants from

the mainland

• If individuals mate more often with individuals bearing the same or different genotypes than would be expected on a random basis, then randommixing of gametes does not occur, and genotypefrequencies change

Nonrandom Mating

Non-random mating


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Non-random Mating:

Sexual Selection

non random mating

Natural Selection

• relative fitness of individuals

• alters the frequency distribution ofheritable traits: directional, stabilizing,

disruptive selection

• is the only evolutionary agent thatadapts populations to their

environments, and may preserve

allele frequencies or cause them to

change with time

Evolutionary MechanismsSummary

All have different consequences

! Mutation introduces new alleles: genetic variation …..

! Genetic drift causes random fluctuations in allele frequencies:

genetic variation tends to be ……..

! Gene flow equalizes allele frequencies between populations:

genetic variation may ……..

! Non-random mating results in a change in allele frequencies

towards the “preferred” type: genetic variation may …….

! Natural selection produces adaptation: genetic variation may

……

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Lecture13 Evol Mechanisms Sv.ppt