23- evolutionofpopulations text.ppt

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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures forBiology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 23

he E!olution o" Populations




• Overview: The Smallest Unit of Evolution

• One common misconception about evolution is

that individual organisms evolve, in the

arwinian sense, during their lifetimes

• !atural selection acts on individuals, but

populations evolve




• "enetic variations in populations

# #ontribute to evolution

Figure 23.1




• #oncept $%&': Population genetics provides a

foundation for stud(ing evolution• )icroevolution

# *s change in the genetic ma+eup of a

population from generation to generation

Figure 23.2




The Modern Synthesis

• Population genetics

# *s the stud( of how populations changegeneticall( over time

# econciled arwin-s and )endel-s ideas




• The modern s(nthesis

# *ntegrates )endelian genetics with thearwinian theor( of evolution b( natural

selection

# .ocuses on populations as units of evolution


http://slidepdf.com/reader/full/23-evolutionofpopulations-textppt 7/61Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Gene Pools and Allele Frequencies

• / population

# *s a locali0ed group of individuals that are capable ofinterbreeding and producing fertile offspring

)/P

/E/

/ L / S 1 /

# / ! / /

2eaufor t Sea

Porcupine

herd r ange

•

.airban+s

•

3hitehorse

.ort(mile

herd range

! O , T 4 3

E S T

T E , , * T O , * E S

/ L / S 1 /

5 U 1 O !

Figure 23.3



• The gene pool

# *s the total aggregate of genes in a populationat an( one time

# #onsists of all gene loci in all individuals of the

population



The Hardy-Weinber Theorem

• The 4ard(63einberg theorem

# escribes a population that is not evolving

# States that the fre7uencies of alleles and

genot(pes in a population-s gene pool remain

constant from generation to generationprovided that onl( )endelian segregation and

recombination of alleles are at wor+



• )endelian inheritance

# Preserves genetic variation in a population

Figure 23.4

"eneration

'

C R C R

genot(pe

C W C W

genot(pePlants mate

/ll C R C W

8all pin+ flowers9

;< C R

gametes;< C W

gametes

#ome together at random

"eneration

$

"eneration

%

"eneration

=

$< C R C R ;< C R C W $< C W C W

;< C R

gametes;< C W

gametes

#ome together at random

$< C R C R ;< C R C W $< C W C W

/lleles segregate, and subse7uent

generations also have three t(pes

of flowers in the same proportions



Preservation of Allele Frequencies

• *n a given population where gametes contribute

to the ne>t generation randoml(, allelefre7uencies will not change



Hardy-Weinberg quilibrium

• 4ard(63einberg e7uilibrium

# escribes a population in which randommating occurs

# escribes a population where allele

fre7uencies do not change



• / population in 4ard(63einberg e7uilibrium

Figure 23.5

"ametes for each generation are drawn at random from

the gene pool of the previous generation:

?;< C R 8 p @ ;&?9 $;< C W 8q @ ;&$9

SpermC R

8?;<9C W

8$;<9

p2

A=<C R C R

'A<C R C W

'A<C R C W

=<C W C W

qp

C R

8 ? ; < 9

E g g s

C W

8 $ ; < 9

pq

*f the gametes come together at random, the genot(pe

fre7uencies of this generation are in 4ard(63einberg e7uilibrium:

q$

A=< C R C R , %$< C R C W , and =< C W C W

"ametes of the ne>t generation:

A=< C R from

C R C R homo0(gotes

'A< C R from

C R C W homo0(gotesB @ ?;< C R @ ;&? @ p

'A< C W from

C R C W hetero0(gotesB @ $;< C W @ ;&$ @ q

3ith random mating, these gametes will result in the same

mi> of plants in the ne>t generation:

A=< C R

C R

, %$< C R

C W

and =< C W

C W

plants

p2

=< C W from

C W C W homo0(gotes



• *f p and q represent the relative fre7uencies of

the onl( two possible alleles in a population ata particular locus, then

# p$ B $ pq B q$ @ '

# /nd p$ and q2 represent the fre7uencies of thehomo0(gous genot(pes and $ pq represents

the fre7uenc( of the hetero0(gous genot(pe




Conditions for Hardy-Weinberg quilibrium

• The 4ard(63einberg theorem

# escribes a h(pothetical population

• *n real populations

# /llele and genot(pe fre7uencies do changeover time




• The five conditions for non6evolving

populations are rarel( met in nature # E>tremel( large population si0e

# !o gene flow

# !o mutations

# andom mating

# !o natural selection




Population !enetics and Human Healt"

• 3e can use the 4ard(63einberg e7uation

# To estimate the percentage of the humanpopulation carr(ing the allele for an inherited

disease




• #oncept $%&$: )utation and se>ual

recombination produce the variation thatma+es evolution possible

• Two processes, mutation and se>ual

recombination # Produce the variation in gene pools that

contributes to differences among individuals




Mutation

• )utations

# /re changes in the nucleotide se7uence of !/

# #ause new genes and alleles to arise

Figure 23.6




Point #utations

• / point mutation

# *s a change in one base in a gene

# #an have a significant impact on phenot(pe

# *s usuall( harmless, but ma( have an adaptiveimpact




#utations $"at Alter !ene Number or %equence

• #hromosomal mutations that affect man( loci

# /re almost certain to be harmful

# )a( be neutral and even beneficial




• "ene duplication

# uplicates chromosome segments




#utation Rates

• )utation rates

# Tend to be low in animals and plants

# /verage about one mutation in ever( ';;,;;;

genes per generation

# /re more rapid in microorganisms




Se!ual Recombination

• *n se>uall( reproducing populations, se>ual

recombination # *s far more important than mutation in

producing the genetic differences that ma+e

adaptation possible




• #oncept $%&%: !atural selection, genetic drift,

and gene flow can alter a population-s geneticcomposition

• Three maCor factors alter allele fre7uencies and

bring about most evolutionar( change # !atural selection

# "enetic drift

# "ene flow




"atural Selection

• ifferential success in reproduction

# esults in certain alleles being passed to thene>t generation in greater proportions




Genetic #ri$t

• Statisticall(, the smaller a sample

# The greater the chance of deviation from apredicted result




• "enetic drift

# escribes how allele fre7uencies can fluctuateunpredictabl( from one generation to the ne>t

# Tends to reduce genetic variation

Figure 23.7

C R C R

C R C W

C R C R

C W C W C R C R

C R C W

C R C W

C R C W C R C R

C R C R

Onl( of

'; plants

leave

offspring

C W C W C R C R

C R C W

C R C R C W C W

C R C W

C W C W C R C R

C R C W C R C W

Onl( $ of

'; plants

leave

offspring

C R C R

C R C R C R C R

C R C R C R C R

C R C R

C R C R

C R C R

C R C R C R C R

Generation 2

p @ ;&

q @ ;&

Generation 3

p @ '&;

q @ ;&;

Generation 1

p 8fre7uenc( of C R 9 @ ;&D

q 8fre7uenc( of C W 9 @ ;&%




$"e &ottlenec' ffect

• *n the bottlenec+ effect

# / sudden change in the environment ma(drasticall( reduce the si0e of a population

# The gene pool ma( no longer be reflective of

the original population-s gene pool

Original

population

2ottlenec+ing

eventSurviving

population

Figure 23.8 A

(a) $ha%ing just a "e& marbles through the

narro& nec% o" a bottle is analogous to a

drastic reduction in the si'e o" a population

a"ter some en!ironmental disaster. By chance,

blue marbles are o!er(represented in the ne&

population and gold marbles are absent.




• Understanding the bottlenec+ effect

# #an increase understanding of how humanactivit( affects other species

Figure 23.8 B

(b) $imilarly, bottlenec%ing a population

o" organisms tends to reduce genetic

!ariation, as in these northern

elephant seals in Cali"ornia that &ereonce hunted nearly to e)tinction.




$"e Founder ffect

• The founder effect

# Occurs when a few individuals becomeisolated from a larger population

# #an affect allele fre7uencies in a population

G Fl




Gene Flo%

• "ene flow

# #auses a population to gain or lose alleles

# esults from the movement of fertile

individuals or gametes

# Tends to reduce differences between

populations over time




• #oncept $%&=: !atural selection is the primar(

mechanism of adaptive evolution

• !atural selection

# /ccumulates and maintains favorable

genot(pes in a population

G ti & i ti




Genetic &ariation

• "enetic variation

# Occurs in individuals in populations of allspecies

# *s not alwa(s heritable

Figure 23.9 A, B

8a9 )ap butterflies that

emerge in spring:

orange and brown

8b9 )ap butterflies that

emerge in late summer:

blac+ and white

( i i Wi "i P l i




(ariation Wit"in a Population

• 2oth discrete and 7uantitative characters

# #ontribute to variation within a population




• iscrete characters

# #an be classified on an either6or basis

• uantitative characters

# Far( along a continuum within a population




• Pol(morphism

• Phenot(pic pol(morphism

# escribes a population in which two or more

distinct morphs for a character are each

represented in high enough fre7uencies to bereadil( noticeable

• "enetic pol(morphisms

# /re the heritable components of characters

that occur along a continuum in a population




• )easuring "enetic Fariation

• Population geneticists

# )easure the number of pol(morphisms in a

population b( determining the amount of

hetero0(gosit( at the gene level and themolecular level

• /verage hetero0(gosit(

# )easures the average percent of loci that are

hetero0(gous in a population

( i ti & t P l ti




(ariation &et)een Populations

• )ost species e>hibit geographic variation

# ifferences between gene pools of separatepopulations or population subgroups

' $&= %&'= &'? A D&'

GG'H'%&'D';&'AH&'$?&''

' $&'H %&? =&'A &'= A&D

GG'&'?'%&'D''&'$H&';Figure 23.1




• Some e>amples of geographic variation occur

as a cline, which is a graded change in a trait

along a geographic a>is

Figure 23.11

!"#!$%&!' esearchers observed that the average si0e

of (arrow plants 8 Achillea9 growing on the slopes of the Sierra

!evada mountains graduall( decreases with increasing

elevation& To eliminate the effect of environmental differences

at different elevations, researchers collected seeds

from various altitudes and planted them in a common

garden& The( then measured the heights of theresulting plants&

$!*+ The average plant si0es in the common

garden were inversel( correlated with the altitudes at

which the seeds were collected, although the height

differences were less than in the plants- natural

environments&

C'C+*%' The lesser but still measurable clinal variation

in (arrow plants grown at a common elevation demonstrates the

role of genetic as well as environmental differences&

) e a n h e i g h t 8 c m 9

/ t i t u d e 8 m 9

4eights of (arrow plants grown in common garden

Seed collection sites

Sierra !evada

ange"reat 2asin

Plateau

A Closer Loo' at "atural Selection




A Closer Loo' at "atural Selection

• .rom the range of variations available in a

population

# !atural selection increases the fre7uencies of

certain genot(pes, fitting organisms to their

environment over generations

volutionary Fitness




volutionary Fitness

• The phrases Istruggle for e>istenceJ and

Isurvival of the fittestJ

# /re commonl( used to describe natural

selection

# #an be misleading

• eproductive success

# *s generall( more subtle and depends on man(factors




• .itness

# *s the contribution an individual ma+es to thegene pool of the ne>t generation, relative to

the contributions of other individuals

• elative fitness

# *s the contribution of a genot(pe to the ne>t

generation as compared to the contributions of

alternative genot(pes for the same locus

*irectional *isruptive and %tabili,ing %election




*irectional+ *isruptive+ and %tabili,ing %election

• Selection

# .avors certain genot(pes b( acting on thephenot(pes of certain organisms

• Three modes of selection are

# irectional

# isruptive

# Stabili0ing




• irectional selection

# .avors individuals at one end of thephenot(pic range

• isruptive selection

# .avors individuals at both e>tremes of the

phenot(pic range

• Stabili0ing selection

# .avors intermediate variants and acts against

e>treme phenot(pes




• The three modes of selection

Fig 23.12 A-C

(a) ire/tiona0 e0e/tion shifts the overall

ma+eup of the population b( favoring

variants at one e>treme of the

distribution& *n this case, dar+er mice are

favored because the( live among dar+

roc+s and a dar+er fur color conceals them

from predators&

(b) iruptie e0e/tion favors variants

at both ends of the distribution& These

mice have coloni0ed a patch( habitat

made up of light and dar+ roc+s, with the

result that mice of an intermediate color are

at a disadvantage&

(/) tabi0iing e0e/tion removes

e>treme variants from the population

and preserves intermediate t(pes& *f

the environment consists of roc+s of

an intermediate color, both light and

dar+ mice will be selected against&

Phenot(pes 8fur color9

Original population

Original

population

Evolved

population

. r e 7 u e n c (

o f i n d i v i d u a l s

The Preser(ation o$ Genetic &ariation




The Preser(ation o$ Genetic &ariation

• Farious mechanisms help to preserve genetic

variation in a population

#i)loidy




#i)loidy

• iploid(

# )aintains genetic variation in the form ofhidden recessive alleles

*alancin Selection




*alancin Selection

• 2alancing selection

# Occurs when natural selection maintainsstable fre7uencies of two or more phenot(pic

forms in a population

# Leads to a state called balanced pol(morphism

Hetero+yote Ad(antae




Hetero+yote Ad(antae

• Some individuals who are hetero0(gous at a

particular locus

# 4ave greater fitness than homo0(gotes

• !atural selection

# 3ill tend to maintain two or more alleles at that

locus




• The sic+le6cell allele

# #auses mutations in hemoglobin but alsoconfers malaria resistance

# E>emplifies the hetero0(gote advantage

Figure 23.13

.re7uencies of thesic+le6cell allele

;K$&<

$&K&;<

&;KD&<

D&K';&;<

';&;K'$&<

'$&<

istribution of

malaria caused b(

Plasmodium falciparum

8a proto0oan9




• .re7uenc(6ependent Selection

• *n fre7uenc(6dependent selection

# The fitness of an( morph declines if it becomes

too common in the population




• /n e>ample of fre7uenc(6dependent selection

P h e n o t ( p i c

d i v e r s i t (

Figure 23.14

#arenta0 popu0ation ap0e

!perienta0 group ap0e

Plain bac+ground Patterned bac+ground

On pec+ing a moth image

the blue Ca( receives a

food reward& *f the bird does not detect a moth

on either screen, it pec+s

the green circle to continue

to a new set of images 8a

new feeding opportunit(9&

;&;A

;&;

;&;=

;&;%

;&;$

; $; =; A; ?; ';;"eneration number

.re7uenc(6

independent control

Neutral (ariation




Neutral (ariation

• !eutral variation

# *s genetic variation that appears to confer noselective advantage

Se!ual Selection




Se!ual Selection

• Se>ual selection

# *s natural selection for mating success

# #an result in se>ual dimorphism, mar+ed

differences between the se>es in secondar(

se>ual characteristics




• *ntrase>ual selection

# *s a direct competition among individuals ofone se> for mates of the opposite se>




• *nterse>ual selection

# Occurs when individuals of one se> 8usuall(females9 are choos( in selecting their mates

from individuals of the other se>

# )a( depend on the showiness of the male-sappearance

Figure 23.15

The ,(olutionary ,nima o$ Se!ual Re)roduction




The ,(olutionary ,nima o$ Se!ual Re)roduction

• Se>ual reproduction

# Produces fewer reproductive offspring than ase>ualreproduction, a so6called reproductive handicap

Figure 23.16

Aeua0 reprou/tion

.emale

eua0 reprou/tion

.emale

)ale

"eneration '

"eneration $

"eneration %

"eneration =




• *f se>ual reproduction is a handicap, wh( has it

persistedM

# *t produces genetic variation that ma( aid in

disease resistance

Why "atural Selection Cannot Fashion Per$ect ranisms




y

• Evolution is limited b( historical constraints

• /daptations are often compromises



• #hance and natural selection interact

• Selection can onl( edit e>isting variations

23- evolutionofpopulations text.ppt

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