Evolution, Adaptation, Natural Selection and Fitness

Dr Pupak Derakhshandeh, PhDAssiss. Prof. of Medical Science of Tehran University

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The Process of Natural Selection

Evolution Evolution

Reproductive Ability

+ Environmental

Restrictions

Reproductive Ability

+Environmental

Restrictions

Struggle for Existence

+Heritable Variations

Struggle for Existence

+Heritable Variations

Natural Selection+

EnvironmentalChanges

Natural Selection+

Environmental Changes

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Model of Selection

• Heritable variation

• Selection

• Other Evolutionary Forces

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Four evolutionary forces

• Mutation• Drift• Isolation• Natural selection• Non overlapping generation

*

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Evolutionary Forces

• Mutation – New allele arises by physical change in structure of DNA

• Genetic drift – Random drift in allele frequency by chance, important mainly in

small populations • Isolation

– Isolated populations can diverge due to drift or natural selection• Natural selection

– The process of differential (survival and reproduction of individuals)

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Natural selection

“ The only force that produces adaptations The only force that produces adaptations ”

Conditions for evolution by natural selection :

• Variation• Heritability (h 2) • Differential fitness

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Genotypic Level Phenotypic Level

Genetic variability exists)alternative alleles exist)

Phenotypic variation)genetic variation is

expressed)

Some variation survive and reproduce more successfully

in given environment

Phenotypic variation passes to offspring

)h 2 ≠ 0)

Distribution of Allele Frequencies will Change Through Time

)Adaptive Evolution)

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Important to recognize

1. Selection and response:

Natural selection is the process of differential survival and reproduction by different individuals in a specific environment

Selection acts on the phenotypeSelection acts on the phenotype

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1. Selection and Response:

Response to selection is different in allele frequencies, if phenotypic variation is heritable, i.e. due to genetic variation. Evolution occurs at level of genotype.

Evolution IS simply change in allele Evolution IS simply change in allele frequencyfrequency

Important to recognize

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2. Selection is caused by all aspects of the environment (Biotic and Abiotic)

> Therefore adaptation is relative to a given context

> An individual’s fitness is relative to its environment

Important to recognize

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3. Whether an allele increases or decreases in frequency:

– depends on what alternative alleles exist in the population

– Fitness is relative to alternative genotypes (phenotypes)

Important to recognize

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4. Heritability: proportion of variation in phenotype that is due to genotype

h2 = 2g /2

p

Important to recognize

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Heritability

• Two identical genotypes, in different environments, may not produce identical

phenotype.

• Two different genotypes, in the same environment, can produce similar or identical phenotype.

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Heritability

• the phenotype is product of interaction between genotype and environment.

• Heritability measures what proportion of variation in the phenotype is due to the genotype.

• There are many ways to estimate heritability.

• Most common is:– Offspring-parent regression heritability

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Offspring-parent regression heritability

Value of trait In offspring

Value of trait in father

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Measuring Natural Selection

1. Fitness is the currency of natural selection– Absolute fitness = W– Probability of survival X reproductive output

• Relative fitness = ω

Absolute fitness of a phenotypeAbsolute fitness of “most fit” phenotype

– Selection coefficient = s = 1 - ω– A measure of the strength or intensity of

selection against a phenotype

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Relative Fitness

• Consider a population of newborns with variable survival among three genotypes:

ω = Relative Fitness

ω= 1 for best performer; others are ratios relative to best performer:

A1A1A1A2A2A2

N100100100

Survival805640

ω11 = 80/80 = 1ω12 = 56/80 = 0.7 ω22 = 40/80 = 0.5

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Average Fitness ω

• Use genotype frequencies to calculate weighted fitness for entire population

ω = P(ω11) + H(ω12) + Q(ω22)

ω = )150/300)2)1) + )0.5))0.7) + )150/300)2)0.5) = 0.93

When fitness varies among genotypes, average fitness of the population is less than 1

Genotype A1A1A1A2 A2A2

ω10.70.5

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Selection Against the Homozygous Recessive Phenotype

• selection that is directed only against the homozygous recessive phenotype

(one of the most common patterns of selection ! )

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Selection Against One of The Homozygotes

• the recessive allele )a) will not completely disappear

• it is still passed on by heterozygous )Aa) parents to the half of their children

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Selection Against One of The Homozygotes

Total population fitness W

GenotypeAAAaaa

Fitness1.01.00.2

Total fnpAAwAA+ pAawAa+ paawaa W

Allelic fn )A))pAAwAA+ 1/2 pAawAa) wA

Allelic fn )a))1/2 pAawAa+ p aawaa) wa

Genotype frequency )A)

)pAAwAA+ 1/2 pAawAa)/W pAA

Genotype frequency )a)

)1/2 pAawAa+ p aawaa )/W paa

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Selection Against One of The Homozygotes

• the allele fitnesses will change – if the allele frequencies change!

• a rare detrimental/lethal allele, as recessive has very little fitness – if it is lethal in the homozygote

because when it is very rare, it is almost never in homozygotes

Note :

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Selection Against One of The Homozygotes

• A population fitness less than 1 does not mean a dead population

• just a population that is not reaching its maximum possible fitness

• the fitness of the heterozygote is represented by a multiplier h

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Selection Against One of The Homozygotes

h=0 , Aa=1

h=1 , Aa=1

When A > a

GenotypeAAAaaa

Fitness1.01 – h s1 - s

When a > A

GenotypeAAAaaa

Fitness1 - S1 )h)1

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Selection Against One of The Homozygotes

• The multiplier h:– a measure of dominance (a). – h=1 means that (a) is dominant:

– h=0 means that (a) is recessive and (A) is dominant

– h=0.5 means that it is perfectly additive

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Directional / Purifying selection

• Additive or Co-dominant (h=1/2)

s = 0.5, h = 0.5

In this case pa will drop smoothly toward 0.

Genotype AAAaaa

Fitness1.01 – h s1 - s

Fitness1.01 – ½ s1 - s

Fitness1.00.750.5

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Directional / Purifying selection

• Natural selection favors a single phenotype

• Allele frequency continuously shifts in one direction

• the advantageous allele will increase in frequency independently of its dominance relative to other alleles

)i.e. even if the advantageous allele is recessive, it will eventually become )fixed)

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Directional selection

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Directional / Balancing selection

• Selection may favor multiple alleles

• It is the same as purifying selection

• Removes deleterious mutations from a population (-)

• Directional selection is a particular mode or mechanism of natural selection

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Directional selection

1. At first, the light-gray form of the peppered moths on light-gray tree

2. industrial pollution:• darkened the tree trunks

3. notice by bird 4. the numbers of dark-gray moths increased 5. In recent years pollution controls have led to decreased

amounts of soot on the trees 6. the light colored moths are increasing in numbers !

Example :

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Directional selection

1 2

3

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Selection Against One of The Homo/Heterozygotes

)Directional selection)

• Albinism (AA/Aa)

• Diabetes (AA/Aa)

• HIV / bubonic Plague (Aa):– CCR5-delta 32

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Selection Against Both Homozygotes

• Complete selection against both homozygotes (AA and aa)

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Selection Against Both Homozygotes

• Nature selecting against both homozygotes was found in Central Africa

• 10% of the world's human population:– infected by malaria– 90% of the cases are in Sub-Saharan

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Selection Against The Heterozygote And One Of The Homozygotes

• For example, if just aa genotype individuals fail to reproduce

• Only AA people will contribute their genes to the next generation

• Genetic testing and counseling: – Discouraging heterozygous carriers (Aa)

of harmful recessive alleles (aa) from reproducing

– Sickle-cell trait and Tay-Sachs disease

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Balancing Selection / Overdominance / Heterozygote Advantage

Genotype AAAaaa

Fitness 0.91.00.2

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Balancing Selection / Overdominance /

Heterozygote Advantage )Aa)

or Selection Against both of The

Homozygotes

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Selection Against both of The Homozygotes ) ≠ Distruptive selection )

• The multiplier h:

– A measure of dominance (A/a).

– h<0 means that (A/a) is dominant:

Or

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Directional selection

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Overdominance!

Genotype AA Aa aa

fitness 1.0 1-hs 1-s

H<0 means:

1-(-1/2)s=1+1/2s (0<s<1)

1.5<1

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Genotype AA Aa aa fitness 0.9 1.0 0.2

• In this case pA will approach a value that maximizes W

• Both A and a will persist in the population

• The equilibrium point depends on wAA and waa(

• in this case, it's p A=0.89, p a=0.11 Examples: Sickle-cell anemia

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Disruptive Selection / Underdominance / Heterozygote Disadvantage

GenotypeAAAaaa

Fitness1.00.81.0

H>1 means that (A or a) is dominant:

Or

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Disruptive Selection

• h >1 means:

1-(1.5)s = 1-1/2s (0<s<1)

-1/2 <w<1

GenotypeAAAaaa

Fitness1.01 – h s1.0

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Selection Against The Heterozygote

• Half will normally be homozygous dominant (AA)

• and half will be homozygous recessive (aa)

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Selection Against All Genotypes

• Completely selects against all genotypes (AA, Aa, aa)

• All genotypes are at a selective disadvantage

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Genetic Change Between Generations

GenotypeAAAaaa

Initial freq.p 2 2pqq 2

Rel. Fitnessw AAw Aaw aa

Freq. After selection

p 2 w AA2pq w Aaq 2 w aa

Rel. freq. after sel.

p 2 w AA

w

2pq w Aa

w

q 2 w aa

w

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Selection Against a Recessive

GenotypeAAAaaa

Initial freq.p 2 2pqq 2

Rel. Fitness111 - s

Freq. After selection

p 22pqq 2 - sq 2

Rel. freq. after sel.

p 2

w

2pq

w

q 2 - sq 2

w

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Relative Fitness

Genotype A1 A1A1 A2A2 A2Total

a405010100

b809010180

b / a 80/4090/5010/10-

Rel. reproductive fitness

2/21.8/21/2

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Relative Fitness:Survival)Tay Sach)

GenotypeA1A1A1A2A2A2

Survival fit.10.90.5

Fertility fit.111

Net fit.1x1 = 11x0.9 = 0.91x0.5 = 0.5

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Relative Fitness:Fertility)CF)

GenotypeA1A1A1A2A2A2

Survival fit.111

Fertility fit.10.90.5

Net fit.1x1 = 11x0.9 = 0.91x0.5 = 0.5

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Modeling directional selection: allelefrequency change is determined by relative

fitness

GenotypeA1A1A1A2A2A2

Viability W 11W 12W 22

Rel. viability1W 12 / W 11W 22 / W 11

Relative 11 - hs1 - s

Where 1 – hs = W 12 / W 11 and 1 – s W 22 / W 11 The parameter s is called the selection coefficient and the parameter h is called the heterozygotous effect.

h = 0A 1 is dominant , A 2 recessive

h = 1A 2 is dominant , A 1 recessive

1 < h < 1Incomplete dominance

h < 0Overdominance

h > 1Underdominance

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Selection against a dominant phenotype

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Selection favoring the heterozygote

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Simple models of selection

rela

tive

fit

nes

s o

f A

1A

11.0

0.5

0.0 0.5 1.0

w11 > w12 > w22

fix A1

w11 > w12 < w22

unstable polymorphism

w11 < w12 > w22

stablepolymorphism

w11 < w12 < w22

fix A2

relative fitness of A2A2

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q = q’ - q

= - q

=

q =

q(1-sq) 1-sq2

q – sq2 – q + sq3

1-sq2

-sq2(1 – q) 1-sq2

How much has the frequency of A2 Change after one generation of

selection ?

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)Roughgarden 1979)

change in the frequency of adeleterious recessive

2

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selection against a deleterious recessive allele

q

q

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change in the frequency of adeleterious dominant

(Roughgarden 1979)

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Selection against a dominant allele

q

q

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Selection favoring heterozygotes

Genotype A1A1A1A2A2A2

Initial genotype freq.

P22pq q2

Rel. fitness1 - s11 - t

w = p2)1 - s) + 2pq)1) + q2)1 - t) = 1 - p2s - q2t

Genotype freq. after selection

p2)1 - s)

1- p2s - q2t

2pq)1)

1- p2s - q2t

q2)1+t)

1- p2s - q2t

s and t = two different fitnesses!

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heterozygote advantage

q

q

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Genotype A1A1A1A2A2A2

Initial genotype freq.

P22pq q2

Rel. fitness1+s11+t

w = p2)1+s) + 2pq)1) + q2)1+t) = 1 + p2s + q2t

Genotype freq. after selection

p2)1+s)

1+p2s+q2t

2pq)1)

1+p2s+q2t

q2)1+t)

1+p2s+q2t

Selection against heterozygotes