increased homozygosity

Inbreeding, continued… 

Evolutionary consequences of inbreeding 

Acting alone, inbreeding does not change allele frequencies! 

inbreeding acting alone does not cause evolution. 

Inbreeding reduces heterozygosity (=increased homozygosity) 

Since relatives share alleles, inbreeding brings together identical copies 

of an allele more often than would occur by chance. 

increased homozygosity 

1

Inbreeding reduces heterozygosity… 

With the most extreme inbreeding (self-fertilization), 

heterozygosity is reduced by half each generation: 

For a population where all individuals self-fertilize: 

Genotypes: 

A 1 A 1 A 1 A 2 A 2 A 2 

All homozygotes 

All homozygotes 

A 1 

A 1 A 2 

A 2 

A 1 A 1 A 1 A 2 

A 1 A 2 A 2 A 2 

A 1 A 1 (25%) 

A 1 A 2 (50%) 

A 2 A 2 (25%) 

Only 50% of the progeny of heterozygotes will be heterozygous 


We can estimate the level of inbreeding within a population by 

comparing observed heterozygosity with the heterozygosity 

expected for random mating (Hardy-Weinberg expectation: 2pq). 

F = 

Inbreeding coefficient 

H expected - H observed 

H expected 

where H expected = 2pq 

(F= 1: complete inbreeding) 

(F< 0: avoidance of inbreeding) 

2


We can estimate the level of inbreeding within a population by 

comparing observed heterozygosity with the heterozygosity 

expected for random mating (Hardy-Weinberg expectation: 2pq). 

F = 

Inbreeding coefficient 

H expected - H observed 

H expected 

where H expected = 2pq 

(F= 1: complete inbreeding) 

(F< 0: avoidance of inbreeding) 

e.g, for B gene: 

H expected = 0.48 

H observed = 0.07 

F = 

0.48 - 0.07 

0.48 

= 0.854 

Futuyma Fig. 9.6 

Avena fatua 

Inbreeding and fitness 

Inbreeding depression: reduced fitness due to inbreeding 

— ***expression of deleterious recessive alleles (e.g., disease) 

— loss of any fitness arising from heterozygote advantage 

(Italy, early 20th Century) 

Inbreeding can exacerbate expression of disease in populations that 

have gone through bottlenecks or founder events. 

3

Inbreeding depression may be most evident during times of stress 

Survivorship 

through the 

1989 blizzard 

‘Genetic rescue’ from inbreeding depression 

e.g., Isolated Swedish adder population (N

Mechanisms that prevent inbreeding in plants 

— Self-incompatibility (S-locus alleles can’t be shared) 

— Dioecy (male and female flowers on different plants) 

— Asynchronous flowering of male and female flowers on a plant 

— Heterostyly: 2 or morphological arrangements of stamens and stigmas; 

only plants with different arrangements will cross-pollinate 

Dioecious species: 

(Holly: Ilex sp.) 

Mechanisms that prevent inbreeding in animals: behavior 

— Dispersal of juveniles 

— Recognition of relatives and avoidance of mating with them 

e.g., mice avoid mating with individuals with shared alleles 

of the major histocompatibility (MHC) locus 

5

Not all populations avoid inbreeding 

e.g., Fig wasps: siblings mate with each other after hatching 

Fig wasp 

Not all populations avoid inbreeding 

Many plant species are partially or entirely self-fertilizing 

Arabidopsis thaliana 

(model genetic system) 

Rice (Oryza sativa) 

and many other crops 

Why self-fertilize 

Advantages: 

1) ***Reproductive assurance (e.g., N=1 founds new population) 

2) Low resource investment in attracting mates/pollinators 

3) Preserve ‘coadapted gene complexes’: optimal combination 

of alleles at different loci in an individual 

Nonetheless, self-fertilizing species may be evolutionary ‘dead ends’ 

Severe inbreeding in a normally outcrossing species 

usually leads to population extinction. 

e.g., Sewall Wright: 35 guinea pig lines 

started from brother-sister matings. 

Within 9 years, half went extinct 

W = ~0.3 relative to control lines 

e.g., Inbred house mice lines 

Only 1 line out of 20 persisted to 12 

generations 

7

An unusual case… 

Inbreeding can potentially purge deleterious recessive 

alleles from a population (none hiding in heterozygotes) 

e.g, Chillingham wild cattle (N300 years of total inbreeding 

Complete homozygosity at 

24 of 25 loci examined 

(Other cattle:>70% 

heterozygous at the same 

loci) 

No drop in viability or 

fecundity relative to other 

cattle. 

Sexual Selection 

• Viability: probability of survival to reproductive age 

• Mating Success 

Sexual selection: differential reproductive success 

due to differential mating success 

• Fecundity: viable offspring 

Bright coloration: favored for mating 

success — attracts females 

Dull coloration: favored for protection from 

predators 

Guppy (Poecilia sp.) 

8

Why is there competition for mates 

Sexes differ in energetic costs of reproduction: 

-sperm are cheap, eggs are limited 

-parenting investment (usually more by mother) 

Why is there competition for mates 

Two consequences: 

Sexes differ in energetic costs of reproduction: 

-sperm are cheap, eggs are limited 

-parenting investment (usually more by mother) 

1) Reproduction is energetically limited for female: she should be choosy 

2) Fewer receptive females than males: females are limited resource, 

so males will vary in their mating success 

Males will compete for females 

Exception: if male’s parenting investment exceeds female’s: 

female 

male 

Phalarope (Phalaropus fulicarius) 

9

Intrasexual selection: competition between males 

1. Direct competition for mates 

Male-male competition during copulation 

10

Evolutionary consequences of direct competition for mates 

Selection for larger size, traits that facilitate fighting success 

- leads to dimorphism between sexes 

Fitness and male body size: viability vs. mating success 

Male marine iguanas are larger than the optimal size for survival, 

given available food resources (algae). 

11

Intrasexual selection… 

Alternative male mating strategy: sneaky males 

e.g., many fish species 

e.g., dimorphism in male Rhinocerous beetles: 

Males that battle for and guard females 

vs. 

Small males that sneak into females’ tunnels 

Disruptive sexual selection on male morphology 

Three male mating strategies in lizard: 

a) ‘Ultradominant’: territory with multiple females 

b) Small, carefully guarded territory: 1 female 

c) No territory, sneak in on ultradominants 

negative frequency-dependent selection 

Side-blotched lizard 

Forms of intrasexual selection… 

2. Sperm competition: 

• Remove competitors’ sperm 

• Plug female genitalia following mating 

• Produce more sperm (larger testes) 

• Repeated matings 

Polygamous primates produce 

more sperm 

Damselfly penis has horn 

that removes other males’ 

sperm 

Futuyma, Fig. 14.7 

12

Forms of intrasexual selection… 

3. Infanticide: 

Accounts for 25% of all lion cub deaths in first year of life 

10% of lion mortality overall 

Puma cub 

13

Intersexual selection: mate choice 

Females often mate preferentially with showy males 

Enlarged, ornamented traits; coloration; vocalization; other 

behavior. 

Showy male 

Drab female 

Like intrasexual selection, leads to evolution of sexual dimorphism 

Mate choice: female widowbirds prefer longer-tailed males 

Futuyma, Fig. 11.9 

14

Intersexual selection: mate choice 

Leads to trait divergence 

among closely related 

species. 

Traits favored by mate choice my compromise a male’s viability 

e.g., guppy coloration with predators 

e.g., Vocalizations in túngara frogs: 

Physalaemus pustulosis 

15

Mate choice: How and why do female preferences arise 

Explanation 1: Direct benefits. Females choose traits with 

direct benefits for her or her offspring. 

e.g., Female birds pick males with best territory, resources. 

Mate choice: How and why do female preferences arise 

Explanation 1: Direct benefits. Females choose traits with 

direct benefits for her or her offspring. 

e.g., Female birds pick males with best territory, resources. 

e.g., Female hangingflies prefer males who provide larger prey during 

mating. 

16

Explanation 2: Sensory bias. Intrinsic preference for particular 

sensory stimuli. Female’s preference precedes the evolution of the 

preferred male trait. 

e.g., Prey orientation: lizard movement 

Explanation 2: Sensory bias. Intrinsic preference for particular 

sensory stimuli. Female’s preference precedes the evolution of the 

preferred male trait. 

e.g., Prey orientation: lizard movement 

e.g., Females of fish species 1 prefer long tails even though no males 

possess them: preference predates the species. 

Species 1 

Species 2 

P+: females prefer long tails 

T+: males possess long tails 

Female preference for long tails 

arose before Species 1 and 2 

diverged from their common ancestor 

17

Sensory bias and female preference… 

Zebra finches females prefer males with an artificial white crest 

Explanation 3: Indirect benefits. Females prefer males with traits that 

indirectly benefit the female or her offspring. 

a) “Sexy son” hypothesis (= Runaway sexual selection) 

(=“Fisherian model”) 

Sons inherit trait that made father attractive to mother (mating success); 

favored by future generations of females 

These successful 

males will pass on the 

alleles for the trait 

(expressed in their 

sons) and the alleles 

for the trait preference 


daughters). 

18

Explanation 3: Indirect benefits. Females prefer males with traits that 

indirectly benefit the female or her offspring. 

a) “Sexy son” hypothesis (= Runaway sexual selection) 

(=“Fisherian model”) 

Sons inherit trait that made father attractive to mother (mating success); 

favored by future generations of females 

These successful 

males will pass on the 

alleles for the trait 


sons) and the alleles 

for the trait preference 


daughters). 

Establishes 

genetic correlation 

between the trait 

(T) and preference 

for the trait (P): 

Offspring of T 2 

males and P 2 

females will carry 

both T 2 and P 2 

alleles. 

(Both sexes carry both genes, but each is only expressed in one sex.) 

Sexy son hypothesis illustrated: sandfly 

Offspring of most vs 

least attractive males do 

not differ in viability or 

fecundity (=no direct 

benefits). 

Lutzomyia 

longipalpis 

But sons of attractive 

fathers have greatest 

mating success. 

19

increased homozygosity

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