1

Genetic omelettes and the death of evolution of new species

Maladaptation

2

Genetic consequences of inbreeding1) decrease in heterozygosity, no change in P (allelic diversity)

(the more related the individuals, the faster the loss of H)

2) increases the probability of a zygote receiving identical alleles (homologous alleles), which will result in increased expression of recessive alleles.

43e-1

3

Genetic consequences of inbreeding3) increased phenotypic expression of deleterious alleles (strongly selected against)

- often results in decreased size, reproduction, vigor, etc., which decrease fitness (i.e., inbreeding depression) -e.g., sickle cell anemia, cystic fibrosis, Tay-Sachs, hemophilia, phenylketonuria, etc. - Genetic load

4) increase in phenotypic variability resulting from a deviation from the mean genotypes in non-inbred individuals

43e-1

4

Inbreeding coefficient

Sewall Wright (1923)

F = the probability that an individual will receive two equal alleles, at a specific locus, that are from the same ancestor.

Autozygous = alleles that are identical by descent

allozygous = not identical by descent

F = probability that an individual will be autozygous at a given locus1 - F = probability that an individual will be allozygous at a given locus

43e-2

5

Calculate Junior’s inbreeding coefficients from this pedigree:

AB CDMom Dad

AC

CC

C = .5

C = .5

C = .5Sis

Junior (or couldbe DD from Dad)

Probability of C from Dad to Sis to Junior = .25Probability of C from Dad (through Sis) to Junior = .50 Probability of Jr. inheriting CC from Dad = .25 X .50 = .125Probability of Junior inheriting DD from Dad = .125

F = .125 + .125 = .25 = probability of Jr. being autozygous

31

6

Calculation of F from sib mating

31e

AB CD

What is F?

A = .5

A = .5

A = .5

A = .5

A = .25

Identical by descent Probability AA .25 x .25 = .0625 BB “ CC “ DD “ F = 4 x .0625 = .25

parents

sibs

-- --

7

Calculating F in a non-inbred population

51-1

Non-inbred gene pool,F1 generation

C1C2 1 grandparent

Non-inbredInbred F2 C1C2C1C1

Ne = number of breeding individuals2 Ne = number of alleles in the gene poolProbability of drawing any first allele, say C1, = 100%Probability of drawing the same allele again = F = 1

2Ne

allozygousautozygous

8

Calculating F in a Non-inbred population, cont.

Fnoninbred = 1 which is approximately 0 in an ideal pop. 2Ne

Probability of drawing autozygous alleles = 1 = Ft

2Ne = p (C1) * p (C1)

Probability of drawing allozygous alleles = 1 - 1 2Ne

51-2

9

Relationship of F and H

When H0 = 1 (i.e., no initial inbreeding), F = 0

so: Ft = 1 - Ht

I.e., inbreeding and heterozygosity are inversely related.

*

*

50

Bottom line: all real-world populations tend to become

• completely homozygous because of genetic drift

• AND completely inbred

10

Outbreeding depression due to regional adaptation

Hunting results in extinction of Czech ibex

Translocation of ibex from nearby Austria

IbexTurkey X IbexCzech-Austria

(fall rut) (spring rut)

fertile hybrids that rutted in fall, gave birth in February (coldest month)

extinction of population 51A

11

OUTBREEDING:

Outbreeding = crossing of unrelated invididuals.

Hybrid vigor = Heterosis = increased fitness due to outbreeding.

which is why:• stray dogs look like mutts and not like AKC poodles• you see wild-type fruitflies on your rotting apple

12

Consequences of inbreeding:Results of an early experiment on inbreeding

in rats (Ritzema-Bos 1894)

Year % of unsuccessfulmatings

Average littersize

% mortality by4 weeks of age

1887 0 7.50 3.91888 2.6 7.14 4.41889 5.6 7.71 5.01890 17.4 6.58 8.71891 50.0 4.58 36.41892 41.2 3.20 45.5

45.5

55-top

13

Juvenile mortality increases after 1 generation

Species Inbred Noninbred Sign test

Ind. elephant 50 15 +

Zebra 50 26 +

Pygmy hippo 40 32 +

Giraffe 60 21 +

Sable 100 18 +

Oryx 100 5 +

Wildebeest 25 14 +

Dorcas gazelle 44 28 +

55-bottom

% juvenile mortality*

* F=0.25; e.g., wild-caught male x daughter

14

Ralls and Ballou: Examination of zoo pedigrees

Infant mortality in 41 of 44 species was higher in the inbred animals

(7 orders, 21 families and 36 genera)

15

Summary

43f3

Inbreeding:1) Inbreeding depression

a) decrease in fertile matingsb) decrease in litter sizec) increase in juvenile mortality

2) Inbreeding does not always result in inbreeding depressiona) selfing plantsb) Tamil tribes of Indiac) European Bison

3) Positive aspectsa) derive offspring without deleterious allelesb) fix alleles (domestic stock)

16

Usual outcome of inbreeding: THE F VORTEX

increased Fdeclining Ne (decreased H)

(increased genetic drift)

Inbreeding depression

decreased N decreased r (reproductive rate)

Extinction

75

17

How much inbreeding is tolerable?

If F = 1 and Ne = 4 M F 2Ne M + F

Then F = 1 F = 1 + 1 2 4 M F 8F 8M

M + F

60a

Important!

18

How much inbreeding is tolerable?

F = 1 + 1 8F 8M

Research on domestic farm animals:

natural selection for performance can balance inbreeding

depression if the ΔF is no more than 1% per generation.

So, F = 0.01 is a tolerable level of inbreeding

60a

19

How much inbreeding is tolerable?

F = 1 + 1 8F 8M

If F = 0.01 is a tolerable level of inbreeding, then

.01 = 1 + 1 so F = 25 and M = 25 8F 8M or, Ne = 50

60a

Magic number!

20

Num

ber

of f

emal

es

Number of males

15.01 tolerance

.005 tolerance

What happens to the ‘magic number’ when sex ratios are unequal?

25

1 1 8Nm 8Nf

+F =

Conclusion:15 = smallest number of effective individuals of one sex

60

21

Population bottlenecks

Pop

ulat

ion

size

Time

bottleneck

H = 1 - 1 = expected proportion of Ho retained after a2Ne 1-generation bottleneck

Ht = Ho 1 - 1 t = proportion of Ho retained t generations after 2Ne a bottleneck

if Ne at t=0 = 4, then Ht=1 = 1 - 1 = 7 i.e. 1/8 of original H 2 x 4 8 was lost in 1

generation 61A