Quantitative Inheritance - Pt.2

1
Quantitative Inheritance - Pt.2

• Chapter 8

2
Offspring-parent regression for height in humans
(and why its called regression) (Fig. 8.11d)
3
Assumptions of offpring-parent regression as an
estimate of heritability

• The most important assumption being made in these
  analyses is that the only cause of resemblance
  between offspring and parents is shared genes
• This assumption may be violated if parents and
  offspring share the same environment and if
  environment has strong effects on the trait

4
Cross-fostering and heritability of beak length
in song sparrows (Fig. 8.12) - 1
5
Cross-fostering and heritability of beak length
in song sparrows (Fig. 8.12) - 2
6
Estimating heritability from twin studies (Fig.
8.14)
If heritability is high both monzygotic and
dizygotic twins should resemble each other, but
monzygotic twins should resemble each other more
closely than dizygotic twins (because the former
share all their genes, while the latter share
only half their genes) If heritability is low,
then neither type of twin should show close
resemblance
7
The heritability (H2 ?) of general cognitive
ability as measured in a study of Swedish twins
is about 0.62 (Fig. 8.1c)
8
Estimating heritability from crosses between
inbred linesCorolla height in longflower
tobacco (see Fig. 8.3)

• F1 individuals all have same heterozygous
  genotype. Therefore F1 variance VE
• F2 individuals have variable genotypes
  (homozygotes and heterozygotes). Therefore, F2
  variance VG VE
• VG (F2 variance) minus (F1 variance)

9
Measuring the strength of directional selection
(Fig. 8.15)Selection for increased tail length
in mice
10
Selection differential and selection gradient

• The directional selection differential, S, is the
  difference between the mean phenotype of the
  selected parents (t in the previous slide), and
  the mean phenotype of the entire population from
  which the parents were selected (t bar in the
  previous slide). It allows us to predict the
  evolutionary response of a population to
  selection.
• The selection gradient is the relationship
  between relative fitness and the phenotypic
  value. It shows how strongly phenotypic
  variation affects fitness.

11
Two-trait analysis of selection on Geospiza
fortis on Daphne Major during the drought of
1976-77 (Fig. 8.16)
Fitness
Beak width
12
Two-trait analysis of antipredator defenses in
garter snakes (Brodie 1992)
For striped snakes, the best survival strategy is
straight-line escape. For unstriped or spotted
snakes, the best survival strategy is to reverse
direction many times
13
The evolutionary response to directional selection

• Evolutionary response (in generation t 1) to a
  directional selection episode (in generation t),
  R h2S
• R is the change in the mean phenotype of the
  population over one (or more) generation(s)
• Note if h2 0, the population will not evolve

14
Response to directional selection, R h2S
15
Response to selection for increased tail-length
in mice

• Di Masso et al. (1991) selected for longer tails
  in mice for 18 consecutive generations.
• Average tail length increased by about 10
• This is a rather modest selection response
• It suggests that the heritability of tail length
  in this population of mice was low, or that the
  intensity of selection, S, was low, or both.
• A selection response, R, indicates that a trait
  is heritable, h2 R/S, and that there is
  additive genetic variance for the trait (in this
  case tail length)
• Closer analysis showed that long-tailed mice had
  more vertebrae in their tails (28 vs. 26-27 in
  controls)
• Therefore, what was actually heritable (had
  additive genetic variance) was number of tail
  vertebrae

16
Selection response in Geospiza fortis, revisited
From the figure at left, R 9.7 - 8.9 0.8
mm Average beak depth of the survivors of the
drought was 10.1 mm S 10.1 - 8.9 1.1
mm Therefore, the realized heritability of beak
length is h2 R/S 0.8/1.1 0.73
17
Heritability and natural selection on flower size
in alpine skypilots (Candace Galen 1989, 1996)

• A perennial Rocky Mountain wildflower
• Flowers are about 12 larger in tundra
  populations vs. timberline populations
• Tundra populations are pollinated almost
  exclusively by bumblebees
• Timberline populations are pollinated by a
  variety of insects
• Questions
• Is flower size in skypilots heritable?
• Do bumblebees select for larger flowers?

18
Is flower size in skypilots heritable?

• Offspring- single parent regression
• Measure diameters of 144 parents from
  small-flowered timberline population
• Collect seeds from parents and germinate 617
  seedlings in laboratory
• Transplant seedlings to random locations in same
  habitat as parents
• Measure flower size in 58 surviving offspring
  seven years later
• The estimate of heritability was h2 1, but this
  has low precision. With more confidence, Galen
  concluded that 0.2 h2 1

19
Estimating the heritability of flower size in
alpline skypilots (Fig. 8.20)
The slope of the regression line is about
0.5 Since this is offspring - single parent
regression, h2 twice the slope, or about 1.0
20
Do bumblebees select for larger flowers?

• Large screen-enclosed cage at study site with 98
  transplanted skypilots bumblebees (but no other
  pollinators)
• Measured flowers and later collected seeds
• Germinated seeds in lab then planted seedlings at
  random locations in natural habitat
• Six years later counted all the surviving
  offspring ( fitness) that had been produced by
  each of the original caged parents
• Calculated selection gradient on parents
  (relative fitness vs flower size)

21
The selection gradient on flower size in alpine
skypilots (Fig. 8.21)
The slope of the line (the selection gradient) is
about 0.13 This corresponds to a selection
differential, S 5 (S VP x selection gradient)
22
Response to selection on flower size in alpine
skypilots

• Using the relationship R h2S, and an estimate
  of S 5, the single-generation response to
  selection would be 1 (h2 0.2) to 5 (h2 1.0)
• Therefore, it would not take very many
  generations for selection by bumblebees to
  produce the 12 difference in flower size seen
  between tundra and timberline populations of
  skypilots

23
Selection on flower size in alpine sky pilots
two questions

• How do we know that bumblebees are doing the
  selecting? Maybe plants with bigger flowers
  produce more offspring even without bumblebees
• Galen (1989) previously documented that plants
  with larger flowers attract more bumblebees and
  plants that attract more bumblebees produce more
  seeds
• Experimental controls when plants are hand
  pollinated or pollinated by other insects, there
  is no relationship between flower size and
  fitness
• If bumblebees are constantly selecting for larger
  flowers, why arent flowers getting bigger and
  bigger?

24
Modes of selection(Fig. 8.23)
25
Modes of selection and genetic variance

• Long-term directional phenotypic selection tends
  to reduce phenotypic and genetic variance (it
  results in fixation of alleles, as in our
  one-locus genetic models of selection)
• Long-term stabilizing selection also tends to
  reduce phenotypic and genetic variance (it is not
  like single-locus overdominant selection, which
  tends to preserve genetic variation)
• Disruptive selection increases phenotypic
  variance in the short-term. However, it is
  generally thought to be uncommon because it will
  be unstable in a random mating population
  (similar to single-locus underdominance), or will
  favor reproductive isolation between alternative
  phenotypes

26
Stabilizing selection on gall size in a
gall-making fly(Weis and Abramson, 1986)

• Fly larva (Eurosta solidaginis) induces host
  plant goldenrod (Solidago altissima) to make a
  gall, inside of which the larva develops
• Parasitic wasps attack fly larvae in small galls
• Birds eat larvae in large galls
• Larvae in medium size galls have highest survival
  rate

27
Stabilizing selection on a gall-making fly (Fig.
8.24)
28
Disruptive selection on beak size in the
black-bellied seed cracker (Smith 1993) (Fig.
8.25)

• Adult birds have either large or small beaks
• Birds in the two groups specialize on different
  kinds of seeds
• Figure shows survival of juveniles in relation to
  beak size

29
Misunderstanding and misusing quantitative
genetics 1

• h2 0 means only that none of the phenotypic
  variation among individuals is due to genetic
  differences among individuals
• h2 0 does not mean that genes do not
  determine the phenotype
• To understand this, consider the example that we
  have used of inheritance of corolla height in
  longflower tobacco
• In a true-breeding (homozygous) parental line,
  all individuals have the same genotype and the
  heritability of corolla height is zero within
  that parental line
• However, the experiment also demonstrates that
  corolla length is under genetic control and
  that the parental lines have genes that influence
  corolla height
• The two parental lines have consistently
  different corolla heights when grown in the same
  environment
• The F2 plants have increased phenotypic variance
  relative to the genetically uniform F1 and the
  homozygous and genetically uniform parental lines
• Starting with the F2, subsequent generations show
  a response to selection

30
Corolla height in longflower tobacco (Fig. 8.3)
31
Misunderstanding and misusing quantitative
genetics 2

• Estimates of genetic variance and heritability
  apply only to the group or population in which
  they are made
• Knowing that a trait has high heritability tells
  us nothing about the causes of differences in
  mean phenotypes between groups or populations
• Several studies indicate that the heritability of
  IQ score is 0.30
• On comparable IQ tests, Japanese children score,
  on average, about 10 points higher than white
  Americans
• Are Japanese genetically smarter than
  Americans?
• What other factors might explain the difference
  in average IQ scores?
• Can you design an experiment to test your
  hypothesis?
• Aside from the obvious ethical issues, what
  problems might such an experiment encounter?

32
All of the difference in average plant height
between these two genetically identical
populations of Achillea is due to environmenal
effects (Clausen, Keck and Heisey) (Fig.
8.26)Mather is in the foothills of the Sierra
Nevada mountainsStanford is low altitude and
near the Pacific coast
33
Populations of Achillea at different elevations
are genetically different - but the direction of
difference depends on the elevation of the
common garden (Fig. 8.29)
Our conclusion about which population is
genetically programmed to have plants with more
stems will depend on where we chose to do the
experiment. This is an example of genotype by
environment interaction