Announcements

1
BIOL223 Genetics Evolution March 23,
2009 Population Genetics Today Population
structure, Intro to Adaptation Wed Adaptation
and Sexual selection Fri Sexual
selection/overview
Announcements Reading Freeman Herron Today
Ch. 10, Wed Ch. 11 Homework HW 6 Key posted
today, March 23 HW 7 after Spring
Break! Practice exam will be posted T, March
24 Take-home exam will be posted Wed, March
25 http//www.cbs.umn.edu/populus/ Review
session, Tues April 7th 530-7PM, HS207
Jumping spider Family Salticidae
2
Relaxing assumptions of Hardy-Weinberg model 1.
Random mating 2. No mutation 3. Infinite
population size (i.e., no genetic drift)
4. No natural selection 5. No migration
(no gene flow) As before, other assumptions
kept in place unless otherwise specified
3

• Objectives
• Understand what we mean by population genetic
  structure
• Understand effects of gene flow (migration)
• Understand evolutionary significance of
  population structure

4
p in Deme A f(A1) 0.5
Deme A
Deme B
p in Deme B 0.5
5
If Nes are small, p can change quickly,
dramatically
p in Deme A 0.25
Deme A
Deme B
NA
NB
p in Deme B 0.60
6
Effect of migration on H-W
Is selection possible? A) Yes B) No
NO! Pop fixed on A1
7
Mathematical model of gene flow
m proportion of individuals in the deme that
have migrated into it that generation
Deme A
Deme B
pB
pA
m
pA (1-m) pA m pB
pA freq(A1) in g1
Just focus on one recipient population for
migration
8
Gene flow makes demes more genetically similar
p
Over multiple generations allele frequencies
converge on p, equilibrium value where net Dp 0
9
What about migration??
A
B
10
pA (1-m)pA mpB
pB (1-m)pB mpA
p
Deme
11
In general, gene flow has two main effects 1.
Gene flow reduces genetic differences between
populations. 2. Gene flow increases genetic
variation within populations.
AA
Aa
Aa
AA
Aa
AA
aa
AA
AA
Aa
AA
aa
AA
Aa
aa
AA
Aa
aa
12
Drift can oppose selection. Can migration?
13
Lake Erie water snakes Islands unbanded cryptic
on rocks Mainland banded cryptic in vegetation
Cryptic hidden
14
King (1993) showed unbanded snakes have higher
survivorship on island. If trait is a simple
Mendelian locus, why arent all island
populations fixed on unbanded allele?
Migration from mainland overwhelms selection
15
Migration Gene flow homogenizes populations
Impact of migration depends on migration rate
(m), of migrants/total N of recipient
population Gene flow can oppose local
selection But gene flow may also introduce
adaptive variation
16
Migration and Drift
Migration and drift both affect ALL loci
(selection only some) Drift and isolation cause
populations to diverge and lose genetic
variation Migration homogenizes populations and
increases variation within populations Gene flow
interacts with drift to produce population
geentic structure visible at neutral marker loci
(many applications in conservation
biology) Together, drift and migration determine
population genetic structure
17
Which population structure is most likely to lead
to long-term persistence?
Evolutionary significance of population structure
(A)
(B)
(C)
(D) all are equal
18
Adaptation An adaptation is an inherited trait
or integrated suite of traits that increases
the likelihood that an individual will survive
and reproduce, relative to individuals lacking
this trait.
Hemeroplanes ornatus Species of sphinx moth C
America
19

• Evolution by Natural Selection
• OBSERVATIONS
• Variation exists.
• Variants experience differential survival and/or
  differential reproductive success.
• Variation is inherited.
• OUTCOME Alleles that confer greater fitness will
  increase in frequency next generation.

Adaptations are products of past natural
selection (i.e. genotypes with these traits had
higher fitness than alternative genotypes)
20
Key Questions in Evolutionary Biology How do
you study adaptation? Are there constraints
(limits) on the adaptive process? How do
different selective mechanisms shape adaptive
traits?
21
How do you study Adaptation?
What are beak hooks for in rock
pigeons? Pickin the nits!
All beaks trimmed
What else needs to be demonstrated?
Ideally, fitness differences
Clayton et al. 2005. Proc Royal Soc London Series
B 272811-817
22

• Testing hypotheses about adaptation
• Experimental approach
• Observational approach
• Comparative approach
• In each, we test observations vs. predictions of
  alternative evolutionary scenarios (esp. null
  hypothesis)
• Null makes least claim on explanation for
  observations

23
Experimental approach to studying
adaptation Example Wing markings and
wing-display in Zonosemata flies
24
Zonosemata vittigera native to Mexico and SW USA
25
Flies have dark wing bands, distinctive
wing-waving behavior. What is the adaptive value
of these traits? Jumping spiders are striped and
wave legs in territorial display Are flies
predator MIMICS like sphinx moth caterpillars?
HOST PLANT Solanum elaeagnifolium Silverleaf
nightshade
26
Flies have dark wing bands, distinctive
wing-waving behavior. What is the adaptive value
of these traits? Jumping spiders striped and
wave legs in territorial display Do flies MIMIC
one predator to scare other predators (like
sphinx moth caterpillar)? OR Are flies
predator MIMICS whose mimicry has been selected
because it scares the predator it mimics?!
Greene et al. 1987. Science 236310-312
27

• Testing hypotheses about adaptation
• Experimental approach
• Test observations vs. the predictions of
  alternative evolutionary scenarios (and vs. the
  null hypothesis)

Hypothesis1 (H1) No mimicry. Many flies have
banded wings and flick them too! Possible
courtship display.
Hypothesis2 (H2) Flies mimic jumping spider to
ward off other predators that seek to avoid
jumping spiders (rather like sphinx moth)
Hypothesis3 (H3) Flies mimic jumping spider to
deter predation by jumping spiders.
Which is null hypothesis?
Big problem No extant Zonosemata vittigera
variants lacking wing bands and wing display!
28
No extant Z. vittigera variants that lack wing
bands and wing-display How do we test for the
Ghost of Predation Past?
29
Testing for the Ghost of Predation Past Fly
surgeon
Greene et al. 1987. Science 236 310-312
30
Zonosemata vittigera
Mustica domestica
Create a variant, as clear-winged Z. vittigera do
not exist
31
The predator line-up
Non-jumping spiders
Whip-tail lizard
Assassin bugs
Mantis