Biology 4250 Evolutionary Genetics

1
Biology 4250 Evolutionary Genetics

• Dr. David Innes
• Dr. Dawn Marshall
• W 2008

2
Announcements

• Monday March 10 afternoon lab.
• Please pass in a page listing your term paper
  topic with a brief outline (abstract) and a few
  references. Presentations begin March 17.

3
Outline of
topics 1. Introduction/History of Interest in
Genetic Variation 2. Types of Molecular
Markers 3. Molecular Evolution 4.
Individuality and Relatedness 5. Population
Demography, Structure Phylogeography 6.
Phylogenetic Methods Species Level
Phylogenies 7. Speciation, Hybridization and
Introgression 8. Human Evolutionary
Genetics 9. Conservation Genetics
Background
Applications
4
Hybridization

• Topics
• Historical background
• Natural hybridization
• Genetic distance and hybridization
• Hybrid zones
• - geography
• - theoretical models
• - examples
• - sexual asymmetries (FA x MB gt FB x MA)
• - cytonuclear disequilibria (mtDNA/nucDNA)
• Evolutionary significance

5
Hybridization

• Artificial Hybridization
• - useful for studying the genetics of species
  differences and reproductive isolating mechanisms
  involved in speciation
• Natural Hybridization
• - a better understanding of speciation
• - interaction of genetics and ecology
• - the role of hybridization in evolution

6
Hybridization

• E. Mayer (1963) Evolutionary role of
  hybridization
• Perfection of isolating mechanisms
  (reinforcement)
• Source of new species
• Increased genetic variability (introgression)
• The total weight of available evidence
  contradicts the assumption that hybridization
  plays a major evolutionary role among higher
  animals

7
Hybridization

• G. L. Stebbins (1950) Evolutionary role of
  hybridization
• the true situation, at least as far as the
  higher plants are concerned, lies somewhere
  between the extremes.
• interspecific hybridization may not be as
  uncommon in animals as is usually believed.
• while hybridization is certainly less common in
  animals than plants, and is correspondingly less
  important as a factor in evolution, its influence
  in certain groups may be considerable.

8
Hybridization

• Burke, J. M. and M. L. Arnold (2001) Genetics and
  the fitness of hybrids. ARES 35 31 52.
• Rieseberg L.H. (1998) Molecular ecology of
  hybridization. Pp. 243 265. In Advances in
  Molecular Ecology. G. R.. Carvalho (ed.)
• Dowling, T. E. and C. L. Secor (1997) The role of
  hybridization and introgression in the
  diversification of animals. ARES 28 253-619
• Rieseberg L.H. (1997) Hybrid origins of plant
  species. ARES 2835989
• Harrison, R. G. (ed.) (1992) Hybrid Zones and the
  Evolutionary Process
• Arnold, M. L. (1992) Natural hybridization as an
  evolutionary process. ARES 23237-261
• Barton, N. H. and G. M. Hewitt (1989) Adaptation,
  speciation, and hybrid zones. Nature 314 497-
  503
• Barton, N. H. and G. M. Hewitt (1985) Analysis of
  hybrid zones. ARES 16113-148.
• Moore W.S. (1977) An evaluation of narrow hybrid
  zones in vertebrates. QRB 52 263277

9
Hybridization
Michael L. Arnold
Read Online!
1997
10
Hybridization

• Definitions
• 1. Systematics (narrow)
• Offspring resulting from a cross
  between species
• 2. Current (broad)
• Crosses between genetically
  differentiated forms
• regardless of their current
  taxonomic status
• Introgression gene movement between species (or
  well differentiated populations) mediated by
  hybridization and backcrossing

11
Hybridization
Hybridization among widely genetically separated
species How genetically divergent can species
be and still hybridize? North
American and European Sycamore 20 Myr
separation ? fertile hybrids Frogs, Birds,
Mammals Immunological Distance
separation times species pairs produce viable
hybrids Frogs and Birds
20 Myrs Mammals
2 3 Myrs Explanation of difference
chromosome evolution, regulatory genes????
12
Sycamore Hybridization
X
Plantanus occidentalis
Plantanus orientalis
American Sycamore
European Plane Tree
London Plane Tree
13
Hybrid Zones

• The geography of hybridization
• Spatial relationship among parental species
• and hybrids
• - sporadic among broadly sympatric
  species
• - contact zone between parapatric
  species
• - linear (hybrid zone width)
• - mosaic (patchy)
• - spatial/temporal stability

14
Hybrid Zones
Species A
Hybrid Zone
Species B
Dr. Stephen A. Karl
15
Hybrid Zones

• Natural experiments
• - many generation of
  hybridization and
• recombination
• - areas of strong selection
• - ecological context
• - processes that cause divergent
  evolution
• 
  (speciation)
• - adaptive evolution
• Windows on evolutionary process

16
Hybrid Zones

• Regions of secondary contact of formally
  allopatric (or parapatric) taxa.
• Evidence
• - cline of allele frequencies through
  hybrid zone
• - concordance among several genetic
  markers
• - deviation from HWE
• - LD among loci

Due to nonrandom mating
17
Postglacial colonization from different refugia
Hybrid Zones
Zones of secondary contact and hybridization
18
Fire and yellow-bellied toads
Bombina bombina
Bombina variegata
19
Bombina bombina
Bombina variegata
20
Allozymes
Clines
Morphology
21
Bombina
Hybrid Zone
Allele freq. 5 loci
Linkage Disequilibrium
22
Hybrid Zone Models

• Models of Hybrid Zone maintenance
• 1. Tension Zone Model
• - balance between dispersal into
  hybrid zone and
• selection against hybrids
• 2. Bounded Hybrid Superiority Model
• - hybrids have high fitness in
  ecological transition
• zones between parental taxa
• 3. Mosaic Model patchy distribution of
  hybrids and
• parental species

23
Hybrid Zone Models

• Models of Hybrid Zone maintenance
• 1. Tension Zone Model
• - selection against hybrids reflects
  intrinsic hybrid
• inferiority regardless of
  environment
• - hybrid zone maintained by
  interactions among
• genes and are not fixed to any
  position

24
Hybrid Zone Models

• Models of Hybrid Zone maintenance
• 2. Bounded Hybrid Superiority Model
• - some hybrid genotypes more fit in
  habitats that
• are novel relative to the habitats
  of the parent
• species position of hybrid zone
  determined by
• local environment
• 3. Mosaic model
• - patchy distribution of habitats.
  Parental species
• and hybrids adapted to different
  environments

25
Genetic Markers

• Nuclear
• Cytoplasmic (mtDNA, cpDNA)
• Sexual asymmetries (which species maternal and
  paternal parent)

26
Hyla cinerea
Hyla gratiosa
27
Hyla cinerea
Hyla gratiosa
Call
Call
28
Example Hybridization

• Tree Frogs
• Hyla cinerea (c)
• Hyla gratiosa (g)
• F
  x M
• From behaviour expect g x c hybrids
• 5 allozyme markers mtDNA

29
Example Hybridization

• Genotype Categories
• Pure cinerea, pure gratiosa
• F1 hybrids 5-locus heterozygote
• Backcross cinerea
• Backcross gratiosa
• Later-generation hybrids (F2)

30
Example Hybridization

• Two markers
• cc cc x gg gg
• cg cg F1
• Backcross
• cg cg x cc cc cg cg x
  gg gg
• cc cc
  cg cg
• cc cg
  cg gg
• cg cc
  gg cg
• cg cg
  gg gg

Mixed Genotypes
31
Example Hybridization

• True backcross mistaken for pure species
• cg x cc -----gt ½ cg ½ cc
• (½)n
• n of markers
• 5 markers 0.03

32
Example Hybridization

• Table 7.5 mtDNA
• gratiosa
  cinerea
• Pure gratiosa 103
  0
• Pure cinerea 0
  60
• F1 20
  0
• cinerea BC 22
  36
• gratiosa BC 52
  1

33
Example Hybridization

• Table 7.5 Interpretation
• F1 hybrids g x c F1 g
  mtDNA
• cinerea BC mtDNA
• F1 x c g
• c x F1 c
• gratiosa BC
• F1 x g g
• g x F1 g

Female Male
Female Male
34
Example Hybridization

• Genetic structure of Hyla hybridization
• Not all individuals participate in hybrid
  matings (high frequency of both parental species)

Bimodal Hybrid Zone
35
Example Hybridization

• Table 7.5 mtDNA
• Allozymes gratiosa cinerea
• Pure gratiosa 103 0
• Pure cinerea 0 60
• F1 20
  0
• cinerea BC 22 36
• gratiosa BC 52 1
• Later generation 9 2
  3

54
7
36
36
Bimodal Hybrid Zones

• Bimodal hybrid zones and speciation
• Chris D. Jiggins and James Mallet
• http//www.mun.ca/biology/dinnes/B4250/Biol4250.ht
  ml