BIOL2007 species Y
EVOLUTION IN SPACE AND TIME cladogenesis
Kanchon Dasmahapatra species Z Time Phylogeny with anagenesis and cladogenesis
anagenesis : evolution within lineages microevolution vs. vs. cladogenesis : splitting of lineages, speciation macroevolution
Genetic divergence of populations Genetic divergence of populations
GEOGRAPHIC LOCAL: sympatric divergence allopatric divergence
e.g. island populations
e.g. Rhagoletis host races
parapatric divergence
Note: Distributions may change! Current distribution ≠ original distribution
1 species X anagenesis Spatial differences in gene frequencies may represent speciation in progress
Parapatric distributions and hybrid zones or contact zones species Y within species: a first step in speciation?
rate of Many intermediates between slight genetic differentiation hybridisation cladogenesis and separate species occur in parapatry
The remainder of the lecture will concern parapatric species Z distributions Time Phylogeny with anagenesis and cladogenesis
Genetic variation across a geographic area Measuring dispersal A consistent change in gene frequency heritable phenotype, If dispersal between across a geographical range is known as a cline birthplace and breeding site is Clines occur because dispersal across a region is limited. random.
Dispersal by individuals leads to gene flow Same distribution as passive diffusion: a two-dimensional normal distribution .
Standard deviation, σσσ, of the dispersal distribution is the most useful measure of dispersal .
A population "neighbourhood" : group of individuals who come from an area 2σ wide
2 1 A Dispersal rate, σ Causes of clines: selection/migration balance
0 .6 0.5 0 .5 extrinsic or environment selection
0 .4
0 .3
Gene frequency Gene σ = 0
0 .2
0 .1
0 a 0 10 5 0 5 10 10 5 0 5 10
1 A
0.5 0 .0 7
0 .0 6
0 .0 5
0 .0 4 Gene frequency Gene
0 .0 3
0 .0 2 Width of a cline a 0 .0 1 0 0 10 5 0 5 10 10 5 0 5 10 is proportional 1 A to dispersal
0.5 0 .0 3 5 divided by
0 .0 3
0 .0 2 5 selection/migration balance
0 .0 2 Gene frequency Gene 0 .0 1 5 √(selection): 0 .0 1
a 0 .0 0 5 0 0 10 5 0 5 10 10 5 0 5 10
1 σ A w=1.7 s
0.5 0 .0 0 4 5
0 .0 0 4
0 .0 0 3 5
0 .0 0 3 extrinsic or environment selection
0 .0 0 2 5
0 .0 0 2 Genefrequency
0 .0 0 1 5 σ = infintity
0 .0 0 1
0 .0 0 0 5
a 0 0 10 5 0 5 10 10 5 0 5 10 Distance Distance
environment A environment a Causes of clines:
Cline in peppered moth melanism Cline in sickle cell anaemia
selection for carbonaria selection against carbonaria selection against typica selection for typica
3 Heterozygote disadvantage can cause clines European shrew chromosome races
Two stable situations: fixed for A OR fixed for a Oxford race Different populations fixed for either A or a Hermitage Oxford Ancestral Hermitage fertile unviable fertile Cline where populations mix. Dispersal balanced by Oxford fertile fertile Ancestral fertile selection against heterozygotes. Hermitage race
Under heterozygous disadvantage, 100% σ 75% w=2.83 , ... where s' is average selection s' against homozygotes. Hermitage homo 50% hybrid genotypes Frequency ancestral homo 25% stronger selection, s ⇒ narrower cline; Oxford homo 0% greater dispersal distance, σ ⇒ broader cline. 239 230 224 217 212 205 199 197 190 185 180 175 174 168 166 157 146
Distance / km
Causes of clines: selection/migration balance Causes of clines: selection/migration balance Intrinsic selection Intrinsic selection
FREQUENCY-DEPENDENT SELECTION Moving clines
Intrinsic selection does not depend on the outside environment.
⇒ No tendency for a cline to remain stationary.
If s ≠ t, cline will move. e.g. warning colour: rare forms non adaptive because predators learn commoner colour pattern. Intrinsic selection again
4 Heliconius erato hybrid zone Causes of clines: drift/migration balance
1.0
0.8
allele 2001 yb 0.6 40 km
0.4 1982 Random drift : no consistent directional changes 0.2
Frequency of of Frequency However, locally, drift may result in a temporary 0.0 monotonic change. 100 0 100 200 300 Distance (km E of Panama City)
MJ Blum 2002. Evolution 56, 1992-1998
Causes of clines: drift/migration balance Hybrid zones time Narrow zones of contact between divergent forms or
even species. “Multiple narrow clines”
Within hybrid zones : few hybrids or many
hybrids themselves may consist only of F1 only, or of F1, F2 and every kind of backcross.
Many species and/or races are distributed in parapatry, and have narrow hybrid zones between them.
STAGE II of Sewall Wright’s SHIFTING BALANCE THEORY
5 Anartia fatima amathea hybrid zone Tarapoto mutliple hybrid zones
N A. fatima A. amathea
Tarapoto
Over 20 species of Ithomiine and Heliconius butterflies show clear subspecies differences 75 km
Dasmahapatra et al. 2002 Evolution
The fire-bellied/yellow-bellied toads (Bombina) Meet in a narrow east west hybrid zone stretching over a large part of eastern Europe.
Bombinabombina Bombinavariegata
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The Bombina hybrid zone
Hybrid zones, then, are places where narrow clines at multiple Clines at a few major loci loci occur together. No differences in allozymes, chromosomes etc. Hybrids viable and fertile. Narrow clines due to strong selection.
Hybrid zones, cline theory and speciation Conclusions Hybridising forms/species indicate speciation in progress Species differ genetically at multiple loci. Reproductive isolation incomplete. To understand their speciation, we need to know about Study genes leading to speciation. species X divergence that took place in the past.
Yet for most genetic studies, we only have the present; a thin film on the surface of time.
species Y Spatially separated populations may be “incipient species”. rate of hybridisation
species Z Time
7 Spatial evolution and cline theory: Spatial evolution and cline theory: understanding “extrinsic” or environmental adaptation “intrinsic” selection
Migration can swamp local adaptation. Intrinsic selected genes (heterozygous disadvantage, frequency dependent selection) will also evolve spatially to But wherever the cline width, w, is substantially smaller than form clines. environmental patch width, adaptation can occur in parapatry, in spite of gene flow. These clines can move.
Differently selected forms can evolve in parapatry. Hybrid zones: regions where multiple narrow clines occur.
Can be used to understand gene flow and selection.
Reading
http://www.ucl.ac.uk/taxome/kanchon/biol2007/lectures/kanchon /Hz/Hz.html
FUTUYMA, DJ 2005. Evolution. Ch 6, Ch 9: 326 9
FUTUYMA, DJ 1998. Evolutionary Biology. Geographic variation and clines: chapter 9 (pp. 257 262). Cline theory: chapter 13 (pp. 381 383). Speciation and natural selection in Hybrid zones: chapter 15 (pp. 454 456, 464 468). anciently asexual bdelloid rotifers Dr Tim Barraclough (Imperial College, London) 5 pm Wednesday, 25th February 2009 Medical Sciences AV Hill Lecture Theatre, UCL
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