week 9 part 2
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The Origin of Species (Ch. 24): Part 2
• Biological species concept‐ Dr. Dorken • Specia?on with isola?on‐ Dr. Dorken • Isola?ng mechanisms‐ Dr. Dorken • Sympatric specia?on‐in part, Dr. Dorken • Hybrid zones • Specia?on speed and number of genes • Hybridiza?on and conserva?on
The Biological Species Concept
A species is a “group of popula0ons whose members have the poten0al to interbreed in nature and produce viable, fer0le offspring, but do not produce viable, fer0le offspring with members of other such groups.” (Mayr
1942)
Note: Not the ONLY defini?on of a species and troublesome for some kinds of organisms who only breed asexually.
The maintenance of species depends on isola?ng mechanisms!
Ernst Mayr (1904‐2005)
Biological Species Concept relies on no?on of Reproduc?ve isola?on
• Barriers that block gene flow between two species and limit the forma?on of hybrids
1. prezygo0c
2. postzygo0c
Evidence for the basis of allopatric speciation: further populations are more genetically distinct: also called ‘isolation by distance’.
Ramachandran S et al. PNAS 2005;102:15942-15947
©2005 by National Academy of Sciences
But then there has to be reproduc?ve isola?on too!
Figure 24.9
Deg
ree
of re
prod
uctiv
e is
olat
ion
Geographic distance (km) 0 50 100 150 200 250 300
2.0
1.5
1.0
0.5
0
Sympatric (“same country”) specia?on Note: Used to be considered controversial and rare!
Can occur through: (1) polyploidy (2) habitat differen?a?on and/or (3) sexual selec?on (Note: these are also factors that can promote allopatric specia?on)
Figure 24.11-1
Species A 2n = 6
Species B 2n = 4
Normal gamete n = 3
Meiotic error; chromosome number not reduced from 2n to n
Unreduced gamete with 4 chromosomes
Figure 24.11-2
Species A 2n = 6
Species B 2n = 4
Normal gamete n = 3
Meiotic error; chromosome number not reduced from 2n to n
Unreduced gamete with 4 chromosomes
Hybrid with 7 chromosomes
Figure 24.11-3
Species A 2n = 6
Species B 2n = 4
Normal gamete n = 3
Meiotic error; chromosome number not reduced from 2n to n
Unreduced gamete with 4 chromosomes
Hybrid with 7 chromosomes
Unreduced gamete with 7 chromosomes
Normal gamete n = 3
Figure 24.11-4
Species A 2n = 6
Species B 2n = 4
Normal gamete n = 3
Meiotic error; chromosome number not reduced from 2n to n
Unreduced gamete with 4 chromosomes
Hybrid with 7 chromosomes
Unreduced gamete with 7 chromosomes
Normal gamete n = 3
New species: viable fertile hybrid (allopolyploid) 2n = 10
Figure 24.12 How mate choice can cause reproductive isolation in cichlids
Normal light Monochromatic
orange light
P. pundamilia
P. nyererei
EXPERIMENT
Seehausen and van Alphen 1998. Behav. Ecol. Sociobiol.
Females strongly preferred own females under normal light, but did not discriminate under orange light
Conclusion about cichlid divergence:
Rapid divergence in this and other African, South America and southern Indian lakes due to ini?al habitat differen?a?on, then, specia?on, which probably occurred rapidly especially if either a single gene (e.g., for habitat characteris0cs) or many are linked with genes for mate preferences.
Concept 24.3: Hybrid zones reveal factors that cause reproduc?ve isola?on
• A hybrid zone is a region in which members of different species mate and produce hybrids
• Hybrids are the result of ma?ng between species with incomplete reproduc?ve barriers (or isola?ng mechanisms)
© 2011 Pearson Education, Inc.
Figure 24.13
EUROPE
Yellow-bellied toad, Bombina variegata
Fire-bellied toad range
Hybrid zone
Yellow-bellied toad range
Fire-bellied toad, Bombina bombina
Freq
uenc
y of
B
. var
iega
ta-s
peci
fic a
llele
Yellow-bellied toad range
Hybrid zone
Fire-bellied toad range
Distance from hybrid zone center (km) 40
0.99
0.9
0.5
0.1
0.01 30 20 10 0 10 20
Poor hybrid survival means the hybrids do not spread into the other species range/habitat
These op?ons will depend on cost of hybridiza?on
Assorta0ve ma0ng for your own species will be strong and reinforced if
hybrids have lower fitness!
Figure 24.15
Females choosing between these males:
Females choosing between these males:
Sympatric pied male
Sympatric collared male
Allopatric pied male
Allopatric collared male
(none)
Female mate choice Female mate choice
Own species
Own species
Other species
Other species
Num
ber o
f fem
ales
28
24
20
16
12
8
4
0
Assorta?ve ma?ng in sympatry prevents hybridiza?on..no selec?on against it in allopatry
Figure 24.16
Pundamilia nyererei Pundamilia pundamilia
Pundamilia “turbid water,” hybrid offspring from a location with turbid water
Figure 24.18
A dune wild sunflower (Helianthus anomalus) originated by hybridiza?on (poten?ally very common in plants)
Typical Mimulus lewisii
(a)
Typical Mimulus cardinalis
(c)
M. lewisii with an M. cardinalis flower-color allele
(b)
M. cardinalis with an M. lewisii flower-color allele
(d)
Figure 24.20
Pollinators (needed for successful reproduc?on) act as pre‐zygo?c isola?ng
mechanisms In hybrid zone, nearly 98% of pollinator visits were to one species or the other (i.e., pollinators preferred either pink or
red flowers)
Schemski and Bradshaw 1999. Proc Natl Acad Sci U S A. 1999 October 12; 96(21): 11910–11915.
Two genes determine pollinator visitaton through colour produc?on
So two muta?ons can lead to specia?on! (Usually more, e.g., Sunflower at least 26 chromosome segments). ‘Yup’ locus is yellow upper locus
Red Yellow and inter.
Conserva?on implica?ons of hybridiza?on (with special reference to
Ontario) Red Mulberry (Morus rubra)
FACTS: Fewer than 300 in province Only 10 sites have > 5 individuals Gene?c assimila?on with more aggressive White Mulberry (Morus alba) Hybrids have higher reproduc?ve success than pure individuals
Solu?on: May need to at least preserve hybrids in the short term to prevent complete loss of Red Mulberry genes; but also destroy White Mulberry trees (shiny leaves on top!)
Threats to Golden‐winged Warbler Main threat is hybridiza?on with Blue‐winged Warbler
Facts: Breed in shrubby areas recently disturbed
But if they are hybridizing naturally what does it mamer if GWWA’s
disappear? Hybrids do not sing intermediate songs so pure GWWA females can easily breed with hybrid (males)
Proportions of ancestry of each geographic region defined with the model-based method STRUCTURE (Pritchard et al. 2000; Falush et al. 2003).
Wilson P J et al. J Hered 2009;100:S80-S89
© The American Genetic Association. 2009. All rights reserved. For permissions, please email: [email protected].
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