Biol 4802 Evolution
Lecture 24, Chapter 17
Topics for today:
1. Process of divergence from populations to species
2. Genetic mechanisms of reproductive isolation
Process of genetic divergence
Analysis of allozyme data & cross-compatibility studies of different Drosophila
populations and species over the last 60 years
Showed that:
1. Genetic differences accrue from population  nonsibling species
Fig. 15.10 old only
 The strength of prezygotic and postzygotic isolation increases gradually with
time
 Time for full reproductive isolation varies (D = 0.30-0.53)
2. How long does it take to evolve reproductive isolation?
 Molecular clock estimate 1.5-3.5 my
 Other study suggested < 1 my in one Drosphila species pair
Fig. 17.10 new 15.11 old
3. Are prezygotic or postzygotic mechanisms more important?
 Prezygotic isolation is a strong barrier to gene exchange especially for:
o Recent speciation events (little genetic distance)
o Species in sympatry (occur in same place)
 Postzygotic isolation (hybrid sterility or inviability) evolves in males before
females
Fig. 17.10 new 15.12 old
How do species differ?
 Kinds of differences
 Genes involved in adaptation that arose by natural selection
 Genes that influence reproductive isolation
 Neutral genetic differences subject to genetic drift
 When did the differences arise?
 In different geographic populations before speciation
 At the time of speciation
 After reproduction isolation
 Difficult to identify specific genes that are responsible for reproductive isolation and
speciation
How to study genes affecting reproductive isolation? (QTL)
 Two species that have diverged relatively recently
 Genetic map for each species with many markers
o Do the autosomes affect male sterility?
 If so, what location?
o Does the X-chromosome affect male sterility?
 If so, what location?
 X chromosome has major effect
 Each chromosome arm carries at least one gene that effects
sperm motility
 This and other studies show ~40 genes on X
 ~120 on autosomes
 Other recently dervied species show as few as 5 gene regions
Fig. 17.12 new 15.13 old
Positive epistatic relationships with X-chromosome disrupted
Fig. 17.13 new 15.14 old
Underlying causes of Haldane’s Rule
Heterogametic shows greater hybrid sterility
 X-chromosome has a greater effect than any of the autosomes on sterility
 Sterility is only expressed when combined with autosomes of other species
 Indicates positive epistasis has developed between genes on the X- chromosome
and autosomes
Why the X?
 Genes on the X evolve more rapidly
 In XY condition, recessive alleles on X are more exposed to natural selection than
alleles on autosomes
 In addition, males are under stronger sexual selection than females
What do these gene do?
We don’t know…
 One gene is known that causes hybrid inviability
 Nup96 encodes a nucleoporin protein
 Regulates the passage of proteins and RNA between nucleus and cyotplasm
 High ratio of nonsynonymous (R, replacement) to synonymous (S) changes indicates
strong selection [this has been corrected from the original version of the notes that
were posted - JRE 12/16/08]
Fig. 15.15 old only
Genetic causes of reproductive isolation?
1. Break up of positive epistasis reduces hybrid fitness (Dobzhansky-Muller incompatibility)
2. Chromosome differences among populations may lead to irregularities in segregation
patterns
Example: Two jimsomweed species
 Differ by five reciprocal translocations
Fig. 17.14 new 15.16 old
3. Break up of positive epistasis between cyotplasmic and nuclear genes may cause
developmental problems
4. Differences in chromosome number prevent introgression
 Polyploidy in plants
 Burrowing mole rat varies in number of chromosome pairs due to chromosome
fusion
 Hybrids presumably have low fitness due to irregular pairing
 Hybrid zone is very narrow
Fig. 17.15 new 15.17 old
5. Genomic rearrangements after hybridization prevents backcrossing the parental species
 Sand dune sunflower species formed by hybrids of two other sunflower species
 Experimental hybridization results in consistent recombination types
 Low compatibility with parental species
 Nonrandom recombination patterns cause speciation
 Three experimental hybrid lineages
 % of individuals with one of the parental linkage blocks
Reisberg et al. 1996 on web page
Reproductive isolation is the defining feature of speciation
Genetic differences accrue between populations by:
 Mutation
 Natural selection
 Genetic drift
However, reproduction isolation necessary to maintain distinctions
 Speciation can be driven by genetic factors
1. Genetic divergence
2. Break up of positive epistasis (Dobzhansky-Muller)
3. Cytoplasmic incompatibility
4. Chromosome divergence (cytology)
5. Recombination in hybrids
 Speciation can be driven geographical factors