Evolution Lecture 24

Chapter 15-16

Topics for today:

1. Process of divergence from populations to species
2. Genetic mechanisms of reproductive isolation

EvoBeaker: Snails

Exercise 4

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 è nonsibling species

Fig. 15.10

· 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. 15.11

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. 15.12

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. 15.13

Positive epistatic relationships with X-chromosome disrupted

Fig. 15.14

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

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. 15.16

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. 15.17

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