Transcript Lecture 26 (4-8-11)

Enhancement of prezygotic barriers to interspecific crosses
Peripatric speciation: founder effect?
Paradise kingfishers
Parapatric speciation in progress across c. 300 feet of ground
Grass: Anthoxanthum odoratum
Self-pollination vs.
cross pollination
• Sex as an Adaptation
• Most species are (gonochoristic) bisexual.
• Therefore, there must be some advantage to sexual
reproduction.
• Previous lecture: gonochoristic lizard species have fewer
parasites than syntopic parthenogenetic congeners.
• Sexual reproduction involves genetic recombination
• The trade-off: separating favorable gene combinations.
• If A1 is always associated with B1 and A2 with B2, the
genes are in linkage disequilibrium.
• Linkage equilibrium is achieved by crossing over.
• Eventual result = association of 2 or more alleles at the
frequency predicted by their individual frequencies.
Bringing together
A1 B1/ A1 B1 and
A2 B2 / A2 B2 in
the same population
A1 B1/ A2 B2
Decay of linkage
disequilibrium
Continues until alleles at
The 2 loci are randomly
associated with each
other
A and B are in linkage equilibrium if
(gA1B1*gA2B2) – (gA1B2*gA2B1) = 0
Countering linkage equilibrium: Primulus vulgaris
Keep genes
together that
promote outcrossing
Heterostylous
G and A closely
linked
Four Reproductive Modes
• An hypothesis (Maynard Smith): Asexual reproduction
(all females) should be twice as advantageous as sexual
reproduction--where half of the population are males.
• Theoretical basis – Female fitness is maximized if her
genes are not “wasted” in the production of males.
• However, when actual asexual populations are studied e.g.,
(parthenogenetic vertebrates) there are some obvious
disadvantages.
• 1. An asexual mechanism in vertebrates is difficult to
produce.
• 2. The two sexual species that hybridize to produce
successful asexual entities:
– A. must be sufficiently divergent genetically to modify
normal meiosis but
– B. can’t be too divergent genetically or development
will be abnormal.
– Even if successful, a individuals can be produced with
developmental deformities.