Pond Boss Forums INTRO Questions & Observations Is this true or a yarn?

Here is another good reference book from AFS





and some info on CENTRARCHID FISHES

TEMPO OF HYBRID INVIABILITY IN CENTRARCHID FISHES
(TELEOSTEI: CENTRARCHIDAE)
DANIEL I. BOLNICK1,2 AND THOMAS J. NEAR3,4
1Section of Integrative Biology, University of Texas at Austin, 1 University Station C0930, Austin, Texas 78712-0253
2E-mail: danbolnick@mail.utexas.edu
3Department of Ecology and Evolutionary Biology, University of Tennessee, 569 Dabney Hall, Knoxville, Tennessee 37996-1610
4E-mail: tnear@utk.edu
Abstract. Hybrid viability decreases with divergence time, a pattern consistent with a so-called speciation clock.
However, the actual rate at which this clock ticks is poorly known. Most speciation-clock studies have used genetic
divergence as a proxy for time, adopting a molecular clock and often far-distant calibration points to convert genetic
distances into age. Because molecular clock assumptions are violated for most genetic datasets and distant calibrations
are of questionable utility, the actual rate at which reproductive isolation evolves may be substantially different than
current estimates suggest. We provide a robust measure of the tempo at which hybrid viability declines with divergence
time in a clade of freshwater fishes (Centrarchidae). This incompatibility clock is distinct from a speciation clock
because speciation events in centrarchids appear to be driven largely by prezygotic isolation. Our analyses used
divergence times estimated with penalized likelihood applied to a phylogeny derived from seven gene regions and
calibrated with six centrarchid fossils. We found that hybrid embryo viability declined at mean rate of 3.13% per
million years, slower than in most other taxa investigated to date. Despite measurement error in both molecular
estimated ages and hatching success of hybrid crosses, divergence time explained between 73% and 90% of the
variation in hybrid viability among nodes. This high correlation is consistent with the gradual accumulation of many
genetic incompatibilities of small effect. Hybrid viability declined with the square of time, consistent with an increasing
rate of accumulation of incompatibilities between divergent genomes (the snowball effect). However, the quadratic
slope is due to a lag phase resulting from heterosis among young species pairs, a phenomenon rarely considered in
predictions of hybrid fitness. Finally, we found that reciprocal crosses often show asymmetrical hybrid viabilities.
We discuss several alternative explanations for this result including possible deleterious cytonuclear interactions.
Speciation-clock studies have been a small cottage industry recently, but there are still novel insights to be gained
from analyses of more taxonomic groups. However, between-group comparisons require more careful molecular-clock
calibration than has been the norm.