Lecture by Tricia Abe
BIOL 606 Session, University of Alberta, March 24, 1999.
Rapporteur: Chris B. Cameron
Tricia Abe began her lecture by asking 'How do genetic drift and selection interact to produce new species?" That is, is speciation a by-product of adaptation or do adaptive differences accumulate after genetic reorganization has occurred in a founder event? Viewing speciation in a historical context we see that Darwin (1859) was the first to recognize lineage splitting in 'The Evolution of Species', but he didn't address the underlying mechanism of speciation. Fisher (1918) documented the accumulation of new and favorable mutations in a population, and that adaptation produces genetic differences as a side effect. In 1931 Wright noticed that the non adaptive drift in small stable populations is sufficient to favor gene combinations that are unlikely to occur in larger populations, and that eventually this drift would become 'hard wired' by natural selection. In 1940 Mayr formulated the biological species concept and by 1954 recognized that random genetic drift and changes in selection (due to bottlenecks or founder events) can cause a rapid shift to a new, coadapted combination of alleles and cause reproductive isolation.
Reznick et al. (1997) interests in the ecological causes of divergence prompted them to measure life history traits of guppies in (experimental) low predation environments and (control) high predation environments. Indeed, predation pressures on the fish determined the evolution of life history traits. Guppies in low predation rivers take longer to become reproductive and are larger when reproductive than fish from the same initial population that are exposed to high predation pressure. Similarly lizards that are transplanted from an initial island population to islands with lower vegetation (Losos et al., 1997) were found to increase hind limb length with perch diameter. These two studies demonstrate that the environment of a population can determine both life history traits (reproductive timing) and the evolution of new morphologies (hind limb length). Ecologically important changes can happen rapidly, but can ecological divergence keep populations reproductively isolated. In other words, can parallel evolution lead to parallel speciation?
Using the example from stickleback fish, Schluter's (1995) introduced four criteria for parallel speciation. They are 1. that separate populations in similar environments must be phylogenetically independent, 2. that ancestral and descendant populations must be reproductively isolated, 3. that separate descendant population inhabiting similar environments must not be reproductively isolated from each other, and finally 4. that the adaptive mechanisms must be identified and tested. Jones and Culver (1989) observed that different populations of amphipods that migrated from light exposed ponds to caves in Virginia independently evolved long antennae and large bodies and reduced their eyes. In a field study they separated an original population of amphipods into a cold, dry, dark environment and into a wet, moist, light environment. This treatment was repeated for another population. After 5 years of adaptation in allopatry, prezygotic isolation evolved only among divergently selected lines. Surprisingly, lab studies of parallel selected populations showed no reproductive isolation, indicating that a pleotropic link existed between the selected traits and prezygotic isolation occured. With convincing evidence that reproductive isolation can occur as a byproduct of adaptive divergence, Miyatake and Shimizu (1999) designed a study wich aimed to pinpoint the cause of reproductive isolation.
Miyatake and Shimizu (1999) selected 8 lines of Drosophila for different traits over a period of 53 generations. What they found was that two characteristics that are controlled by timing (timing of mating and timing of larval development) are pleiotropically linked. Finally, a significant, positive correlation between larval development period and timing of mating was the cause of divergence in the different lines.
Discussant: Grant McIntyre
Mark Wilson began the discussion by asking 'Why are independently isolated populations of sticklebacks not reproductively isolated?' Tricia Abe indicated that the isolation in sticklebacks may be behavioral, and S. Graham added that the time since isolation of the fish may be too short for sufficient divergence to have occurred.
Jackson felt that the key paper (Orr and Smith, 1999) was preaching to the converted selectionist, he also assumes that selection is the normal mode of speciation and that drift is secondary.
Jackson began a debate by implying that intrinsic barriers to gene flow (genetic traits that increase pre- or postzygotic reproductive isolation) is the same as reinforcement. Strobeck pointed out that reinforcement is different in that it only occurs when two populations come back together. Palmer agreed, stating that reinforcement is the accumulation of differences in allopatry.
Graham asked (in reference to Schluters stickleback populations), 'What caused the biological difference in sympatry, drift or selection?' Palmer pointed out the Schluter's parallel speciation example emphasizes whether selection for drift is due to the environment that they are in versus selection for differences is due to gene flow.
Bergstrom (a stickleback biologist) pointed out that similar forms may be due to multiple invasions of the environments.
Graham pointed out that gene flow alone is not likely to explain the differences in morphology because gene flow is a homogenizer.
Strobeck pointed out that prezygotic isolation, whether it be behavioral or due to morphological differences, results from changing rapidly to a new environment.
Palmer asked if chromosome differences are responsible for the isolation of species or alternately, was the isolation responsible for the chromosome differences? Strobeck answered that in Hawaiian Drosophila populations (where high speciation rates are reported) it was the isolation (due to multiple lava flows) that led to chromosome changes via translocation or inversion.
Palmer asked if there was something peculiar about Drosophila in that it can tolerate chromosome inversions? The answer to this question is not clear (at least to us), but Strobeck commented that Drosophila cannot tolerate inversions where the chromosomal centromere is located.
Jackson commented that the paper (Orr and Smith, 1999) equated the test for ecology in speciation with parallel speciation.
Jackson was surprised to learn that benthic stickleback mate with benthics (between ponds) easier than benthics mate with limnetics (within ponds). Palmer assumes that the reason for nonmating within ponds is behavioral and/ or morphological, assuming that there hasn't been long enough time for isolation to occur.
Abe shared an example in plants where an environmental divergence promoted speciation. the presence of a 'copper resistance gene' in a plant enables it to live in copper containing soil, and although there is no reason (initially) that speciation should occur, eventually it does because plants without the 'copper resistance gene' are isolated from plants with the gene.