INFERRING PROCESS FROM PATTERN
The mechanics of population genetics attempts to describe how alleles
and genotypes change from one generation to the next. Often evolutionary
biologists cannot study these transitions in real time, either because
the generation time is too long (redwoods, elephants) or the species are
fossils or other necessary information cannot be measured. In some cases
the patterns of variation can suggest or indicate what processes
have been acting. Ideally we would want to look at the patterns of variation
and be able to say unequivocally that one or another process had produced
that pattern. This is not always possible since identical patterns can
be produced by very different processes. BUT: most patterns suggest obvious
experiments that might tease apart various processes contributing to those
Example: trajectory of allele frequency change shows a gradual but rough
increase to fixation. If we had this information alone we could not determine
whether drift or selection drove the allele to fixation. Design testable
hypothesis to distinguish possible causes: measure effective population
size if pq/2Ne << p each generation
might conclude selection drove allele frequency change.
Biston betularia: pattern suggests selection; what is hypothesis and
how would you test it? Bird predation test: mark-release-recapture
experiment in different areas: more dark preyed upon in rural areas (see
figure 5.5, pg. 108), more light form preyed upon in industrial areas.
Patterns of frequencies of dark form is very different in moths with different
population structures. Biston betularia has low density and
flies far to find mates: frequency of dark form is quite uniform
in industrial areas. Gonodontis bidentata has higher density
and is more localized: more variation in frequency of melanic forms.
Different balance of gene flow and selection. Two points: 1) increased
migration rate (m) leads to more homogenization of allele frequencies,
2) increases effective population size (Ne) means that selection
will be more effective (weaker drift).
Heavy metal tolerance in plants. Selection inferred: mine populations
have evolved resistance to high concentrations of copper, zinc and lead.
Test: for selection of genetic basis: transplant plants across
boundary and the "pasture" form cannot grow on the heavy metal
soil: implies true genetic response. Pattern of distribution: cline
in frequency of resistant forms as one moves across boundary. Suggests
gene flow between tolerant and non-tolerant forms, but despite gene
flow divergence is maintained: must be strong selection to overcome effects
of gene flow. Examine patterns carefully: both the presence of a
cline (frequency transition) and the "steepness" of the
cline can say thing about process.
Shell polymorphism in Cepaea nemoralis: find different
colors (pink, yellow, brown) and different banding pattern (five banded,
two-banded, unbanded, etc.). Genetics worked out pretty well: one locus
for color, another for banding number and other modifier loci. Pattern:
find different morphs in different habitats. Is it selection or not? Hypothesis?
= predation. Test=? look at so called "thrush-anvils" stones
where the song thrush who preys on the snails breaks them open to eat.
Look at the frequency of types of shells around thrush anvils and compare
these frequencies to the frequencies in the general population. Apparent
that there is differential predation by habitat: pink and unbanded snails
more common in woods; yellow and banded snails more common in fields and
hedgerows. Implies that visual predation is important character.
Allele frequency clines: Alcohol dehydrogenase locus (Adh) and
a-glycero-phosphate dehydrogenase locus (a-GPDH)
have cline along the east coast of N. America. Hypothesis=? selection differs
with latitude. Test=? compare southern hemisphere: get same cline in opposite
direction implies selection but difficult to identify a specific mechanism
Leucine amino peptidase locus (Lap) in Mytilus edulis polymorphism
varies with habitat and position in Long Island Sound Lap94
allele found in higher salinity waters. Hypothesis=? salinity. Test=? sample
at different time of year with different runoff; find predicted results.
Physiology: determine how the different alleles work in the cell and differ
in performance. Results lead to a prediction of which should be in certain
environments. Test prediction with additional sampling. Test model
of action of selection: phenotype could be additive, mutliplicative or
dominance. Each genetic model gives different fitnesses, hence predicts
different deviation from Hardy Weinberg genotype frequencies. Data fit
predictions of one model (dominance) well.