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Lecture 11: Phenotypic Variation and Review
Reading: Economy of Nature, pp. 241-253.
Reminder: Examination I (on lectures
1-11)
Ecotypic Variation
Ecotypes are local specializations within
a species that have evolved as a result of natural selection of subpopulations
within a species. Ecotypes are genetically different from each other
and reflect an evolved response to local environmental variation. This
term was first used to describe variation among plants.
In any species, observed variation in
size, shape, coloration, behavior, physiology, or biochemistry may be
a product of current environmental differences between the sites at
which individuals are living, a product of heritable differences genotypic
differences = ecotypic differences) between individuals from different
sites, or a combination of both causes for phenotypic variation.
Transplanting individuals possessing
different traits to a constant environment or performing cross transplants
between natural sites is a means of evaluating the relative importance
of environmental and genetic variation in producing the observed phenotypic
variation. The finding of persistent differences between subpopulations
independent of environmental conditions suggests that genetic variation
underlies observed phenotypic variation.
The cause for variation in yarrow (Achillea)
is evaluated by growing seeds from different subpopulations in a garden
near sea level. The persistence of subpopulation differences in growth
form, height, and amount of seed production indicates that subpopulation
differences are ecotypic (have a genetic basis) (Ricklefs, 1996, p 322,
Fig. 14.16).
Clinal Variation
Often the geographic variation observed
within a species is gradual and continuous (for example biomass and
linear body measures). Gradual geographic variation in phenotypes is
termed clinal variation. The causes for such variation can be either
environmental differences, genotypic differences, or both. The causes
for clinal variation and discontinuous variation are evaluated in the
same manner.
Clinal variation caused by genetic differences
(=ecotypic variation) is seen in a species of field cricket in Japan.
The duration of nymphal development and adult head width remain different
even when cricket nymphs from different environments are raised under
constant laboratory conditions (Ricklefs, 1996, p 323, Fig. 14.17).
Reproductive Life Cycle Patterns (Variation)
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Semelparous:
single life-time reproduction, "big-bang" life cycle
reproduction leads to death of adults because all effort goes to
offspring production at one time salmon, bamboo, annual desert
plants |
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Iteroparous:
repeated reproduction during life-time reproduction
does not typically lead to death of adults most mammals and birds |
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