Download Chapter 8 Population genetics and natural selection

Chapter 8 Population genetics
and natural selection
 Variation within populations
--Variation within plant populations
Phenotypic variation: Variation
responded to the environment, but
not
hereditary changes.
Example: Bonniet’s survey and experiment:
Potentilla
grown at higher elevation
grown at lower elevation
different stem length,
different internode length
different leaf area. Fig.8.4;
Kerner’s planting experiment,
plants of a same species when
grown at higher and lower
altitudes, also show different
morphological difference. Fig.8.6.
 But further experiment presented
that plants from alpine seeds and
from low elevation site (180m)
seeds are indistinguishable. This
means that the morphological
differences are morphological
adjustment to environmental
conditions and not hereditary
changes.
 --Genetic variation: Variation is heritable
even though in a different environment.
Example: Turesson’s planting
experiment.
Plants from nine different areas planted
in the same garden show different
morphology. Fig.8.8
Another example: Fig.8.10.
 Variation within animal populations
Example: Sauromalus obesus, a large herbivorous
lizard of the southwestern United States. The
lizard at higher elevations has larger body than
that at lower elevations. Because rainfall is higher
and more constant at higher elevations, and food
availability is much more constant. Tracy (1999)
raised juvenile lizards in a same exp. Environment
of the species from six populations living at
different elevations (200-890m). At the end of the
experiment, lizard from higher elevations grew to
a larger size. Fig.8.14.
 Change due to chance
--Relation between genetic
variation/diversity and population size:
Larger population generally have higher
genetic diversity than smaller populations.
Example: Chihuahua spruce.
Chihuahua spruce is restricted to small,
highly fragmented cool moist areas of
subalpine environment, at elevations
between 2200m and 2700m. Relationship
between population size and genetic
diversity is showed in fig.8.18.
--Isolated island populations esp.
endemic island populations have lower
genetic variation/diversity than mainland
populations. Fig.8.19, Fig.8.20.
 Natural selection
Natural selection by environment can
result in evolutionary change in
population, result in adaptation to the
environment. Example: Lizard exp. of
Losos et al. (1997).
Losos et al. (1997) experiment:
Lizards from home island (forest height
10m)
14 islands (with no lizard before)
with different forest height (1-3m)
10-14 years later, morphological measurement
Result: Larger differences from the forest
in Staniel Cay presented larger
morphological
variations. Fig.8.23.
 Estimating genetic variation in populations
Methods: (1) Transplant experiments.
Example: Transplanting of Potentilla
glandulosa.
Plants from lowland and alpine –Mid-elev.
garden
Plants from lowland and mid-elev. – alpine
garden.
Plants from mid-elev. and alpine – lowland
garden.
 If there were no genetic differences
among the populations, all plants would
grow equally well in all gardens.
 If there were significant differences in
growth among the populations in a
same garden, there were genetic
differences among the populations. Fig.
8.28.
(2) Molecular method:
a. Measuring allozymes (等位基因酶).
Fig.8.18.
b. Measuring numbers and lengths of
DNA
fragments.
c. Measuring DNA sequencing.
Fig.8.29