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Lecture 4: Ecology and Evolution
Reading: Economy of Nature, pp. 369-395.
Ecological Niche (Hutchinson, 1957)
The set of conditions and resources minimum
and maximum values that are limiting to a population of a given species.
This is an n-dimensional hyperspace.
The fundamental niche is the niche defined
by abiotic factors alone.
The realized niche is the fundamental
niche with biotic factor limitations superimposed. The realized niche
is typically smaller than the fundamental niche.
Two or three-dimensions are typical in
niche descriptions (see Ricklefs, 1996, p 107, Fig 5.4)
Limiting Conditions and Resources
Generalizations
- Lethal conditions may limit distributions
but such conditions need only occur occasionally.
- Distributions are more often limited
by regularly suboptimal conditions (rather than lethal) leading to reductions
in growth, reproduction, or increased predation.
- Sub-optimal conditions act by altering
outcomes of biological interactions.
- Sub-optimal conditions often interact
with other factors (determining which single condition is critical can
be difficult). At the edge of a species distribution, individuals
occupy patches most like the conditions in the center of that species
range.
- Evolutionary responses tend to modulate
effects of suboptimal conditions.
Ecology and Evolution
What is changing as a result of evolution?
Phenotypes
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Changes
in phenotypes is reflected in qualitative and quantitative aspects
of interactions between organisms
and their environment. |
What is the phenotype? Any aspect of
an organism except the information encoded in the genetic materials
(genotype).
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Genotype
+ Environment >>>> yields>>>>>>>Phenotype |
Phenomena of special interest:
Organic diversity: number of different
species
Patterns in nature: distribution and
abundance
Adaptation: close fit between form and
function
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aspects
of phenotypes seem formed for specific
functions |
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products
of evolution by natural selection |
Evolution is change in phenotypes over
time, long term change
Involves changes in gene (allele) frequencies
in a population
Evolution is measured in population change,
cumulative genetic change is the result which is reflected in phenotypic
change.
Importance and Sources of Phenotypic Variation
Natural selection occurs when there is
phenotypic variation regardless of the source, but only variation with
a genetic basis (phenotypic differences caused by genotypic variation)
can be the source of evolutionary change.
What causes phenotypic variation?
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Since
the phenotype is the product of an interaction
between information, the genotype, and the environment,
variation in either genotypes or environments can cause
variation in phenotypes. |
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All traits
have a genetic background but not all differences
between individuals are due to genetic differences. |
Imagine a plant species with mono-hybrid
genetics for height, semi-dominant allelic interactions (dosage effects),
and strong environmental effects on height.
Given a population that is isogenic and
homozygous, there will still be variation in height, but the variation
will be due entirely to environmental variation.
Another population with two alleles,
A and a, in equal frequency and given random mating within
that population, there should be three genotypes produced in ratios
of 1:2:1 as given below:
Alleles for plant height: (A)
tall information (a) short information
Genotypes:
AA Aa
aa
tallest information shortest information
Allele frequencies: f(A) = 0.5
f(a) = 0.5
Genotype frequencies: f(AA)
= (0.5)(0.5) = 0.25
f(Aa) = 2(0.5)(0.5) = 0.50
f(aa) = (0.5)(0.5) = 0.25
Frequency histograms for plant height
in isogenic and polygenic populations with strong environmental influences
on phenotypic expression.
*Note that both populations exhibit very
similar phenotypic variation, frequencies of specific heights, indicated
by the darker outline curve.
Selection can and will occur in both isogenic
and polygenic populations. No phenotypic change can occur in the
isogenic population as a result of selection. No evolution can occur in
the isogenic population.
Only selection on traits whose variation
has a genetic basis (polygenic population, heritable variation) can result
in phenotypic change = evolutionary change.
Forms of selection and resulting forms
of evolution. The frequency distributions show how populations respond
to selection assuming that the observed phenotypic variation results,
in part, from genotypic variation (polygenic populations). The shaded
area of each distribution indicates the negative action of selection,
for example predation on the tallest individuals in a population.
The causes for natural selection, the causes
for differential survival and reproduction, are the hostile forces of
nature (limiting resources and conditions).
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