Ecology – Biology 3500
Dr. Adams
INDIVIDUAL ECOLOGY UNIT
Chapter 4: Population Genetics and Natural Selection
You must know definitions of the following:
evolution, natural selection, adaptation,
genes/alleles, and genetic
terminology (dominant/recessive, homozygous/heterozygous,
incomplete dominance, loci, etc.)
Evolution by Natural Selection -- (VALID) Assumptions:
(includes time)
1. Reproduction with variation.
2. At least some of the
variation is heritable
3. Overproduction of offspring
(leads to competition, more predation/disease, death)
4. The variation means that
some will have a higher chance of survival than others.
Leads to adaptations allowing "survival of the fittest" -- what
IS fitness?
Variation within populations
Plants -- Cinquefoil examples
Potentilla glandulosa --
significant variation across altitudinal gradient.
If completely
environmental, variation would disappear in common garden
experiments.
When grown in
common gardens, not all variation disappears, and those adapted to a
particular altitude largely grew best at their own altitude, though the middle
elevation
individuals appeared to excel at all altitudes -- clearly some
selected genetic differences
that enhanced survivorship within certain habitats
- these are called ecotypes.
Potentilla nivea and
pulchella complexes (on Spitsbergen Island, Norway)
P.
pulchella shows significant differences (three morphs) in different
habitats; common
garden studies, however, indicate that the differences are due to a plastic
genome
The P.
nivea complex, thought to include three species, indeed does when genetic
analysis taken into account.
So, in one genus there is a species (pulchella)
that shows variation that is virtually completely
environmental, another species (glandulosa) which has some genetic basis
for the
morphological
differences between populations, and a species complex (nivea) where
the
differences are significant enough that there are three distinct species
Animals -- Whitefish (Coregonus sp.) in isolated
rivers and lakes in the Alps
Phenotypic and genetic analysis of
specimens from 19 described populations indicate that
using the
data collectively gives a roughly 80% ability to assign individuals to
appropriate
source
populations -- in other words, there is some significant genetic distinctness.
Enough
to be called
species? The investigators went so far as to call them "evolutionarily
significant
units," enough so that they should be managed separately (not mixed).
Hardy-Weinberg -- for a trait with two alleles within a population:
Equations: p = frequency of A
q = frequency of a p + q = 1 (obviously)
p2 = frequency of AA 2pq = frequency of
Aa q2 = frequency of aa
Again, clearly p2 + 2pq + q2 = 1
(I will show you the derivation of
this equation in class, even though you probably know
how it is
derived already; we will also do an example or two or . . . ?)
The idea here is that a population would be considered to be
in H-W equilibrium if the
equations resulted in actual
representation of a real population, and the frequencies didn't
change from generation to generation.
To be in H-W equilibrium, however, the population
would have to exhibit the following:
1. Random mating
2. No mutation
3. Large population size -- prevents chance
events (genetic drift) from altering frequencies
significantly.
4. No immigration or emigration -- in other
words, populations are in isolation, and have NO
gene flow.
5. No selection -- in other
words, all organisms have equal fitness
How many of these conditions are met
in actual populations? VERY few are met, and most
certainly not all, in ANY population. As such, genetic frequencies will
change, which means . . .
EVOLUTION IS OCCURRING!
Natural Selection (H-W requirement #5): understand that selection is
not necessarily happening
continuously in one direction -- what is favorable now may
not be favorable later; it is not a
process of perfection; it may act on different populations of
the same organism differently
(different selective pressures)
Types of selection:
1. Stabilizing -- extremes selected against
2. Directional -- entire curve shifts to one side or other; probably most common
3. Disruptive -- both extremes selected for; typically doesn't happen within one
population
(though it can); this usually happens in different populations of a species, which can
lead to divergence and new species
Heritability -- The fraction of the variation is due to
variation in genes, represented by
h2 = Heritability = VG =
VG__
P = phenotypic, G = genetic, E = environmental
VP VG + VE
If heritability is near 0, then that means . . . ? If heritability
is .50, then that means . . . ?
If heritability is near 1, then that means . . . ?
Understand that natural selection
working on traits with low heritability will NOT result in
any significant genetic change, certainly not in the short term.
Which brings us to an important
question. Can adaptation take place quickly in a trait with
significant heritability? This, of course, is the meat of the idea of
evolution.
Examples:
Stabilizing Selection: Egg Size in Ural owls (and other birds); human birth weight
Directional
Selection: Beak lengths of Soapberry Bugs and introduced foodplants;
you will be
responsible for the story in both the U.S. and Australia.
Turns out that heritability is high
-- juveniles reared on one hostplant retained beak length
when switched
to another. Natural selection has adapted different populations of these
bugs within
30 to 100 years.
Disruptive Selection: Darwin's Finches on the
Galapagos, butterfly species
Change Due to Chance
Genetic Drift -- more potent in smaller populations (as you
will see in lab)
(remember that mutation is, in
essence, another chance event)
Genetic Variation in island populations -- almost always less
than variation in mainland pops.
WHY?
Remember that "islands" can also apply to (semi-) isolated
populations in any kind of habitat.
In reduced patches of habitat with
small populations, inbreeding also reduces variation.
Example:
Glanville Fritillary Butterfly in Finland (see pages 94-95)
Chapter 5: Temperature Relations
Local variation in temperatures due to: altitude, aspect,
vegetation, ground color,
topographic relief
(boulders/burrows), nearby
water (riparian habitats and vegetation)
In aquatic environments, depth in the water minimizes temperature fluctuations,
and,
as indicated previously, water in general buffers changes in temp.
An evolutionary trade-off: Adapting to one set of circumstances typically
minimizes the
ability of
organisms to succeed in other environments. They must choose to allocate
resources so that they can maximize their performance in their chosen environment.
Organismal performance -- most organisms adapted to a rather narrow
range of conditions
for their activities, including a
rather narrow range of temperatures (though homeothermy
provides much greater temperature
tolerance by providing a narrow internal temperature
range). Organisms can allocate
only so much for each activity, and therefore less
temperature stress leaves more energy
for other activities. Why is a narrow temperature
range useful?? Enzyme
function.
Questions to
ponder: 1.Can different organisms have different enzymes to do the
same process but function at different temperatures? and 2. Can the SAME org.
have more than one enzyme to do the same process but function at dif. temps.?
3. Can the same organism acclimate to different environmental
conditions?
Photosynthetic efficiency peaks in
plants from dif. latitudes/altitudes at dif. temps.
This trend is
repeated for virtually any group of organisms.
Endothermy increases range, but
requires more energy input (see below)
Regulating temperature -- an attempt to balance heat
gain vs. heat loss
Sources of
heat/heat loss: metabolism (g), conduction (g or l), convection
(g or l),
radiation
(g or l), evaporation (transpiration for plants) (l)
Poikilothermy (varies with ambient), Homeotherms (constant TB)
Ectothermy and Endothermy
Plants: Different strategies
for different habitats --
Deserts:
little transpiration (why?); leaves narrowed/reflective/off the ground (why?)
Arctic/Alpine: opposite of deserts in many respects:
Leaves flattened/darkened/near ground; can reach temps far above ambient
Tropical Alpine plants and giant rosettes
Thermogenic
plants (skunk cabbage; see page 119)
Animals:
Many
ectotherms (eg., lizards, beetles) bask in cool environs, stand "tall" and
"dance" in hot locations. Insects tend to be darker in cool climes, lighter in
warm
(dif. broods may vary with seasons and with dif. tempse in the environment in which
they grow; see grasshoppers in Fig. 5.20).
Endotherms do
have dif. (but higher than ectotherms) metabolic rates depending on
preferred habitat. Aquatic endotherms typically have significant
insulation.
Interestingly, insects (and others) can act as endotherms with muscular thermo-
genesis (shivering). Know the concept of thermoneutral
zones.
Endothermic aquatic animals (some fish [tuna/sharks], mammals, penguins)
Mammals and penguins -- no large respiratory surface exposed to the water; thick
feathers/fur/blubber protect them from heat loss
Fish endotherms can swim faster and greater distances -- more access to prey
Apparently can maintain temp with heat produced by highly active muscles
Interestingly, some insects can use muscular thermogenesis to raise body temps
significantly above the surrounding air temps (sometimes as much as 50 deg C)
Surviving the extremes: inactivity -- torpor, diapause, hibernation/estivation
Special adaptation in invertebrates -- antifreeze.
Chapter 6: Water Relations -- water moves down concentration/pressure
gradients
Life is a never ending attempt at balancing water loss with
water gain
For terrestrial organisms, especially
in arid environments, it can be the #1 factor
determining existence in a particular
biome
Water availability
Atmospheric water -- relative
humidity/vapor pressure
100% humidity
= precipitation
<100% = vapor
pressure deficit; when low, water leaves organisms into the air
Aquatic environments: You
should understand osmosis, osmotic pressure, and
hypo-/hyperosmotic
conditions. Will briefly discuss invertebrates and bony fish
in
fresh/marine environments,
and cartilaginous fish in marine environment.*
From soil to plants -- follows a
water potential gradient; from soil through xylem to
leaves and
out (transpiration) through stomates/lenticels (a continuous water
column).
Though stomates are for CO2/O2 exchange, stomates WILL be
closed
to prevent
excessive water loss -- so water is the ultimate controller of stomates.
Water regulation in animals/plants on land
Again, water losses (how?) must be
balanced by water gains (how?)
secretions (l), evaporation/transpiration (l), absorption (g)
Examples (see
book, pgs. 133-134)
Modifications
for acquisition/conservation under certain conditions:
Plants: water gain from soil (or water); water loss by transpiration and
secretions
1. more root growth in plants when water stressed, moreso in species found
in drier climates
2. heavier cuticle on/narrowing of leaves in drier climates
3. C4/CAM photosynthesis and stomate narrowing in drier climates
4. broad, shallow root in drier climates to acquire water when available
(cactus)
Animals: water gain by eating/drinking/metabolism; water loss by
urination/feces/
evaporation (sweating in some)
1. more armor (turtles) in drier climates
2. similarly, thicker, more waterproofed cuticle (tiger beetles) in drier
climates
3. activity at night (many mammals, scorpions, etc.); some so efficient at
water
conservation (Merriam's Kangaroo Rats) that they do not need to drink in
desert habitats --subsist entirely on food and metabolic water.
4. drink large amounts (camel) when water available in dry climates
5. evaporative cooling, even possible in some small arthropods
Camels and
Saguaro cactus
Scorpions and
Cicadas -- WHY does the specific *Cicada species discussed
"want" to
be active during the daytime in the desert?
Water and salt balance in Aquatic environments
*As indicated above, you will
need to know what is going on with:
Marine
Fish (bony and cartilaginous) and marine invertebrates
Invertebrates largely isosmotic -- no energy expended to maintain body water,
though may need to expend some energy to balance certain solutes
Cartilaginous -- slightly hyperosmotic; gain water through osmosis (across gut,
gill
membranes), eliminate excesses through (dilute) urine; sodium, too, diffuses in,
with excesses eliminated through a rectal salt gland.
Bony fish -- hypoosmotic; gain water by constantly drinking, but must rid body
of
salt picked up with water -- do so with specialized salt glands associated with
gills, and by excretion of (concentrated) urine
Fresh
water bony fish and invertebrates -- hyperosmotic
Bony fish -- like cartilaginous fish in marine environment; easily gain excess
water
from (and lose salts by diffusion to) external environment. Get rid of
excess water
through large amounts of dilute urine; have cells in gills that actively pick up
salts.
Invertebrates -- tissues are between one-half and one tenth as concentrated as
marine relatives -- limits of dilution are determined by the minimal levels of
solutes
in body fluids that must exist for normal nerve, muscle, etc. function.
Like for the
fresh water bony fish, must actively pump out water and actively pump in salts.