Ecology – Biology 3500
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:
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 "survival of the fittest" -- make sure you know what fitness means
Variation within populations
Plants -- Cinquefoil examples
Potentilla glandulosa -- significant variation across altitudinal gradient.
If completely environmental, variation would disappear in common garden.
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
4. No immigration or emigration -- in other words, populations are in isolation
5. No selection -- in other words, all organisms have equal fitness
How many of these conditions are met in actual populations? Probably not many, 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:
3. Disruptive -- 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 =
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.
Example: Beak lengths of Soapberry Bugs and introduced foodplants.
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.
Change Due to Chance
(remember that mutation is, in essence, another chance event)
Genetic Variation in island populations -- almost always less than variation in mainland pops.
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.
Chapter 5: Temperature Relations
Local variation in temperatures due to: altitude, aspect, vegetation, ground color,
topographic relief, nearby water (riparian habitats and vegetation)
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 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, conduction, convection, radiation,
evaporation (transpiration for plants)
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 "giant" rosettes
Thermogenic plants (skunk cabbage)
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).
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-
Surviving the extremes: torpor, diapause, hibernation/estivation
Special adaptation in invertebrates -- antifreeze.
Chapter 6: Water Relations -- water moves down concentration 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 -- check out the Cicada story at
the beginning of the chapter.
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 can be closed
to prevent excessive water loss.
Water regulation in animals/plants on land
Again, water losses (how?) must be balanced by water gains (how?)
Examples (see book, pgs. 133-134)
Modifications for acquisition/conservation under certain conditions:
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 spread in drier climates to acquire water when available (cactus)
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.