Population-all
of POPULATIONS
POPULATIONS
the individuals of a
species that live together in one place at
one time.
Demography-the statistical study of
populations. It is used to predict how
the size of a population will change.
KEY FEATURES OF
POPULATIONS
POPULATIONS
1.
1.
Population size
Population
is the number of individuals in a population.
is
has an important effect on the ability of the population to survive.
has
A small population is more likely to become extinct:
-in the case of random events or natural disaster
-due to inbreeding where the population is more genetically alike.
-due
Recessive traits are more likely to appear.
Recessive
-with reduced variability it is harder to adapt to changes.
KEY FEATURES OF
POPULATIONS, cont
POPULATIONS,
2. Population density
2. Population
the number of individuals in a given area.
the
if they are too far apart they may only rarely
encounter one another resulting in little
reproduction.
reproduction.
KEY FEATURES OF POPULATIONS, cont
KEY
Population size is limited by:
densitydependentfactors
densitydependentfactors
densityindependentfactors
densityindependentfactors
Disease
Competition
Predators
Parasites
Food
Crowding
Thegreaterthepopulation,
thegreatereffectthese
factorshave.
Ex.Blackplagueinthe
MiddleAgesmoredeaths
incities
Volcaniceruptions
Temperature
Storms
Floods
Drought
Chemicalpesticides
Majorhabitatdisruption(as
intheNewOrleans
flooding)
Mostareabioticfactors
KEY FEATURES OF
POPULATIONS, cont
POPULATIONS,
3. Dispersion
the way in which the individuals are
arranged.
arranged.
Most common
PREDICTINGPOPULATION
PREDICTINGPOPULATION
GROWTH
Model:
Ahypotheticalpopulationthathaskey
characteristicsoftherealpopulationbeing
studied.
Usedbydemographerstopredicthowa
populationwillgrow.
PREDICTINGPOPULATION
PREDICTINGPOPULATION
GROWTH,cont
Nearlyallpopulationswilltendtogrow
exponentiallyaslongasthereare
resourcesavailable.
Twoofthemostbasicfactorsthataffect
therateofpopulationgrowtharethebirth
rate,andthedeathrate.
r(rateofgrowth)=birthratedeathrate
PREDICTINGPOPULATION
PREDICTINGPOPULATION
GROWTH,cont
Exponentialgrowthcurve:
populationgrowthplotted
againsttime.
Asapopulationgetslarger,it
alsogrowsatafasterrate.
Thisisthemaximum
populationgrowthunder
idealcircumstances.
Includesplentyofroomfor
eachmember,unlimited
resources(food,water)and
nohindrances(predators).
FACT:Nopopulationexhibitsthistypeofgrowthforlong.
PREDICTINGPOPULATIONGROWTH,
PREDICTINGPOPULATIONGROWTH,
cont
Logisticmodel:Thismodelaccountsforthe
decliningresourcesavailabletopopulationsas
theygrow.
Itassumesthebirthanddeathratesarenot
constant.
Asthepopulationgrows,birthsdeclineand
deathrises.
Eventuallybirth=deathsothepopulationstops
growing.
Carryingcapacity(K):Thenumberoforganisms
ofonespeciesthatanenvironmentcansupport
indefinitely.
PREDICTINGPOPULATIONGROWTH,
PREDICTINGPOPULATIONGROWTH,
cont
Twomodesofpopulationgrowth.
TheExponentialcurve(alsoknownasaJ
curve)occurswhenthereisnolimitto
populationsize.
The Logistic curve (also known as an S-curve)
shows the effect of a limiting factor (in this case
the carrying capacity of the environment).
POPULATION GROWTH
STRATEGIES
There are 2 ways a population can
prosper:
1. Depends on the rate of growth (r)
2. Influenced by the carrying capacity
(K)
POPULATION GROWTH
STRATEGIES, cont
r-strategists: characterized by exponential
growth, which results in temporarily large
populations, followed by sudden crashes in
population size. Ex. Insects, bacteria,
some plants
live in unpredictable and rapidly changing
environments
Reproduce quickly when conditions are
favorable
Many offspring: small, mature rapidly, no
parental care
r = rate of growth
POPULATION GROWTH
STRATEGIES, cont
K-strategists: characterized by a high degree
of specialization. Ex. Trees, whales, tigers,
etc.
Live in stable and predictable environments
Can compete effectively
Reproduce late in life
Few offspring: large, slowly, mature often much
parental care
K = carrying capacity
POPULATION GROWTH STRATEGIES, cont
Human Populations:
K- strategist characteristics
In recent times however, man has learned
to expand the carrying capacity of his
environment by increasing food supply,
combating pests and curing diseases.
Can Earth support this increase?
Damage to the planet will eventually
reduce the carrying capacity for humanity
and slow the growth of the human
population.
HOW POPULATIONS EVOLVE
Charles Darwin:
Natural selection causes biological diversity.
Modern version:
Populations contain individuals with different
versions of genes called alleles.
Alleles that improve the chances of survival
and reproduction are favored and become
more common.
Changes are caused by mutations in DNA.
Hardy-Weinberg Principle
In 1908 G.H. Hardy and Wilhelm
Weinberg showed that dominant
alleles do not replace recessive
ones.
The Hardy-Weinberg Principle states:
populations do not change unless
evolutionary forces act upon them.
Hardy-Weinberg Principle
Background Information
Recall, it is at the population level that evolution occurs.
A population is a group of individuals of the same species in a
given area whose members can interbreed.
Because the individuals of a population can interbreed, they share
a common group of genes known as the gene pool.
Each gene pool contains all the alleles for all the traits of all the
population.
For evolution to occur in real populations, some of the gene
frequencies must change with time.
The gene frequency of an allele is the number of times an allele
for a particular trait occurs compared to the total number of alleles
for that trait.
Gene frequency = the number of a specific type of allele / the total
number of alleles in the gene pool
Hardy-Weinberg Principle, cont
An important way of discovering why real
populations change with time is to construct a
model of a population that does not change.
This is just what Hardy and Weinberg did.
Their principle describes a hypothetical situation
in which there is no change in the gene pool
(frequencies of alleles), hence no evolution.
Hardy-Weinberg Principle, cont
The frequencies of the alleles will remain unchanged
generation after generation if the following conditions are
met:
1. Large population. The population must be large to
minimize random sampling errors. Genetic drift, the
random change in allele frequency in a population, can
cause great change in small populations.
2. Random mating. There is no mating preference.
3. No mutation. The alleles must not change.
4. No migration. Exchange of genes between the population
and another population (gene flow) must not occur.
5. No natural selection. Natural selection must not favor
any particular individual.
Natural selection is the process by which populations
change in response to their environment.
Natural Selection Shapes Populations
Natural selection is a powerful agent of genetic
change.
HOWEVER: there are limits to what it can
accomplish because selection does not act
directly on genes.
Natural selection acts on phenotype, NOT
geneotype.
THEREFORE: selection against unfavorable
recessive alleles is SLOW.
Natural Selection Shapes
Populations, cont
Polygenic
trait: A characteristic
influenced by several genes.
There are three types of selection on
polygenic traits.
1. directional
2. stabilizing
3. disruptive
Natural Selection Shapes
Populations, cont
NATURAL SELECTION
DIRECTIONAL
SELECTION
STABILIZING
SELECTION
DISRUPTIVE
SELECTION
Favors one extreme
phenotype
Favors the average
phenotype
Favors both extreme
phenotypes
Possible reason:
Predators can identify
easier and eat the
average type organism
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