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Unformatted text preview: POPULATIONS POPULATIONS Population-all of 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
1. Population size
Population – is the number of individuals in a population.
– 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.
Recessive traits are more likely to appear.
-with reduced variability it is harder to adapt to changes. KEY FEATURES OF
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. KEY FEATURES OF POPULATIONS, con’t
Population size is limited by:
densitydependent factors densityindependent factors
densityindependent factors Disease
Food Crowding The greater the population, the greater effect these factors have.
Ex. Black plague in the Middle Ages – more deaths in cities Volcanic eruptions
Major habitat disruption (as in the New Orleans flooding) Most are abiotic factors KEY FEATURES OF
POPULATIONS, 3. Dispersion
– the way in which the individuals are
arranged. Most common PREDICTING POPULATION PREDICTING POPULATION GROWTH Model: A hypothetical population that has key characteristics of the real population being studied.
Used by demographers to predict how a population will grow. PREDICTING POPULATION PREDICTING POPULATION GROWTH, con’t Nearly all populations will tend to grow exponentially as long as there are resources available. Two of the most basic factors that affect the rate of population growth are the birth rate, and the death rate. r(rate of growth)=birth rate – death rate PREDICTING POPULATION PREDICTING POPULATION GROWTH, con’t Exponential growth curve: population growth plotted against time.
As a population gets larger, it also grows at a faster rate. This is the maximum population growth under ideal circumstances. Includes plenty of room for each member, unlimited resources (food, water) and no hindrances (predators). FACT: No population exhibits this type of growth for long. PREDICTING POPULATION GROWTH, PREDICTING POPULATION GROWTH, con’t Logistic model: This model accounts for the declining resources available to populations as they grow.
It assumes the birth and death rates are not constant.
As the population grows, births decline and death rises. Eventually birth=death so the population stops growing. Carrying capacity (K): The number of organisms of one species that an environment can support indefinitely. PREDICTING POPULATION GROWTH, PREDICTING POPULATION GROWTH, con’t Two modes of population growth. The Exponential curve (also known as a J
curve) occurs when there is no limit to population size. 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
There are 2 ways a population can
1. Depends on the rate of growth (r)
2. Influenced by the carrying capacity
(K) POPULATION GROWTH
r-strategists: characterized by exponential
growth, which results in temporarily large
populations, followed by sudden crashes in
population size. Ex. Insects, bacteria,
live in unpredictable and rapidly changing
Reproduce quickly when conditions are
Many offspring: small, mature rapidly, no
r = rate of growth POPULATION GROWTH
K-strategists: characterized by a high degree
of specialization. Ex. Trees, whales, tigers,
Live in stable and predictable environments
Can compete effectively
Reproduce late in life
Few offspring: large, mature slowly, often much
K = carrying capacity POPULATION GROWTH STRATEGIES, con’t 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
– 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
• 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
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, con’t
• 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, con’t
• The frequencies of the alleles will remain unchanged
generation after generation if the following conditions are
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
HOWEVER: there are limits to what it can
accomplish because selection does not act
directly on genes.
Natural selection acts on phenotype, NOT
THEREFORE: selection against unfavorable
recessive alleles is SLOW. Natural Selection Shapes
Populations, con’t Polygenic trait: A characteristic
influenced by several genes. There are three types of selection on
– 1. directional
– 2. stabilizing
– 3. disruptive Natural Selection Shapes
SELECTION Favors one extreme
phenotype Favors the average
phenotype Favors both extreme
phenotypes Possible reason:
Predators can identify
easier and eat the
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This note was uploaded on 11/23/2011 for the course BIO 100 taught by Professor Robinson during the Fall '08 term at BYU.
- Fall '08