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How Populations Grow
What is a population?
 A population consists of all the
individuals of a species that live
together in one place at one time.
• What is demography?
 Demography is the statistical study
of all populations.
3 Key Features of Populations
1.
Population size is the number of individuals in a
population and can affect the population’s ability to
survive.
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2.
Population density is the number of individuals that
live in a given area.
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3.
Small populations are more likely to become extinct b/c
of inbreeding, a natural disaster or disease.
If few and spread out, they may not encounter one
another to breed.
Dispersion is the way individuals of a population are
arranged in a space.

They can be evenly spaced, clumped together or in a
random distribution.
Three Patterns of Population
Dispersion (p. 321)
Modeling Population Growth
 A population model is a hypothetical
population that attempts to exhibit the
key characteristics of a real population.
 Allows demographers to predict what
might occur in a real population.
 Describes the rate of population growth
as the difference between the birth and
death rates expressed as the # per 1,000
people.
Growth Rate & Population Size
 Exponential growth curve
 Curve in which the rate of population growth
stays the same, as a result the population size
increases steadily.
 This is called the J-shaped curve. (p. 322)

Limiting Factors of Population
Growth
Examples include:
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Carrying capacity (K)
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Predators
Disease
Availability of resources
The population size that an environment can
sustain.
Density-dependent factors include limited
resources such as food and water, because the
rate at which they become depleted depends
upon the population density of the population
that uses them.
Resources and Population Size

Logistic model
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Population model in
which exponential
growth is limited by a
density-dependent
factor
It’s a population model that takes into account
the declining resources available and assumes
that birth & death rates vary.
When a population is below carrying capacity,
the growth rate is rapid, however as it
approaches the carrying capacity, death rates
rise and birth rates decline.
Growth Patterns in Real Populations
 Density-independent factors

Weather & climate
• r-strategists
Grow exponentially when
environmental conditions allow
them to reproduce.
 Ex: bacteria, some plants, many
insects such as cockroaches &
mosquitos

Growth Patterns in Real Populations
•
K-strategists
Organisms that grow slowly, such as
whales, and have small population sizes
 They are called K-strategists because
their population density is near the
carry-capacity of their environment.
 Typically larger, slow maturing, long life
span and dependent newborns.
 Ex: Many endangered species such as
tigers, gorillas, and whales
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How Population Evolve
•
Allele Frequencies

Alleles are different forms of a gene
• Hardy-Weinberg Principle
The frequencies of alleles in a
population do not change unless
evolutionary forces act on the
population.
 It’s a process that favors particular
alleles. For example, a dominant lethal
allele will not become common just
because it’s dominant.

•
How Population Evolve
Five principle evolutionary forces
Mutations
 Gene flow
 Nonrandom mating
 Genetic drift
 Natural selection
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Mutations
•
Mutations are a source of variation
that makes evolution possible.
Mutations of alleles can change allele
frequencies.
 However, most genes mutate only about
1-10 times per 100,000 cell divisions.
 Not all give phenotypic changes
(outward appearance).
 Some will change protein function.
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Gene Flow
•
•
Gene flow is the movement of
alleles into or out of a population.
Migration of individuals to or
from a population either adds
alleles or removes alleles.
Nonrandom Mating
•
Individuals who mate with others that live
nearby or are of their own phenotype.
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Inbreeding, self-fertilizing plants, and
choosing mates based on certain traits
Increases the number of homozygous
individuals
Female widowbirds
prefer to mate with males,
such as the one shown, that
have long tails over males that
have short tails. This increases
the proportion of alleles for
long tails in the population.
Genetic Drift
 The random change in allele frequency in a population
 In small populations, the frequency of an allele can be
greatly changed by a chance event, such as a fire or
landslide
 When an allele is found in only a few individuals, the loss
of even one individual can drastically affect the allele
frequency. This can result in genetic uniformity which
leads to reduced disease resistance.
Cheetahs are endangered.
Cheetahs have gone through at
least two drastic declines in
population size.
Natural Selection
 The process by which individuals that have favorable
variations and are better adapted to their
environment survive and reproduce more
successfully than less well adapted individuals do.
 The frequency of an allele will increase or decrease,
depending on the allele’s effects on survival and
reproduction.
 Ex: the allele for sickle cell is declining in frequency in the U.S.
because individuals who are homozygous rarely have children.
 Ex: Hemophilia in a family. The last Tsar of Russia’s only son, Alexei
Nikolayevich (yellow circle), had hemophilia, a
blood-clotting disorder that affects males who
have a single copy of a recessive gene.
•
Natural Selection & the
Distribution of Traits
Normal distribution

Polygenic traits that exhibit a range of
phenotypes clustered around an average
value – forms a bell-shaped curve
• Directional selection
 The frequency of a particular trait moves in
one direction in a range.
• Stabilizing selection

The distribution of a particular trait becomes
narrower, tending to stabilize the average by
increasing the proportion of similar
individuals.
Natural Selection & the
Distribution of Traits
• Directional selection
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The dashed line represents the average height for this
population
When selection eliminates one extreme from a range of
phenotypes, the alleles promoting this extreme become less
common in the population.
Directional selection has a role in the evolution of single-gene
traits, such as pesticide resistance in insects.
Normal
distribution
Natural Selection & the
Distribution of Traits
• Stabilizing selection
 When selection reduces extremes in a range of phenotypes, the
frequencies of the intermediate phenotypes increase.
 As a result, the population contains fewer individuals that have
alleles promoting extreme types.
 In stabilizing selection, the distribution becomes narrower,
tending to “stabilize” the average by increasing the proportion of
similar individuals.