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Chapter 15: Populations
• Section 1: How populations grow.
– What is a Population?
– Modeling Population Growth.
– Growth Patterns in Real Populations.
• Section 2: How populations evolve.
– The Change of Population Allele Frequencies.
– Action of Natural
Selection on Phenotypes.
– Natural Selection and
the Distribution of Traits.
Section 1: How populations grow
• What is a population?
– We learned this last chapter.
– This chapter we will learn:
• What causes populations to grow?
• What determines how fast they grow?
• What factors can slow their growth?
• A population consists of all the individuals of a
species that live together in one place at one time.
– Examples
• All the E. coli living in your intestine.
• All of the same species of fish swimming in a pond.
• Populations grow b/c organisms normally reproduce multiple
times.
• Population growth is limited due to limits of natural resources.
• Demography: the statistical study of all populations.
– Demographers study the composition of a population and try
to predict how the size of the population will change.
Three Key Features of Populations
1. Population Size:
– it is the number of individuals in a population
– one of the most important features of any population
– can affect the populations ability to survive.
– Studies show that very small populations are among those most
likely to become extinct.
• Random events and natural disturbances endanger small
populations
more than large populations.
•
Experience more inbreeding
– This leads to less genetic
variability.
Example: worldwide Cheetah
population
Three Key Features of Populations
2. Population density: the number of individuals that
live in a given area.
– If the individuals of a population are few and are
spaced widely apart, they may seldom encounter one
another,
making
reproduction
rare.
– This would
be low density.
– Population
density in USA as of
1990.
Three Key Features of Populations
3. Dispersion: the way the individuals of the
population are arranged in space.
– Three patterns of dispersion are possible.
1. Self determined or determined by chance: occurs when
individuals are randomly spaced.
2. Regular intervals: occurs if individuals are evenly spaced.
3. Clumped distribution: individuals are bunched together in
clusters.
– Each pattern reflects the interactions between the
population and its environment.
Modeling Population Growth
• The first step to determining how a population will
grow is to make a create a hypothetical population
and to attempt to show key characteristics of a real
population. This is a population model.
• This is useful, b/c demographers can use the model
to predict what might occur to a real population if
certain changes were made to the real population.
• There are three stages of population growth that
are included in the model.
3 stages of population growth
1. Growth rate: a population grows when more
individuals are born than die in a given period.
2. Growth rate and population growth: when
population is plotted against time on a graph, the
population growth curve resembles a J-shaped
curve and is called an exponential growth curve.
• A curve in which the rate of
population growth stays the
same, as a result the population
sizes increases steadily.
• Exponential curve: to calculate the number of
individuals that will be added to a population as it
grows, multiply the size of the current population
(N) by the rate of growth (r).
• Normally, as we learned last chapter, populations
do not always grow unchecked (ie death). The
population size that a given environment can
sustain is called the carrying capacity (K).
• When the carrying capacity is considered, the
population is modeled with a logistic growth curve
or logistic model (example on next slide).
K is the carrying capacity
K
Logistic Growth Curve
• Logistic model- a population model in which
exponential growth is limited by a density-dependent
factor.
• It is a population model that takes into account the
declining resources available to populations
– Assumes that birth and death rates vary with population size.
– When the population is below carrying capacity the growth
rate is rapid.
– As the population approaches the carrying capacity, death
rates begin to rise and birthrates begin to decline because
competition is increasing.
What is the difference between this
two growth curves?
Which has limited natural
resources and therefore
increased competition?
Which one has the strongest
natural selection pressure?
Growth Patterns in Real Populations
-Things aren’t always as simple as suggested in the
growth models.
-Often, growth is not limited by density- dependent
factors, but instead growth is limited by densityindependent factors. For example, mosquitos are
most common in the summer (more rain = more
places to lay eggs).
-Density-dependent factors: for example: food & water.
Resources that become depleted based on population
size.
-Density-independent factors: for example: the climate.
Factors that aren’t affected by population size.
• The growth of fast growing plants and organisms is
often described by an exponential growth model (rstrategist).
• The growth of slower growing plants and organisms
is often described by a logistic growth model (Kstrategist).
• Most species are located somewhere in the middle.
• Some species switch between the two, as their
environment changes.
R- strategists
• R- strategists grow exponentially when the
environmental conditions allow them to.
– This results in temporarily large populations (mosquitoes in
summer).
– When environmental conditions are not ideal, the population
quickly declines.
– Reproduce early in life.
– Have many offspring each time the reproduce.
– Small offspring that mature rapidly with little or no parental
care.
K-strategist
• Mature slowly.
• Often have small population sizes.
• Their population density is usually near the carrying
capacity (K) of their environment.
• Long life span
• Few young
• Reproduce
late in life
• Care for young
• Live in stable
Environments.
Section 2: How Populations Evolve
• Scientists wondered if dominant alleles would
spontaneously replace recessive alleles within
populations.
• 1908- G.H. Hardy & Wilhelm Weinberg
demonstrated that this is not the case. Dominant
alleles do NOT automatically replace recessive
alleles.
• Using algebra, they showed that the frequency of
alleles in a population does not change.
• The ratio of heterozygous to homozygous
individuals does not change from generation to
generation unless the population is acted on by
processes that favor particular alleles.
• This discovery is called the Hardy-Weinberg
principle.
– It states that the frequencies of alleles in a population do
not change unless evolutionary forces act on the
population
• It is an equation that can be used to predict genotype
frequencies in a population.
– For example, a dominant allele that is lethal will not
become more common just because it is dominant.
(Actually, it kills people, so if those people haven’t
reproduces, the allele could become less common).
– The principle holds true for any population as long as:
1) the population is large enough that its members are
not likely to mate with relatives and as long as
2) evolutionary forces are not acting on the population.
5 principle evolutionary forces: these
forces can’t be used with HardyWeinberg.
1.
2.
3.
4.
5.
Gene flow:
Mutation
Nonrandom mating
Genetic drift
Natural Selection
- These forces cause the ratios of genotypes in a
population to differ significantly from those
predicted by the Hardy-Weinberg principle.
Evolutionary Force: 1) Mutation
• This is why HW does not hold true for mutations:
• Mutation from one allele to another can
eventually change allele frequencies, but it
happens very slowly.
• Not all mutations result in phenotypic changes.
• Mutation is however a source of genetic variation
and makes natural selection possible.
Evolutionary Force: 2) Gene Flow
• Movement of individuals from one population to
another can cause genetic change.
• This movement is called migration and creates
gene flow.
• Gene flow: the movement of alleles into or out of
a population.
• It occurs because new individuals (immigrants)
add alleles to the population and departing
individuals (emigrants) take alleles away.
Evolutionary Force: 3) Nonrandom
mating
• Occurs when individuals prefer to mate with
others that live nearby or are of their own
phenotype. Therefore, mating is not random.
• Example: mating with relatives- causes lower
frequency of heterozygotes than would be
predicted with the H-W principle.
• Example: choosing a mate based on size, color,
abilities, etc.
Evolutionary Force: 4) Genetic Drift
• In small populations, the frequency of an alleles
can be greatly changed by an unexpected event
(ie natural disaster, fire, etc).
• Small populations that are isolated from one
another can differ greatly as a result of genetic
drift. (The same disaster that happens to one
population may not also happen to the next
population).
• This leads to a decrease in mutations, which
could mean that alleles frequencies can’t adapt
to disease and an individual will be more likely to
die from the disease.
Evolutionary Force: 4) Natural
Selection
• Causes deviations from the H-W principle by
directly changing the frequencies of alleles.
• The frequency of an allele will increase or
decrease, depending on the allele’s effects on
survival and reproduction.
• Natural selection is one of the most powerful
agents of genetic change.
Action of Natural Selection on
Phenotypes
• Natural selection enables individuals who express
favorable traits to reproduce and pass those traits
on to their offspring. Therefore, natural selection
acts on phenotypes, not genotypes.
• Natural selection shapes populations affected by
phenotypes that are controlled by one or by a
large number of genes.
• A trait that is influenced by several genespolygenic trait.
– Ex: human height and hair color
Normal & Directional Selection
This is called normal distribution.
This distribution is often seen by
polygenic traits in a population.
The range of phenotypes is
clustered around an average
value.
This is direction selection. It is
when the frequency of a
particular trait moves in one
direction in a range. Plays a role
in single-gene traits like pesticide
resistance. It eliminates one
extreme from a range of
phenotypes. So, the alleles
promoting this extreme become
less common in a population.
Stabilizing Selection
• Occurs when selection reduces extremes in a range
of phenotypes, the frequencies of the intermediate
phenotypes increase. The population ends up
containing fewer individuals that have alleles
promoting extreme types. The distribution
becomes narrower, stabilizing the average, by
increasing the proportion of similar individuals.
Very common in nature.