Chapter 15: Populations

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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.

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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.

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• 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.

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Three Key Features of Populations

• 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

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Three Key Features of Populations

• 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.

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Three Key Features of Populations

• Population density in USA as of 1990.

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Three Key Features of Populations

• Dispersion the way the individuals of the
  population are arranged in space.
• Three patterns of dispersion are possible.
• Self determined or determined by chance occurs
  when individuals are randomly spaced.
• Regular intervals occurs if individuals are
  evenly spaced.
• Clumped distribution individuals are bunched
  together in clusters.
• Each pattern reflects the interactions between
  the population and its environment.

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Modeling Population Growth

• The first step to determining how a population
  will grow is to make or 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.

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Population growth

• Growth rate a population grows when more
  individuals are born than die in a given period.
• 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.

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• 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, 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).

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K is the carrying capacity
K
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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.

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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?
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Growth Patterns in Real Populations

• Things arent always as simple as suggested in
  the growth models.
• Often, growth is not limited by density-
  dependent factors, but instead growth is limited
  by density-independent 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 arent affected by
  population size.

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• The growth of fast growing plants and organisms
  is often described by an exponential growth model
  (r- strategist).
• The growth of slower growing plants and organisms
  is often described by a logistic growth model
  (K-strategist).
• Most species are located somewhere in the middle.
• Some species switch between the two, as their
  environment changes.

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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.

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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.

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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.

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• 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 havent 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.

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5 principle evolutionary forces these forces
cant be used with Hardy-Weinberg.

• 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.

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Evolutionary Force 1) 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.

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Evolutionary Force 2) 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.

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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.

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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 cant adapt
  to disease and an individual will be more likely
  to die from the disease.

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Evolutionary Force 5) 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 alleles effects on
  survival and reproduction.
• Natural selection is one of the most powerful
  agents of genetic change.

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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 genes-
  polygenic trait.
• Ex human height and hair color

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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.
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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.