Evolution of Populations

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Evolution of Populations

• Chapter 11

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Warm Up 1/21 1/22

1. Compare Larmark and Darwin in their theories of
   evolution.
2. List the 5 evidences of evolution.
3. Define the following Vestigal, fitness, mutation
   and adaptation

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KEY CONCEPT A population shares a common gene
pool
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Genes and Variation

• Gene pool - all the genes that exist within a
  population

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Genetic variation in a population increases the
chance that some individuals will survive

• Genetic variation leads to phenotypic variation.
• Phenotypic variation is necessary for natural
  selection.
• Genetic variation is stored in a populations
  gene pool.
• made up of all alleles in a population
• allele combinations form when organisms have
  offspring

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Allele frequencies measure genetic
variation.measures how common allele is in
populationcan be calculated for each allele in
gene pool
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Genes and Variation

• Relative (allelic) frequency - the percentage of
  a particular allele in a gene pool.

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KEY CONCEPT Populations, not individuals,
evolve.
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Natural Selection in Populations

• Natural Selection- In nature, unequal ability to
  survive and reproduce... Survival of the fittest.
  
• Natural Selection ACTS ON PHENOTYPE but
  influences genotype (thus, allelic frequency)
• Artificial Selection- Mankind selects for desired
  traits. Also known as selective breeding

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Over time, the zebra herd becomes faster as the
slower zebra (and their genes) are removed from
the population survival of the fittestDarwins
Theory Evolution by means of natural selection
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• ADAPT OR DIE!
• Camouflage- organisms blend-in with surrounding
  environment
• Mimicry- species copy another to insure their own
  survival
• NatGeo

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Natural Selection effects Genetic Change in
Populations

• Natural Selection has three affects on phenotype
  distribution
• Directional Selection
• Stabilizing Selection
• Disruptive Selection

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Artificial Selection
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Normal Distribution of traits

• A normal distribution graphs as a bell-shaped
  curve.
• Phenotypes near the
• middle range tend to
• be most common.
• Examples- height and
• weight

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Directional Selection

• This type of selection favors phenotypes at
• one extreme of a traits range.
• An extreme phenotype that was once rare becomes
  more common.
• Ex. Drug resistant bacteria

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Directional Selection

• Directional Selection- Individuals on one end of
  a curve are better fitted than the middle or
  other end
• Peccaries naturally choose to consume those
• cactus plants with the fewest spines As a result,
• at flowering time there are more cacti with
• higher spine numbers thus, there are more of
• their alleles going into pollen, eggs, and seeds
• for the next generation.

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Stabilizing Selection

• Stabilizing selection favors the intermediate
  phenotype.
• Selection against both extremes decreases the
  genetic diversity of a given population.

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Stabilizing Selection

• Stabilizing Selection- Individuals near center of
  a curve are better fitted (have highest
  fitness) than both ends
• Peccaries are consuming the low-spine
• number plants, and the insects are killing
• the high-spine-number plants. As these
• gene combinations are removed from the
• cactus gene pool, there is less and less
• variety possible in subsequent
• generations.

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Disruptive Selection

• This type of selection occurs when both extreme
  phenotypes are observed.
• Individuals with the intermediate type are
  selected against.
• By favoring both extreme phenotypes, disruptive
  selection can lead to the formation of a new
  species.

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Disruptive Selection

• Disruptive Selection- Individuals at upper and
  lower ends are better fitted than the ones in
  the middle
• Years of collecting have left their toll on
• the roadside cacti. In this environment, it is
• maladaptive to be good looking and have
• a reasonable number of spines. Low
• spine-number plants are not picked
• because they don't "look right", and high
• spine-number varieties are left alone
• because they are too hard to pick.
• Gradually, the gene pool changes in favor
• of the two extreme spine number types.

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Other mechanisms of Evolution

• Natural selection is not the only mechanism
  through which populations evolve
• 3 other mechanisms of Evolution
• Gene Flow
• Genetic Drift
• Sexual Selection

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• Gene flow movement of alleles into or out of a
  population
• Immigration new alleles move IN
• Emigration alleles move OUT

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• Genetic drift - change in allelic frequencies by
  chance
• Ex sudden extinction of a dominant species
  small populations most affected

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Genetic drift is a change in allele frequencies
due to chance.

• Genetic drift causes a loss of genetic diversity
  It is most common in small populations.
• A population bottleneck can lead to genetic
  drift.
• It occurs when an eventdrastically
  reducespopulation size.
• The bottleneck effect isgenetic drift that
  occursafter a bottleneck event.

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• The founding of a small population can lead to
  genetic drift.
• It occurs when a few individuals start a new
  population.
• The founder effect is genetic drift that occurs
  after start of new population

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Sexual selection occurs when certain traits
increase mating success.

• Sexual selection occurs due to higher cost of
  reproduction for females.
• males produce many sperm continuously
• females are more limited in potential offspring
  each cycle

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• Genetic equilibrium - when alleles stay the same
  from generation to generation

• The Hardy Weinberg Principle Allele frequencies
  will remain constant under five conditions
• Random Mating
• Large Population
• No movement (immigration or emigration)
• No Mutations
• No Natural Selection equal change of survival

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5 Factors that can lead to evolution

• Genetic Drift
• Gene Flow
• Mutation
• Sexual Selection
• Natural Selection

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Genetic drift changes allele frequencies due to
chance alone
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Gene flow moves alleles from one population to
another
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Mutations produce the genetic variation needed
for evolution.
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Sexual selection selects for traits that improve
mating success.
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Natural selection selects for traits advantageous
for survival
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KEY CONCEPT New species can arise when
populations are isolated

• The isolation of populations can lead to
  speciation
• speciation - evolution of a new species
• .
• Reproductive isolation can occur between isolated
  populations
• Populations can become isolated in several ways
• 1. Behavioral
• 2. Geographic
• 3. Temporal

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• 1. Behavioral Isolation Two populations capable
  of breeding but cannot because of courtship
  rituals

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• 2. Geographic Isolation Two populations are
  separated by geographic barriers
• Ex Rivers, Oceans, Mountains
• 3. Temporal Isolation Two or more populations
  reproduce at different times

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Patterns of Evolution

1. Extinction
2. Divergent Evolution (adaptive radiation)
3. Convergent Evolution
4. Coevolution

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1.Extinction

• Why do species go extinct?

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Extinction

• Natural selection, climate changes, and
  catastrophic events have caused 99 percent of all
  species that have ever lived to become extinct.
• Mass extinctions caused by continents moving,
  sea level changing, volcano eruptions, large
  meteors

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Predict what each ecosystem will look like after
the event.
Catastrophic Event
Catastrophic Event
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Question

• When a mass extinction happens, what do you think
  will happen next?

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Divergent Evolution (adaptive radiation)

• Divergent evolution natural selection causes 1
  species to evolve into many species with many
  different adaptations (homologous structures)
• After mass extinctions, many environments will be
  open for inhabitation
• Species will migrate to that area and new
  environmental pressures will cause the population
  to change over time
• This is also known as Adaptive Radiation

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Adaptive Radiation in honeycreepers
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Evidence of Evolution

• Homologous structures - similar structures found
  in related organisms that are adapted for
  different purposes.
• Ex human arm and bat wing or whale flipper
• ---DIVERGENT EVOLUTION---
• the process of two or more related species
  becoming more and more dissimilar.

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Homologous structures ? Divergent evolution
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Convergent Evolution

• Convergent Evolution when unrelated organisms
  come to resemble one another (analagous
  structures)

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• Analogous structures - structures found in
  unrelated organisms that have a similar function
  but may be structurally different
• Ex bird wing and insect wing
• ---CONVERGENT EVOLUTION---
• independent evolution of similar features in
  species of different lineages

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Analogous structures ? Convergent evolution
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Coevolution

• When 2 species evolve in response to one another

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Coevolution can occur in competitive
relationships, sometimes called evolutionary.
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Speciation occurs in patterns

• Punctuated equilibrium species show little
  evolutionary change for millions of years,
  followed by periods of rapid speciation
• Gradualism- Species evolve slowly, over long
  periods of time.