The Process of Evolution

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The Process of Evolution
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Populations and Variations

• I. Populations and Variations
• Population - a group of individuals belonging to
  the same species that occupy a given area.

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Populations and Variations

• In most natural populations, the manifestations
  of traits are not the same from one individual to
  the next. (variations)

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Populations and Variations

• Variations in traits are inherited.
• Information about traits resides in hundreds of
  genes.
• The genetic variation results in different
  phenotypes within a population.

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Populations and Variations

• more than 10600 combinations of genes are
  possible in human gametes.
• Far more genetic variation is possible than can
  ever be expressed in the individuals of any
  population.

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Allele Frequencies Gene Pools

• II. Allele Frequencies and Gene Pools
• Gene pool - all the genotypes that exist in a
  population

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Allele Frequencies Gene Pools

• Remember that the different forms of a gene are
  called alleles.
• Allele Frequencies - The relative abundance of
  each kind of allele in a population.

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Allele Frequencies Gene Pools

• The number and type of alleles in a population is
  constantly changing.
• Evolution occurs when there is a change in the
  allele frequency of a population

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Evolution

• Changes in allele frequencies can occur in a
  number of different ways
• 1) Mutation 3) Gene Flow
• 2) Genetic Drift 4) Natural Selection

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Evolution

• III. Mutation
• Mutation - a heritable change in the genetic
  code (DNA)
• Mutations are random events, they may be helpful
  or harmful

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Evolution

• Harmful Mutations
• Most mutations are harmful. Why?
• Each individual inherits a combination of many
  genes that are already fine tuned by selection.
• Therefore, the product of a mutant gene is likely
  to be less functional not more so.

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Evolution

• Helpful and Neutral Mutations
• (mutations that survive in the population)
• Every once in a great while a helpful mutation
  may arise and be beneficial to the organisms.

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Evolution

• Some mutations have neutral effects or
  sometimes the effect of the mutant gene is
  masked by a dominant allele.
• These genes are passed on by chance.

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Evolution

• IV. Genetic Drift
• Allele frequencies can change randomly through
  generations because of chance. This process is
  called genetic drift

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GENETIC DRIFT
Start with five different alleles
Due to random events, only two different alleles
are left after several generations.
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Evolution

• Genetic drift is most rapid when population size
  is small.
• During extreme cases of genetic drift, a
  population originates or is rebuilt from very few
  individuals

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Evolution

• Founder Effect - a few individuals leave a
  population and establish a new one. By chance
  the allele frequencies for many traits may not be
  the same as in the original population.

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Evolution

• Bottlenecks - disease, starvation, or some other
  disaster can nearly wipe out large populations.
  Even though the population recovers, the relative
  abundance of alleles has been altered at random

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Evolution

• V. Gene Flow 
• Allele frequencies change when individuals leave
  a population (migration) or enter a population.
• This movement of individuals is called gene flow.

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Evolution

• VI. Natural Selection
• Natural Selection - the differential survival and
  reproduction of individuals in a population based
  on the traits they posses

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Evolution

• Natural selection may have a directional,
  stabilizing, or disruptive effects on the range
  of traits (phenotypes) in a population

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Evolution

• a)Directional selection - shifts allele
  frequencies in a constant direction in response
  to the environment

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Evolution

• b) Stabilizing selection - favors the most common
  phenotype in a population
• Peccaries are eating those
• plants with low-spine-number
• causing their alleles to
• vanish from the gene pool

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Evolution

• STABILIZING SELECTION
• Preferring densely spined cacti, egg-laying
  parasites
• more often destroy varieties of plants with
  larger numbers of spines. An infested cactus
  rarely survives.

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Evolution

• c)Disruptive Selection- favors characteristics at
  both ends of the range of traits in the
  population and operates against intermediate
  forms.

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Evolution

• DISRUPTIVE SELECTION
• 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.

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Speciation

• VII. Speciation
• Speciation - the process by which species
  originate
• How does speciation occur?

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Speciation

• Sometimes barriers arise between the parts of the
  population and create local breeding units.
  Then, two or more gene pools exist when there was
  only one.

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Speciation

• If over time there is no gene flow between the
  two populations then selection, mutation, and
  genetic drift can operate differently in each
  population.

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Speciation

• This can cause different traits to arise in each
  population. This is called divergence

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Speciation

• When the divergence is great enough that the two
  populations will no longer interbreed, the two
  population have become different species.

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Speciation

• Geographic Isolation -Populations can be
  separated by geographic barriers ex. mountains,
  rivers
• Ex. The Blue-headed wrasse was separated from the
  Rainbow wrasse when the Isthmus of Panama was
  created.

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Speciation

• Reproductive Isolation - any aspect of structure
  or behavior that prevents interbreeding
• ex. the development of different breeding seasons
  or different mating behaviors

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• Over time, two species of frog have developed
  different mating calls through isolation.
• Females respond only to the calls of their
  species.