Natural Selection and Variation

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Natural Selection and Variation
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Natural Selection Mechanism for Evolution

• Works on existing natural genetic variation in
  the population.
• But where does the variation come from?

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Variation Source Mutations

• Spontaneous changes in DNA
• Can be inherited
• Only vulnerable to selective environmental
  pressure if the gene is expressed
• Not all mutations are bad
• Most mutations are neutral

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Mutations continued

• Mutations are random!
• If the environmental conditions change the impact
  of the mutation can change (become positive,
  negative, or neutral)
• If a variation present in a population helps an
  organism survive in current environmental
  conditions then the trait may be passed on and
  the allele frequency of the population can change

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Variation Source Sexual Reproduction

• Sexual Reproduction increases variation through
  in 3 ways
• Independent Assortment
• Crossing over can swap DNA from one chromatid to
  another to form recombinant chromosomes
• Random Fertilization combination of particular
  egg and sperm

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Meiosis

• Produces gametes
• 2 rounds of division
• Results in reduction in chromosome number from
  diploid to haploid
• 4 cells not identical to each other or the parent
  cell

http//www.accessexcellence.org/RC/VL/GG/meiosis.p
hp
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Mendels Law of Independent Assortment
http//kvhs.nbed.nb.ca/gallant/biology
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Independent Random Assortment
2n gives number of possible combinations N
number of pairs of chromosomes (haploid
number) This example 3 chromosomes 8 possible
unique combinations Human 23 pairs of chromosomes
gives 8 million possible unique combinations
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Crossing Over

• Occurs during Prophase 1 of meiosis
• Homologous chromosomes have paired up to form
  tetrads
• Synapsis is the process during which tetrads are
  formed held together by a protein complex
• Pieces of non-sister chromatids within the tetrad
  may be swapped at intersection points called
  chiasma

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Crossing Over
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Multiple Cross Overs
Humans average 1-3 crossing over events per
chromosome
http//www.informatics.jax.org/silver/figures/figu
re7-4.shtml
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Gamete Formation
1 in 8 million possible combinations in a single
human sperm
1 in 8 million possible combinations in a single
human egg
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Random Fertilization
8 million X 8 million possible combinations
equals 64 trillion possible diploid
combinations. Now add in variation created by
crossing over 1-3 times per chromosome and the
possible combinations are astronomical!
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Darwin Couldnt Explain

• Why offspring resembled but were not identical to
  parents?
• Recognized that heritable variation allowed
  evolution to occur.
• Mendels discoveries were not recognized by
  biologist until after the deaths of both Darwin
  and Mendel.

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Asexual Reproduction

• Does not involve the fusion of gametes
• Offspring are identical to parents
• Budding, Fragmentation (Regeneration), Fission
• Lower chance of population surviving severe
  environmental change
• But - lower energy costs, large number offspring
  possible, no need to find mate