Points to remember about Natural Selection

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Points to remember about Natural Selection

• Populations evolve, but individuals do not.
  I.E., Natural selection acts on the level of the
  individual, but populations are the smallest unit
  that can evolve.
• Natural selection only works on heritable
  variations, not acquired traits.
• Natural selection can only work with what its
  given. Variations are produced by different
  genetic mechanisms.
• Natural selection is situational to a given
  environment in a given time and place.
• No ideal organism.
• Fitness of a given trait changes.

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Population Genetics Microevolution
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The historical context of Darwins life and ideas
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Evolution is.

• For Darwin (1859) Evolution is gradual change of
  heritable traits in a population across
  generations, eventually generating species over
  time.
• For the Modern Evolutionary Synthesis (early 20th
  c.) Evolution is a change in allele frequency
  over time

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Population Genetics - Terms

• Population Localized group of individuals of the
  same species
• Species Group of populations whose individuals
  can potentially interbreed
• Gene Pool Total aggregate of genes in a
  population at one time
• Allele an alternative form of a gene
• Homozygous have identical alleles for a given
  trait (dominant or recessive) (e.g. AA or aa in a
  diploid)
• Heterozygous have gt1 different alleles for a
  given trait (e.g. Aa or aA in a diploid)
• Why arent all individuals of one species
  identical?

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Genetic Diversity
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Genetic Diversity is the product of

• Sexual reproduction
• Genetic recombination
• Mutation- key form of genetic variation in
  asexual reproduction, substrate for new genetic
  material
• Provides alternative alleles that may or may not
  be useful with changes in environment raw
  material for evolution
• R.A. Fisher's Fundamental Theorem of Natural
  Selection the rate of natural selection is
  proportional to the degree of variation within a
  population

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The Modern Evolutionary Synthesis in action

• Hardy-Weinburg Equilibrium
• p2 2pq q2 1

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Figure 23.3a The Hardy-Weinberg theorem
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Figure 23.3b The Hardy-Weinberg theorem
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The Modern Evolutionary Synthesis in action

• Hardy-Weinburg Equilibrium
• p2 2pq q2 1
• Assumptions
• 1. Large population size
• 2. No gene flow
• 3. No mutation
• 4. Random mating
• 5. No selection

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The Modern Evolutionary Synthesis in action

• Microevolution (shift in allele frequencies)
  happens when the H-W assumptions fail !
• Small population size gt Genetic Drift
• Migration gt Gene Flow
• Mutation
• Natural Selection
• Nonrandom mating

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Genetic Drift random changes in allele (and
genotype) frequencies from generation to
generation due to sampling error
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Genetic Drift, part A The bottleneck effect
Alleles in original population
Alleles remaining after bottleneck
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Cheetah
Bottleneck effect
www.petsdoc.com/pics/funpages/ wildlifephotos/chee
tah.jpg
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Northern Elephant Seal
Bottleneck Effect

• http//www.icsi.berkeley.edu/dbailey/gallery/imag
  e/elephant.jpg

• Reduced to 20 individuals in 1896
• Now 30,000 individuals, with no detectable
  genetic diversity

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Genetic Drift, Part B The founder effect
Change in allele frequencies when a new
population arises from only a few
individuals e.g., only a few fish are introduced
into a lake e.g., only a few birds make it to an
island
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www.fishbase.org
Scorpaenidae Lionfish Pterois volitans
Founder Effect

• New Atlantic population, maybe from only 10
  individuals

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Gene Flow
addition or removal of alleles due to
individuals entering or leaving a population from
another population
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• On average, two humans differ by 0.1 (1 in 1000
  bp).
• Is race a genetic reality?
• NO 90 of genetic variation occurs within any
  single "race". LOTS OF OVERLAP
• YES only 10 of the variation can be explained
  by Asian, African, or European origin.

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Gene Flow
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Bluefin Tuna
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http//www.chambers-associates.org/Big-Marine-Fish
/map_bluefin_spawning_areas.html
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Mutation
random, heritable changes in DNA that
introduces new alleles into a gene pool
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random, heritable changes in DNA sequences that
introduce new alleles into a gene pool
Mutation

• members.tripod.com/Alphacentaur/ X-men.JPG

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Modes of natural selection
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Modes of selection
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Directional selection
Beak size selection in a Galápagos population of
the medium ground finch
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Diversifying selection
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Diversifying selection
Small bill size for soft seeds, large bills for
hard seeds selection against intermediate sized
bills which crack both types of seeds
inefficiently
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Stabilizing selection
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Stabilizing selection
http//www.hummingbirds.net/gallery0.html
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What maintains genetic variation?

• Diploidy
• Balancing selection
• Heterozygote advantage
• Frequency-dependent selection
• Neutral variation

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Sicle cell anemia is a good example of
heterozygote advantage
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Sex
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The two-fold disadvantage of sex
BUT, genetic diversity allows adaptation to
changing environments
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Sexual Selection

• non-random mating where individuals have sexually
  dimorphic traits that make them more attractive
  to mates

Broad-billed hummingbirds

• http//www.hummingbirds.net/

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Sexual Selection - how can this be favored?
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Sexual selection and the evolution of male
appearance
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