Natural Selection: Fundamentos e Consecuências, Notas de estudo de Genética

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NATURAL SELECTION:

Fundamentals

Natural selection

• “This preservation of favourable variations and the rejection

of injurious variations, I call Natural Selection.”

--Darwin (1859)

• Presented simultaneously with A. R. Wallace
• Emphases differed, though both correct–
  • Darwin: emphasized competition within species
  • Wallace: emphasized environmental pressures

Requirements for evolution

by natural selection

• Variation in traits
• Heritability of traits
• Trait variants affect

survival/ reproduction

Quantitative Traits vs. Single Locus

• Already discussed selection in the context of heritability:

Heritability = Response/Selection

• Genetic component of variation dictates selection’s response
• Response often from change in allele frequencies at multiple loci
• Can also be studied at single locus/ gene
• We’ll come back to phenotypes shortly

Strong selection in humans:

Single loci

• Spontaneous bad mutations are common
• Half of pregnancies never detected because spontaneously

abort very early

• Half of spontaneous abortions result from genetic problems
  • ~25% of all human fertilizations immediately eliminated by natural selection!

Weak(er) selection in humans:

Single loci

• Historically, all humans adult lactose intolerant
• Estimate ~5% fewer kids if lactose intolerant
• New mutation arose– now most people “lactase persistent”

(lactose tolerant) as adults

• What is effect of 5% more kids???

Can simulate with AlleleA

• Fitness of “AA” (intolerant) is 0.
• Fitness of “Aa” and “aa” (tolerant) is 1.
• Time: 5000 years
• All were “AA” and then new mutation (a) arose in Africa ~5000 years ago

Selection uses

“ relative fitness ” of genotypes

• In lactase example:
  • AA: 0.95 Aa: 1.00 aa: 1.
  • AA has “5% fewer kids” successfully on average than Aa or aa
• Because something is “selected against”, doesn’t mean it’s

“bad” by itself, just not as good as the alternative...

• Humans survived for a long time as AA (lactose intolerant)...

What is relative fitness? Example.

• BB genotypes produces on average 3.2 surviving offspring
• Bb genotypes produce on average 3.0 offspring
• bb genotypes produce on average 2.4 offspring

What is relative fitness? Example.

• BB genotypes produces on average 3.2 surviving offspring
• Bb genotypes produce on average 3.0 offspring
• bb genotypes produce on average 2.4 offspring
• Most fit genotype: BB
  • Call it “100% of maximum”, fitness = w (BB) = 1.
  • Others are percentage of maximum
    • w (Bb) = 3.0 / 3.2 = 0.94 (~6% less fit than BB)
    • w (bb) = 2.4 / 3.2 = 0.75 (~25% less fit than BB)

Effects on Hardy-Weinberg

• Assume all “aa” individuals die at age 10
• At age 8:
  • AA: 490 Aa: 420 aa: 90

Effects on Hardy-Weinberg

• Assume all “aa” individuals die at age 10
• At age 8:
  • AA: 490 Aa: 420 aa: 90 p(A) = 0.
• At age 25:
  • AA: 490 Aa: 420 aa: --dead--

Effects on Hardy-Weinberg

• Assume all “aa” individuals die at age 10
• At age 8:
  • AA: 490 Aa: 420 aa: 90 p(A) = 0.
• At age 25:
  • AA: 490 Aa: 420 aa: 0 p(A) = 0.
• Deviation from Hardy-Weinberg
  • Selection altered genotype frequencies
• Selection also altered allele frequencies

Effects on Hardy-Weinberg

• Assume all “aa” individuals die at age 10
• At age 8:
  • AA: 490 Aa: 420 aa: 90 p(A) = 0.
• At age 25:
  • AA: 490 Aa: 420 aa: 0 p(A) = 0.
• Deviation from Hardy-Weinberg
  • Selection altered genotype frequencies
• Selection also altered allele frequencies
• Are aa’s gone for good? What happens next?