Download Non-adaptive Evolution Natural Selection Natural Selection

2/10/2011
Non‐adaptive Evolution
• Gene Flow
– Movement into or out of a population
– Movement into is only gene flow if immigrant successfully breeds
if immigrant successfully breeds
– More likely than drift to be beneficial
– However, possibility of outbreeding depression
Natural Selection
• Only mechanism that consistently leads to adaptation
• Fitness is Relative
– The most fit individuals aren’t perfectly adapted
– Better adapted than others
• Well‐adapted population genetically “tainted”
Natural Selection
Modes of Selection
• Phenotype determines fitness
• Underlying genotype adapts only by the effects of natural selection on phenotype
• Indirect connection; not 1:1 correspondence di
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of phenotype to genotype
• Environmental and Genetic Context:
– Detrimental allele may increase in frequency by being linked with beneficial allele
Adaptation Characters
• Easy to focus on obvious characters
– Body size, color, height, etc.
• Anything genetically determined can evolve
http://www.veoh.com/browse/videos/category/educational/watch/v7061199tb6F4Dw7
http://www
veoh com/browse/videos/category/educational/watch/v7061199tb6F4Dw7
http://www.youtube.com/watch?v=p‐_RHRAzUHM
– Protein sequences, physiological responses, intelligence, visual acuity, odor, etc. Sexual Selection
• Intrasexual Selection
– Example: Males fight over access to females
SEFI
• Intersexual Selection
– Example: females choose mates BOP
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Sexual Selection
• Preferred traits often correlate with health and vitality
– Honest indicators of quality
of quality
Preserving Genetic Variation
• Why doesn’t selection result in one “best” genotype?
• Diploidy
– Recessive alleles hide behind dominants
• Balancing Selection
B l i S l ti : maintaining two or more forms
i t i i t
f
– Heterozygote Advantage
• Sickle‐cell disease
– Frequency‐dependent Selection
• Common phenotype selected against
Preserving Genetic Variation
• Neutral Variation
– Much DNA doesn’t affect phenotype
– Change due to genetic drift, mutations
– Presently neutral may become expressed
Natural Selec on ≠ Perfec on
1. Existing variation
Alleles don’t arise due to “need”
2. Historical constraints
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Arms became wings
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3. Compromise
Sexual selection
4. Chance, natural selection, environment interact
Large‐scale random events can have huge effects
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