BSC 196 Lecture 3: Bio- Lecture 3

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8 Feb 2017
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Wednesday, January 25, 2017
1
Evolution of beak size cont.
Evolution by natural selection
- More offspring produced than can survive
Corollary: maximum population size is limited (carrying capacity)
- Even in normal years, most birds died before breeding (89%)
- Severe drought in 1977 (mortality of adults - 84%)
- Fitness differences
Non-random survival and/or reproduction
Individuals with deeper beaks exhibit higher survival
What causes the fitness difference?
- Change in composition of seeds
- During drought, increase in relative abundance of large hard fruits
- Individuals with deeper beaks better to eat hard seeds
- Has the population evolved?
Chicken hatched after the drought had deeper beaks than chicks hatched before
the drought
Take Home Lessons
- Selection acts on phenotypes, but evolution occurs through changes in gene
frequencies
- Natural selection does not adapt organisms to future changes, only past changes in
the environment
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