BIOL 2040 Lecture Notes - Lecture 3: Brassica Oleracea, Brussels Sprout, Kohlrabi
3 May 2018
School
Department
Course
Professor
Natural Selection
September 17, 2015
Artificial Selection:
• Brassica oleracea – cabbage, kohlrabi, Brussels sprouts, broccoli and cauliflower all
created from artificial selection from this wild cabbage.
• The cherry tomatoes are genetically manipulated domestic tomatao. Inserting
allele fw2.2 will cause the tomato to become smaller
The logic of evolution by Natural Selection: 4 Postulates
1. Individuals within populations are variable
2. Variations are, at least in part, passed to offspring (heritability)
3. Fitness varies within each generation
4. Survival and reproduction are not random; relate to those characteristics that vary
among individuals
Whenever these 4 conditions exist, then the genetic composition of the population changes
each generation through evolution by natural selection.
Natural Selection – differential survival and/or reproductive success of individuals caused
by differences in their phenotypes
Evolution by Natural Selection – change in mean (or variance) in a population over
generations
Fitness:
• A measure of the ability to survive and reproduce
• Relative, not absolute
Testing the 4 Postulates:
a. SS x Ss x ss in Snapdragon flowering plants
• Testing 75% white and 25% yellow
• Yellow flowers have fewer bee visits and relative seed set (therefore, lower
fitness than white)
• 2nd Generation: 77% white and 23% yellow
• Therefore, Evolution by Natural Selection
b. Finches in Galapagos Islands – Geospiza fortiz
• (1) Individuals vary with beak depth
• (2) Trait is heritable (midparent and midoffspring beak depth regression
• (3) Drought in 1977 caused seed characteristics to change which caused
drastic changes in the population – fitness varies
• (4) The depth of beaks from finches were larger after the drought; this shift
is caused by natural selection
• Result: Evolution by Natural Selection as finches in next generations also
displayed larger beaks
Changes in means:
• Natural Selection: mean trait value after selection – mean trait value before
• Evolution: mean trait value in generation 2 – mean trait value in generation 1
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