BIOEE 1780 Lecture Notes - Lecture 4: Directional Selection, Aposematism, Natural Selection
28 Apr 2018
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Lecture 4 – Natural Selection II
Narrow Sense Heritability (h2): the proportion of phenotypic variance that is transmitted from parents
to offspring
• Can be measured with offspring-parent regression
o Scientists measure the phenotypic resemblance of a trait between parents and their offspring
They regress the mean trait value of offspring against the mean trait value of the parents
• The parent mean is called the midparent value, because there are only 2 parents
o When these values are plotted, the relationship indicates how closely the offspring trait values
resemble those of their parents
• Significant positive relationship (slope is greater than 0) = parents with unusually large trait
values produce offspring with unusually large trait values
• The graph on the left has a steeper slope, indicating a stronger resemblance between
offspring and parents.
• To predict how much the offspring generation will evolve, use the parent-offspring
regression.
• Follow the mean value of selected parents on the x-axis until it intersects with the
regression line.
• Read across to the corresponding y value.
• This is the new mean trait value expected for the offspring.
• When the slope of the offspring-parent regression line is steep, the phenotype
experiences a bigger shift.
• Determines how much the population evolves
o Without a mechanism of evolution, no evolution will occur. Offspring phenotypes should be
similar to the parental phenotypes.
Types of Natural Selection
1. Directional Selection: favors phenotypes at one end of a distribution
a. Mean of the population changes
b. Population evolves in that direction
2. Stabilizing Selection: favors values toward the middle of the distribution
a. Fitness of organisms at either end is lower
b. Mean remains the same
c. Variation in the population decreases
d. Ex) human birth weight
3. Disruptive Selection: favors phenotypes towards the extremes of the distribution
a. Ex) small fish are able to hide from predators, large fish are too big for predators
How quickly the population evolves in response to selection depends on the amount of variation, and
how much of the variation is heritable (h2).
• When h2 = 1, all of the variation is due to allelic differences among individuals. Offspring have
similar traits as their parents.
• Evolutionary Response (R) = h2 x S
o Phenotypic variation that influences fitness = S
o The ability to transmit phenotypic characteristics to offspring = h2
Horned Lizards Example
• Horns = a defense against predators
Document Summary
Narrow sense heritability (h2): the proportion of phenotypic variance that is transmitted from parents to offspring: can be measured with offspring-parent regression, scientists measure the phenotypic resemblance of a trait between parents and their offspring. They regress the mean trait value of offspring against the mean trait value of the parents: when these values are plotted, the relationship indicates how closely the offspring trait values. The parent mean is called the midparent value, because there are only 2 parents resemble those of their parents. Significant positive relationship (slope is greater than 0) = parents with unusually large trait values produce offspring with unusually large trait values. The graph on the left has a steeper slope, indicating a stronger resemblance between offspring and parents. To predict how much the offspring generation will evolve, use the parent-offspring regression. Follow the mean value of selected parents on the x-axis until it intersects with the regression line: read across to the corresponding y value.
