Download Unit 3 - Section 9.1 Types of Selection Overheads

Grade 11 University Biology – Unit 3 Evolution
Mechanisms of Evolution – Types of Selection Overheads
Section 9.1 Pages 356
Natural Selection
Populations have many phenotypes and genotypes. If a single allele
improves survivability, even slightly, it gives a selective advantage. As a
result, the frequency of that allele will increase in the population AND in
subsequent generations. Natural selection causes changes in the allele
frequencies of a population...and thus, evolutionary change.
There are three types of selection.
1. Stabilizing Selection
 Selection against
individuals exhibiting
variations in a trait that
deviates from the current
population average; rather,
favours intermediate traits
 The most common
phenotype is the favoured
phenotype
 Most common form of
selection
 Once a species adapts to its environment, selective pressures
maintain the evolved feature
 Example is human birth weight. Low birth weight babies tend
to be sicker, while high birth weight babies are difficult to
deliver. Survivability in both cases is low. Thus, medium
weight babies survive more, and thus, the intermediate trait is
favoured.
2. Directional Selection
 Favours an increase or
decrease in the value of a
trait from the current
population
 Occurs when the
environment favours one
EXTREME trait
 Causes average to move in
one direction...in the illustration, the average has moved
toward an extreme
 Example – fishing with nets captures certain sized fish.
Smaller fish escape. This change moves the average size of
fish in future generations to smaller size...and that size
becomes the norm
3. Disruptive Selection
 Favours two or more variations of a trait that differ from the
current population average
 Favours individuals at BOTH extremes
 Produces
distinctive forms
within a population
(e.g., Darwin’s
finches – small
beaked ate small
seeds and big
beaked ate large
seed; yet, medium
beaked could not eat or compete for either seed size
(maladaptive))
Sexual Selection
 Selects for any trait that influences mating success
 Leads to sexual dimorphism (i.e., differences between male and
female)
 Most common, female selects male AND males compete
 Some traits, while sexually attractive, are detrimental (e.g., bright
coloured plumage of the male peacock)
For more information – See Figure 9.9 on Page 356