Slide
1
In this tutorial we are discussing the different modes of natural
selection. Recall that natural selection is best defined as differential
reproduction - which we now know means that those individuals with greater
fitness pass on more copies of their genes to the next generation.
This results in an adaptive allele frequency change within the population.
Remember that natural selection requires variation in the population - the trait that is being selected for must be heritable and must contribute to an increase in fitness.
Slide 2
A classic example of natural selection in action
is the pepper moth and industrial melanism. Melanin in the pigment that
results in dark coloration in the moth and other organisms,
There are two forms of the moth: one mottled (light-colored) and one black. In natural conditions, the mottled phenotype is more common because it mimics lichen on trees and thus birds tend to prey on the black moth.
You can see on this image that in the light colored background the light colored moth is hard to see. In a dark colored background, the dark colored moth is difficult to see.
Slide 3
During the Industrial Revolution,
soot from factories darkened the trees and poor air quality destroyed the
lichen. This resulted in a shift of the phenotype frequency curve because the
mottled phenotype became more visible to predators, and thus became the prey
of choice for birds. The environment selected for dark colored moths.
This graphic shows what happened to the frequency of dark colored moths in the population after clean air regulations were enacted. Better air quality allowed the lichens to recolonize the tree trunks and the moths environment became predominantly light colored again. Once again, the environment favored light colored moths and the percentage of dark colored moths has decreased significantly.
Slide 4
Natural selection results in
adpative change in populations. But how is this change noticed? By
observing the changes that take place the distribution of the phenotype in the
population.
We can describe the type of selection that is happening by observing these changes in the population.
The types of selection are:
Stabilizing Selection,
Directional Selection, Diversifying Selection,
Balancing Selection, and Sexual Selection.
Slide 5
To discuss
this issue we will think about traits that are distributed around a median
phenotype. In the original population the phenotype is distributed in a
normal distribution or a bell curve. The different types of selectoin
affect which part of the distribution is selected.
Slide 6
Stabilizing
selection occurs in stable environments where there is very little change
occuring in the environment. in this type of selection the mean
phenotype is selected for while the extreme phenotypes are selected against.
This culls out variants and results in a population with a more limited
range of phenotypes.
Slide 7
A nice example of stabilizing
selection is birth weight in humans. The birth weight of nearly 7000
human female babies were recorded in London.
After one month, survival
of these babies was determined as a function of birth weight
Overall
survival was 95.9%
-Babies less than 4.5 lb had 41.4% survival
-babies
7.5 -8.5 lb had 98.5 survival
-Babies greater than 10.0 lb had 90.5%
survival
Babies that are born too small or too large are selected against and babies between 7.5 and 8.5 pounds are selected for. This is an example of stabilizing selection.
Slide 8
Directional selection is
common in environments that are changing and an extreme phenotype is selected
for. We have already learned about the Galapagose finches evolving a
thicker, stronger beak in response to a drought and the peppered moth evolving
to a darker color morph in response to a darkening of their environment.
These are examples of directional selection.
Slide 9
In
Diversifying (also referred to as disruptive) selection both extremes are
selected for and the mean phenotype is selected against. This results in
a bimodal distribution in the population.
Slide 10
An
amusing example of disruptive selection can be seen in the Horned Dung beetles
of Australia. In this species, male dung beetles have either large horns
or no horns.
Males with large horns guard the tunnels in which female beetles lay eggs. Mate with females and keep other males away.
Males with no horns dig side tunnels to sneak into the female tunnels, bypass guarding males and mate with females
Males with intermediate horns would not be as good at guarding tunnels, and would not be as good at sneaking matings. The result is there are two male phenotypes - large horns or no horns.