Slide
1
Natural selection tends to remove alleles from the population - this
decreases genetic variation in the population.
Balancing selection works
to maintain deleterious/alternative alleles in a population
Examples
of balancing selection are Heterozygote advantage
and Frequency-dependent
selection.
Slide 2
We have already discussed the idea of heterozygote
advantage. This refers to a situation where the heterozygotes have
increased fitness when compared to either homozygous dominant or homozygous
recessive indivdiuals.
An example of this is sickle cell trait - in an environment where malaria is present - the heterozygous individuals have increased resistance to malaria when compared to homozygous dominant individuals (homozygous recessive individuals have sickle cell disease).
Please recognzie that this heterozygote advantage only occurs in places where malaria is common. Heterozygotes do not have an advantage in the US where malaria is not a problem.
Slide 3
Frequency-dependent selection
refers to a situation where the more common phenotype is selected against.
Water boatmen are aquatic insects that occur in three color morphs - dark brown, medium brown, and light brown. Fish like to eat water boatmen and they tend to develop a search preference. When the dark brown form is most abundant - fish tend to eat the dark brown form. This causes the dark brown form to become less common and one of the other color morphs becomes most abundant (let's say light brown for example). Now the light brown becomes the fish's favorite form and the light brown becomes selected against. The result is that the abundance of the three forms changes consistently in response to fish predation.
Slide 4
Another
form of selection is sexual selection. Sexual selection describes the
selection of a mate based on a specific heritable trait.
It is not a mode of natural selection because natural selection increases the adaptation of a population to a specific set of environmental conditions whereas sexual selection does not
Selection is typically for a secondary sexual
characteristic - something like coloration or behavior. And this type of
selection often results in sexual dimorphism - when males and females look
different from
each other.
Slide 5
Sexual selection can
take one of two main forms. Either intersexual selection where males
perform elaborate rituals, sing elaborate songs, build elaborate structures,
or have elaborate appearances all in an attempt to attract a female.
An example of intersexual selection is bower birds where the male builds an elaborate bower to attract a female. This is not a nest and it has no purpose other than to impress a female. Females choose the male who builds the best bower. Male bower birds are known to rob camp sites and garbage cans looking for brightly colored bits of plastic and metal to decorate their bowers. In this image here, the male has decorated his bower with pieces of blue plastic.
Slide 6
Intrasexual selection is
when a male competes with another male for access to females. This type
of system usually evolves when all of the females live in a centralized
location and a dominant male can fight to maintain access to them.
An example of this is the elephant seal - male seals fight for access to a group of females. The winner will get to mate with all of the females while the loser gets no mating opportunities (unless of course, he can sneak in while the dominant male is not looking).
Because the stakes are so high - competitions between male seals can become quite violent. Once a male wins - he has to work hard to maintain his position. Dominant male elephant seals will often go months without feeding because to do so would mean leaving his females unguarded.
Slide 7
In many species,
sexual selection results in sexual dimorphism (a difference in appearance
between males and females)
Sexual dimorphism is often most noticeable in birds - males usually have bright colors and females are often more drab. However, it does occur in other species such as some mammals, fish, reptiles and lizards.