Mechanisms of Evolution
Genetic Drift –
The
Bottleneck
Effect
Example: Northern elephant seals. They have reduced genetic variation. In 1980
humans hunted the elephant seals and reduced their population to just 20 in numbers.
Then the population has been rebounded to about 30,000. But because of the
bottleneck effect they still have the less genetic variations.
Genetic Drift – The Founder Effect
Example:
16-18,000 Old Order Amish people of Pennsylvania are descended from a ~
200 individuals belonging to Pennsylvania during the early 1700's.
Because of their closed population stemming from a small number of
German immigrants, the Amish carry unusual concentrations of of gene
mutations that cause a number of otherwise rare inherited disorders,
including forms of dwarfism.
One form, Ellis-van Creveld syndrome, involves not only short staure but
polydactyly, (extra fingers and toes). This syndrome is common in the Amish
because of the founder effect.
Why such elaborate displays?
Why such bright plumage or markings?
Why the head gear?
Why do males and females look
different?
Sexual Selection
Definition:
Sexual selection is the process by which a
character increases in frequency in a
population because it enables the organism to
acquire more mates.
That the beneficial sexual
trait is heritable.
Two Kinds of Sexual Selection:
(1). Male competition  males compete
among each other for access and time to
females
- competition can be direct (e.g. fighting)
or indirect (e.g. via visual displays, call
rate, song, or even sperm removal).
Two Kinds of Sexual Selection cont:
(2). Female Choice  creates traits that
evolved in males because they attract
the attention of females.
- females choose the largest males or
males with the most elaborate/complex
displays
Sexual Dimorphism
Definition:
The difference between phenotypes (physical
differences) in males and females of the same
species.
Why did sexual selection evolve?????
Because of the characteristics of sexual
reproduction.
1) Females carry a limited number of eggs
(limiting factor)
2) Males have an unlimited amount of sperm
-
These 2 characteristics have created
intense competition between males and
choosiness in females
Do you think a peacock with a large tail is
better adapted to the environment?
-
Not necessarily
Sexually selected traits may not be
adaptive in the sense that they help an
individual survive in the environment