BIOL 3445 Lecture Notes - Lecture 12: Neonatal Intensive Care Unit, Stabilizing Selection, Directional Selection
BIOL 3445.001 | Lecture #12 | 2/22/2018
The Evolution of Phenotypes
EXAM #1
― Getting your exams back today
― Mean was a 76% which is pretty good, so this exam will not be curved
― If you aren’t happy with your score, there is time to bounce back – don’t freak
out
o If you’re serious and want to improve your score, go to office hours
o Study with someone from the class
▪ You might know understand they don’t and vice versa
▪ Make sure you don’t remember or study something the wrong way
o The Learning Resource Center has drop-in tutoring
▪ Their tutors are very smart
▪ No tutors specific to this class but they can help you approach the
information with a different perspective which might help
― If you don’t understand something or think your exam was unfairly graded, don’t
be afraid to ask questions
o The graders are human, so they could have added something incorrectly
o Only come with legitimate concerns – please don’t just beg for a better
score
― There is extra credit available, so it is possible to get better than 100%
OBJECTIVES
― Understand the different ways selection can act to change phenotypes in a
population
― Use the “breeder’s equation” to estimate R, S, and h2
SELECTION ON PHENOTYPES
― A phenotype will evolve if:
o There is variation in the phenotype
o There is selection on the phenotype
o Differential reproduction with respect to the phenotype is present
o The phenotype is heritable
find more resources at oneclass.com
find more resources at oneclass.com
BIOL 3445.001 | Lecture #12 | 2/22/2018
Stabilizing Selection
― Individuals with trait values close to the mean are
favored and selection acts against extreme trait values
― The most common example in nature
― Narrows down population variation
― Example: Human Birth Weights
o Babies born too small are less likely to survive
o Babies that are too big may get stuck and kill
the mother and baby
o Now we have NICU and C-sections, so this
example doesn’t have as big of an effect on the human population
Directional Selection
― Individuals with trait values at one extreme are favored
while selection acts against the other extreme
― The population will eventually reach fixation
o A gene can only go so far (you can’t get fur that
is blacker than black)
o If extreme A is eroded away the phenotype will
fixate on extreme B
― Mostly just moves the mean to a new center
o Can switch directions and sill be directional
o May sound like disruptive or negative frequency selection but they’re
different
― Example: Melanic Peppered Moths
o As dark soot covered trees in England during the industrial revolution, the
lighter moths were suddenly more visible to predators
o Darker moths were able to hide so the phenotype shifted to select for
them
o After the people realized soot was bad for the
environment and they cleaned up their factories,
the soot was washed away, and the lighter moths
made a comeback
Disruptive Selection
― Individuals with traits at either extreme are favored while
the mean is selected against
― Increases variance in the population
― Example: Black-Bellied Seedcracker Finches
o Experience disruptive selection when food is scarce
o Big-beaked birds can open tough shells of large seeds
o Small-beaked birds can open small seeds
o Medium-beaked birds will starve after the average-size seeds are all
eaten
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Mean was a 76% which is pretty good, so this exam will not be curved. If you aren"t happy with your score, there is time to bounce back don"t freak out. Their tutors are very smart: no tutors specific to this class but they can help you approach the information with a different perspective which might help. There is extra credit available, so it is possible to get better than 100% Understand the different ways selection can act to change phenotypes in a population. Use the breeder"s equation to estimate r, s, and h2. A phenotype will evolve if: there is variation in the phenotype, there is selection on the phenotype, differential reproduction with respect to the phenotype is present, the phenotype is heritable. Individuals with trait values close to the mean are favored and selection acts against extreme trait values. Individuals with trait values at one extreme are favored while selection acts against the other extreme.
