BIOL 3445 Lecture Notes - Lecture 20: Red Queen Hypothesis, Sexually Transmitted Infection, Sexual Selection
BIOL 3445.001 | Lecture #20 | 3/29/2018
Sex and Sexual Selection
ANNOUNCEMENTS
➢ In-Class Group Work
o Graded over the weekend – results are up on Canvas
o Will do 13 total, 3 will be dropped
➢ Exam #2
o Grading is not going well but she’s only graded the hardest parts
o “How bad is it?” “So far, the parts I’ve graded have been pretty bad”
o Will curve the grade if she needs to
o Grader is pretty speedy so grades should be up in the next day or too
OBJECTIVES
― Explain why sex evolved even though it is costly
― Why is Muller’s Ratchet a problem for asexual species?
― How does the “Red Queen” hypothesis explain the evolution of sex?
― Explain why sexual selection exists and how it differs from natural selection
― Describe the differences between intrasexual and intersexual selection
WHY SEX?
Cons
― The Two-Fold Cost of Sex:
o A single, asexual female can produce twice
as quickly as a sexual female and a male
▪ She doesn’t need to find a male
▪ Produces all daughters who produce all daughters
▪ Sexual females produce ½ males and ½ females – nothing against
males but they’re basically a dead-end because they can’t
reproduce on their own
o Natural selection should favor asexual reproduction
― Search Cost: males and females must find each other to mate
o If you breed one time a year in a specific place, mistakes can be costly
― Reduced Relatedness: sexual organisms can only pass half of their genes on to
their offspring
o Asexual parents are 100% related to their offspring
o Natural selection is based on reproduction and fitness
o If you’re only 50% related, you’re ½ as “successful” as an asexual
reproducer
find more resources at oneclass.com
find more resources at oneclass.com
BIOL 3445.001 | Lecture #20 | 3/29/2018
― Risky Partners: interacting with another individual can be dangerous
o Risk of attack or damage from mate
o Risk of sexually transmitted diseases and infections
o Risk of attack from rivals
o Example: female newts have a very high cost
▪ Must breathe air but breed in the water
▪ Females are so desirable that males swarm
them during breeding season
▪ Males form a ball around her and just pee all over her while she’s
held under water
▪ Real risk of drowning and suffocating
Pros
Generating Genetic Novelty
― Combining Beneficial Mutations
o Most asexual organisms can’t combine novel beneficial mutations that
evolve independently in two different individuals
o Sexually reproducing organisms can do it more easily
― Clearance of Deleterious Mutations
o Muller’s Ratchet: asexual lineages tend to accumulate deleterious
mutations
o They are unable to separate good and bad mutations in their offspring
o Over time, the genetic load (burden of deleterious mutations) of these
lineages increases and can never be purged
o Once a mutation appears, it can’t be selected against because all
offspring are clones and they each have all of the mutations
― Shuffling genes during recombination and crossing over
― Independent assortment
― Group Exercise: Why is Muller’s Ratchet not a problem for sexually reproducing
organisms?
o The good genes coming in outweigh the bad mutations
o There is a higher turnover
o Not all offspring will be the same, so even if a few offspring die because of
a bad mutation, some will live to reproduce
o Natural selection can’t act on clones
▪ If everyone is identical, there is no “better” or “worse” to select
for/against
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
In-class group work: graded over the weekend results are up on canvas, will do 13 total, 3 will be dropped. Explain why sex evolved even though it is costly. Explain why sexual selection exists and how it differs from natural selection. Describe the differences between intrasexual and intersexual selection. Search cost: males and females must find each other to mate. If you breed one time a year in a specific place, mistakes can be costly. Reduced relatedness: sexual organisms can only pass half of their genes on to their offspring: asexual parents are 100% related to their offspring, natural selection is based on reproduction and fitness. If you"re only 50% related, you"re as successful as an asexual reproducer. Combining beneficial mutations: most asexual organisms can"t combine novel beneficial mutations that evolve independently in two different individuals, sexually reproducing organisms can do it more easily. Shuffling genes during recombination and crossing over.
