how and why sexual reproduction increases the speed at which
favorable mutations can be combined
What are the advantages of reproducing sexually?
One is: that sex can break Muller’s ratchet “concentrate
harmful mutations” in some offspring, while at the same
time producing some offspring that have very few harmful
mutations this is an advantage b.c. being able to produce
at least some offspring that don’t have many mutations is
essentially a big fitness advantage and it can prevent a
sexually producing population from going extinct.
o HOWEVER, remember that mutations have a small fraction of
mutations that are beneficial.
Adaptive evolution is by which these helpful mutations “alleles”
spread out in a population.
(Ex there are 2 large populations that each have 3 helpful mutations,
A B C, and one of the populations produce asexually, the other one
In sexually producing populations you reach a point where all 3
mutations, A B C, reach fixation! (but in the beginning
individuals cannot have more than one mutation in one
individual but the next generations start having more than one,
ex AB or AC or BC, and then in next generations, eventually
the 3 mutations reach fixation) (quickly and efficiently)
In asexually producing populations, the 3 mutations will
compete until one of them will increase in frequency and
outcompete the other ones and then after long time after having
A mutation for the whole population an individual will have
another mutation, B, and then it going to spread over population
and then after very long time the 3 mutation, C, arise in this
population, so eventually they reach the 3 helpful mutations,
but it takes really long, long time! Much slower than sexual
relationship between sexual reproduction and extinction risk
• sexual recombination benefits the population by increasing speed of
– rate at which disadvantageous mutations can be discarded – rate at which advantageous mutations can be brought together
in the same individual
• Then after time, this decreases the likelihood of extinction and this is
the main advantage of reproducing sexually despite the major
costs(mate, male, meiosis costs)
why mutational (long-term) explanations for sex are not sufficient to
explain its persistence
Problem with this view? Is it enough?
Imagine a population in which reproduction is sexual, almost
individual in the population produces sexually and sucks up the
costs, then there is an individual migrating from somewhere
else who is going to behave selfishly, and reproduce asexually,
she is going to outcompete all individuals in the population that
reproduce sexually, she is going to be successful!
o What is the individual benefit?
The speed in which harmful mutations can be removed and the speed
in which helpful mutations are combined.
This of course lead to the prevention of extinction.
bet-hedging in a changing environment (when an environment is
changing very rapidly, then you should take a chance and hope for
getting offspring with low than average of harmful mutation “lottery
Which is more important in maintaining sex? Long or short term advantage?
Remember that selection doesn’t anticipate “predict” the future,
instead, selection offers individual advantage right here and right now!
So, long term explanations provide REDUCED risk of extinction, but
they are not sufficient to explain why sex persists! The ecological
explanations (in a changing environment, sex benefits individuals and
the Red Queen principle) of sex are the only reasons why sex isn’t
completely outcompeted by individual asexual reproducing.
role of environmental stability in determining whether sexual or
asexual reproduction is favored
if the environment is very stable is better to colon your self or produce as much diversity as possible?
Asexually production will be favored by selection in a stable
selection. But in a changing, unpredictable environment, sex is
favored to make sure that at least some of your offspring are
going to be well adaptive in the future “ Lottery principle” buy
many copies of the same ticket or hedge your bets? In reality,
environment is not stable, so producing asexually is as foolish
as buying the same copy of lottery ticket!
lottery principle and Red Queen principle as explanations for
advantages of sex
mutational explanation for sex:
• lottery principle: buy many copies of the same ticket, or badge your bets
o In reality, environment is not stable, so producing asexually is
as foolish as buying the same copy of lottery ticket!
• Red Queen principle: running to stay in the same place.
-co-evolutionary arms race between natural enemies (hosts/parasites)
put frequency dependent selection (ex. Negative dependent
frequency when parasites, an organism that lives in or on another
organism, put the negative selection on their hosts, this is done by
living on or in the common genotype not the rare one!( if we say 90%
of population have genotype A and the rest 10% are B, parasites will
choose the 90% one)
-this is one good individual advantage of producing sexually where
you can produce new (rare) genotypes that have high fitness
-when parasites are common, producing new combinations of alleles
why sex places different selective forces on males vs. females
Sexual selection refers to the process by which Individuals are
competing with other individuals of the same sex for access to a
mate, the opposite sex. Sexual Selection is another outcome of the existence of sexual
reproduction stems from the fact that sexual reproduction places
fundamentally different selection pressures on males as
opposed to “in contrast with” females.
A lot of the traits that look beautiful or colorful or attracting in
the nature can be explained by sexual selection! These traits
seems to interfere with survival ship of this particular
A trait favored by female choice intersexual selection (this
when individuals of one sex are choosing amongst the opposite
Other traits may be favored through sexual selection, but
slightly different variant of sexual selection. Some traits like
large body size in male elephant seals, or horns in many
mammals, insects, and fish, these traits are actually favored
through intrasexual selection. That is “”direct”” competition
between members of one sex (fight over the opposite sex)
Imp. Some traits are favored by intersexual selection, others
favored by intrasexual selection.
The distinction between the two kinds is not always to draw!
Intrasexual and intersexual selections can be both operating at
the same time! Because what we mean by sexual selection is
competition between members of one sex, usually males, for