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Lecture 17

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Western University
Biology 1001A
Tom Haffie

 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 produce sexually:  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 production.  relationship between sexual reproduction and extinction risk • sexual recombination benefits the population by increasing speed of evolution – 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? -Long-term advantages:  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. -Short-term advantage: 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 principle”)  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. Organizes  -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 is advantageous.  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 individual!  A trait favored by female choice  intersexual selection (this when individuals of one sex are choosing amongst the opposite sex.  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
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