anisogamy_handout

# anisogamy_handout - Some clariﬁcations on anisogamy and...

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Unformatted text preview: Some clariﬁcations on anisogamy and the evolution of the sexes: 1. Don’t get confused by the axes on the graphs shown below! The graphs depict the distribution of gamete sizes in populations examined before and after the evolution of anisogamy. These graphs are intended to show how the distribution of gamete sizes changed from a normal (bell shaped) curve to a bimodal curve that reﬂects the clear size difference between male and females gametes. BUT, our arguments about the selective pressures leading to anisogamy were (as always!) based on costs and benefits to individuals. We will use this type of graph multiple times in class, so you need to know what is going on. The Y—axis depicts the number of animals in the population that have gametes of a certain size. It does not represent the number of gametes of that size! To make this graph, we would go to our study population and measure the size of gametes produced by each individual. For each size of gamete, we would then count up how many individuals make that size to produce the value that we would plot on the Y-axis. "* CF mm- NDNlDDM-‘B ______,_> ‘1le 9mm LIME. o 0 S y Bog/5'49 AM (SDGAI’IV 2. To understand the evolution of anisogamy, there are several assumptions that need to considered: V a. An individual has only so much energy that can be devoted to making gametes. As a result, there is a tradeoff between the number of gametes produCed and the size of each gamete — if an individual makes a lot of gametes they must be small, While producing large gametes means that the individual can make only a few. b. There is a minimum zygote size that must be met for a zygote to be viable. In other words, the union of two gametes must produce an entity that is larger than some threshold size if the zygote is to survive. 3. To understand the selective pressures favoring anisogamy, it’s important to think in'terms of costs and beneﬁts: a. For individuals that produce lots of small gametes, there is a cost if two small gametes combine to produce a zygote that is too small to survive. However, individuals with small gametes can produce so many of them that it is quite likely that some will result in viable zygotes, so there is a beneﬁt to producing so many gametes. b. For individuals that produce just a few large gametes, there is a cost in that they only have a few chances to produce offspring, but there is a beneﬁt in that any size combination of gametes will be successful. 0. While there are beneﬁts to both extremes of gamete size that offset the associated costs, individuals that produce middle sized gametes (and, accordingly, medium numbers of gametes) face the same costs but with no sure beneﬁts. Mid-size gametes are not large enough to insure that they will always produce viable zygotes and they aren’t numerous enough to insure that some of an individual’s gametes will result in offspring. As a réu‘lt, selection should act against individuals that produce medium sized gametes, leading to disruptive selection and the formation of two distinct size classes of gametes, or anisogamy. 4. While the above scenario describes the evolution of anisogamy, it doesn’t explain why, in sexually reproducing species, males (small gametes) always mate with females (large gametes). Answering this question relies on the same arguments made above for anisogamy. Once we have two distinct size classes of gametes, male gametes are too small to produce a viable zygote if they combine with each other. In contrast, female gametes are ﬁne if they combine with each other. But, male gametes are so much more numerous that the most likely combination is male-female. Because the male—female combination should be more successful than the male-male combination, any heritable traits that favor male gametes combining with those of females should be strongly selected for and, over time, we would expect this to become the dominant pattern. ...
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## This note was uploaded on 02/11/2010 for the course IB Animal Beh taught by Professor Lacey during the Fall '09 term at Berkeley.

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anisogamy_handout - Some clariﬁcations on anisogamy and...

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