IB31Lecture+15+Outline+Evolution+of+Sex+II+Sp08 - IB31...

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IB31, Lecture 16 March 14, 2008 SEX: EVOLUTIONARY MECHANISMS II I. Given that sex is good for something, why Male and Female? Why not three or 11 sexes? Some Paramecium (Protozoa) species have up to 14 sexes (mating types)! A. Ask class what is a male? Sex involves the fusion of two gametes produced by meiosis. In most animals we recognize male and female by external characteristics such as genitalia, size, secondary sexual characteristics (beards, horns, etc.) The real difference is RELATIVE SIZE OF THE GAMETES. Females produce large, immobile, food-rich "eggs". Males produce small, mobile "sperm" that are nothing more than DNA + motor + cap of enzymes to penetrate egg. B. Why? Sexual reproduction without eggs and sperm occurs in some single celled organisms such as protozoa and the 'gametes' which fuse are the same size. This is referred to as ISOGAMY – having the same sized gamete. When animals have eggs and sperm, it is called ANISOGAMY. Parker proposed the following evolutionary pathway to explain anisogamy. 1. Assume isogamy, but some heritable variation in size. Larger zygotes have greater chance of surviving because they have more stored food reserves. 2. Large gametes produce large zygotes, but fewer can be made. 3. If the survivorship of large gametes is sufficiently great to more than compensate for their reduced numbers (the bigger they are, the fewer that can be produced), they will be favored. 4. This produces immediate selection for small gametes to find and fuse with large gametes. Otherwise, they have no chance. Larger gametes should try to avoid fusing with small ones, but the penalty is
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not as great. Selection acts more strongly on small gametes - and because they are small - even more of them can be produced – they will evolve faster and succeed in parasitizing larger "eggs". Intermediates lose out. They are not big enough to contain the reserves needed and they are too big and slow to compete with "sperm". 5. This model proposed by Parker has been pretty much accepted and unchallenged for 40 years. However, recently biologists have started to take a second look. The basic assumption that larger gametes more than proportionally increase the fitness of the resulting zygote may not always be true, so alternative models are being suggested. A couple of such hypotheses relate to situations where sperm are limited and there is a low probability of two gametes meeting.
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  • Spring '08

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