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Lecture 4

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University of Toronto St. George
De Sousa

Lecture 4 – October 2 nd PHL243 Summary of Week 3 - Aquinas and Natural Law: merges two senses of Law - There is by nature just one natural end of sex - But that natural end has to be enforced - It is not merely natural, but backed by God’s command - Non-reproductive use of sex is per-version, and sin - Problems o How do we discover what nature intends o What’s good about natural anyway The economic point of view in Biology - The economic point of view consists in evaluating an action in terms of the value of its consequences o Look at the different consequences an action might have and weight each one by its probability of its occurrence - A typical version: Bayesian Decision Theory. The “expected utility” U of an option is defined as the sum of the values of each possible outcomes di weighted by its probability P Biology and Evolution - Key tenets of modern evolutionary biology o All living things have a common ancestor o Diversification is largely random o Natural selection shapes adaptation o The mechanism of heredity is (mostly) genetic o Phenotypes (something that is apparent)are selected, but what are inherited are features of genotypes Evolution by natural selection - Three assumptions o A source of variation or diversity  Have to start with a variety of things that they have to be capable of being passed on o Heritability, almost but not quite perfect  Must be highly reliable so that copying errors is rare, the errors are called mutations o A differential rate of success in survival  Some variants may be more successful in leading more offspring than others - The pattern applies equally to learning and to evolution, as well as to cultural transmission and the progress of technology - Only the items selected are different Lecture 4 – October 2 nd PHL243 A paradox • The economic point of view applies more clearly to evolution by natural selection than it does to economics. • Because in economics, you have to assume that agents are rationally self-interested. (Which is unrealistic) • In biology both interest and probability interest have In biology, both interest and probability interest have objective measures: – how probable is it that a given gene will be passed on, and – how many copies of itself a gene will leave in the longish run. What does not follow: perfection of organs • The process of evolution is “satisficing” and competitive. • You get amazing results, but not perfect ones • Expect Rube Goldberg devices and compromises. • Countless things in nature don’t work as if they were “designed” to be the best possible – Eg: human eyes; breathing & feeding tubes; simultaneous blinking; selfish transposons (MS&Sz 97)….. – And sex. What is sex? • At the most primitive level (bacteria), gene exchange without reproduction. • One hypothesis: gene exchange allowed repair. • When involved in reproduction of metazoans: - When involved in reproduction of metazoans: o Division of labour: different cells do their thing; specialized cells take care of replication. o Gametes contain half an individual’s genes o 2 gametes fuse, for a full complement of genes. Meiosis: duplication + double division III Drawbacks of sexual reproduction. Lecture 4 – October 2 nd PHL243 • NB: “the sexual process is the exact opposite of reproduction” (Maynard-Smith) (sex- taking two to make one, reproduction- start with one and end up with two) • The cost of meiosis: for any given gene, the chances of being transmitted is only 50% • The cost of mate search: some luck needed. • “Two fold cost”: Compared to parthenogenesis maintenance costs are exactly double. o Need two people to produce the same number of offspring if you were just a female • Novelty: Why throw out a proven winner? o (note that chromosomes aren’t even always intact) • Why trust a lottery? Lots more luck needed. o (“lotteries discriminate against the unlucky….”) IV Theories about the benefits of sex • First ask: cost, or benefit to what or whom? o Gene (in some sense it’s the ultimate beneficiary, because only genes replicate) – Individual? (but [contra MS&Sz] why is having offspring an advantage to the individual? – Group? – Species? • At some level, some of the disadvantages listed (for another level) turn out to be advantages. • Eg. Parthenogenesis is an advantage for the lineage in short term, but disadvantage for the species long term Advantage strictly to gene: • Transposons might favour their own replication by fusion (The Hickey and Rose theory). • This would favour the replication of the gene in question, but not necessarily be good for the individual. - Novelty vs. Stability o The “proven winner” and “lottery” point: yes, but only if conditions stay just the same. o Sex is like buying many different lottery tickets. o This does nothing to the individual, and actually nothing to this or that genotype either: but it gets some genes transmitted and may keep the group or species going through change. When “two fold cost” pays for itself: Lecture 4 – October 2 nd PHL243 • Two parents are better than one if they more than double offsprings’ chances of survival. • This can happen at the level of gametes, compared to simple replication of a unicellular organism. • Or at the level of organisms (e g mammals requiring at the level of organisms (e.g. mammals, requiring intensive nurturing) Other advantages at the population level • Getting rid
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