Lecture 4 – October 2 nd
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
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
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
o Heritability, almost but not quite perfect
Must be highly reliable so that copying errors is rare, the errors are called
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
• 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
– 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
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
• 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
• 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?
• At some level, some of the disadvantages listed (for another level) turn out to be
• 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
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
• Two parents are better than one if they more than double offsprings’ chances of
• This can happen at the level of gametes, compared to simple replication of a unicellular
• 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