1. Stabilizing
(indicates tradeoffs)
b. Fertility
c. Interbirth intervals Blurton Jones, Kung
c. Ache height and hunting ability
d. Human
brain size
2. Directional
The human
brain
The
changing importance of IQ
3. Disruptive
Two or more morphologies
Sexual
selection - sexual dimorphism
Humans,
red deer vs. gibbons
Orangutans,
males come in 2 morphologies
Intensity of selection
For humans it would take only
10,000 years for a trait with a 1% disadvantage to change from 99% frequency to
less than a 1% frequency as long as selection is consistent and unidirectional.
Even trivial traits can be
associated with reproductive success
Head roundness and differential
survival
No trait is adaptive in all
environments
The brain
Reaction
to heat and body fat cf. reaction
to cold and body fat
Common objections to applying
evolutionary theory to human behavior
1. Natural selection works fine on morphology
and physiology, but the interesting thing to study about humans is their
behavior and natural selection does not work on behavior
2. humans are unique: they have heavy
reliance on learning, behavior varies culturally and not genetically, humans
possess language and language is related to culture -through language, people
pass on culture from one generation to next.
Free us from the tyranny of our genes
3. Biol. Evolution based upon differential propagation of genes - slow process, many generations for small amounts of genetic change - human change very rapid - e.g. demographic transition - Differential propagation of genes cannot explain demographic transition (or can it?)
4. Humans engage in many behaviors counter to their genetic
fitness: suicide, smoking, reproductive restraint to avoid overpopulation
Class today will show that these
objections are not valid
Can behavior evolve?
The dung fly story - Geoffrey Parker
Dungfly females - mature a large
quantity of eggs, store until ready for oviposition
look for suitable dung to place
larvae in, first flies to and past to upwind side, than cautiously moves to
dung surface to oviposit
store sperm but no problem many
males, site for sperm competition
males -flock to dung 4-5 males to
every female
male sights female, leaps, mounts,
orients and mates immediately - penis like a plunger tries to force out old
sperm, success partly based on how long male copulates
After ejaculation, hangs on while
female is ovipositing. other males
attack mated pair to displace male.
Male guards and fights other male off with middle legs while he holds on
with first and third pairs
what is a male to do about all
those other males?
three types of females from male
point of view: arriving, copulating and ovipositing
arriving females (free for
copulation) best bet
copulating (struggle but haven't
oviposit)
ovipositing (only some eggs left
to be fertilized)
1st 20 minutes - females are in
five zones
a. dung surface b. grass area
surrounding within 20 cm (8 inches) c. 20-40 cm from edge d. 40-60 f. 60-80
Males will place themselves in
zones so that all achieve relative equal numbers of copulations (ideal-free
distribution) (figure 8.3)
The longer a male mates with a
female the more sperm from the previous male he displaces, but at a diminishing
rate. There is a tradeoff between the
proportion of the females eggs he fertilizes and the possibility of finding
another female. Given an average
expected search time for the next female, there is an optimum amount of time to
spend in copula with the female. Flies
spend very close to the optimum (fig 8.6).
Can we show that natural selection can act on behavior?
It is possible to select for
behavioral traits
William Cade and Cricket calling times
He took males with different calling times and selected from the
distribution two groups: short and long callers. He mated them with females at random. After one generation, there were significant differences between
the two groups. Those differences
became increasing large with each generation of selection
Genetics of behavioral differences
among humans - twin studies reared apart in figure
Identical
twins are more similar to one another than fraternal twins, both
morphologically and behaviorally. IQ as
well (Table 5.1).
Even environmentally flexible
behavior is predictable according to the effects of natural selection
The case of blue gill sunfishes.
There are two major types of males:
Parentals and cuckolders.
Parentals delay maturation until 7 years of age. They build nests to protect offspring and
attract females. females release eggs
by nest and males release sperm to fertilize eggs, and males guard the eggs.
Cuckolders start reproducing at age 2 and provide no care. They wait near a parental male’s nest and
dart in ejecting sperm during the spawning with the parental male; thus gaining
matings and parasitizing the care of parental males.
The payoff to cuckolders depends on the environment. In weedy areas, cuckolders do better when
they are alone. They can hide and if
there are few other cuckolders, most of the additional fertilizations are
theirs. When there is low cover, they
do better in bigger numbers, because they can distract the bigger parental
males. The number of cuckolders
systematically corresponds to % cover in predicted fashion.
It is also probably the case that many cuckolders become parentals after
they grow.
Distinction between genotype and phenotype
1. Genotype is the particular combination of
genes possessed by an individual
2. Phenotype is the suite of actual
characteristics or traits exhibited by the living organisms.
3. Sometimes phenotypes vary predictably with genotype as in the case of cricket calling and personality characteristics
4. Often phenotypes can vary according to the
environment, even when genotype is held constant
Example of stature in room, perhaps 95% genetically
determined, can bet that everyone here had adequate nutrition during
development, if room was filled with representative sample of the world’s
population, would have to change the percentage determined by genes, perhaps
down to 50% or less. Nutritional
differences come into play. Cannot make
statements about what percentage is environmentally or genetically determined
without specifying what population and what environment-
examples are a) distribution of other dungfly males affects the best zone
to be in b) % cover in sunfish; c) age in sunfish
5. Give rise to the concept of evolved reaction norms. What evolves is not the behavior itself but
the response to given environmental and personal conditions.
6. Environment comes in at two points: it turns genotypes into
phenotypes and selects among phenotypes: nutrition very important in
determining phenotype
Tinbergen’s 4 levels of causation:
1. function or adaptive value, male rage
2. mechanism, physiological,
neurotransmitters, male rage
3. ontogeny, development, expression can be suppressed, punished or modeled in environment of growth
4. phylogenetic or species history, sets range of options, humans and other higher primates can’t choose to have litters or external fertilization under normal conditions, only in fertility clinics
