BIOL2007 TUTORIAL EXERCISE: Due tomorrow, Fri. 4:30 Kevin in office 2:30-4:00 pm TODAY

1
BIOL2007 TUTORIAL EXERCISE Due tomorrow, Fri.
430Kevin in office 230-400 pm TODAY

• May be some help if you are having problems with
  game theory exercise

2
BIOL2007 EVOLUTION OF SEX AND SEXUAL SELECTION
EVOLUTIONARY QUESTIONS ABOUT SEX TodayA)
The evolution of sex 1) What is the advantage of
sex?   B) Ev. of sexual dimorphism - sexual
selection 2) Why are there "secondary sexual
characteristics"
3
OTHER interesting questions could be asked 4)
Why meiosis? 5) Why mitochondria c asexual? 6)
Why only 2 sexes? 7) What is the optimal sex
ratio? (Kevin) 8) Why does the Y chromosome
become degenerate (non-coding)? 9) How does
sex determination evolve? 10) Sexual conflict
why and how?   See BIOL2011 (Behavioural Ecology
Sociobiol) and BIOL3012 (Sex, Genes and
Evolution).
4

• WHAT IS SEX?
• Recombination all of life
• Meiosis - eukaryotes only Anisogamy -
  unequal gamete size - multicell eukaryotes
  Dioecy - Separate sexes
• - vs. hermaphrodites and monoecy
•  
• These are complex energy-requiring traits
• ? ADAPTATIONS
• Recombination seems to be a lowest common
  denominator of all sex.

5
EVOLUTION OF SEX Costs of sex   Recombination
and sex is complicated, costly, predation-prone
(even if fun).
6
Two-fold cost of sex Populations grow faster if
parthenogens. Suppose each female produces 2.4
progeny
... and so on (but it works for any reprod.
rate)
7
Males contribute little males are useless! SO
parthenogens should outdo sexual females and
increase in frequency. (But advantage not always
twofold)   If such strong disadvantages to
dioecy, must be some pretty hefty
advantages. Recombination is the primary feature
of sex. Advantages of sex must have something to
do with recombination.
8
Advantages of sex a) recombination increases
evolutionary rate Muller Recombination allows
advantageous mutations to combine in the same
individuals.
9
D'you know what dandelions are?
Higher evolutionary rate can be adaptive for a
species. May work against individual
disadvantages for sex within populations so
group selection. Asexual "cheats" gain
individual advantage, spread to fixation within
species, not to other species. Asexual species
do exist (e.g. dandelions). Why do they not take
over? If other species evolving/adapting faster,
competition may cause extinction of asexuals ?
one of the few cases where group selection may
actually be operating. (Conversely, why don't
dandelions die out?)
10
(No Transcript)
11
Evidence asexual species trapped on twigs in
phylogenies.
12
b) Individual selection Survival in a
coevolutionary "arms race Environment of
offspring may be very different from that of
their parent, parents adaptations may not be
sufficient (e.g. Daphnia, aphids). Sex as a
lottery at least some progeny with reshuffled
adaptations - direct individual selection
advantage to parents in producing recombined,
variable offspring. Other ideas. There are
plenty, but we will leave that to other courses.
13
EVOLUTION OF SEXUAL DIMORPHISM - SEXUAL SELECTION
Darwin secondary sexual characteristics due to
a struggle for genetic representation between
members of the same sex. Sexual selection
(struggle for mating success) vs. natural
selection (struggle for survival and reproduction
after mating). Nowadays sexual selection a
special form of natural selection, but a very
important one.
14
Birds of paradise (Paradiseidae)
15
Western grebe Aechmophorus occidentalis
16
Jacksons widowbird Euplectes jacksoni
17
Satin bowerbird Ptilinoorhynchus violaceus
18
Stag beetle Lucanus cervus
19
Stalk-eyed fly Cyrtidopsis dalmanii (Diopsidae)
20
Why are males more extreme? Males usually more
flamboyant than females, with horns, bright
colours, or displays. Why?  Darwin's view
males are more "passionate" than females. So
females can be more choosy. Males have to
sedulously display their charms before the
females Maybe (!) but why are males more
passionate?
21
Males cheap gametes can produce lots little
parental investment ? potentially father many
offspring Females expensive gametes produce
few often show more parental care, at least as
far as nutrients in the egg ? more limited no.
offspring than males
22
? Males compete for access to females more
indiscriminate, less to lose ? Females plenty
of willing males around worthwhile to be choosy,
because number of matings doesnt limit offspring
number
23
Elephant seals gt 90 males father no offspring
fittest fathered 93. But gt 50 females have ?1
offspring.  Males higher variance in offspring
number.
24
Sexual selection can lead to increased matings
at the expense of reduced survival. Darwin
sexual selection might outweigh natural
selection, and explain conspicuous coloration in
the animal kingdom. e.g. sexual selection may
antagonize natural selection for camouflage.
25
Two major types of sexual selection Intrasexual
selection - male-male competition in which males
compete by fighting other males for females And
Intersexual selection - sexual selection by
female choice in which males compete to obtain
the most interest from females.
26

• Intrasexual selection - male-male competition
• a) Selection for fighting ability
• Examples
• Red deer males harems of females, defend by
  roaring, displaying, and finally fighting
  injuries and death may result
• Salmon males fight to the death with enlarged
  hooklike jaws, useless for feeding.
• Fig wasps specialized males with huge jaws
  hatch out and kill other males, then mate with
  all other females in the fig.

27
b) Sperm competition Male-male competition
after mating. Sexual selection between sperm of
rival males for fertilization in reproductive
tract of females. In primates, males from
species with polyandry have larger testes than
males from monandrous species.  e.g. Gorillas
monandrous, chimps polyandrous. See also other
courses
28
RA Fisher his analyses rehabilitated female
choice Today, explosion of work showing 1)
females do indeed choose, and 2) sexually
selected traits often costly.
29
Widowbird Euplectes progne
Before
Elongated
Control I
Control II
Shortened
0.0
0.5
1.0
1.5
2.0
After
Elongated
Control I
Control II
Shortened
0.0
0.5
1.0
1.5
2.0
Average no. mates
30
(No Transcript)
31

• Why SHOULD females choose males with exaggerated
  and costly traits?
• Three major theories
• Sensory bias. Exaggeration is a supernormal sign
  stimulus.
• Pre-existing preferences Physalaemus frog
  phylogeny shows that "chuck" sound has evolved in
  a terminal branch of the genus.
• Females of related spp. which lack "chuck" still
  like it. (Mike Ryan).

32
a) Sensory bias b) Direct benefits of female
choice e.g. avoiding disease by mating with
uninfected males. Or getting help at the nest
c) Indirect benefits of female choice Indirect
benefits are passed on to the offspring via
genetic contribution from male.
33
i) sexy sons Mathematical models, by Russ Lande
and Mark Kirkpatrick (1980s) runaway models of
sexual selection.
34
Imagine male trait evolution and female choice
females can choose whatever they want males have
a natural-selection optimum. Coevolution of male
trait and female response ? potential runaway
process of evolution.
35
ii) "good genes" hypothesis Exaggerated, costly
traits ? high fitness.   Parasite hypothesis of
Hamilton and Zuk Male birds and mammals bare
patches of skin (e.g. baboons bottoms, wattles
on cockerels) Diseases, parasites alters blood
colour ? Bare flesh honest signals of overall
health. Hamilton Zuk sexually dimorphic
species of birds suffer more from parasites.
36
Zahavis "handicap principle". Not very costly
male badges prone to cheating Zahavi only very
costly traits uncheatable A costly "handicap" is
selected, paradoxically, because it guarantees
"honest signalling".
37
Which is correct? Direct selection? Clearly this
occurs "Good genes" some evidence, easy to
test Non-adaptive "sexy sons" idea appealing,
but difficult to prove Evolutionists havent yet
resolved these arguments, but they are fun to
think about
38
Sexual selection in humans Darwin "The Descent
of Man and Selection in Relation to Sex".
Sexual selection may explain the extraordinary
racial differences in animals and humans
39

• Desmond Morris, Jared Diamond,
• evolutionary psychologists
• similar conclusions
• Peculiarly human features such as
• bare skin, hairlessness
• lips
• hair colour, skin colour
• enlarged breasts in females
• beard in males
• copulation when not in oestrus
• weapons, war?
• ornaments, jewellery?
• music?
• art? ... etc.

40

• TAKE HOME POINTS
• Evolution of sex
• The basis of sex is recombination
• Sex is usually costly
• Separate sexes may exist because of
• asexual lineages die out (group selection)
• offspring variability (individual selection)

41

• TAKE HOME POINTS (contd.)
• Sexual selection/sexual dimorphism
• Male-male competition
• fighting
• sperm competition
• Female choice
• sensory bias
• direct benefit to self/offspring
• indirect (inherited benefits)
• sexy sons
• good genes

42
FURTHER READING FUTUYMA, Evolution. Ch 14
329-339, Ch 17 417-426. Science Library View
B242 Teaching Collection by going to eUCLid use
Keyword, Basic Search, All Fields B242