Transcript A) The evolution of sex

BIOL2007 TUTORIAL EXERCISE:
Due tomorrow, Fri. 4:30
Kevin in office 2:30-4:00 pm
TODAY
May be some help if you are having
problems with game theory exercise
BIOL2007 EVOLUTION OF SEX AND
SEXUAL SELECTION
EVOLUTIONARY QUESTIONS
ABOUT SEX
Today:
A) 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"
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).
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.
EVOLUTION OF SEX
Costs of sex
Recombination and sex is complicated,
costly, predation-prone (even if fun).
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)
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.
Advantages of sex
a) recombination increases evolutionary rate
Muller: Recombination allows advantageous mutations
to combine in the same individuals.
Higher evolutionary rate can be adaptive for a species.
D'you
know
what
dandelions
are?
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?)
Evidence:
asexual species trapped on “twigs” in phylogenies.
b) Individual selection
Survival in a coevolutionary "arms race“
Environment of offspring may be very different from
that of their parent, parent’s 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.
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.
Birds of
paradise
(Paradiseidae)
Western grebe
Aechmophorus
occidentalis
Jackson’s widowbird
Euplectes jacksoni
Satin bowerbird
Ptilinoorhynchus
violaceus
Stag beetle
Lucanus cervus
Stalk-eyed fly
Cyrtidopsis
dalmanii
(Diopsidae)
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?
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
 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 doesn’t limit offspring number
Elephant seals:
> 90% males father no
offspring;
fittest fathered 93.
But > 50% females have
1 offspring.
Males: higher variance in
offspring number.
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.
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.
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.
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
RA Fisher:
his analyses rehabilitated female choice
Today, explosion of work showing:
1) females do indeed choose, and
2) sexually selected traits often costly.
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
Why SHOULD females choose males with
exaggerated and costly traits?
Three major theories:
a) 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).
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.
i) “sexy sons”
Mathematical models, by Russ Lande
and Mark Kirkpatrick (1980s): runaway
models of sexual selection.
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.
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.
Zahavi’s "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".
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 haven’t yet resolved these
arguments, but they are fun to think about
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
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.
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)
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
FURTHER READING
FUTUYMA, Evolution.
Ch 14: 329-339, Ch 17: 417-426.
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