Transcript Chapter 10: Life Histories and Evolutionary Fitness

CHAPTER 8: SEX AND
EVOLUTION
Stalk-eyed flies
2
Stalk-eyed flies
 Both M &F have these
stalks
 In some species – they are
up to twice as long in
males as they are in
females (see pic ->)
 Why does this difference
between the sexes exist?
3
Background
 Among the most fascinating
attributes of organisms are
those related to sexual
function, such as:
 gender differences
 sex ratios
 physical characteristics
and behaviors that
ensure the success of an
individual’s gametes
4
Sexual reproduction mixes genetic material
of individuals.
 In most plants and animals reproduction is accomplished by
production of male and female haploid gametes (sperm and
eggs):
 gametes are formed in the gonads by meiosis
 Gametes join in the act of fertilization to produce a diploid
zygote, which develops into a new individual.
5
Asexual Reproduction
Progeny produced by asexual
reproduction are usually identical to one
another and to their single parent:
This fern sprouts a fully formed
plant from the tip of a leaf
 asexual reproduction is
common in plants (individuals
so produced are clones)
 many simple animals (hydras,
corals, etc.) can produce
asexual buds, which:
 may remain attached to
form a colony
 may separate to form new
individuals
6
Other Variants on Reproduction
 Asexual reproduction:
 production of diploid eggs (genetically identical)
without meiosis (common in fishes, lizards and some
insects)
 production of diploid eggs (genetically different) by
meiosis, with suppression of second meiotic division
 self-fertilization through fusion of female gametes
 Sexual reproduction:
 self-fertilization through fusion of male and female
gametes (common in plants)
7
Sexual reproduction is costly.
 Asexual reproduction is:
 common in plants
 found in all groups of animals, except birds and
mammals
 Sexual reproduction is costly:
 gonads are expensive organs to produce and
maintain
 mating is risky and costly, often involving elaborate
structures and behaviors
8
Sexual reproduction is
costly.
So why does sexual reproduction exist at all?
9
Cost of Meiosis 1
 Sex has a hidden cost for organisms in which sexes
are separate:
 only half of the genetic material in each offspring comes
from each parent
 each sexually reproduced offspring contributes only 50%
as much to the fitness of either parent, compared to
asexually produced offspring
 this 50% fitness reduction is called the cost of meiosis
 for females, asexually produced offspring carry
twice as many copies of her genes as sexually
produced offspring:
 thus, mating is undesirable
10
Cost of Meiosis 2
 The cost of meiosis does not apply:
 when individuals have both male and female
function (are hermaphroditic)
 when males contribute (through parental
care) as much as females to the number of
offspring produced:
 if male parental investment doubles the number of
offspring a female can produce, this offsets the
cost of meiosis
11
So why have sex?
12
Sex and Pathogens
 The evolution of virulence by parasites that cause disease
(pathogens) is rapid:
 populations of pathogens are large
 their generation times are short
 The possibility exists that rapid evolution of virulence by
pathogens could drive a host species to extinction.
13
The Red Queen Hypothesis
 Genetic variation represents an opportunity for hosts to
produce offspring to which pathogens are not adapted.
 Sex and genetic recombination provide a moving target
for the evolution by pathogens of virulence.
 Hosts continually change to stay one step ahead of their
pathogens, likened to the Red Queen of Lewis Carroll’s
Through the Looking Glass and What Alice Found There.
 ‘it takes all the running you can do, to keep in the same
place.’
14
Sex vs Asex
 One of the main proponents of the Red Queen hypothesis
was the late W. D. Hamilton.
 In the late 1970s, with the help of two colleagues from the
University of Michigan, Hamilton built a computer model of
sex and disease, a slice of artificial life. It began with an
imaginary population of 200 creatures, some sexual and
some asexual. Death was random. Who won?
 As expected, the sexual race quickly died out. In a game
between sex and "asex," asex always wins -- other things
being equal. That's because asexual reproduction is easier,
and it's guaranteed to pass genes on to one's offspring.
15
Now add parasites
 Next they introduced 200 species of parasites, whose power depended on "virulence
genes" matched by "resistance genes" in the hosts.
 The least resistant hosts and the least virulent parasites were killed in each
generation.
 Now the asexual population no longer had an automatic advantage -- sex often won
the game. It won most often if there were lots of genes that determined resistance
and virulence in each creature.
 In the model, as resistance genes that worked would become more common, then
so too would the virulence genes. Then those resistance genes would grow rare
again, followed by the virulence genes. As Hamilton put it, "antiparasite adaptations
are in constant obsolescence." But in contrast to asexual species, the sexual
species retain unfavored genes for future use. "The essence of sex in our theory,"
wrote Hamilton, "is that it stores genes that are currently bad but have promise for
reuse. It continually tries them in combination, waiting for the time when the focus
of disadvantage has moved elsewhere." 16
Real-world evidence
 asexuality is more common in species that are little troubled by
disease: boom-and-bust microscopic creatures, arctic or high-altitude
plants and insects.
 The best test of the Red Queen hypothesis, though, was a study of a little
fish in Mexico called the topminnow. The topminnow, which sometimes
crossbreeds with another similar fish to produce an asexual hybrid, is
under constant attack by a worm that causes "black-spot disease." The
asexually reproducing topminnows harbored many more black-spot
worms than did those producing sexually.
 That fit the Red Queen hypothesis: The sexual topminnows could devise
new defenses faster by recombination than the asexually producing ones.
17
Individuals may have female function, male
function, or both.
 The common model of two sexes, male
and female, in separate individuals, has
many exceptions:
 hermaphrodites have both sexual functions
in the same individual:
 these functions may be simultaneous
(plants, many snails and most worms) or
 sequential (mollusks, echinoderms, plants,
fishes)
18
Sequential hermaphroditism
 Check it out: more on the web
 Some organisms are male first and then become female later
in their lives
 Some organisms are female first and then become male later
in their lives
19
Whether a sequential hermaphrodite is first male or first
female depends on how reproductive success through male
or female function potentially increases with increasing
body size. Larger females have larger reproductive organs
and can lay more eggs. Small males can have high
reproductive success where fertilization is internal and
males do not contest social status, as in the case of the
slipper shell. Where males compete for territories, as in the
wrasse, large size is prerequisite to successful reproduction.
20
For others…
 Sequential hermaphroditism reflects changes in the costs
and benefits of male and female sexual function as an
organism grows. In some marine gastropods having
internal fertilization, such as the slipper shell Crepidula,
insemination requires the production of only small
amounts of sperm. Hence male function consumes few
resources and has little effect on growth. Consequently,
individuals of many such species are male when they are
small and become female when they are large and thus
able to produce correspondingly large clutches of eggs
21
Check it out on the web for more explanation…
22
Mechanisms of sex determination
 Determined by the physical environment
 Several species of turtles, lizards, alligators: sex determined by the
temperature at which it develops in the egg
 Embryos that develop at lower temp produce males; higher temp:
females; for turtles. Opposite for alligators and lizards.
 Hmm… ?
 Determined by the social environment
 The wrasse (discussed earlier) is a sequential hermaphrodite
 Raised in isolation  females; raised in small groups, at least one
develops initially into a male w/o passing through a female phase
 Females may become males later when they grow large enough to
compete for territories ; primary males never change their sex
23
Sex ratio and pollution
 Recent study: “Lower oxygen levels in polluted waters could lead to a higher
ratio of male fish that may threaten certain species with extinction”
 hypoxia (O2 depletion) can affect sex development, sex differentiation and
the sex ratio in fish species. hypoxia can inhibit the activities of certain
genes that control the production of sex hormones and sexual
differentiation in embryonic zebra fish.
 In his study, Wu found that 61 % of zebra fish - a universal freshwater fish
widely used in scientific and pollution research - spawned into males under
regular oxygen conditions. Under hypoxia conditions, the ratio of males
increased to 75 %.
 Hypoxia can be a naturally occurring phenomenon, particularly in areas
where salt and fresh waters meet in estuaries such as the Pearl River Delta.
It can also be caused by pollution.
24
So? What do we know?
 in-utero exposure to pollutants could affect a child's sex.
 There are more than 80,000 chemicals in production today,
many of which are known to be persistent or to disrupt
hormone systems, and most of which haven't really tested
for their impact on human health.
 A 2007 study from the University of Pittsburgh found that
during the past thirty years, the number of male births has
steadily decreased in the U.S. and Japan. The study found a
decline of 17 males per 10,000 births in the U.S. and a decline
of 37 males per 10,000 births in Japan.
25
Human sex ratio and pollution
 The steepest sex ratio declines observed in the world have occurred on the
3,000-acre Aamjiwnaang (pronounced AH-jih-nahng) First Nation reservation in
Canada.
 The ratio of boys to girls there began dropping in the early 1990s. Between 1999
and 2003, researchers found, only 46 boys were born out of 132 recorded births.
(35%)
 Dozens of petrochemical, polymer and chemical plants border the reservation
on three sides. Mercury and PCBs contaminate the creek that runs through the
land, and air-quality studies show the highest toxic releases in Canada, said Jim
Brophy, executive director of Occupational Health Clinics for Ontario Workers,
based in Sarnia, the nearest city.
 Boys made up only 42 % of the 171 babies born from 2001 to 2005 to
Aamjiwnaang living on the reserve or nearby.
26
Promiscuity: is a mating system for
which the following are true
 males mate with as many females as they can
locate and induce to mate
 males provide their offspring with no more than a
set of genes
 no lasting pair bond is formed
 it is by far the most common mating system in
animals
27
Promiscuity 2 …
 it is universal among outcrossing plants
 there is a high degree of variation in
mating success among males as compared
to females:
 especially true where mating success depends
on body size and quality of courtship displays
 less true when sperm and eggs are shed into
water or pollen into wind currents
28
Polygamy: occurs when a single individual of one
sex forms long-term bonds with more than one
individual of opposite sex
a common situation involves one male
that mates with multiple females,
called polygyny: (eg: elephant
seals)
 polygyny may arise when
one male controls mating
access to many females in a
harem
 polygyny may also arise
when one male controls
resources (territory) to
which multiple females are
attracted
29
polyandry
 Rare cases of a single female having more than one male
mate
 Some human communities
 1% of birds..
 A common example of this can be found in the Field Cricket
Gryllus bimaculatus of the invertebrate order Orthoptera
 Widely shown in frogs (Agile frogs, Rana dalmatina), polyandry
was also documented in polecat (Mustela putorius) and other
mustelids
 Why?
30
Monogamy: the formation of a lasting
pair bond betw one male &one female:
 the pair bond persists through period required to rear
offspring
 the pair bond may last until one of the pair dies
 monogamy is favored when males can contribute
substantially to care of young
 monogamy is uncommon in mammals (why?), relatively
common among birds (but recent studies provide
evidence for extra-pair copulations in as many as a 1/3 of
the broods leading to mate-guarding)
31
Real monogamy?
 Extra-pair copulations (EPC)
 1/3 or more of the broods produced by some monogamous
species contain 1 or more offspring sired by a different male
 Mate guarding behavior on the part of males during their mates’
periods of fertility
32
The Polygyny Threshold
 When should polygyny replace monogamy?
 For territorial animals:
 a female increases her fecundity by choosing a territory with
abundant resources
 polygyny arises when a female has greater reproductive
success on a male’s territory shared with other females than
on a territory in which she is the sole female
 the polygyny threshold occurs when females are equally
successful in monogamous and polygynous territories
 polygyny should only arise when the quality of male territories
varies considerably
33
Sexual Selection
In promiscuous and polygynous mating
systems, females choose among potential
mates:
if differences among males that influence female
choice are under genetic control, the stage is set
for sexual selection:
 there is strong competition among males for
mates
 result is evolution of male attributes evolved
for use in combat with other males or in
attracting females
35
Consequences of Sexual Selection
 The typical result is sexual
dimorphism, a difference in
the outward appearances of
males and females of the
same species.


Charles Darwin first proposed in 1871
that sexual dimorphism could be
explained by sexual selection
Females of many spider species are
larger than males
 Traits which distinguish sex
above primary sexual organs
are called secondary sexual
characteristics.
36
Pathways to Sexual Dimorphism
Sexual dimorphism may arise from:
 (1) life history considerations and ecological
relationships:
 females of certain species (e.g., spiders) are larger than
males because the number of offspring produced varies
with size
 (2) combats among males:
 weapons of combat (horns or antlers) and larger size
may confer advantages to males in competition for
mates
 (3) direct effects of female choice:
 elaborate male plumage and/or courtship displays may
result
37
Female Choice
Evolution of
secondary sexual
characteristics in
males may be under
selection by female
choice:
in the sparrow-sized male
widowbird, the tail is a
half-meter long: males
with artificially elongated
tails experienced more
breeding success than
males with normal or
shortened tails
38
39