Transcript Chapter 11: Sex and Evolution

Chapter 11: Sex and
Evolution
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
(c) 2001 W.H. Freeman and
Company
Stalk-eyed flies, 有柄眼果蝇
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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
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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.
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Asexual Reproduction
Progeny produced by asexual
reproduction are usually identical to
one another and to their single parent:
asexual reproduction is common in plants
(individuals so produced are clones)
many simple animals (hydras水螅, corals
珊瑚虫, etc.) can produce asexual buds,
which:
may remain attached to form a colony
may separate to form new individuals
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(c) 2001 W.H. Freeman and
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Other Variants on
Reproduction
Asexual reproduction:
production of diploid eggs (genetically identical)
without meiosis (common in fishes, lizards and
some insects) parthenogenesis孤雌生殖
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)
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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
So why does sexual reproduction exist at all?
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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
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(c) 2001 W.H. Freeman and
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Figure 11.5
Cost of Meiosis 2
The cost of meiosis does not apply:
when individuals have both male and female
function (are hermaphroditic雌雄同体)
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
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Advantages of Sex
One advantage to sexual reproduction is
the production of genetically varied
offspring:
this may be advantageous when
environments also vary in time and space
Is this advantage sufficient to offset弥补
the cost of meiosis?
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(c) 2001 W.H. Freeman and
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(c) 2001 W.H. Freeman and
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(c) 2001 W.H. Freeman and
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Who’s asexual?
If asexual reproduction is advantageous, then it
should be common and widely distributed among
many lineages:
most asexual species (e.g., some fish, such as
Poeciliopsis若花鳉鱼) belong to genera that are sexual
asexual species do not have a long evolutionary
history:
suggests that long-term evolutionary potential of
asexual reproduction is low:
• because of reduced genetic variability, asexual lines
simply die out over time
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Sex: A Short-Term
Advantage?
Theoretical models based on
environmental variability fail to find an
advantage to sexual reproduction!
A promising alternative is that genetic
variability is necessary to respond to
biological changes in the environment.
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藤黄科书带木属
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Sex and Pathogens
The evolution of virulence致病力 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.
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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.
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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软体动物, echinoderms棘皮
动物, plants, fishes)
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Sexual Functions in Plants
 Plants with separate sexual functions in separate
individuals are dioecious:
this condition is relatively uncommon in plants
 Most plants have both sexual functions in the same
individual (hermaphroditism):
monoecious plants have separate male and female
flowers
plants with both sexual functions in the same flower are
perfect (72% of plant species)
most populations of hermaphrodites are fully outcrossing
 Many other possibilities exist in the plant world!
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(c) 2001 W.H. Freeman and
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Separate Sexes versus
Hermaphroditism
When does adding a second sexual function
(becoming hermaphroditic) make sense?
gains from adding a second sexual function must not
bring about even greater losses in the original sexual
function
this seems to be the case in plants, where basic
floral structures are in place
for many animals, adding a second sexual function
entails承受 a net loss in overall sexual function
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(c) 2001 W.H. Freeman and
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Sex ratio of offspring is
modified by evolution.
When sexes are separate, sex ratio may
be defined for progeny of an individual or
for the population as a whole.
Humans have 1:1 male:female sex ratios,
but there are many deviations from this in
the natural world.
Despite deviations, 1:1 sex ratios are
common. Why?
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(c) 2001 W.H. Freeman and
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1:1 Sex Ratios:
Background
Every product of sexual reproduction
has one father and one mother
if the sex ratio is not 1:1, individuals
belonging to the rarer sex will experience
greater reproductive success:
such individuals compete for matings with
fewer individuals of the same sex
such individuals, on average, have greater
fitness (contribute to more offspring) than
individuals of the other sex
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(c) 2001 W.H. Freeman and
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1:1 Sex Ratios: An
Explanation
Consider a population with an unequal sex
ratio...
individuals of the rare sex have greater fitness
mutations that result in production of more offspring
of the rare sex will increase in the population
when sex ratio approaches 1:1, selective advantage
of producing more offspring of one sex or another
disappears, stabilizing the sex ratio at 1:1
this process is under the control of frequencydependent selection
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Why do sex ratios deviate
from 1:1?
One scenario involves inbreeding:
inbreeding may occur when individuals do not
disperse far from their place of birth
a high proportion of sib matings leads to local
mate competition among males
from the parent’s standpoint, one male offspring
serves just as well as many to fertilize his female
siblings, while production of more female offspring
will lead to production of more progeny
the result is a shift of the sex ratio to predominance
of females, the case in certain parasitic wasps
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(c) 2001 W.H. Freeman and
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(c) 2001 W.H. Freeman and
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Mating Systems: Rules for
Pairing
There is a basic asymmetry in sexually
reproducing organisms:
a female’s reproductive success depends on her
ability to make eggs:
large female gametes require considerable resources
the female’s ability to gather resources determines her
fecundity
a male’s reproductive success depends on the
number of eggs he can fertilize:
small male gametes require few resources
the male’s ability to mate with many females determines his
fecundity
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Promiscuity 1
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
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Promiscuity 2
Promiscuity is a mating system for which
the following are true:
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
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Polygamy
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:
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(c)attracted
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(c) 2001 W.H. Freeman and
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Monogamy
Monogamy involves the formation of a lasting
pair bond between one male and 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, relatively
common among birds (but recent studies provide
evidence for extra-pair copulations结合 selecting
for mate-guarding)
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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
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W.H. Freeman and
territories varies
considerably
Company
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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
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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
Traits which distinguish sex above primary
sexual organs are called secondary sexual
characteristics.
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Pathways to Sexual
Dimorphism
Sexual dimorphism may arise from:
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
combats among males:
weapons of combat (horns or antlers) and larger size may
confer advantages to males in competition for mates
direct effects of female choice:
elaborate male plumage and/or courtship displays may
result
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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
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Runaway Sexual Selection
When a secondary sexual trait confers
greater fitness, the stage is set for
runaway sexual selection:
regardless of the original reason for female
preference, female choice exaggerates fitness
differences among males:
leads to evolution of spectacular plumage (e.g.,
peacock) and other seemingly outlandish plumage
and/or displays
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The Handicap Principle
Can elaborate male secondary sexual
characteristics actually signal male quality to
females?
Zahavi’s handicap principle suggests that
secondary characteristics act as handicaps -- only
superior males could survive with such burdens
Hamilton and Zuk have also proposed that showy
plumage (in good condition) signals genetic factors
conferring resistance to parasites or diseases
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(c) 2001 W.H. Freeman and
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Summary
Sexual reproduction is widespread, yet its
benefits are not entirely clear. Genetic diversity
among offspring of sexual unions may confer 授
予fitness in the face of environmental variation
and rapidly-evolving diseases.
Sex ratios, mating systems, and secondary
sexual characteristics arise in sexually
reproducing organisms in response to selective
pressures affecting both males and females.
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