Transcript Chapter 11: Sex and Evolution

+
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
Chapter 11: Sex and
Evolution
+
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
+
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.
+
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, corals, etc.) can produce asexual
buds, which:

may remain attached to form a colony

may separate to form new individuals
+
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)
+
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?
+ 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
+
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
+
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 the cost of meiosis?
+
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) 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
+
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.
+
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.
+
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.
+
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)
+
Sexual Functions in Plants

Plants with separate sexual functions in separate
individuals are dioecious:


Most plants have both sexual functions in the same
individual (hermaphroditism):




this condition is relatively uncommon in plants
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!
+
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 a net loss in overall sexual function

+
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?
+
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
+
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

+ 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

+
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
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
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
+
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 attracted
+
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 selecting for mate-guarding)

+
The Polygyny Threshold
 When
 For
should polygyny replace monogamy?
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
+
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
+
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.
+
Pathways to Sexual Dimorphism
 Sexual

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:


dimorphism may arise from:
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
+
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
+
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
+
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
+
Summary
 Sexual
reproduction is widespread, yet its
benefits are not entirely clear. Genetic
diversity among offspring of sexual unions
may confer 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.
+
As usual…

Quizzes. Do the quizzes.

