Chapter 11: Sex and Evolution

1
Chapter 11 Sex and Evolution

• Robert E. Ricklefs
• The Economy of Nature, Fifth Edition

2
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 individuals gametes

3
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.

4
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

5
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)

6
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?

7
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

8
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

9
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?

10
Whos 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

11
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.

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 Carrolls Through the Looking Glass and
  What Alice Found There.

14
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)

15
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!

16
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

17
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 11 malefemale sex ratios, but there
  are many deviations from this in the natural
  world.
• Despite deviations, 11 sex ratios are common.
  Why?

18
11 Sex Ratios Background

• Every product of sexual reproduction has one
  father and one mother
• if the sex ratio is not 11, 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

19
11 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 11, selective
  advantage of producing more offspring of one sex
  or another disappears, stabilizing the sex ratio
  at 11
• this process is under the control of
  frequency-dependent selection

20
Why do sex ratios deviate from 11?

• 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 parents 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

21
Mating Systems Rules for Pairing

• There is a basic asymmetry in sexually
  reproducing organisms
• a females reproductive success depends on her
  ability to make eggs
• large female gametes require considerable
  resources
• the females ability to gather resources
  determines her fecundity
• a males reproductive success depends on the
  number of eggs he can fertilize
• small male gametes require few resources
• the males ability to mate with many females
  determines his fecundity

22
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

23
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

24
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

25
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)

26
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 males 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

27
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

28
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.

29
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

30
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

31
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

32
The Handicap Principle

• Can elaborate male secondary sexual
  characteristics actually signal male quality to
  females?
• Zahavis 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

33
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.

34
As usual

• Quizzes. Do the quizzes.
• ?