... ornaments in the monogamous swallow. Nature ... Swallow

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Sexual Selection

• Selection for traits which are solely concerned
  with increasing mating success is usually
  referred to as sexual selection

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Can Work in Two Ways

• 1. By favoring the ability of one sex (usually
  males) to compete directly with one another for
  fertilizations, for example by fighting
  intra-sexual selection
• 2. By favoring traits in one sex which attracts
  the other sex inter-sexual selection

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The intensity of sexual selection depends on the
degree of competition for mates this in turn
depends on two factors

• 1. The difference in parental effort between the
  sexes
• 2. The ratio of males to females available for
  mating at any one time (known as the operational
  sex ratio)

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Strength of Sexual Selection

• When parental effort is more or less equal, as in
  monogamous birds, for example, where both male
  and female feed the young, sexual selection is
  less intense than in species with very different
  levels of parental effort
• If equal numbers of the two sexes come into
  breeding condition at the same time, the degree
  of sexual selection is reduced because there is
  less chance for a few males to control access to
  very large numbers of females

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Strength of Sexual Selection

• In contrast, when females come into breeding
  condition asynchronously theres a chance for a
  small number of males to control many females one
  after the other
• With such high potential payoffs, sexual
  competition should be intense

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Summary
P.E.
M.E.
P.E.
M.E.
M.E.
M.E.
P.E.
P.E.
Mating Sexual Selection
Promiscuous/Polygamous
Monogamous
Very Strong Less Strong
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If sexual selection is to explain the differences
between the sexes, it will have to act on the
sexes differently

• A.J. Bateman (1948) predicted that sexual
  selection variation in mating success will
  usually be a more potent force in the evolution
  of males than in the evolution of females
• In other words, access to mates will be a
  limiting resource for males but not females
• Of course this prediction is central to the
  theory of sexual selection

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Direct Tests

• Jones, A.G., J.R. Arguello, and S.J. Arnold.
  2002. Validation of Batemans principles a
  genetic study of sexual selection and mating
  patterns in the rough-skinned newt. Proceedings
  of the Royal Society of London 2692533-2539.
• Jones, A.G., G. Rosenqvist, A. Berglund, et al.
  2000. The Bateman gradient and the cause of
  sexual selection in a sex-role-reversed pipefish.
  Proceedings of the Royal Society of London
  267677-680.

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Rough-Skinned Newt
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Broad-nosed Pipefish
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Sexual Selection Theory

• Jones et al.s study on pipefish shows that
  greater sexual selection on males than on females
  is not inherent in the identity of the sexes
  themselves
• When access is limiting for females instead of
  males we predict that females will compete with
  each other over access to males and males will be
  choosy

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Competition for Mates

• The most dramatic and obvious way in which males
  compete for mates is by fighting and ritualized
  contests, and males often have evolved weapons
  for fighting
• Males may dispute over direct access to females
  or over places where females are likely to go

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Male-Male Competition Intrasexual Selection

• Wikelski, M. and F. Trillmich. 1997. Body size
  and sexual size dimorphism in marine iguanas
  fluctuate as result of opposing natural and
  sexual selection an island comparison. Evolution
  51922-936

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Although intense fighting over females can occur,
males often compete in less conspicuous ways

• Abele, L.G. and S. Gilchrist. 1977. Homosexual
  rape and sexual selection in acanthocephalan
  worms. Science

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Abele and Gilchrist (1977)

• The males of Moniliformes dubius, a parasitic
  acanthocephalan worm found in the intestine of
  rats cements up the females genital opening after
  copulation to prevent other males from
  fertilizing her
• In addition to sealing up the female after
  copulation, the male sometimes copulates with
  rival males and applies cement to their genital
  region to prevent them from mating again

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Carayon (1974)

• Shows that the male hemipteran Xylocoris
  maculipennis pierces the body wall of the female
  and injects sperm during copulation fertilizing
  her eggs
• Like with acanthocephalan worms, males sometimes
  engage in homosexual copulation. A male
  injects his sperm into a rival male where they
  wait to be passed on to a female next time the
  victim mates

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Female Choice

• Since females in the majority of species invest
  more than males in each offspring, they might be
  expected to be choosier in selecting their mates
• Females often select males on the basis of
  material resources that they can offer or females
  may select males on the basis of genetic benefits
  for their offspring

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Female Choice

• Non-genetic benefits or material resources might
  include male defended breeding territories
  containing resources that play a crucial role in
  the survival of a females eggs or young
• Females may also choose whether or not to mate
  with a male on the basis of his ability to
  provide food. In some birds and insects, for
  example, males may provide food for the female
  during courtship making a significant
  contribution to her eggs

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For example, female hanging flies, Hylobittacus
apicalis, will mate with a male only if he
provides a large insect for her to eat during
copulation

• The larger the insect the longer the male is
  allowed to copulate and the more eggs he
  fertilizes. The female gains from a large insect
  by having more food to put into her eggs

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Genetic Benefits

• If some males have better genes than others,
  could a female improve the success of her progeny
  by choosing males with good genes?
• Good genes are the ones which increase the
  ability of her offspring to survive, compete and
  reproduce

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Good Genes

• Welch, A., R.D. Semlitsch and H. C. Gerhardt.
  1998. Call duration as an indicator of genetic
  quality in male gray tree frogs. Science
  2801928-1930.

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Fitness of long-calling frogs vs. short-calling
frogs
1995 1996
Fitness Measure HF LF
HF LF Larval Growth
NS LCB LCB LCB Time to
Metamorphosis LCB NS LCB
NS Mass at Metamorphosis NS LCB
NS NS Larval Survival
LCB NS NS NS Postmetamorphic
growth - - NS LCB
HF High food LF Low food
NS non-significant LCB Long call better
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Elaborate Displays

• The theory of sexual selection is most famous as
  an attempt to explain the evolution of
  exclusively elaborate adornments and displays
• Although some elaborate displays may have evolved
  for use in contests between males, some have
  certainly evolved as a result of selection by
  females for genetic benefits

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Two Hypotheses to Explain how Selection for
Genetic Benefits might Produce Elaborate Traits

• 1. Fishers hypothesis (also called the runaway
  process)
• 2. The Handicap hypothesis

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Many studies have shown a relationship between
male mating success or female preference and male
sexual displays

• Andersson, M. 1982. Female choice selects for
  extreme tail length in a widowbird. Nature

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Andersson (1982)

• Showed that females of the long tailed widow bird
  in Kenya prefer males with long tails
• This is a highly polygynous species making it an
  ideal candidate for sexual selection the male is
  a sparrow sized bird with a tail up to 50cm long
• The females tail is about 7cm long, presumably
  close to the optimum for flight purposes

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Andersson (1982)

• Studied 36 males which he divided into 4 groups
• One group he docked the tails to about 14cm
• Another group he increased tail lengths by 25cm,
  gluing on cut feathers
• The remaining two groups served as controls - one
  left untouched the other has their tails cut
  and glued without altering length

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Conclusion

• Male ornaments are favored by female mate choice
  and probably evolved through it
• Not due to intra-sexual selection among males
  competing for territories or hierarchy ranks -
  all males held territories equally

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Hypotheses

• Fishers Hypothesis (1930) - postulates runaway
  feedback between female preference and male
  displays
• Imagine at the start there was a range of tail
  lengths and female preferences in the population
• Females with a preference for slightly longer
  than average tails would be mated to males with
  longer tails
• The crucial fact to note is that offspring of
  these mating would have both the tail and
  preference genes

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Hypotheses

• Fishers Hypothesis
• The preference is expressed only in females and
  the tail in males, but everyone carries both
  kinds of genes
• In short, there will arise an association or
  covariance between tail and preference genes

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Hypotheses

• The Handicap Hypothesis - Zahavi (1975) suggested
  an alternative view of elaborate male sexual
  displays
• He suggested that females prefer long tails (or
  other equivalent traits) because they are
  handicaps and therefore act as reliable signals
  of a males genetic quality
• The tail demonstrates a males ability to survive
  in spite of the handicap, which means that he
  must be extra good in other respects

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Hypotheses

• Zahavis Hypothesis
• If any of this ability is heritable, then the
  tendency to be good at surviving will be passed
  on to the offspring
• Therefore females select for good genes by
  selecting to mate only with males whose displays
  honestly indicate their genetic quality

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It is important to note that in this hypothesis
the good genes are genes for survival and
reproduction, rather than genes purely for
attracting females, as assumed in Fishers
hypothesis
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Evidence for the Fisher and Handicap Hypotheses

• To demonstrate that a trait had evolved by
  Fishers process, it would be necessary to show
  that there is genetic variation for both female
  preference and the male trait and that genes tend
  to covary
• Because Fishers hypothesis assumes that the only
  benefit of the selected trait is increased mating
  success, it would be necessary to show that
  expression of the male trait did not correlate
  with any inherited aspect of fitness

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Support for Fishers Hypothesis

• Houde, A.E. and J.A. Endler. 1990. Correlated
  evolution of female mating preferences and male
  color patterns in the guppy Poecilia reticulata.
  Science

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Houde and Endler (1990)

• Males from different populations differ greatly
  in the extent to which they develop bright orange
  and blue spots, which are a stimulus for females
  during courtship
• Females from streams with large spotted males
  have stronger preferences for males with large
  orange spots than streams with small-spotted males

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Houde and Endler (1990)

• The difference between populations in both male
  sexual color pattern and female preference are
  genetic they persist in the lab for many
  generations
• The fact that the differences persist under lab
  conditions suggests that the expression does not
  depend on the ability to gather food or on
  disease resistance - i.e., results are consistent
  with Fishers hypothesis

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Handicap Hypothesis

• Mainly focused on
• Hamilton, W.D. and M. Zuk. 1982. Heritable true
  fitness and bright birds a role for parasites?
  Science
• - namely that sexual displays are reliable
  indictors of genetic resistance to disease

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One of the most detailed studies to date

• Moller, A.P. 1988. Female choice selects for male
  sexual tail ornaments in the monogamous swallow.
  Nature
• Moller, A.P. 1990. Effects of a haematophagous
  mite on the barn swallow a test of the Hamilton
  and Zuk hypothesis. Evolution

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Moller (1988)

• Moller demostrates that females prefer males with
  longer tails
• Males with experimentally elongated tails paired
  up more quickly and were also preferred by
  females seeking extra-pair matings

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Why do females prefer males with longer tails?

• Moller (1990) - Could the ornament signal a males
  genetic quality in terms of his ability to resist
  parasites?
• Swallows carry a blood-sucking mite,
  Ornithonyssus bursa, which infects both adults
  and nestlings

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Moller (1990)

• The life cycle of the mite, from egg to adult,
  lasts just 5-7 days so one reproductive cycle of
  the swallow provides time for 8-10 generations of
  mites (up to 14,000 mites)
• Moller showed that nestlings reared in nests with
  lots of mites were lighter and smaller and
  suffered increased mortality. Removal
  experiments confirmed that mites were the cause
  of the reduced growth

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Moller (1990)

• There was large variation in the population in
  the degree of parasite infection
• To test whether parasite resistance was
  heritable, Moller exchanged half the nestlings
  between pairs of nests

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Nestling parasite burden was correlated with that
of its parents, even when it was reared in
anothers nest - suggests that resistance is at
least partly genetic
of mites on own offsping in other nest
of mites on male parent
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Link with the swallows tail

• Moller found that parents with longer tails had
  offspring with smaller mite loads, even when
  their offspring were raised in another nest

mites on own offspring in other nest
males tail length (mm)
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Moller (1990)

• This suggests that the length of a males tail
  signals his degree of parasite resistance
• This fits the Hamilton-Zuk/Handicap hypothesis
  that females are choosing males able to pass on
  good genes to their offspring

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Female Choice Sexual Selection
Seehausen, O., J.M. van Alphen Jacques, and F.
Witte. 1997. Cichlid fish diversity threatened by
eutrophication that curbs sexual selection.
Science 2771808-1811.
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Female Choice
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Female Mate Choice in Plants
Waser, N.M., M.V. Price, A.M. Montalvo, and R.N.
Gray. 1987. Female mate choice in a perennial
herbaceous wildflower, Delphinium nelsonii.
Evolutionary Trends in Plants 129-33.
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Waser et al. (1987)

• The success of a cross between two plants should
  depend in part on their genetic similarity -
  i.e., an optimal separation or optimal
  outcrossing distance an index of genetic
  similarity
• the optimal outcrossing distance for Delphinium
  nelsonii is 3-10m

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Offspring of crosses over the optimal outcrossing
distance survive best under field conditions
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Waser et al. (1987)

• These effects of outcrossing distance on fitness
  components are mirrored in the performance of
  male gametophytes
• In a series of controlled crosses, pollen donors
  10m from the recipient had a significantly higher
  probability of delivering a pollen tube to the
  ovary than did pollen from donors 1m or 100m away

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Waser et al. (1987)
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Waser et al. (1987)

• This suggests a physiological interaction between
  pollen and pistil that increases the probability
  of producing offspring with high viability
• Such interaction can be interpreted as adaptive
  discrimination or choice of mates on the part of
  the female