Sexual selection

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Sexual selection Sexual dimorphism difference
in phenotype between males and females of same
species results from sexual selection,
differences in selection Based on sex Thoughts
How is this accomplished when the only difference
between males and Females Is the Y chromosome?
Note that we compete most strongly with those we
most Overlap with in our needsour own species,
and our own sex. Sexual selection is A powerful
force in nature.
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• How can we explain sexual dimorphism
• if a phenotype is good for one sex, shouldnt
  it be good for both?
• Sometimes the phenotype doesnt seem good at all
  peacock feathers
• Individuals do not vary only in their success at
  surviving and reproducing, but
• Also in their success at attracting mates
• inasmuch as the act of courtship appears to be
  with many birds a prolonged
• And tedious affair, so it occasionally happens
  that certain males and females
• Do not succeed during the proper season, in
  exciting each others love, and consequently do
  not pair

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Sources of sexual selection

• Parental Investment
• Increases fitness of offspring
• Decreases fitness of parent
• Most mammals, males court females choose, high
  parental investment, in particular on part of
  females
• Invertebrates, eggs expensive, neither parent
  invests in parental care

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Limits to fitness

• In mammals, male reproductive success limited by
  fertilizations, access to females
• Female reproductive success limited by pregnancy,
  eggs
• These different pressures on males and females
  result in sexual dimorphism
• The stronger the pressures, the greater the
  dimorphism

5
Rough skinned newts Males gather at ponds in
winter Females saunter in during Jan. and
Feb. High male/female ratio on any given day No
parental care, cost of eggs and sperm drive
sexual selective regime Most males do not mate
all females mate Sexual selective pressures
stronger on males because there are more males
than females, males are under pressure to
compete for females
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Pipefish Males provide parental care At any
time, there are more females available With eggs
than males available to brood them, As brooding
takes longer than maturing eggs Here, female
fitness is dictated by access to Males, the
reverse of the newts Generally, whichever sex
bears the burden of Reproduction will be less
available to mate, Due to time investment (in
terms of maturing Eggs, brooding young) The sex
that sits waiting will compete strongly For the
less available sex, allowing the less Available
sex to choose among a number of mates
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High male/female ratio Not all males
reproduce Male success based on access to mates
Low male/female ratio Fewer females than males
reproduce Female success based on access to mates
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Sexual selection on Males Male-male
competition -combat (pre-conception)
-sneaker males (while the big boys are busy
fighting) -sperm competition
(post-mating) -infanticide (post-birth)
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Responses to sexual selection Size and combat in
iguanas females choose males able to occupy a
breeding territory. Females put 20 of their
mass into eggs! Larger males more likely to
copulate
Is sexual selection stronger than natural
selection in this example?
10
Sperm competition -multiply mated
females -large ejaculates -meiotic drive (fast
sperm) -male guarding -copulatory
plug -blocking female storage -removal of
competitors' sperm
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Infanticide -young male lions under strong
sexual selection -brief time to hold a
pride -kill cubs not weaned so They can mate
females more Quickly (8 months) -25 cub
mortality -the cost to female fitness Makes this
all the stronger
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Sexual selection on males female choice
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Long feathers increase fitness With respect to
sexual selection
Why? Have to be extra fit to carry this tail
burden
Long feathers reduce fitness with respect To
natural selection
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Females choose males with long calls
why??
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Long calls indicate Better fitness How did the
call gene Get linked to the Fitness aspect
(larval Growth, time to Metamorphosis,
etc.) Gene? Or Are they the same genes Are
long calls heavy lifting
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Females choose based on male donation bigger
food, longer sex, more Sperm transferred
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Sensory biases

• Females have preferences (red, fast, etc.) that
  already exist
• Males exploit this, evolving cues that stimulate
  these biases
• Evolution of human intelligence?

-Net stance for catching copepods -Copepods make
water vibrate -Male water mites make water
vibrate -Female captures spermatophore
prey -Net stance and Male trembling co-evolved
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Sexual selection human intelligence

• only runaway sexual selection as described by
  Fisher is sufficient to explain the huge increase
  in brain size. Being intelligent, witty and
  entertaining was sexy to our ancestors! This
  hypothesis requires only that there was an
  initial preference for more intelligent males

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Sexual selection human intelligence

• The main evolutionary pressure for human beings
  to increase in intelligence was competition with
  other people, in particular, sexual competition
  between individuals of the same sex. The argument
  runs thus the primary function of most animal
  communication is to manipulate others, not just
  to impart information the ability to deceive and
  to detect deception in others is highly important
  for many social animals thus it is reasonable to
  assume that this principle underlies the
  evolution of our highly developed communicative
  ability. What is more, in order to be really
  convincing to others in our deception it became
  necessary to develop the ability for individuals
  to deceive themselves Robert Trivers has argued
  that this may be why we developed a subconscious.
  In a similar vein Nicholas Humphrey argued that
  the need to guess the likely actions of other
  individuals required us to develop the capability
  to imagine what is in others minds a theory of
  mind, which was a key factor in the development
  of self-consciousness

20
Runaway sexual selection male feature and
female preference genetically linked, Results in
positive feedback loop Question what puts the
breaks on this system?
21
Sexual selection on females

• Polyandry multiple mating by females

22
Plant sexual dimorphism In animal pollinated
species (pollination is not random, like with
wind) Flowers should be dimorphic Perianth
products reproductive parts and showy part of
flower
2) Male flowers Contain an inhibitor That keep
bees From visiting Subsequent Male flowers
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Human sexual selection since females invest
more matter and energy into producing each egg
than males invest in producing each sperm, eggs
form more of a limiting resource for males than
sperm do for females. Thus, males should
compete more intensively to fertilize eggs than
females do to acquire sperm, while females should
be choosier than males. Males compete for
quantity of females, and females compete for
quality of males. In short, males court, and
females choose. -Bateman (1948) and Trivers
(1972)
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Cost of reproduction and access to mates
Maternal investment was obligatory in hominids
paternal investment was not. In female mammals
the costs of internal fertilization, gestation,
and long-term lactation are especially high. For
example, the minimum parental investment by
female humans under ancestral conditions would
have been a harrowing 9-month pregnancy followed
by at least three years of breast-feeding and
baby-carrying (Shostak, 1981) whereas the
minimum paternal investment would have been a few
moments of copulation and a teaspoonful of semen
(Symons, 1979). The result is an enormous
difference in maximum lifetime reproductive
success. King Moulay Ismail the Bloodthirsty, a
medieval despot of Morocco, sired over 800
children by the women in his harem, and the first
emperor of China, around 3000 years ago, was
reputed to have sired even more through his much
larger harem (Betzig, 1986). By contrast, the
world record for a woman is 69 children, many of
which were triplets (Daly Wilson, 1983). Even
under relatively egalitarian tribal conditions,
some men can father several dozen children by
several different women, whereas no woman bears
more than 10 or so children (Chagnon,
1983). Thus, a mans reproductive success
generally increases with his number of sexual
partners (in the absence of contraception),
whereas a woman reaches her reproductive limit
rather quickly as her number of sexual partners
increases.
25
Human evolutionary history and sexual
selection Humans were much less monogamous early
in evolution, allowing sexual selection to have a
greater impact on the evolution of sexual
dimorphism (why?) Hunter Gatherers primary
subsistence method involves the direct
procurement of edible plants and animals from the
wild, foraging and hunting without significant
recourse to the domestication of either
Primate mating systems Females forage alone
monogamy (gibbons) Females forage together,
one male can control access polygamy -male
competition, aggressiveness, canine teeth, size
(gorillas) Females forage together, group of
competing males control access multimale
polygamy -status competition and
coalition-formation (chimps, humans)
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History of human evolution
Fossils and genetic markers suggest that hominids
diverged from other anthropoid apes 6 million
years ago (mya) with increasing bipedalism, group
size, and omnivory in the sequence
-Australopithecus ramidus , A. afarensis, and A.
afiicanus. By two mya, hominids had divided
into two main branches (see Foley, 1987)
Paranthropus (also known as AustraZopithecus),
including P. robustus and P. boisei, and Homo,
including successively Homo habilis (2.0- 1.8
mya), Homo erectus (1.8-0.5 mya), and Homo
heidelbergensis (400,000-120,000 years ago).
This latter type split into two species (see
Stringer Gamble, 1993) the Neanderthals (Homo
neanderthalensis, 200,000-40,000 years ago), and
modern Homo sapiens (120,000 years ago to the
present).
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Rise of agriculture and weakening of sexual
selection Some assume that opportunities for
mate choice would have been severely
limited under ancestral conditions (arranged
marriages, exchange of women as chattel,
marriage, generally low status of women under
patriarchy). Question why are opportunities
for mate choice important? But there is good
archaeological and ethnographic evidence that
many of these factors arose within the last 10,
000 years, because agriculture requires long-term
investment in preparing and maintaining a plot
of land, and thereby reduces the physical and
social mobility that underlay the free choice of
sexual mates in hunter-gatherer tribes.
28
Human Sexual Dimorphism -indicates sexual
selection -must be shared across cultures (not
environmental) -size Fig. 11.40
among military men of equal status, taller
have more kids -aggressiveness Fig.
11.39 most violence is male-male of reproductive
age
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Neoteny and human evoltion

• neoteny -- or the retention of childlike
  characteristics in mature members of a species.
  This process appears so amplified in humanity
  that we have been called the neotenous clan of
  apes. Humans much more closely resemble chimp or
  gorilla infants than adults of either species,
  e.g., in the smooth, vertical dome of the
  forehead and the relative ease of bipedality
  displayed by very young apes.

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Our essential somatic properties, i.e. those
which distinguish the human body form from that
of other primates, have all one feature in
common, viz they are fetal conditions that have
become permanent. What is a transitional stage in
the ontogensis of other Primates has become a
terminal stage in man -Bolk 1926
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1. Our flat faced orthognathy 2. Reduction of
lack of body hair. 3. Loss of pigmentation in
skin, eyes, and hair 4. The form of the external
ear. 5. The epicanthic (or Mongolian) eyefold. 6.
The central position of the foramen magnum (it
migrates backward during the ontogeny of
primates). 7. High relative brain weight. 8.
Persistence of the cranial sutures to an advanced
age. 9. The labia majora of women. 10. The
structure of the hand and foot. 11. The form of
the pelvis. 12. The ventrally directed position
of the sexual canal in women. 13. Certain
variations of the tooth row and cranial
sutures. 14. Absence of brow ridges. 15. Absence
of cranial crests. 16. Thinness of skull
bones. 17. Position of orbits under cranial
cavity. 18. Brachycephaly. 19. Small teeth. 20.
Late eruption of teeth. 21. No rotation of the
big toe. 22. Prolonged period of infantile
dependency. 23. Prolonged period of growth. 24.
Long life span. 25. Large body size
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Brain Development

• No learning in chimps post maturation
• Even aged humans often retain a plasticity of
  behavior that is typically found among animals
  only in the young.
• Human emphasis on learned, rather than inherited,
  behavior, has been widely accepted as a chief
  driver of this trend, requiring our minds to
  remain supple and receptive for ever-longer
  spans.

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Ch. 11 questions 1, 2, 3, 4, 5, 6, 7, 8, 10, 12,
13, 14
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• The sudden drop in puberty onset over the last
  100 years shows evidence of an evolutionary
  dynamic in contemporary times, and emphasizes the
  speed under which evolution can unfold.
• The curtailing of the final stage of cerebral
  development by early puberty could have mental
  implications

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• accumulated knowledge played a crucial role in
  enabling humans to develop a rich, varied diet,
  which in turn required the capacity for language
  and for a large memory? Or was it that a sexual
  preference for juvenile features drove us towards
  prolonged retention of such features (neoteny)
  which in turn allowed the development of a larger
  brain as a secondary effect of a longer period of
  growth?

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• Three million years ago our ancestor the upright
  ape Australopithecus afarensis, also known as
  Lucy, had a brain size of about 400cc. Modern
  humans have a brain that is a remarkable 3½ times
  that size, at 1400cc. This inordinately large
  brain is very costly to run the brain consumes
  18 of our energy expenditure. From a Darwinian
  perspective this suggests that there must have
  been significant and immediate advantages to
  possessing a larger brain which outweighed the
  expense.