REPRODUCTIVE ECOLOGY

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REPRODUCTIVE ECOLOGY
Why Sex? Why Sexes? Parental
Investment Sexual Selection Intrasexual
Selection Intersexual Selection
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WHY SEX?

• We assume that reproduction requires two
  individuals.
• In many organisms that is not true. Life
  originated without sex, so sexual reproduction is
  something that had to evolve.

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BIOLOGICAL REPRODUCTION

• Asexual reproduction is the biological process by
  which an organism creates a genetically-similar
  or identical copy of itself without a
  contribution of genetic material from another
  individual.

• Sexual reproduction is a biological process by
  which organisms create descendants that have a
  combination of genetic material contributed from
  two (usually) different members of the species.
  Each of two parent organisms contributes half of
  the offspring's genetic makeup by creating
  haploid gametes.

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WHAT IS SEXUAL REPRODUCTION?

• Production of haploid gametes by meiosis, a
  reduction division
• Fusion of these gametes produces a zygote and
  restores the full diploid complement of
  chromosomes.
• Sex evolved 1-2 billion years ago in the early
  eukaryotes the reason is unclear but it its
  likely that it is maintained in the current day
  by selection.

Sperm entering egg
Fertilized egg with 2 pronuclei
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MEISOSIS AND COST OF SEXUAL REPRODUCTION

• Recombination An event, occurring by the
  crossing-over of chromosomes during meiosis, in
  which DNA is exchanged between a pair of
  chromosomes of a pair. Thus, two genes that were
  previously unlinked, being on different
  chromosomes, can become linked because of
  recombination, and linked genes may become
  unlinked.
• Recombination probably evolved 3 billion years
  ago as a mechanism of DNA repair
• Recombinational Load? Why doesnt the genome
  congeal?

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OTHER COSTS OF SEX Mating Costs

• Finding a mate
• Attracting a mate
• STDs sexually transmitted diseases
• Attracting predators
• Injury during
• mating

Representation of the Human Immunodeficiency
Virus
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OTHER COSTS OF SEX sexually transmitted
diseases
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WHY SEX?

• In many ways, asexual reproduction is a better
  evolutionary strategy Only one parent is
  required, and all of that parent's genes are
  passed on to its progeny. In sexual reproduction,
  only half of each parent's genes are passed to
  the next generation. What's more, a mate must be
  found.
• Yet sex persists.

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WHY SEX?
A variety of theories have been proposed over
the years to explain why sexual reproduction may
be more advantageous than asexual reproduction,
and, for that matter, why sexual reproduction
even exists at all. Sex may be advantageous
good because it creates genetic variety, which,
in turn, is useful in adapting to constantly
changing and challenging environments.
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WHY SEXES?

• We assume that sexual reproduction requires two
  individuals.
• One individual is called a female, the other a
  male
• What is the
• fundamental
• distinction?

Field Cricket copulatory organ
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WHY SEXES?

• What is the
• fundamental
• distinction?
• One sex produces a large gamete Female
• One sex produces a small gamete Male

WHAT ARE THE IMPLICATIONS?
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WHY SEXES?

• Unequal gamete size?
• ANISOGAMY
• Equal gamete size?
• ISOGAMY
• How did anisogamy evolve?
• DISRUPTIVE SELECTION

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WHY SEXES?

Advantage of larger Gamete? EGG Advantage of
smaller Gamete? SPERM
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ANISOGAMY IMPLICATIONS?

• FEMALE Sex that contributes the most
  (investment) in gametogenesis
• MALE Sex that contributes the least (investment)
  in gametogenesis

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PARENTAL INVESTMENT?

• Energetic demand
• Time commitment
• Risk of predation
• Decreased longevity
• Any investment by parent in an individual
  offspring that increases the offsprings change
  of survival and reproductive success at the
  expense (cost) of parents ability to invest in
  other offspring.

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ANISOGAMY IMPLICATIONS?

• FEMALE Sex that contributes the most
  (investment) in gametogenesis
• MALE Sex that contributes the least (investment)
  in gametogensis

ASYMMETRY BETWEEN SEXES IN INVESTMENT
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DIFFERENCES BETWEEN SEXES

• FEMALE Sex that contributes
• the most (investment) in gametogenesis.
• MALE Sex that contributes the least
• (investment) in gametogensis
• Observed differences between sexes?

Mandrill
Northern Elephant Seal
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DIFFERENCES BETWEEN SEXES

• Observed differences between sexes
  Morphological, behavioral, physiological.
  Explain?
• Darwin advanced theory of sexual selection in his
  book entitled
• The Descent of Man and Selection in Relation to
  Sex. 1871.

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Darwin, C. 1871. The Descent of Man and
Selection in Relation to Sex.

• Sexual selection depends on the success of
  certain individuals over others of the same sex,
  in relation to the propagation of the species
  whilst natural selection depends on the success
  of both sexes, at all ages, in relation to the
  general conditions of life. The sexual struggle
  is of two kinds
• in the one it is between individuals of the same
  sex, generally the males, in order to drive away
  or kill their rivals, the females remaining
  passive
• whilst in the other, the struggle is likewise
  between the individuals of the same sex, in order
  to excite or charm those of the opposite sex,
  generally the females, which no longer remain
  passive, but select the more agreeable partners.
• The belief in the power of sexual selection rests
  chiefly on the following considerations. Certain
  characters are confined to one sex and this
  alone renders it probable that in most cases they
  are connected with the act of reproduction.

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The sexual struggle is of two kinds

• Competition between individuals
• of the same sex, generally the males,
• in order to gain opportunities to mate
• with individuals of other sex, generally
• females. INTRASEXUAL SELECTION
• Struggle between the individuals
• of the same sex, in order to excite or
• charm those of the opposite sex,
• generally the females, who select
• the more agreeable partners.
• INTERSEXUAL SELECTION

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DIFFERENCES BETWEEN SEXES?

• Differential investment FEMALE Sex that
  contributes the most (investment) in
  gametogenesis. MALE Sex that contributes the
  least (investment) in gametogenesis.
• Lets graph the relationship between the costs
  (and benefits) of parental investment (PI)
  against the number of offspring produced (total
  amount of parental investment).

Costs Benefits Of PI
Total amount of PI (offspring produced)
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DIFFERENCES BETWEEN SEXESIMPLICATIONS?
How do males increase reproductive success?
Finding and securing additional fertilizations
Turn eggs into offspring at a faster rate.
How do females increase reproductive success?
RS
Costs Benefits Of PI
Total amount of PI (offspring produced)
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DIFFERENCES BETWEEN SEXESIMPLICATIONS?
Males under intense selective pressure (sexual
selection) to seek/compete for females because
they have greater reproductive potential.
Females under selective pressure (sexual
selection) to exercise female choice that is,
guard against wasting investment.
RS
Costs Benefits Of PI
Total amount of PI (offspring produced)
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PREDICTIONS OF SEXUAL SELECTION THEORY?

• Male reproductive success should increase
  substantially when more mates
• Obtained.

2. Individual reproductive success should vary
more among males.
RS
Costs Benefits Of PI
Total amount of PI (offspring produced)
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PREDICTIONS OF SEXUAL SELECTION THEORY HOW TEST?
Males
Females
Mean Offspring produced

0 1 2 3
0 1 2 3
1 2 3 1 2 3
FEMALES
MALES
Number of Offspring
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DIFFERENCES BETWEEN SEXES

• FEMALE Sex that contributes
• the most (investment) in gametogenesis.
• MALE Sex that contributes the least
• (investment) in gametogensis
• Observed differences between sexes?

Mandrill
Northern Elephant Seal