LIFESCI 2D03 Chapter Notes - Chapter 11: Anisogamy, Guppy, Isogamy

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Chapter 11: Mating Behaviour
11.1 Sexual Selection Favours Characteristics That Enhance
Reproductive Success
1. Darwin proposed natural selection as a mechanism to explain the evolution of
adaptive traits in species.
2. Primary sexual characteristics: the genitalia and organs of reproduction.
1. Darwin made copious notes on the morphological differences between the
male and female genitalia.
3. Secondary sexual characteristics: morphological differences between the sexes
that are not directly involved in reproduction.
1. For example, in birds such as widowbird, only males have colourful plumage
and a very long tail feathers.
1. Darwin observed that such elaborate and exaggerated traits, in
conjunction with complex behavioural displays and vocalizations, are
often involved in conspecific interactions during the mating season.
4. Secondary sexual traits puzzled Darwin because they did not seem to fit into his
theory of natural selection.
1. Exaggerated morphological and behavioural traits should be energetically
expensive to produce and maintain, and they can make individuals more
obvious to predators and so reduce their survivorship.
5. In On the Origin of Species, Darwin first proposed that exaggerated male traits
might be advantageous for reproduction rather than for survival.
6. In The Descent of Man and Selection in Relation to Sex, Darwin in fact
hypothesized that these traits might arise from a different form of selection.
7. Sexual Selection: a form of natural selection that acts on heritable traits that
affect reproduction.
8. Mate Competition: selection in which one sex competes with other member of
the same sex for access to the other sex for reproduction.
9. While such competition is an important aspect of sexual selection, Darwin
recognized another aspect as well: females may be choosy.
1. Thus proposed that males may also compete among themselves to increase
their attractiveness to females. In this way, he envisioned that females often
play and active role in reproductive decisions in mate choice.
1. Mate choice: selection by one sex for member of the other sex for
10. One fundamental difference between males and females is the size of their
1. Anisogamy: Males tend to produce many small, motile gametes (sperm),
while females tend to produce much larger, nutrient-rich, and nonmotile
gametes (eggs).
2. Isogamy: in many algae, fungi, and unicellular protozoans; however, all
individuals produce similar-sized gametes. Appears to be the ancestral form.
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11. Geoff Parker and colleagues developed a model to answer this question. This
model is based on the following the assumptions:
1. In the ancestral marine environment, individuals in a population produce
different-sized gametes.
2. Each parent has a fixed amount of energy to allocate to gamete production,
resulting in a size-number trade off: as the number of gametes produced
increases, their size will decrease
3. Zygote viability is related to its size. Larger zygotes have higher viability
because they contain more resources for survival.
12. Parker considered the fitness of small, large, and intermediate-sized gametes.
Small gametes have a numerical advantage: they will create the most zygotes.
13. On the other hand, large gametes always produce zygotes with the highest
survival. Intermediate zygotes have neither advantage and so have the lowest
1. There is disruptive selection against intermediate-sized gametes. The result is
high fitness for either “proto-males”, in which produce many small gametes,
or “proto-females” which produce fewer large gametes.
14. Bateman was one of the first researchers to examine sexual selection in males and
females using fruit flies,
15. In a series of experiments, Bateman paced equal numbers of virgin females and
males in milk bottles for three or four days and allowed them to mate.
1. Adults varied in age from one to six days old. Females can take up to four
days to become sexually mature, while males are sexually mature within 24
hours of eclosion (emergences of the adult from the pupal case).
1. Each adult was heterozygous for a different dominant mutation, and so
Batemans could determine the parents of three-fourths of the offspring by
their phenotype.
16. Across all experiments, 96% of the females produced progeny. In contrast only
79% of the males successfully produced progeny.
1. From these data, Bateman concluded that males had a higher variation in
reproductive success than females.
1. Bateman inferred that the intensity of sexual selection, as measured by the
variation in reproductive success, was, in general, higher on males than
2. He also inferred that this increased intensity of sexual selection was due
to male-male competition.
17. His experiment suggested that a males reproductive success increases more
strongly with the number of mates obtained than does female reproductive
1. Bateman concluded that reproductive success for a female is primarily limited
by egg production, because female can obtain sufficient sperm to fertilize all
her eggs from a single mate.
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2. In contrast, sperm production should not limit male reproductive success.
Instead, the number of matings a male obtains primarily limits male
reproductive success.
18. This situation sets the stage for greater variation in male reproductive success-
and thus more intense sexual selection on males.
1. In sum, the evolution of male-male competition and female choice follows
from competition for access to gametes of the opposite sex.
19. Bateman’s hypothesis: female reproductive success is most strongly limited by
the number and success of eggs that she can produce, while male reproductive
success is limited by the number of mates he obtains.
1. This difference between the sexes has become a foundation of sexual selection
20. Trivers expanded on the concept by identifying all forms of parental investments
as a key difference between sexes, in addition to gamete size.
21. Parental Investment Theory: the sex that has a greater investment in offspring
production should be choosier when it comes to mates.
1. The other will then experience more intense sexual selection.
1. As a result of sexual selection, we expect that males will often exhibit
exaggerated traits used in competition for females.
2. The traits ay be morphological or involve behaviours such as vocalization
and courtship rituals.
22. Weapon: exaggerated morphological traits used in male-male competition.
23. Ornaments: exaggerated morphological traits used to attract females.
24. Within this population, individual males will defend and mate with several
females each year. Antlers are present only in males and are used in aggressive
contests to defend females from rival males.
25. Researchers take blood samples from calves and adults for genetic analysis to
determine individuals reproductive success.
1. They also collect and weigh the antlers shed by males each year to determine
their mass (size). When individuals die, post mortem analyses provide
information on body size (hind leg length).
1. These researchers determined that males with larger antlers had higher
breeding success in any given year, as well as over their lifetime.
1. In fact, male body size per se had little effect on breeding success.
26. These data indicated that antler size is significantly correlated with male
breeding success in this population: males with larger antlers sire more offspring
because they obtain more mates.
1. In red deer we see a positive correlation between weapon size and
reproductive success.
27. In many beetles, males possess a horn-like projection that females lack.
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