Sexual Selection!
- Survival of the fittest, reproduction of the sexiest
I. The struggle to reproduce. In addition to the struggle for existence- finding food, not
getting eaten, surviving diseases and parasites- organisms compete for mates
II. Competition between members of the same sex. Animals may fight each other for access
to the opposite sex (typically females)
III. Mate choice and courtship. One sex may court the other sex (typically the male courts the
females)
IV. Strategies reflect investment. The nature of the competition relates to the nature of
investment of the two parents. Sexual selection is most intense when there is a very high
investment in offspring. Sexual selection can result in sexual dimorphism when the nature of
investment by males and females is different
V. Sexual conflict. The winning strategy for one sex may be at the expense of the other.
Natural selection: an incomplete theory?
The idea :
• organisms vary
• variation is heritable
• there is a struggle for existence between organisms, and only some will survive
• variations that ensure survival will be more likely to be passed on
• this leads to cumulative changes in the species (adaptive evolution)
Darwin felt that natural selection was the primary, and perhaps the exclusive, cause of adaptation in the natural world.
But not all structures were clearly adaptive.
, Adaptive structures: the dense fur of a fox keeps it warm, the huge flukes of a whale help it dive deep, the long wings of a
gull let is soar, the sharp teeth of a crocodile help it feed. These structures benefit their owners – they are ADAPTIVE. One can
see how mutations to incrementally improve swimming, flying, feeding and staying warm would be favored
However, animals with wild colours and bizarre structures (Peacock, Bird of Paradise, butterfly, etc.) are difficult to explain
with natural selection: Why should survival of the fittest favor a mutation that makes you iridescent, or gives you a long tail?
Costly in terms of resources used to build a large tail- resources that could be spent on other things.
Iridescence makes you more conspicuous to predators, a long tail hinders your escape, increasing mortality
It seems outright maladaptive.
Was Darwin’s theory of natural selection wrong… or is there something missing?
E.g. why do animals (such as birds, frogs and whales) sing? Time spent singing is time not spent on other things: finding
food, building nests. Singing is energetically costly. Singing attracts attention of predators. Why would mutations for singing
be favoured by natural selection?
E.g. horns and antlers: many animals have elaborate horns, antlers and other weapons (such as the Chameleon, Rhinoceroa
beetle and Bighorn sheep). Bighorn sheep live high in the mountains and have few predators. Usually, a bighorn sheep will
just run away from a predator- not fight. Blunt, not pointy- not good for killing a predator. Females are smaller, more
vulnerable- need defense, but lack big horns. Why would natural selection favor horn evolution?
Darwin’s idea: Sexual selection:-
Darwin’s idea was based on observation and deduction
Darwin noticed that some of these nonadaptive or maladaptive characters - bright colors, songs, and weapons- are used in
mating
Survival is only part of the story. Even if you find enough food, survive the weather, don’t get eaten, and don’t die of a
disease, you’ve got another problem
You’ve got to get a mate
Differential reproduction
Combat: Animals with elaborate horns or other weapons use them in combat against other members of the species
Usually, horns are larger in -or only found in- males, which fight
- Survival of the fittest, reproduction of the sexiest
I. The struggle to reproduce. In addition to the struggle for existence- finding food, not
getting eaten, surviving diseases and parasites- organisms compete for mates
II. Competition between members of the same sex. Animals may fight each other for access
to the opposite sex (typically females)
III. Mate choice and courtship. One sex may court the other sex (typically the male courts the
females)
IV. Strategies reflect investment. The nature of the competition relates to the nature of
investment of the two parents. Sexual selection is most intense when there is a very high
investment in offspring. Sexual selection can result in sexual dimorphism when the nature of
investment by males and females is different
V. Sexual conflict. The winning strategy for one sex may be at the expense of the other.
Natural selection: an incomplete theory?
The idea :
• organisms vary
• variation is heritable
• there is a struggle for existence between organisms, and only some will survive
• variations that ensure survival will be more likely to be passed on
• this leads to cumulative changes in the species (adaptive evolution)
Darwin felt that natural selection was the primary, and perhaps the exclusive, cause of adaptation in the natural world.
But not all structures were clearly adaptive.
, Adaptive structures: the dense fur of a fox keeps it warm, the huge flukes of a whale help it dive deep, the long wings of a
gull let is soar, the sharp teeth of a crocodile help it feed. These structures benefit their owners – they are ADAPTIVE. One can
see how mutations to incrementally improve swimming, flying, feeding and staying warm would be favored
However, animals with wild colours and bizarre structures (Peacock, Bird of Paradise, butterfly, etc.) are difficult to explain
with natural selection: Why should survival of the fittest favor a mutation that makes you iridescent, or gives you a long tail?
Costly in terms of resources used to build a large tail- resources that could be spent on other things.
Iridescence makes you more conspicuous to predators, a long tail hinders your escape, increasing mortality
It seems outright maladaptive.
Was Darwin’s theory of natural selection wrong… or is there something missing?
E.g. why do animals (such as birds, frogs and whales) sing? Time spent singing is time not spent on other things: finding
food, building nests. Singing is energetically costly. Singing attracts attention of predators. Why would mutations for singing
be favoured by natural selection?
E.g. horns and antlers: many animals have elaborate horns, antlers and other weapons (such as the Chameleon, Rhinoceroa
beetle and Bighorn sheep). Bighorn sheep live high in the mountains and have few predators. Usually, a bighorn sheep will
just run away from a predator- not fight. Blunt, not pointy- not good for killing a predator. Females are smaller, more
vulnerable- need defense, but lack big horns. Why would natural selection favor horn evolution?
Darwin’s idea: Sexual selection:-
Darwin’s idea was based on observation and deduction
Darwin noticed that some of these nonadaptive or maladaptive characters - bright colors, songs, and weapons- are used in
mating
Survival is only part of the story. Even if you find enough food, survive the weather, don’t get eaten, and don’t die of a
disease, you’ve got another problem
You’ve got to get a mate
Differential reproduction
Combat: Animals with elaborate horns or other weapons use them in combat against other members of the species
Usually, horns are larger in -or only found in- males, which fight
