Download Sexual Selection

Territoriality
• Territoriality is where individuals organisms maintain exclusive use of an area
that contains some limiting resource
• Individuals actively defend their territory, which is costly
• Benefits gained from maintaining territory, must outweigh costs
Competition for Space
Sexual Selection
• Sexual selection is a type of natural selection, driven by male/female
relationships
Sexual Selection
• Secondary sexual characteristics: horns, long feathers, coloration
Reproductive competition
• Sexual selection occurs when individuals compete for mating opportunities
 involves both intrasexual and intersexual selection
 leads to evolution of secondary sexual characteristics
Intersexual selection
• Intersexual selection
 Benefits of mate choice
 Males may help rear young, gather food, defend nest, etc..
• Indirect benefits
 Females may choose healthiest or oldest males
 overall genetic or physiological health
 more vigorous offspring
Reproductive strategies
• Animals use different reproductive strategies to increase reproductive success
 Mate choice
 Parental investment
 Mating systems
Parental investment and mate choice
• Mate choice occurs when individuals do not mate at random, but appear to
make decisions on mates base on quality
 common in females
• Parental investment refers to contributions each sex makes in producing and
rearing offspring
 usually higher in females
 different selective pressures
 gametes
Parenting behavior
• Enhancing the survival of offspring can increase parents’ reproductive success
• Parental behavior comes at a cost; drains time and effort that could be spent
producing additional offspring
Male reproductive strategy
• Males produce energetically inexpensive sperm and often provide no parental
care
• Males maximize reproductive success by mating with as many females as
possible (quantity)
Female reproductive strategy
• Females produce large, energetically expensive eggs and often provide
parental care
• Females increase reproductive success by increasing the quality of their
mates
Choosy females
• Female choice can dictate rules of male competition and shape male behavior
• Selects for traits in males that appeal to females, and relate to their “vigor”
• Male hangingflies offer nuptial gifts (better chances with a large gift!)
• Some controversy as to why females are choosy, e.g handicap theory
Sexual selection leads to sexual dimorphism
• Secondary sexual characteristics evolve to attract members of the opposite
sex
Intersexual Selection
 Handicap hypothesis
 Only genetically-superior males can survive with a handicap
 Sensory exploitation involves evolution in males of an attractive signal that
exploits preexisting biases
Intrasexual selection
• Intrasexual selection
 Individuals of one sex compete for the opportunity to mate with individuals
of the other sex
 Selection will strongly favor sexual dimorphism.
 Sperm competition
Male Contests
• Females of some species (e.g. elk) cluster in groups
• Males of such species may fight one another for access to harems
• Selects for large males that can defeat other males in contests
• Lekking is form of male contest
Mating systems
• Three types
 Monogamy
 Polygyny
 Polyandry
• Behavioral ecologists are using DNA fingerprinting to find the benefits of
different mating system strategies
 Are finding that extra-pair copulations are important!
Mating systems
• Extra-pair copulations
 Researchers found that in one study, 20% of red-winged blackbird offspring
were a result of extra-pair copulations.
 May be very pervasive
 Males benefit by increased mating success.
 Females may benefit by increased rearing assistance.
Mating systems
• Number of mates
 monogamy - one male and one female
 polygyny - one male and many females
 polyandry - one female and several males
• Needs of offspring
 altricial - require extensive, prolonged care
 precocial - require little parental care
Sociality and altruism
Benefits of group living
• Improved detection and repulsion of predators
 Sawflies feed gregariously
 Produce a noxious regurgitant when irritated
Selfish herd
• A group held together by self-interest
• Other members of the group form a living shield against predators
• Individuals may compete for the safest spots
Dominance hierarchy
• Some individuals accept subordinate status to others
• Dominant members have higher reproductive success than subordinates
• Subordinates wait their turn, or become “sneaky”
Costs of group living
• Increased competition for food, mates, and other limiting resources
• Increased vulnerability to disease and parasitism
• Risk of exploitation by other group members
Selection for altruistic behavior
• Nonbreeding helpers are found in mammals, birds, and insects
• Altruists apparently sacrifice their reproductive success to help others
• How are genes for altruism perpetuated?
Factors favoring altruism
• Altruism - performance of an action that benefits another individual at a cost to
the actor (nest helpers)
 Natural selection would seem to argue against altruism
 Such acts may not be truly altruistic, and may be benefiting the actor
 Nest helpers may gain parenting experience or inherit territory
Altruism
• Reciprocity - Individuals may form partnerships in which mutual exchanges of
altruistic acts occur.
• Kin selection - By directing aid toward close genetic relatives, an altruist may
increase reproductive success of its relatives enough to compensate for the
reduction in its own fitness.
 The more closely related the individuals, the more likely the potential
genetic gain.
Theory of kin or indirect selection
• Proposed by William Hamilton
• Genes associated with caring for relatives may be favored by selection
• Altruists pass on genes indirectly, by helping relatives who share close genetic
similarity to survive and reproduce
Kin selection
(or, why some organisms are nice to their relatives)
• Premise: altruism can increase in frequency if altruistic individual is related to
recipient
• Kin selection is natural selection that favors the spread of alleles that increase
the indirect component of fitness
Inclusive fitness
• Includes direct fitness and indirect fitness
• Fitness can accrue two ways:
 Descendant kin (own offspring)
 Collateral kin (cousins, nieces, nephews)
• Helping either descendant or collateral kin will lead to copies of own alleles
surviving
Inclusive fitness
• Inclusive fitness = fitness via descendant kin + fitness via collateral kin
= own offspring +
offspring
sum effects on r =
relatedness
production
to others
of others
• Organisms should maximize inclusive fitness
Coefficient of relatedness
• The probability that homologous alleles in two individuals are identical by
descent
Quantifying relatedness
rparent-offspring = 1/2
rsiblings = 1/2
raunt-nephew = 1/4
General case: (1/2)n,
where n = number of links
Hamilton’s rule
• An allele for altruistic behavior will spread if
rB > C or Br-C > 0
 B is benefit to recipient
 C is cost to the actor
 B and C are measured in units of surviving offspring
Evolution of social systems
• Society - group of organisms of the same species organized in a cooperative
manner
• Insects
 All ants, some bees, some wasps, and all termites are eusocial.
 Social insect colonies are composed of different castes of workers that
differ in size and morphology and have different tasks to perform.
Social insects
• Workers in colonies of social insects are sterile
• These colonies are extended families
• Workers pass on their genes indirectly by helping relatives reproduce
Termites
• Workers and soldiers are sterile
• A single queen and one or more kings are the parents of the entire colony
Eusocial hymenoptera
• Haplo-diploid sex determination
• Females diploid, males haploid
• r queen—worker = ½
• r worker—worker = ¾ (share all genes from dad)
Eusocial hymenoptera
Mother
Sister
Father
Brother
• Raising sisters increases inclusive fitness more than reproducing yourself, or
raising brothers
Naked Mole Rats
• Only mammals known to have a sterile worker caste
• Single queen mates with one to three males
• DNA evidence shows clan members are all closely related
Naked mole rats also exhibit altruism