Individuals Actively Defend Their Territory, Which Is Costly

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

n  involves both intrasexual and intersexual selection

n  leads to evolution of secondary sexual characteristics

Intersexual selection

•  Intersexual selection

n  Benefits of mate choice

n  Males may help rear young, gather food, defend nest, etc..

•  Indirect benefits

n  Females may choose healthiest or oldest males

n  overall genetic or physiological health

n  more vigorous offspring

Reproductive strategies

•  Animals use different reproductive strategies to increase reproductive success

n  Mate choice

n  Parental investment

n  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

n  common in females

•  Parental investment refers to contributions each sex makes in producing and rearing offspring

n  usually higher in females

n  different selective pressures

n  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

n  Handicap hypothesis

n  Only genetically-superior males can survive with a handicap

n  Sensory exploitation involves evolution in males of an attractive signal that exploits preexisting biases

Intrasexual selection

•  Intrasexual selection

n  Individuals of one sex compete for the opportunity to mate with individuals of the other sex

n  Selection will strongly favor sexual dimorphism.

n  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

n  Monogamy

n  Polygyny

n  Polyandry

•  Behavioral ecologists are using DNA fingerprinting to find the benefits of different mating system strategies

n  Are finding that extra-pair copulations are important!

Mating systems

•  Extra-pair copulations

n  Researchers found that in one study, 20% of red-winged blackbird offspring were a result of extra-pair copulations.

n  May be very pervasive

n  Males benefit by increased mating success.

n  Females may benefit by increased rearing assistance.

Mating systems

•  Number of mates

n  monogamy - one male and one female

n  polygyny - one male and many females

n  polyandry - one female and several males

•  Needs of offspring

n  altricial - require extensive, prolonged care

n  precocial - require little parental care

Sociality and altruism

Benefits of group living

•  Improved detection and repulsion of predators

n  Sawflies feed gregariously

n  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)

n  Natural selection would seem to argue against altruism

n  Such acts may not be truly altruistic, and may be benefiting the actor

n  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.

n  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:

n  Descendant kin (own offspring)

n  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 + sum effects on r = offspring 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

n  B is benefit to recipient

n  C is cost to the actor

n  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

n  All ants, some bees, some wasps, and all termites are eusocial.

n  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 Father

Sister 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