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Chapter 13
LIFE HISTORY PATTERNS
PATTERNS OF REPRODUCTION
There are two forms of reproduction, sexual and asexual.
Asexual reproduction takes several forms: budding, mitosis in unicellular organisms, runners,
parthenogenesis, etc.
1. Dioecious: male and female individuals; sexes separate; e.g. most animals and some
plants.
2. Monoecious: plants with male flowers and female flowers on the same individual.
3. Hermaphroditic: plants with male and female organs in the same flower; in animals,
hermaphroditic individuals have testes and ovaries, a condition common in
invertebrates.
MATING SYSTEMS
1. Monogamy: a persistent pair bond between one male and one female: common in birds.
2. Polygyny (polygamy): one male has a harem of several females: some birds, many
mammals.
3. Polyandry (polygamy): one female has several males; rare, in some birds like rails,
cranes, jacana and spotted sandpiper.
4. Promiscuity (polygamy): no bonds are established: common in mammals.
Female birds are tied to a nest and eggs require incubating. Female mammals are not tied to
the nest, do not incubate eggs, and produce milk for the young. The continued presence of a
male is not a necessity in mammals.
Cuckoldry and anticuckoldry.
Cuckoldry occurs when the females of monogamous pairs mate with males other than their
partners.
Males of many species have developed anticuckoldry behavior to prevent their females from
mating with another male, e.g. guarding the female
The female invests a large amount of energy in producing a clutch of eggs and incubating them.
Mating with several males assures her that some eggs will be fertilized.
Cuckoldry is common among many animal species.
SEXUAL SELECTION
Males are not selective with whom they mate.
Females are selective and males must prove their fitness. This is called sexual selection.
There is intense rivalry between males for female attention. This is called intrasexual
competition.
MODELS OF SEXUAL SELECTION
“Sexual selection is selection for characters that enhance mating success. Darwin was
impressed by the fact that qualities of sexual attractiveness were often the reverse of qualities
leading to individual survival. He thought that gaining a higher chance to win mates was worth
the risk conferred by such characters. Bright colors, long tails, plumes, antlers and horns
threaten the survival of the animal but they also give them an advantage in fighting other males
or attracting females. To Darwin, sexually selected characters were of no use other than being
attractive to the females. He described sexual selection as selection in relation to sex. Wallace,
however, thought that all those characters were more than ornaments with some utilitarian
quality which females benefited for choosing. In his view, the ornamentations are used to
advertise genuine quality as only the healthiest males can afford doing so; mating with them will
generate more and healthier offspring.”
M.Tevfik Dorak, http://dorakmt.tripod.com/evolution/sselect.html
Sexual selection favors traits that enhance mating success even if they handicap the male by
making him more vulnerable to predation.
Females invest a lot in reproduction. It is to their advantage to be selective in choosing a mate,
one who will pass on the best genes to the next generation.
1) “Run-away Selection Theory (RA Fisher, 1958): According to Fisher, if a majority of females
prefer a particular kind of male, other females would be favored if they mate with the same kind
of males because their sons will be attractive to many females. Every individual will tend to
inherit its mother’s genes for preferring its father, and its father’s genes for the qualities
preferred. These two (groups of) genes will then segregate together and under certain
circumstances, due to positive feedback, may lead to runaway selection of more and more
exaggeration of the quality preferred. This would continue until the disadvantage, in terms of
male survival, exceeds the reproductive advantage for males. Eventually, all the males in the
population will end up with a tail length at the optimum point. When all males come to have the
same trait, there would be no genetic advantage to a female in choosing one rather than the
other. Because of this major problem with this theory, more elaborate forms of it have been
developed.” M.Tevfik Dorak, http://dorakmt.tripod.com/evolution/sselect.html
Simple version of the Runaway Selection Model:
A. Assume two genes are involved: Female choice gene (C = choose males with exaggerated
trait, c = mate randomly) and Male trait gene (T = have exaggerated trait, t = normal trait)
1) When C allele is common, T is favored - C females do better since their offspring have both
C allele(s) for daughters and T trait for sons.
2) T increases not because it directly benefits females but because it gets chosen (it pays to
have sons with trait and daughters who chose it).
3) This leads to runaway selection and the T allele increases until:
- It is fixed in the population
- Natural selection against T carriers halts it
T mutants may appear and lead to further exaggeration.
It is the co-evolution of the trait and choice loci that drives the process. They become “linked”
and self-reinforcing. The result is an ongoing exaggeration of the trait; held in check only by
natural selection. Females gain success by just having the tendency to choose for certain
traits.
http://www.brown.edu/Courses/Bio_45/PDF/b4502L213.pdf
2) Handicap Theory. According to the handicap theory, males develop secondary sexual
characteristics (e. g. bright plumage) that could reduce the male’s survival.
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This theory presumes the evolution of three traits: a male handicap, a female attraction
for the handicap, a viability trait.
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Males who have survived in spite of these handicaps are the stronger
3) Revealing Signal Theory (Hamilton & Zuk, 1982): This theory proposes that only males
resistant to parasites would be able to display conspicuous features -not necessarily costly to
them- to attract females. Thus, the ornaments (such as long tails, inflated throat poaches or
bright plumage) simply reveal the state of health without damaging it so they constitute
revealing handicaps. Female choice for males with the best-developed sexual characters would
result in offspring that are likely to inherit genetically-determined resistance to parasites from
their father. M.Tevfik Dorak, Ph.D. http://www.dorak.info/evolution/sselect.html
PROCESS OF SELECTION
Intrasexual selection or male competition increases selective pressure for the evolution of
horns, larger size, exaggerated plumage and bright colors.
It is presumed that in some way these characteristics influence female choice.
Intersexual selection involves female choice among males offering resources or offering only
genes.
The selection of a resource, usually a territory, appears to be a criterion for selection. The
question is, does the female selects the territory and accepts the male that goes with it? Or
does she selects the male, and accepts his territory as a consequence?
The selection process comes down to salesmanship on the part to the male and sales
resistance on the part of the female.
Leks are aggregations of males for the purpose of display and attraction of females.
Four criteria should be verified to identify a lekking species:
1. there is no male parental investment beyond the sperm;
2. males aggregate at specific sites for display;
3. the only resource females find on the lek is the male, i.e. the male genes;
4. females can select her mate(s),
There are several hypotheses advanced to explain lek behavior:
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Female choice: an unusual opportunity to find a mate among displaying males.
Hotpot model: males cluster in places where encounters with females are potentially
high.
Hotshot model: the dominant male has the best territory and mates with the largest
number of females; subdominant males may be able to steal mating opportunities.
REPRODUCTIVE EFFORT
Parents allocate certain amount of time and energy to reproduction.
PARENTAL CARE
Caring for young is a major reproductive expenditure: providing food, shelter, protection, etc.
The degree of development at birth or hatching varies between species.
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Precocial young are able to move about at or shortly after birth, although they may have
along infancy and grow slowly.
Altricial animals are born helpless, naked or nearly so, and often blind. They grow
rapidly and mature early.
Most mammals are altricial or semiprecocial.
PARENTAL INVESTMENT
Parents have only limited amount of energy. They have to allocate certain amount of energy to
their own survival.
Parents may invest a maximum amount of energy into a single reproductive effort in a lifetime.
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Semelparous organisms breed only once in their lifetime.
They produce many young.
They may be annual or perennial.
Salmon, some squid, 17-year cicada, agave, bamboo, some palms.
Organisms may invest reproductive effort into many small offspring and provide a minimal
amount of parental care.
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Iteroparous breed several times in their life.
They have fewer young and extend parental care.
Most vertebrates, perennial herbaceous plants, shrubs and trees.
PARENTAL ENERGY BUDGETS
Plants that grow in adverse environments allocate more energy to reproduction than those in
more moderate habitats.
As parents divide available energy among an increasing number of offspring, the fitness of
individual offspring declines.
Studies suggest that physiological and ecological factors probably set clutch size among birds
at an optimum level that results in maximum lifetime reproduction.
Brood reduction is another way of concentration the energy of reproduction into fewer offspring
in order to increase their fitness. This occurs often when food is scarce.
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Asynchronous hatching.
Siblicide: the larger sibling kills the runt.
Birds in higher latitudes have larger clutch size than those in the tropics; mammals have larger
litters. The mortality of adults is also higher in higher latitudes.
The availability of resources has been advanced as the explanation: longer days allow for
longer time foraging; greater mortality of adults in winter provides more food in the spring to
survivors.
Most hermaphroditic animals are not self-fertilizing.
Hermaphroditic plants have the advantage of a single individual being able to colonize a new
habitat.
Gender change among animals, notably fish species, appears to be stimulated by a social
change involving sex ratios in the population. Sex reversal can occur is some individuals of the
previous sex are removed.
Among plants, perennials delay flowering until they have attained a sufficiently large leaf area to
support seed production.
Fecundity in fish increases with size, which in turn increases with age. Because early fecundity
reduces both growth and later reproductive success, fish obtain a selective advantage by
delaying sexual maturation until they grow larger.
GENDER ALLOCATION
Resource allocation may influence sex ratios in the offspring because of the differential costs of
producing males and females.
Natural selection often favors those parents who invest equally in sons and daughters.
When both are equally expensive, the sex ratio will be 1:1.
The cost of rearing may not be equal. The sex ratio will become skewed toward the less
expensive sex.
R-SELECTION AND K-SELECTION
r and K selection
The theory of r- and K- selection predicts that species in different environments will differ in life
history traits such as size, fecundity, age at first reproduction, number of reproductive events
during the lifetime, and total life span.
r-selected species tend to...
Early maturity (increases r)
Many young (increases r and dispersal).
Small young so they can make many.
survival
Short life or annuals.
Less parental care.
Less competitive ability.
Very good dispersal ability.
K- selected species tend to...
Late maturity
Few young.
Large young to increase
Long life or perennial.
Extended parental care.
Good competitors.
Good dispersal ability.
In stable habitats, populations tend to stay at or near K, the carrying capacity.
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A tendency towards becoming established and remaining in place.
They can cope with physical and biotic pressures.
Conditions are stable and K varies little; no growth.
E.g. Climax forests, deserts, caves, ocean depths.
In unstable or temporary habitats, populations will grow very fast to reach K and their biotic
potential, r, is fully realized.
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Space, light, water, etc. is rarely a problem at the beginning (little competition).
Opportunity for the population to grow.
When conditions shift (unstable habitat!), the population becomes overcrowded and
declines; K varies greatly.
E. g. abandoned fields, sandbars, seasonal ponds, openings in the forest.
HABITAT SELECTION
Quality of the habitat is essential to reproductive success: “rich” habitats improves reproductive
success…fitness.
The selection of a habitat is an important part of an organism’s life history pattern.
Structure and diversity of the habitat in relation to food appears to be important in habitat
selection.
Nesting sites, hiding places, perches, etc. also contribute to the quality of the habitat.
Habitats range from optimal to submarginal. The optimal habitats fill up faster.
All species show some plasticity in the selection of habitats. Latecomers or subdominant
individuals are left with the poor habitat where they may have little chance of reproducing
successfully.
The only recourse plants have in habitat selection is to send seeds in anticipation that they will
arrive at some place suitable for seedling germination and survival.
For example, in the northwestern United States males of both Red-winged and Yellow-headed Blackbirds set up
territories in open marshes. The Redwings arrive earlier in the spring and occupy the entire marsh. When the
Yellowheads fly in, they take over the best territories (areas of cattails and other plants in deep water that harbor the
richest insect life) and force the Redwings into the shallower, drier, more marginal habitats. The Redwings are able to
breed successfully in these areas, however, while the Yellowheads are unable to exploit the less productive sites
successfully. http://www.stanford.edu/group/stanfordbirds/text/essays/Bird_Communities.html
More information: http://www.stanford.edu/group/stanfordbirds/text/essays/Habitat_Selection.html