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Chapter 7 Life History strategies
鄭先祐生態主張者： Ayo 工作室
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Life History StrategiesThe effects of body sizeMetamorphosis Diapause and resting stagesSenescence Reproductive strategiesConstrains and ambiguities in the study
of the life history strategiesEnvironmental application (Life histories
and conservation)
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Consider the following life histories
1. A salmon, having spent five years in the North pacific, enters the Yukon River and swims upstream some 2,000 miles to a small tributary. By the time it reaches the small stream where its parents mated, it is near death. Finally, it spawns and dies.
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Consider the following life histories
2. A female red kangaroo in the desert of Australia cares simultaneously for three young at different stage of development. The oldest has left its mother’s pouch and lives independently, although it remains near its mother’s side. The second, a newborn, is attached to a teat in the pouch. It is helpless and incompletely developed. The third is a fertilized egg in the uterus, where it will remain, unattached to the placenta, for 204 days.
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Consider the following life histories
3. A mayfly egg hatches in a small stream in the Rockies. After feeding under the surface of the water for a few weeks, the nymph swims to the surface and hatches into the first adult stage. This winged form flies off and conceals itself in the vegetation along the stream. In a few hours, it sheds its skin and becomes a sexually mature adult. After males and females fly over the water and mate, the females lay eggs on the surface, and both sexes die.
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Consider the following life histories
4. A bamboo plant in Patagonia reproduces vegetatively for 100 years. Along with other individuals, it forms a dense stand of plants. Then, in one season, all the individuals in the population flower simultaneously, reproduce sexually, and die. Another 100 years later, the process is repeated.
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Consider the following life histories
A dandelion seed lands in a well-manicured lawn and germinates that same day. Within a week, the plant has a small rosette of leaves and has produced a flower only a few inches tall. The flower asexually produces a huge number of seeds that are scattered by the wind. A few days later, the plant flowers again.
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Life historyThe term life history refers to any aspect
of the developmental pattern and mode of reproduction of an organism.
Five fundamentals aspects: 1. Body Size. ( 體型大小 ) 2. Metamorphosis. ( 蛻變 ) 3. Diapause. ( 休眠 ) 4. Senescence. ( 老化 ) 5. Reproductive patterns. ( 繁殖類型 )
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The effects of body sizeThe body sizes of organisms span a
tremendous range. In linear dimension this scale extends
from bacteria 1 micrometer in length to redwoods 100 meters tall (not including the roots), a span of eight orders of magnitude.
Even within a taxon, there is a great range of body sizes (Fig. 7.1).
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Fig. 7.1 (a) and (b)
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Fig. 7.1 Frequency distributions of number of species with respect to log body mass for North American mammals (a), birds(b), and freshwater fish ©.
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The effects of body size Body size has an important influence on life. These effects can be ecological, physiological, or
both. An organism’s total food requirements increase with
increasing size, while per-gram food requirements decrease.
Larger organisms have lower risks of predation. Vulnerability to physical factors also varies with size. Larger organisms generally have longer life spans
and thus longer generation times, which affect the potential rate of evolution via natural selection.
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Cope’s law Within a particular taxonomic group, size
tends to increase over evolutionary time, this notion is referred to as Cope’s law.
For example, in minnows, those species closest to the phylogenetic root of the family are the smallest.
The common ancestor of equines, Hyracotherium, the dawn horse, was a small animal. The fossil record indicated steadily in size. But other lines of horse-like mammals did not show this trend.
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100 grams hypothesis The distribution of mammalian body sizes has
a mode at about 100 grams. 100 grams represents an optimal body size in
the mammals. The optimal body size is a balance between
these two limitations. (1) the acquisition of energy, which increases with
mass raised to the 0.75 power, and (2) the rate of conversion of energy to offspring,
which changes as a function of mass to the –0.25 power.
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Reproductive power is maximal for animals of about 100 grams.
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Allometry The situation in which different morphological
characters change at different rates in referred to as allometry.
The general equation for allometric relationships is Y = aXb.
In Fig. 7.3 the relationship between body weight and brain weight for mammals is plotted on a log-log scale. The relationship is described by the equation
Y=0.16 X 0.67
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Y=0.16 X 0.67
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Allometric changesY = aXb
Log Y = log a + b log x 假若 b < 1 , y changes more slowly than x. When b>1, the reverse is true.
譬如：圖 7.3 brain mass vs. body wt. 10,000kg animal the brain/body ratio is
0.0056, For a 100gram animal, the ratio is 0.018. The brain weights change at a slower rate.
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Kleiber’s law Surface area varies as the square of the
linear dimension, whereas the volume varies as its cube.
In this case, the allometric equation would be Y = a X 2/3 Where Y is the surface area and x is the volume.
Mammals show a consistent allometric relationship between metabolic rate and body mass in which the slope is 0.75, a phenomenon known as Kleiber’s law.
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Quarter power scaling In many ecological and physiological
allometries, the value of b is 0.25 or 0.75, a phenomenon known as quarter power scaling.
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0.375 powerEnquist et al. (1998) showed that in
trees, the basal stem diameter scales to mass to the 0.375 power, a fascinating result because in mammals, the diameter of the trachea and aorta scale to the same power.
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Metamorphosis Organisms that metamorphose undergo
radical changes in morphology, physiology, and ecology over the course of their life cycle.
Species that metamorphose must undertake complex genetic and physiological processes in the transformation.
What sorts of ecological advantages could outweigh these complications?
One hypothesis is that metamorphic species specialize so as to exploit habitats with high but transient productivity – and hence high potential for growth.
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Transient productivity hypothesis
Part of this strategy is that specializations for feeding, dispersal, and reproduction are separated across stages.
A frog tadpole occupies an aquatic environment with extremely high potential for growth.
But an aquatic larva is not capable of dispersal to new ponds, the adult frog is.
The energy that adults obtain from feeding is dedicated to dispersal and reproduction.
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Transient productivity hypothesis
Many insects benefit from the same strategy. 譬如：毛毛蟲快速的成長，蛻變為蝴蝶後，不
再成長。蝴蝶將其能量保留用於 dispersal and reproduction.
In many marine invertebrates, the pattern is reversed. The larvae are specialized for dispersal, whereas the adults grow and reproduce. 譬如： barnacle.
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Neoteny ( 幼體化 ) Under certain ecological conditions, it is
apparently advantageous for reproduction to occur in the larval stage.
Neoteny, a life cycle in which the larvae of some populations or races become sexually mature and no longer metamorphose into adults.
Neoteny is an example of a secondary adaptation because it could evolve only secondarily– after metamorphosis evolved.
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Diapause and Resting stages If conditions occasionally or regularly
become harsh, it may be advantageous for the organism to have a resistant stage built into the life cycle.
This genetically determined resting stage, characterize by cessation of development and protein synthesis and by suppression of the metabolic rate, is called diapause.
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Variety of resistant stages The variety of resistant stages among living
organisms is huge. Bacteria form highly resistant spores in response
to desiccation, heat, and certain chemical environments.
spores of fungi, seeds of plants, pupae of insects are highly resistant.
Some of these resistant stages can be extremely long-lived. 譬如：於極地的 lupine (pea family), recovered from ancient lemming burrows in the Arctic, germinated in three days even though they were more than 10,000years old.
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Face two important abiotic problems The severity of the abiotic regime, including
temperature extremes, insufficient or excess water, wind, and so forth.
The unpredictability probably pose greater difficulties for organisms than harsh, predictable conditions.
譬如： many seeds require a period of stratification, exposure to low temperatures for some minimum period, before they will germinate. ( 避免因為環境的 unpredictability造成致命的傷害 ) 。
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範例： red kangaroo (Megaleia rufa)
The marsupial inhabits the deserts of central Australia, where the onset of rains and the resulting flush of vegetation are extremely unpredictable.
It is advantageous for a kangaroo female to produce young at a time when plant productivity is sufficient to support her offspring.
However, gestation is so long that if a female waited to mate and carry the young until after the rains came, the favorable period might be past.
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Use of embryonic diapause during gestation.
At any one time, the female has three young at various stages of development: one in diapause, one in the pouch, and one on the hoof.
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範例： Couch’s spadefoot toad
The spadefoot toad inhabit some of the most severe deserts in North America.
Adults of this species burrow deeply into the substrate where it is cooler and perhaps more moist.
When it rains, the adults emerge and congregate to mate at temporary ponds.
Development is greatly accelerated: The eggs hatch within 48 hours, and the tadpoles metamorphose at 16-18 days.
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範例： plants in the deserts In the Mojave and Sonoran Deserts, rainfall is
infrequent, and even though it may occur in a predictable season, it often falls over a narrow and unpredictable geographic area in any one year.
Many of the plants in these regions have adopted an annual habit: They complete their life cycle in a single year (or season) and die.
The seeds are highly resistant to desiccation and can remain dormant in the desert for many years.
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Senescence Why must organism die? Is death truly inevitable?Could the timing of death evolve?Why do some organisms live so much
longer than others?Recent findings suggest the possibility
that life span has some genetic basis and may, in fact, evolve.
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Senescence and life span Senescence, as the degenerative changes
that result in an increase in expected mortality with age. Eventually, the probability of survival reaches zero.
The range of life spans in plants, from a few days to more than 5,000 years.
Sturgeon( 鱘魚 ) 150 years, earthworm 10 years, captive tarantulas( 毒蜘蛛 ) 數十年。
A clonal species Huge living fungi (Armillaria bulbosa) 可能有 10,000 or more years old.
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Does life span evolve? Small animals are known to have shorter life
spans than large ones. Because the per gram metabolic rate, heart
rate, and so on are much higher in small mammals, their shorter life spans were believed to be a consequence of this rapid physiology– the organism simply wears out sooner.
However birds generally live longer than mammals of comparable size, even though birds have higher metabolic rates, heat production, and heart rate.
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Two main hypotheses1. The mutation accumulation hypothesis:
The accumulation of damage ultimately results in a decrease in survivorship with age. Senescence per se do not evolve, rather it is the inevitable result of exposure to the environment.
2. The evolutionary senescence hypothesis: the pattern of senescence evolves in organisms.
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The concept of reproductive value provides a selective basis for different life history strategies.We have a way to compare the trade-offs associated with different life histories.
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The reproductive value
V = (lt/lx)(mx)(Nt/Nx)
Where lx is the age-specific survivorship
mx is the age-specific birth rate
N is population size x is the current age
(lt/lx) represents the probability of surviving from
age x to age t. A low probability of survival should result in a
low reproductive value.
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Fig. 7.7 Age-specific changes in (a) mean annual survival of female sparrowhawks.
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Fig. 7.7 Age-specific changes in (b) mean annual production of young of female sparrowhawks.
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Fig. 7.7 Age-specific changes in (c) reproductive value of female sparrowhawks.
Natural selection will have less impact on deleterious mutations whose effects are manifest after the peak in reproductive value.
Consequently, senescence may result from the accumulation of such mutations.
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Transgenic DrosophilaCells contain certain enzymes, such as
superoxide dismutase, that scavenge oxygen radicals and thus protect cells from damage.
Orr and Sohal (1994) developed transgenic Drosophila that contained three copies of one of the genes for these enzymes, the flies with extra copies of the genes had 33% longer life spans than normal flies.
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Artificial selection on Drosophila
Partridge and Fowler (1992) used artificial selection to generate two genetic lines: an “old line” generated by allowing only the oldest individuals to mate, and a “young line” generated by mating only young individuals.
Individuals from the “old line” jad longer life spans (Fig. 7.8)
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Fig. 7.8 Survivorship curves for (a) male and (b) female Drosophila from old line(dashed lines) and young line (solid lines) selected strains.
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Fig. 7.8 Survivorship curves for (a) male and (b) female Drosophila from old line(dashed lines) and young line (solid lines) selected strains.
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Altitudinal variation in Melanoplus grasshoppers The evolutionary hypothesis predicts that
earlier senescence should evolve at high elevations where late-age reproductive opportunities are limited by the shorter season and harsh conditions.
Indeed, low-elevation populations haf lower senescence, as depicted by the survivorship curves in Figure 7.9.
The differences in senescence between high and low-altitude populations have a genetic basis.
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These data were generated from populations in laboratory cultures under the same conditions.
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The effect of reproduction on mortalityVanvoorhies (1992) compared the age-
specific mortality rates of two mutant nematodes – one that does not produce mature sperm and one that does not mate – with wild-type animals.
The wild-type strain had higher age-specific mortality and a shorter life span than both mutants (Fig. 7.10).
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Fig. 7.10 (a) survivorship curves for nematodes. Open circles are wild-type males; closed circles are mutant males that do not produce sperm.
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Fig. 7.10 (b) survivorship curves for mated wild-type males (open circles) and mutant males that do not mate (closed circles).
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Reproductive Strategies Both sexual and asexual strategies are
successful, for they are found among virtually all taxa of both plants and animals.
Asexual reproduction can have advantages in a uniform or unchanging environment.
Sexual reproduction can produce different offspring with different genotypes via shuffling of genes by sexual recombination.
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Two basic assumptions:1. Because the amount of energy for
reproduction is finite, the species must make an evolutionary “decision” about how to apportion that energy in the reproductive process.
A relationship exists between the demography of a species, particularly its mortality schedule, and its reproductive pattern.
Energetics of reproduction
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Trade-offs in reproductive strategies
Number of offspring per reproductive event ( 每次要生幾個 )
Present versus future reproduction( 現在生殖的投入，與未來生殖的投入 )
Age at sexual maturity ( 成熟年齡 )
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Number of offspring per reproductive event
Females can product a few large offspring, or a large number of smaller individuals.
There is an upper bound to the optimal number in a clutch.
譬如： kittiwake ( 三趾鷗 ) 從一窩兩個蛋，增加到三個蛋， no parents were able to raise the enlarged broods to fledging.
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The weight at weaning, probably an important factor in survivorship, is significantly lower in large litters.
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Mean birth weight declines with increasing litter size.
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緯度愈大， clutch/litter size 愈大。(mammals and birds)但是hibernating mammals and hole-nesting birds, which have litter or clutch sizes smaller than expected.
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Hypotheses to explain this trend
1. Lack proposed that the longer days at higher latitudes allow parents more time to gather food to feed more young, enabling them to support larger clutches.
2. A shorter breeding season at high latitudes selects for larger clutches because fewer can be produced.
3. The higher mortality associated with the more severe northern climates selects for larger clutches or litters.
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Present versus future reproduction
Vx = mx + (lt/lx)(mt)
Where mx represents
the current reproductive value.
Survivorship curves, type I, II, and III
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Iteroparity vs. semelparity Type I survivorship curves are typical of
mammals. Mammals are likely to have several reproductive efforts over the course of their life span. We refer to this strategy as iteroparity.
Species with a Type III curve are more likely to have a single, large reproductive event in their lifetime, a strategy referred to as semelparity.
Massive semelparous reproduction is sometimes referred to as big-bang reproduction.
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Semelparity vs. annual plantThe terms semelparity and annual are
not strictly synonymous.For example, a desert annual plant is an
organism with a semelparous life history strategy.
Similarly, perennial ( 多年生 ) 未必然就是 iteroparity. 譬如：竹子。
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Semelparous reproductive strategy 的古典範例 Sockeye salmon. They are hatched in freshwater, and after a
few weeks there, the juveniles migrate to salt water.
There they exploit the tremendous productivity of the temperate and arctic marine environment.
After several years in the open ocean, the salmon begin a grueling migration back to the very stream in which their parents reproduced.
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Fig. 7.13 (a) Migrating sockeye salmon, (b) death follows a massive spawning event.
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Anadromous fish Anadromous fish such as salmon tend
to occur in temperate latitudes, where the oceans are more productive than the freshwater systems.
The frequency of anadromous fish increases abruptly when the oceans surpass freshwater in productivity (Fig. 7.14)
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Age at sexual maturity It seems reasonable that selection
would push the age at which sexual maturity occurs to the lowest possible age.
However, if reproductive success depends on age, size, status, or experience, delayed reproduction may be advantageous.
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The theory of r- and K- selection
r-selected species are commonly found at low population densities, where growth is exponential.
K-selected species face intense intraspecific competition for scarce resources.
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範例： two vole species These species are very closely related. Microtus breweri is endemic to Muskeget
island and it evolved recently from the mainland species, M. pennsylvanicus.
The island form does not experience the high-amplitude cycles that are characteristic of M. pennsylvanicus (Fig. 7.16)
Instead, it remains at relatively high density. M. breweri has chracteristics associated with K selection, where M. pennsylvanicus exhibits a more r-selected strategy (Table 7.3).
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Fig. 7.16 Log population size over time for (a) M. breweri . M. breweri remains at high density.
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Fig. 7.16 Log population size over time for (b) M. pennsylvanicus.
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M. breweri has chracteristics associated with K selection, where M. pennsylvanicus exhibits a more r-selected strategy
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Intraspecific 範例 Common dandelion in Massachusetts. The species was found in three distinct
environments: a well-traveled pathway, a lawn that was regularly mowed, and a successional field that was mowed once a year.
The three habitats represented different degrees of density-independent disturbance.
Electrophoretic studies on plants revealed four distinct genotypes that were associated with particular environments.
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Genotype D was characterized by adaptations that favored competition over reproduction: fewer flowering heads per plant and larger photosynthetic surface area.
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Box Life Histories and Conservation Some characteristics of life histories are key
determinants of the vulnerability of a species to extinction.
Body size: the size of an organism is correlated with a number of features related to its probability of extinction, including generation time (Fig. 7B.1).
The populations of the species with larger body size are more vulnerable to accidents or chance decreases in population size that may lead to extinction.
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R0 and G
R0 measures the average number of female offspring expected over the life span of a female.
G is the generation time.R0 = NG/N0 = erG
lnR0 = rG
r = lnR0/G
These equations demonstrate that he intrinsic rate of increase, r, is negatively associated with the generation time.
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r and K-selected speciesWe expect K-selected species to be
vulnerable because of a number of interacting factors.
Being large, and thus requiring greater resources, they find themselves near K.
The optimal strategy for K-selected species emphasizes quality of offspring rather than quantity.
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r-K continuum and bet-hedging strategy Species can generally be placed somewhere
along this continuum. However, not all species fall neatly onto this
continuum. A bet-hedging strategy combines elements of
r and K selection. If juvenile mortality is variable and
occasionally high , neither a classic r nor a classic K strategy is optimal.
生殖的能量分散投入，以減少完全失敗的風險。
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C-S selectionsS-selection means specialist selection,
which favors the present success. Under s-selection, the species evolves toward to be a confined and endemic species.
C-selection means colonizer selection, which favors the future success. These species are high starvation tolerance, and wide distribution, a kind of colonizers.
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(1) body growth C-selection
(+) or K-selection
(2) reproduction S-selection
(1) body growth C-selection
(-) (2) reproduction S-selection
or r-selection
Fig. 11 Evolutionary mechanism of C-S selection.
Environmental
condition
Constant (+) or
Variable (-)
Defense against
limiting factor(s)
Body (1) or
Offspring(2)
Body (1) or
Offspring(2)
Energetic
priority
Types of
selection
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Variable climate
Sub-variable climate Constant climate
Fig. 12. Climates and types of selections
Climates and types of selections
r-selection C-selection
K-selection S-selection
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Constraints and ambiguities in the study of life history strategies
The theories of the evolution of life history strategies are optimization theories.
What do we conclude if the life history does not match the predictions?
It is possible that the poor match results from one or more of the following constraints:
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Constraints 1. Evolution is an ongoing process. 2. Fitness is a relative property. Although our
theory might suggest the optimal life history, selection cannot choose the solution with absolute fitness, it can only choose the fittest of the options available.
3. It may be difficult for us to ascertain the appropriate time scale over which to consider an organism’s life history. (Fig. 7.18)
4. Allometric relationships may confound our analysis of life history strategies. (Fig. 7.19)
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Fig. 7.18 Demonstration of how the choice of time scale can lead to ambiguity in life history depicted by the product of age-specific birth and mortality rates (lxbx) . (a) The life histories of two intertidal barnacles graphed in absolute time.
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Fig. 7.18 (b) The same life histories graphed in generation time.The first graph (a) emphasizes the differences between the two species; the second (b), the similarities.
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