1Ecology 2000

Chap.32Evolution of Life histories

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Introduction Organisms have limited time, energy, and

nutrients at their disposal. Adaptive modifications of form and

function serve two purposes in this regard. One is to increase the resources available

to individuals. The other is to use those resources to their

best advantage, this is in a manner that maximizes the survival and reproduction.

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Every modification involves a trade-off. Therefore, each individual is faced with

the problem of allocation of time and resources.

In this chapter, we shall consider some general rules governing the allocation of time and resources in life strategies.

Each life history has many components, maturity, parity, fecundity, and termination of life.

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Evolution of life histories 32.1 Interest in life history adaptations has b

een stimulated by their variation among species.

32.2 Life history theory developed rapidly during the 1960s.

32.3 Natural selection adjusts the allocation of limited time and resources among competing demands.

32.4 Age at first reproduction generally increases in direct relation to adult life span.

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32.5 Perennial life histories are favored by high and relatively constant adult survival.

32.6 Optimal reproductive effort varies inversely with adult survival.

32.7 When survival and fecundity vary with age, models of life history evolution must be based on the life table.

32.8 Bet hedging minimizes reproductive failure in an unpredictable environment.

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32.9 Extensive preparation for breeding and uncertain or ephemeral environmental conditions may favor a single, all-consuming reproductive episode.

32.10 Senescence evolves because of the reduced strength of selection in old age.

32.11 Life history patterns vary according to the growth rate of the population.

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32.1 Interest in life history adaptations has been stimulated by their variation among species.

Fig. 32-1 Relationship between annual fecundity and adult mortality in several populations of birds.

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Table 32-1 Life history traits of several populations of the fence lizard

Clutch size ( 平均值 ) ： 6.2 - 11.8 Clutches per year ： 1 - 4 Eggs weight (g) ： 0.22 - 0.42 Relative clutch mass ： 0.21 - 0.28 Age at maturity ( 月 ) ： 12, 21, 23 Survival to breeding ： 0.03 - 0.11 Annual adult survival ： 0.11 - 0.49

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Varied life histories may be found even among different populations of the same species, as illustrated by fence lizards (Table 32-1).

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Table 32-2 Typical life histories of plants in environments with different selective factors. Competitors ( 競爭型 ) Ruderals ( 荒地型 ) Stress tolerators ( 耐壓型 ) 於不同的環境狀況下，各有不同適應的

策略。 兩類影響因素： disturbance 和 stress

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Life history variation in plants

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32.2 Life history theory developed rapidly during the 1960s.

Fisher (1930) The genetical theory of Natural selection

Lack (1940s)suggested that because of the longer day length at higher latitudes in the season when offspring are reared, birds living at temperate and arctic latitudes could gather more food, and therefore rear more offspring, than birds breeding in the Tropics, where day length remains close to 12 hours year-round.

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Lack 的觀點 Lack made three important points.

(1) he related life history traits to reproductive success and thus to evolutionary fitness.

(2) he demonstrated that life histories vary consistently with respect to factors in the environment.

(3) he proposed a hypothesis that could be subjected to experimentation.

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during the 1960s.

The early 1960s marked a turning point in population studies and saw the birth of modern evolutionary ecology.

Life history study burst in 1966 with the publication of papers by Martin Cody and George C. Williams.

K-selected and r-selected traits Hamilton (1966) , Gadgil and Rossert

(1970), Schaffer (1974)….

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32.3 Natural selection adjusts the allocation of limited time and resources among competing demands. Fig. 32-2 Proportion

al distribution of dry weight among different plant parts of the groundsel during its life cycle.

The development of reproductive parts ar the expense of leaves and roots toward the end of the growing season.

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Allocation of limited time and resources Fig.32-2 Proportional distribution of dr

y weight among different plant parts of the goundsel, Senecio vulgaris, during its life cycle.

於開花結果過程，是 at the expense of leaves and roots. ( 葉子和根的部份縮小 ) 。

這就是 trade-offs

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Fig. 32-3 Number of chicks fledged from nests of European magpies in which seven eggs were laid, but manipulated clutches of between five and nine eggs.

歐洲鵲，每窩產 7 個蛋。

運用人為的方法，添加蛋數或減少蛋數。

再看其蘊育成功子代的數量。

最成功的是原來的每窩 7 個蛋。

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Fig. 32-4 clutch size and young surviving

大山雀，英國， 1960-1982 年間。 統計 4,489 鳥窩的蛋數。 灰色的是 clutch size 的 frequencies 淡紅色是 每窩存活的 young 之數目，至

少存活至下一個季節。 存活數最高的，並不是 the most comm

on clutch size.

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Fig. 32-4 The frequencies of clutch sizes (gray bars) of the great tit between 1960 and 1982, and the number of young per clutch surviving at least to the next season (green bars) as a function of clutch size.

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Fig. 32-5 Decrease in the rate of larval survival to emergence with increasing numbers of larvae deposited per bean in the bruchid beetle.

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Fig. 32-6 female guppies( 孔雀魚 ) 於 3 種不同食物供應量下，生殖、脂肪

和身體的能量分配情況。 分成兩組，一組是有與雄的接觸 (R) ，另

一組是沒有與雄的接觸 (N) 。 與雄的接觸，增加生殖的投入，但並沒

有因而減少脂肪和身體的能量投入。 這裡並沒有發現是 trade-offs 的現象。

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Fig. 32-6 Content of energy in somatic tissues, fat deposits, and reproductive tissues (including eggs) in female guppies raised at three food levels and which were either permitted to be (R), or prevented from being(N), courted and inseminated by males.

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Evolution of life historiesThree questions: (1) When should an individual begin to pro

duce offspring? ( 成熟年齡 ) (2) How often should it breed? ( 多少次數 ) (3) How many offspring should it attempt t

o produce in each breeding episode? ( 每次生多少個 )

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32.4 Age at first reproduction generally increases in direct relation to adult life span.

Fig. 32-7 Relationship between age at maturity and annual adult survival rate, which is directly proportional to life span, in a variety of birds.

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Age at first reproduction Figure 32-3 age at maturity and adult

survival rate 的關係。 壽命較長的生物，成熟的年齡通常是較晚。

Why should this be so? Table 11.3 假設每年可生 10 個蛋，但

若延後一年才生，則可生 20 個蛋，延後到第三年才生，則一年可生 30 個蛋。 如此可看出其壽命長短，對何時開始生殖的

影響。

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若壽命只有 5 歲，那第 3 年開始生殖，所得的子代數最高。但壽命若有 8 歲，那第 4 或第 5 年才開始，才可有最多的子代數。

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32.5 Perennial life histories are favored by high and relatively constant adult survival. Plants and animals either

reproduce during a single season and die (annual reproduction), or have the potential to reproduce over a span of many seasons (perennial reproduction).

A theoretical comparison of annual and perennial reproduction

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A theoretical comparison of annual and perennial reproduction Annual plant 的種子產量 (Ba) x their surviv

al to reproductive age (S0) a = BaS0

perennial plant 還要多加一項，個體存活下去的機率 (S)

p = BpS0 + S Ba - Bp > S / S0 ，如此就會保持在 annual

reproduction

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32.6 Optimal reproductive effort varies inversely with adult survival. the balance between fecundity and survi

val = BS0 + S SR

SR 是與生殖相關的 survival d = S0 dB + S dSR

B 的增加，牽動使 SR 減少

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Table 32-4 slow growth/high fecundity and rapid growth/low fecundity 的比較 假設兩種魚，都是在體重 10克時性成熟。 但是其中一種是運用較多能量在持續的成長，

另一種則用較多用於生殖上。 Fish A allocates 20% to growth, 80% to e

ggs. Fish B allocates half to growth and half to eggs.

發現於壽命 4 年的， high fecundity 較有利。 多於 4 年的， low fecundity 較有利。

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Each weighing 10 grams at sexual maturity, but which allocate resources to growth and reproduction differently.

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32.7 When survival and fecundity vary with age, models of life history evolution must be based on the life table.

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32.8 Bet hedging minimizes reproductive failure in an unpredictable environment.

As a general rule, selection shifts allocation away from stages of the life cycle with the greatest uncertainty.

This strategy is sometimes referred to as bet hedging. (賭預防 )

Fig. 32-8 mosquitofish ( 大肚魚 ) 於變動大的水域，或是在穩定的水域 生殖投資量不同。

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Fig. 32-8 Life history characteristics of populations of mosquitofish introduced into reservoirs in Hawaii indicate greater total reproductive investment. Reproductive allocation is the proportion of the dry mass constituting embryos.

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32.9 Extensive preparation for breeding and uncertain or ephemeral environmental conditions may favor a single, all-consuming reproductive episode.

Semelparity ，如鮭魚， big-bang reproduction ，一生只生一次。一生可生多次，則稱 iteroparity

the occurrence of semelparity or iteroparity (1) variable environments might favor iteroparity.

但事實上， semelparity 通常發生在更 variable (usually drier) 的環境。

(2) variable environments might favor semelparity. (3) 集體開花更可以吸引 pollinators ，有利於 semel

parity.

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Fig. 32-9 Kaibab agave in the Grand cayon.An plant grows as a rosette of thick, fleshy leaves (a) for up to 15 years. Then it rapidly sends up its flowering stalk (b) and sets fruit, after which the entire plant dies.

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Table 32-6 兩種植物的 life histories Semelparity is associated with dry habitat

s that are highly variable in both space and time.

In summary, semelparity appears to arise either when preparation for reproduction is extremely as in the undertaking of long migration to breeding grounds, or when the payoff for reproduction is highly variable and predictable.

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It suggests that semelparity is associated with dry habitats that are highly variable in both space and time.

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32.10 Senescence evolves because of the reduced strength of selection in old age. The rates of most physiological function

s in humans decrease in a roughly linear fashion between the age 30 and 85 years, by 15-20% for nerve conduction and basal metabolism, 55-60% for volume of blood circulated through the kidneys, and 63% for maximum breathing capacity.

Birth defect 發生的機率，隨婦女的年齡而增加 (Fig. 32-10)

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Fig. 32-10 In humans, the risk of abnormal numbers of chromosomes in offspring increase with the mother's age.

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How can senescence evolve?

Senescence may reflect the accumulation of molecular defects that fail to be repaired. (mutation-accumulation theory of senescence)

selection 的作用：倘若基本死亡率高，那 selection 將應會使其老化慢。但事實上，卻剛好相反 (Fig. 32-11)

antagonistic pleiotropy. 有些基因於年幼時是可增加 fitness ，但於年老時反而是減少其 fitness 。 (Table 32-7)

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Fig. 32-11 species of birds and mammals with a high baseline mortality rate appear to have a high rate of aging as well.

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Selection for later egg laying also increased longevity

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disposable-soma theory of senescence Kirwood and Rose (1991) suggested that

the most important trade-off involved in senescence is between allocating energy to the repair and maintenance of germ cell DNA and somatic DNA).

This idea, known as the disposable-soma theory of senescence.

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32.11 Life history patterns vary according to the growth rate of the population. Eric Pianka (1970) listed a variety of traits that

could be considered either r-selected or K-selected (Table 32-8).

The concept of r- and K-selection has been useful in helping to formalize the definition of fitness in density-regulated populations, but attempts to transfer the concept to actual populations without regard to the realities of the complexities in life history have probably been detrimental rather than helpful.

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Suggested readings Bazzaz, F. A., N. R. Chiarello, P. D. Coley, and L. F.

Pitelka (1987) Allocating resources to reproduction and defense. BioScience 37:58-67.

Fleming, I. A. and M. R. Gross (1989) Evolution of adult female life history and morphology in a Pacific salmon (Coho: Oncorhynchus kisutch). Evolution 43:141-157.

Gross, M. R. (1996) Alternative reproductive strategies and tactics: Diversity within sexes. Trends in Ecology and Evolution 11:92-98.

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Janzen, D. H. (1976) Why bamboos wait so long to flower. Annual Review of Ecology and Systematics 7:347-391.

Reznick, D.N. (1985) Costs of reproduction: An evaluation of the empirical evidence. Oikos 44:257-267.

Reznick, D.N., H. Bryga, and J. A. Endler (1990) Experimentally induced life-history evolution in a natural population. Nature 346:357-359.

Schlichting, D. C. (1989) Phenotypic integration and environmental change. BioScience 39:460-464.

Strathmann, R. R. (1990) Why life histories evolve differetly in the sea. American zoologist 30:197-207.

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