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Life history evolution 1 Some Important Concepts 1.1 Life History Life history – the adaptations of an organism that influence aspects of its biology such as the number of offspring it produces, its survival, and its size and age at reproductive maturity. Adaptation _ A process of genetic change in a population whereby, as a result of natural selection, the average state of a character becomes improved with reference to a specific function, or whereby a population is thought to have become better suited to some features of its environment . OR a feature that has become prevalent in a population because of selective advantage in the improvement in some function. The life history of a hypothetical female Virginia opossum (负鼠) 1.2 Fitness: 适应度(适合度) Fitness— ① The relative competitive ability of a given genotype conferred by adaptive morphological, physiological or behavioral characters, expressed and usually quantified as the average number of surviving progeny of competing genotypes; ② a measure of the contribution of a given genotype to the subsequent generation relative to that of other genotypes. 1.3 Reproduction Value (生殖价值) (i) Reproductive value (RV) at a given age or stage is the sum of the current reproductive output and the residual (i.e. future) reproductive value (RRV); (ii) RRV combines expected future survival and expected future fecundity (生殖力); (iii) RV takes account of the contribution of an individual to future generations, relative to the contribution of others; (iv) the life history favored by natural selection from amongst those available in the population will be the one for which contemporary output and RRV is highest. Measuring Reproductive Value: the reproductive value of an individual of age x is: ly x y RVx m x my R y x 1 l x y y max future reproductive output mx – the birth rate of individual in age-class x, as contemporary reproductive output lx – the probability that the individual will survive to age x R – the net reproductive rate of the whole population per unit time (age interval). 1.4 Methodology for Life History Study Every life history, and every habitat, is unique. We must find ways in which life histories might be grouped, classified and compared, so as to search for association between one life history trait and another or between life history traits and features of the habitats in which the life histories are found. The idea of optimization (最优思想): observed combinations of life history traits are those with the highest fitness. The idea of ‘bet-hedging’(‘两面下注’思想): when fitness fluctuates, it may be most important to minimize the setbacks from periods of low fitness rather than evolving to a single optimum. 2. Three types of questions (1) Individual life history traits: Why some birds produce clutches of three eggs, while others produce larger clutches? (2) Links between life history traits: Why is it that the ratio between age at maturity and average lifespan is often roughly constant within a group of organism but markedly different between groups?. (3) Links between life histories and habitats: Why is it different in fecundity between populations from subtropical and temperate regions? 2.1 Life History Traits ① Individual size — Being large or small? Increasing competition Decreasing survival of a shortage of food, or of defense Increasing survival Increasing offspring production Increasing success in predation or in defense Large body size ??? So, an intermediate size might be optimal ? The relationship between seed size, seedling mass, and seedling recruitment (增员) among herbs and grasses living in seminatural grasslands in southeastern Sweden. Seedling dry mass Larger seeds produce large seedlings. Seed mass(mg) Seed mass and seedling mass among grassland plants in Sweden On average, larger seeds were associated with a higher rate of recruitment (增员). Seed mass and recruitment rates in grassland plants There is also a positive relationship between seed size and seedling height among trees. Relationship between seed mass and seedling height among trees. Predicted optimum size Sampled optimum size Adult male damseflies (豆娘) ② Development — Being rapid or slow? Early reproduction Rapid Development Slow Development Early emerging from host More storage of energy, longer lifespan and reproduction period; Large body size 3 Trade-offs (得失,权衡) A ‘trade-off’ is a negative relationship between two life history traits in which increase in one are associated with decrease in the other as a result of a compromise. A trade-off Life history trait 2 Life history trait 1 3.1 How to detect trade-offs? (1) Correlation between the means of two or more traits in different populations or species can strongly suggest a trade-off, although such correlations might result from other, unknown different among the populations. (i) Egg size and number in fish 鲈鱼科物种丰富,1个属有100多种,生活在溪流底,体色鲜 艳(尤其在繁殖季节),是理想的研究生活史特征的材料 15 darter species Turner & Trexler(1998) sampled 64 locations on streams and rivers in US. (ii) Seed size and number in plants Plants vary widely in the number of offspring they produce,ranging from those that produce many small seeds to those that produce a few large seeds. A small sample of the great diversity of seed sizes and shapes 在4个大科(菊科、禾本科、十字花科和豆科)植物中, correlation between seed mass and number of seeds. Number of seed per plant The negative Average seed mass (2) Phenotypic (表型) or, better, genetic correlations between traits within populations can be useful indicator of the extent to which enhancement of one component of fitness would be immediately accompanied by reduction of another. (ii) Development time and body size in parasitic wasps The alternative approach is to use experimental manipulation to reveal a trade-off directly from a negative phenotypic correlation. 发育时间 体型 寄主龄期 死亡率 Developmental characteristics of M. pulchricornis on different instars of S. exigua larvae. Graphical representations (A–C) of the three most frequently reported empirical relationships (trade-offs?) between host size (age) and egg-to-adult development time (left column) and adult body size (right column) in koinobiont parasitoids (寄生蜂) . (3) Correlated responses to artificial or natural selection provide some of the most consistent evidence of trade-offs. ‘young’ population has a higher mortality rate, but … has a higher egg production (4) Experimental manipulation of one trait and observation of the effect on other traits often reveals trade-offs. The costs of reproduction increase mortality The difference in mortality rate between ‘old’ and ‘young’ populations disappeared when a gene that prevent females reproduction was crossed into the populations. 3.2 Adult survival and reproductive allocation (成体生存与生殖分配) ①Where adult survival is lower, organisms begin reproductive at an earlier age and invest a greater proportion of their energy budget into reproduction; ②where adult survival is higher, organisms defer reproduction to a later age and allocate a smaller proportion of their resources to reproduction. ①成体存活率低的生物在年轻时就生殖,而且投入较大能量 到生殖中; ② 相反,成体存活率高的生物则推迟生殖,而且投入较小能 量到生殖中。 Age at maturity (year) Lizards and snakes that have higher survival mature at a later age or … Adult survival …fish with higher mortality rates reach reproductive maturity at an early age Adult fish mortality Species with higher mortality 生殖-身体质量指数 would show higher relative reproductive effort (fig.) (reproductive efforts were measured as GSI (Gonadsomatic Index), which was taken as the ovary weight of each species divided by the species body weight and adjusted for the number of batches of offspring produced by each species per year.) Relationship between adult fish mortality and reproductive efforts 4. Life History Classification 可以根据几个重要的种群参数来划分生活史类型,如生殖 力或后代数量(mx)、存活率(lx)、性成熟年龄(a)等 4.1 r and K selection (MacArthur and Wilson, 1967) Characteristics favored by r and K selection Population attribute rm (内禀增长率) Competition ability r selection High Not strongly favored K selection Low Highly favored Development Rapid Reproduction Early, single Late, Repeated Body size Small Large Offspring Many, small Slow Few, large r selection 蚜虫 K selection 天牛 4.2 Plant Life Histories 竞争型物种 Grime(1979) select two variables as important selective pressure on plants: intensity of disturbance (扰动 程度) and intensity of stress (胁迫程 度). 桦 1年生草 胁迫忍耐型物种 山毛榉 杂草型物种 4.3 Opportunistic,Equilibrium and Periodic life histories (机会型、平衡型 和周期型生活史) High juvenile survival, Guppy, finch, Anole fecundity Low juvenile survival, low fecundity, early maturity Opportunistic Life history Juvenile survivorship high fecundity, late maturity Periodic life history Ocean sunfish Age of reproductive maturity Equilibrium life history High juvenile survival, low fecundity, late maturity Shark, human (4) Reproductive effort, Offspring size, and Benefit-cost ratios Charnov (2002) developed a classification free of the influences of size and time by using relative variables (dimensionless): I/m: is the mass of offspring at independence from the parent ,(I), divided by the average adult mass (m). E/α: is the average length (e.g. years) of a species’s reproductive life ,(E), divided by the average length of time required to reach reproductive age ,(α). C· E: is the proportion of adult body mass allocated to reproduction per unit time (C), multiplied by the adult lifespan (E). It is a benefit-cost ratio without dimensions, as high reproductive effort, benefit, is associated with high rates of mortality, a cost. Life history cub classification of fish, mammals, and altricial (晚成雏的)birds 思考题 1、举例说明研究生活史特征的理论或实践意义。 要求: 1)何种生物?描述基本生物学特征。 2)有哪些生活史特征?为什么是这些而不是其他特征? 3)阐述这些生活史特征的理论意义或可能的实践价值