Zoology 445 Fall 2006Reading: Chapter 12Life Histories: OutlineI: IntroductionLife History = characteristics of growth, survivorship and reproduction of an organism.Life history is a group of traits that are all under selection. Life history options are limited by historical (evolutionary) constraints and trade-offs.Figure 12.2The evolution of a life history can viewed as a series of trade-offs that can affect fitness.Three basic questions of life history evolution:1. Why do organisms senesce & die?2. How many offspring should an individual produce in a given year?3. How big should each offspring be?AM Jarosz 1Zol 445 JaroszI. Why do organisms senesce and die?Three hypotheses that have been proposed to explain death.A. Rate of living hypothesis B. Deleterious mutations hypothesisC. Antagonostic Pleiotropy Hypothesis Pleiotropy = a single gene influences multiple phenotypic traitsA. Rate of living hypothesis on aging: Aging & death are caused by the accumulation of irreparable damage to cells & tissues. Damage caused by errors in replication, transcription & translation.Selection acts to resist damage to the extent physiologically possible.Two predictions:a. Life spans should be tied to metabolic rate.b. It is not possible to evolve longer life spans.a. Life span tied to metabolic rateFor mammals, metabolic rate is correlated with size. Smaller animals have higher metabolic rates than larger animals.Consequently, small animals should have shorter lifespans than larger animals.If lifespan was limited by metabolic rate, then all animals should have similar lifetime energy expenditures per gram of tissue.2Zol 445 Jaroszb. It is not possible to evolve longer life spans.Experiment for early and late reproduction in Drosophila melanogaster.Life span increased by about 70% from 35 days in the initial population to 60 days after selection.B. Deleterious mutations hypothesis = Mutations that reduce early survivorship will be more strongly selected against than mutations reducing survivorship only at older ages.1. Life Tables = the schedule of survivorship and reproduction of females within a populationSay we have a hypothetical organism with the following life table: a. The probability of survivorship from one year to the next is 80% (0.80), b. Individuals reach reproductive maturity at age three. c. Produce one offspring every year it survives beyond maturityd. The maximum lifespan is 16 yearsLife Table:Wildtype Matures at age 3, 80% survival per year; 1 offspring per yearAgeFraction Surviving RS of SurvivorsExpected RS for individuals0 1.000 01 0.800 02 0.640 03 0.512 14 0.410 15 0.328 16 0.262 17 0.210 18 0.168 19 0.134 110 0.107 111 0.086 112 0.069 113 0.055 114 0.044 115 0.035 1Expected lifetimeRS =(Absolute fitness)3Zol 445 JaroszNow say we have two deleterious mutations enter the population.One causes organisms to die at 5 while the second causes organisms to die at age 14.Mutation that causes individuals to dies at age 5AgeFraction Surviving RS of SurvivorsExpected RS for individuals0 1.000 0 0.0001 0.800 0 0.0002 0.640 0 0.0003 0.512 1 0.5124 0.410 1 0.4105 0.000 1 0.0006 0.000 1 0.0007 0.000 1 0.0008 0.000 1 0.0009 0.000 1 0.00010 0.000 1 0.00011 0.000 1 0.00012 0.000 1 0.00013 0.000 1 0.00014 0.000 1 0.00015 0.000 1 0.000Expected lifetimeRS =(Absolutefitness)Mutation that causes individuals to die at age 14AgeFraction Surviving RS of SurvivorsExpected RS for individuals0 1.000 0 0.0001 0.800 0 0.0002 0.640 0 0.0003 0.512 1 0.5124 0.410 1 0.4105 0.328 1 0.3286 0.262 1 0.2627 0.210 1 0.2108 0.168 1 0.1689 0.134 1 0.13410 0.107 1 0.10711 0.086 1 0.08612 0.069 1 0.06913 0.055 1 0.05514 0.000 1 0.00015 0.000 1 0.000Expected lifetimeRS =(Absolutefitness)4Zol 445 JaroszC. Antagonostic Pleiotropy Hypothesis = A mutation that simultaneously increases reproductive success early in life and causes early death late in life could be favored by selection.Mutation that causes individuals to mature at age 2, but die at age 10AgeFraction Surviving RS of SurvivorsExpected RS for individuals0 1.000 0 0.0001 0.800 0 0.0002 0.640 1 0.6403 0.512 1 0.5124 0.410 1 0.4105 0.328 1 0.3286 0.262 1 0.2627 0.210 1 0.2108 0.168 1 0.1689 0.134 1 0.13410 0.000 1 0.00011 0.000 1 0.00012 0.000 1 0.00013 0.000 1 0.00014 0.000 1 0.00015 0.000 1 0.000Expected lifetimeRS =(Absolutefitness)Text on pages 467 – 474 has examples of data that test these hypotheses.Scorecard:A. Rate of living hypothesis ------ Proven to be falseB. Deleterious mutations hypothesis ---- Some data support this hypothesisC. Antagonostic Pleiotropy Hypothesis --- Other data support this hypothesisII. How many offspring should be produced?Fig 12.165Zol 445 JaroszTest of Lack’s hypothesis. Parus major In reality clutch sizes seem to be lower than the optimum predicted by the Lack hypothesis.Reasons:1. Reproduction this year appears to negatively affect the ability to reproduce young in the next year. Fig 19.52. Clutch size (number of siblings in the nest) appears to affect a female offspring's ability to produce eggs. (Fig 12.18)6Zol 445 JaroszIII. How big should each offspring be?Fig 12.21b. Relationship between egg volume and egg number. Data are for 26 different fish families.Say we have the relationships depicted in Fig 12.22 a & b shown below.Part c is obtained by multiplying values in a with values in b.7Zol 445 JaroszIn reality, multiple life history traits respond to changes in the environment. Conclusion:I. Life history traits appear to be shaped by a series of trade-offs imposed by having to choose how a limited amount of energy can be utilized for growth, maintenance, and & reproduction.II. These trade-offs must be optimized to maximize fitness within a particular environment.\Evolution Zol 445\2006\Life History