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Lecture 2

BIOL 3170 Lecture 2: Lecture 2 - LIFE HISTORY BIOLOGY

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York University
BIOL 3170
Mark Vicari

LIFE HISTORY BIOLOGY (Ch. 8) Life history biology analyzes all the components of fitness (features which make an individual successful) Life history • The set of strategies an organism uses to allocate its energy, materials and time o In particular, the allocation of those resources between growth and maintenance, and reproduction Fitness • A relative measure of reproductive success of an organism in passing its alleles to the next generation o Based on the number of successful offspring an individual leaves Main Question of Life history biology: • Given environmental constraints and opportunities, how can the evolution of a species’ traits be explained • How do a specie’s traits - i.e., its phenotype – maximize its fitness (reproductive success)? o Morphological o Physiological How does Natural Selection optimize these things in order to o Behavioural maximize lifetime reproductive success? o Developmental Principle of Allocation • The principle that if an organism allocates energy to one function, such as growth or reproduction, it reduces the amount of energy available for other functions, such as defense What finite things do organisms budget? • Time • Matter (i.e. nutrients) • Energy What interests do organisms allocate these 3 finite things among? • Foraging (acquisition of nutrients/energy) • Growth • Maintenance • Reproduction Allocations are context-dependent • Consider a female migratory songbird o April: high energy allocation to migration o June: high energy allocation to egg production o August: high energy allocation to moulting feathers o January: high energy allocation to foraging • Consider breeding sea lions o Male: High time allocation to territoriality and fighting o Female: High time allocation to parental care and foraging • Consider Rock pipits o Harsh Winter: less time defending nest and resting, more time feeding o Mild Winter: more time defending nest and resting, less time feeding • Consider Warbler o Less nearby males: more time foraging; less defending nest o More nearby males: less time foraging; more time defending nest Organisms are input-output systems o Proximate goal of input: get energy and materials o Ultimate goal of input: get progeny Input: Foraging, or Photosynthesis Life History makes “trade-off” decisions here Black box: (lots of things happening) Output: Growth and Maintenance Output: Progeny The following is generally true about reproductive success in populations: • Each individual, on average, helps produce (i.e. along with a partner) 2 offspring that live to reproduce themselves o In other words, a male and female couple will replace themselves with offspring o If this did not occur, populations would decrease over time There are many strategies that different species use to optimize reproductive success/maximize fitness (trade-offs), which vary based on the following factors known as LIFE HISTORY TRAITS: • Age and size at maturity (when they can reproduce) o Incl. Growth rate • Average number and size of offspring • Frequency of reproduction • Lifespan • Investment into reproduction e.g. Parental care, migration to spawn Sexes of the same specie often exhibit different life history traits as well • E.g. in some turtle or fish species, females reach sexual maturity at a much later age Life history traits – traits that influence the schedule of birth, growth, reproduction and death Traits can vary between: 1. Species a. E.g. age at reproductive maturity: b. E. coli bacterium: 20 minutes c. Fruit fly: 9 days d. Gray squirrel: 1 year e. Galapagos tortoise: 25 years 2. Populations of same sp. a. E.g. Atlantic salmon: age at reproductive maturity for populations that breed in rivers in b. Southern Nova Scotia: 1 year c. Northern Quebec: 10 years 3. Indvls within a population a. E.g. two types of coho and chinook salmon males: b. Hooknoses – large; mature at 3 years; fight other males for territories in which females lay eggs c. Jacks – smaller; mature at two years; don’t fight; fertilize females’ eggs when hooknoses are fighting d. Hooknoses are most fit when jacks are common and hooknoses are rare e. few fights; jacks have few mating opportunities f. Jacks are most fit when hooknoses are common and jacks are rare g. Hooknoses engage in frequent fights; frequent mating opportunities for jacks. h. Negative frequency-dependent selection  polymorphism First major trade-off theme: Two main components contribute to Fitness 1. Traits contributing to survival success 2. Traits contributing to reproductive success • Increased investment (i.e. energy, materials, time) in one entails reduced investment in the other Survival vs. Reproduction Trade-off o E.g. Foxglove o 2-yr life cycle (“biennial”) o First year: growth o Second year: flowering & death o If flowering prevented in second year, will survive another year, attempt to flower again ▪ Thus, it is the investment into flowering for reproduction that kills the plant o E.g. octopus o Female will guard eggs 24 hours a day until they hatch o She will die of exhaustion after they are born o This behaviour is controlled by the Optic Gland, which causes the octopus to lay eggs and then guard them o If removed after laying eggs, the octopus will abandon the eggs and resume living, but won’t be able to reproduce again Darwinian Demon • Theoretical organism which can maximize all aspects of fitness (i.e. no trade-offs) • This species would dominate the world and outcompete all others • Its life history: o Capable of reproducing at birth a large number of large offspring, infinite lifespan Various life history strategies and ways species utilize trade-offs results in the great diversity we see today • E.g. indviduals who have a later age at maturity tend to produce more offspring which are also larger, but they risk not surviving until maturity Limited resources  Trade-offs E.g. Swifts: two female morphs 1. Lay 2 eggs / season 2. Lay 3 eggs / season • Why don’t 3-egg females replace those laying only 2 eggs? o David Lack: followed success of different females in fledging offspring from the nest o In harsh breeding seasons (fewer insects to feed on): o Females with 3 chicks spread limited resources too thinly --> few offspring fledge o Those with only 2 chicks had higher average fledging rate o Favorable seasons: 3-egg females fledged more chicks o Fluctuating conditions maintain polymorphism amongst females Determinate egg-layers – always lay the same number of eggs • E.g. swifts Indeterminate egg-layers – can alternate size of clutch depending on environmental conditions • E.g. chickens Phenotypic plasticity – the ability of one genotype to produce more than one phenotype when exposed to different environments • Ability of an organism to change its phenotype in response to changes in the environment • E.g. chickens when farmer takes eggs away- produces more Consider age and size at maturity • What is an optimal age for reproduction? o Answer: when the difference between Benefits and the Costs (B – C) is maximal o Same for optimal size… • Prediction: species mature at the age and size where there is the greatest fitness payoff How do we predict age and size at maturity? • Older age at first reproduction  larger size  higher quality offspring and more offspring • However, counterbalanced by disadvantages of longer generation time • Natural selection  optimal balance Optimal “strategies” can change with conditions • In good times, growth can be rapid • In bad times, growth can be slow • Possibility #1: mature at a particular weight, regardless of age o Under slow growth conditions, it takes a long time to mature, and the cost is the risk of dying before reproduction • Possibility #2: mature at a particular age, regardless of weight o Under slow growth conditi
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