An organism’s life history is a record of events relating to its growth, development, reproduction, and survival.

Life history characteristics include:

• Age and size at sexual maturity.
• Amount and timing of reproduction.
• Survival and mortality rates.

Life History Diversity

• Individuals within a species show variation in life-history traits due to genetic variation or environmental conditions.
• The life history strategy of a species is the overall pattern in average timing and nature of life-history events.
• It is shaped by the way the organism divides its time and energy between growth, reproduction, and survival.
• Ideal or optimal life histories maximize fitness (genetic contribution to future generations).
• But none are perfect; all organisms face constraints and ecological trade-offs.
• Phenotypic plasticity: One genotype may produce different phenotypes under different environmental conditions. [For example, growth and development may be faster in higher temperatures.]
• Changes in life-history traits can cause changes in adult morphology.
• Phenotypic plasticity may result in a continuous range of sizes, or discrete types called morphs.
• Polyphenism—a single genotype produces several distinct morphs.
• Spadefoot toad tadpoles have small omnivore morphs and larger carnivore morphs.
• Allometry: Different body parts grow at different rates, resulting in differences in shape or proportion. (Isometry- opposite, entire body growing proportion)

Modes of Reproduction

• Asexual reproduction: Simple cell division (binary fission)—all prokaryotes and many protists.
• Some multicellular organisms reproduce both sexually and asexually (e.g., corals).
• Benefits of sexual reproduction: Recombination promotes genetic variation and increased ability to respond to environmental challenges.
• Disadvantages: An individual transmits only half of its genome to the next generation; the population growth rate is slower.
• Isogamy: Gametes are equal in size.
• Anisogamy: Gametes of different sizes. Usually, the egg is much larger and contains nutritional material.
• The most multicellular organism produces anisogametes.
• Some species have direct development—the fertilized egg develops into a juvenile without passing through a larval stage. (i.e humans)
• Complex life cycles have at least two stages, with different body forms and that live in different habitats.
• Metamorphosis: Abrupt transition in form between the larval and juvenile stages.
• Most vertebrates have simple life cycles without abrupt transitions.
• But complex life cycles are common in insects, marine invertebrates, amphibians, and some fishes.
• Many parasites have evolved complex life cycles with specialized stages for each host.
• The different stages are specialized for different functions, e.g., colonization or sexual reproduction.

Life History Continua

• Classification schemes for reproductive patterns place the patterns on continua with extremes at each end.
• Number of reproductive events during the organism’s lifetime:
• Semelparous species reproduce only once.
• Iteroparous species can reproduce multiple times.
• r-selection and K-selection describe two ends of a reproductive strategy continuum.
• r is the intrinsic rate of increase of a population.
• r-selection: For high population growth rates; an advantage in newly disturbed habitats and uncrowded conditions.
• r-selected species (“live fast, die young”)- small organism:
• Short life spans, rapid development, early maturation, low parental investment, high reproduction rates.
• Most insects, small vertebrates such as mice, weedy plant species.
• K is the carrying capacity for a population.
• K-selection species: For slower growth rates in populations that are at or near K; this is an advantage in crowded conditions; efficient reproduction is favored.
• K-selected (“slow and steady”):
• Long-lived, develop slowly, late maturation, invest heavily in each offspring, low reproduction rates.
• Large mammals, reptiles such as tortoises and crocodiles, and long-lived plants such as oak and maple trees.

• Most life histories are intermediate between these extremes.
• The two species found in more predictable wet forest habitats had K-selected characteristics. R-selected species often grow in rapidly changing and unpredictable regions.
• One classification scheme for plant life histories is based on stress and disturbance (Grime 1977).
• Stress—any abiotic factor that limits growth.
• Disturbance—any process that destroys plant biomass.

• Low stress/low disturbance:
• Competitive plants with superior ability to acquire light, minerals, water, and space—have a selective advantage.
• High stress/low disturbance:
• Stress-tolerant plants with phenotypic plasticity, slow rates of water and nutrient use—not palatable to herbivores.
• Low stress/high disturbance:
• Ruderal plants (weed-r selected plants) with a short life span, rapid growth rates, heavy investment in seed production.
• Can exploit habitats after the disturbance has removed competitors.
• Seeds can survive a long time until conditions are right for rapid germination and growth.

Trade-Offs

Trade-offs: Organisms allocate limited energy or resources to one function at the expense of another.

Trade-offs between size and number of offspring:

• The larger the investment in each individual offspring, the fewer offspring can be produced.
• Investments: Energy, resources, and loss of time for other activities such as foraging.
• “Lack clutch size”: Maximum number of offspring a parent can successfully raise to maturity.
• Named for David Lack (1947) who noticed that bird’s clutch size increases with latitude; longer daylight hours may allow parents more time to forage and feed more offspring.
• In species without parental care, resources are invested in propagules (eggs or seeds).
• The size of the propagule is a trade-off with the number produced. (inverse relation; large seeds= small amount and vice versa)
• In plants, seed size is negatively correlated with the number of seeds produced.
• The size–number trade-off can also occur within species.
• Northern populations of western fence lizards have larger average clutch size, but smaller eggs, than southern populations.
• Trade-offs between current and future reproduction:
• For an iteroparous organism, the earlier it reproduces, the more times it can reproduce over its lifetime.
• But the number of offspring produced often increases with the size and age of the organism.

• It may be advantageous to delay reproduction and invest more energy in growth and survival in order to increase lifetime reproductive output.
• Senescence: Decline in physiological function with age. (cellular death)
• Onset can set an upper age limit for reproduction.
• Though human may experience this after reproduction; we live on because we are social beings and have the grandparents roles.
• Senescence may occur earlier in populations with high mortality rates or predation.

Life Cycles

• Complex life cycles may result from stage-specific selection pressures, and help minimize the drawbacks of small, vulnerable early stages.
• Functional specialization of stages is a common feature of complex life cycles.
• In many insects, the larval stage stays in a small area, such as on a single plant.
• The larvae are specialized for feeding and growth. The adult is specialized for dispersal and reproduction.
• Even in species with gradual morphological change, individuals may have different ecological roles depending on size and age—niche shift.
• The ecological niche is the physical and biological conditions that an organism needs to grow, survive, and reproduce.
• Niche shifts should occur when the organism reaches a size at which conditions are more favorable in the adult habitat than in the larval habitat.
• In the Nassau grouper, small juvenile fish hide in algae clumps; larger ones stay in rocky habitats.
• Dahlgren and Eggleston (2000) found that smaller juveniles are very vulnerable to predators in the rocky habitats, but larger ones were not.
• The niche shift was timed to maximize growth and survival.
• If the larval habitat is very favorable, metamorphosis may be delayed or eliminated. (see below)
• Some salamanders mature sexually while retaining larval morphology and habitat (paedomorphic).
• Mole salamander has both aquatic paedomorphic adults and terrestrial metamorphic adults in the same population.
• Sequential hermaphroditism: Change in sex during the course of the life cycle.
• Common in fish and invertebrates.
• The timing should take advantage of the high reproductive potential of different sexes at different sizes.