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 change 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; 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.
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 condition.
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 region.s
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 short life span, rapid growth rates, heavy investment in seed production.
Can exploit habitats after disturbance has removed competitors.
Seeds can survive a long time until conditions are right for rapid germination and growth.
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).
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 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.
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.
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 have both aquatic paedomorphic adults and terrestrial metamorphic adults in the same population.
Sequential hemaphroditism: Change in sex during the course of the life cycle.
Common in fish and invertebrates.
The timing should take advantage of high reproductive potential of different sexes at different sizes.