7 Life History Chapter 7 Life History CONCEPT

7 Life History

Chapter 7 Life History CONCEPT 7. 1 Life history patterns vary within and among species. CONCEPT 7. 2 Reproductive patterns can be classified along several continua. CONCEPT 7. 3 There are trade-offs between life history traits. CONCEPT 7. 4 Organisms face different selection pressures at different life cycle stages.

Introduction 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

Concept 7. 1 Life history patterns vary within and among species.

Concept 7. 1 Life History Diversity 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. Such differences result from genetic variation, differences in environmental conditions, or both.

Figure 7. 3 Life History Strategy

Concept 7. 1 Life History Diversity Some life history traits are determined genetically. Ideal or optimal life histories maximize fitness (genetic contribution to future generations). But none are perfect; all organisms face constraints and ecological trade-offs.

Concept 7. 1 Life History Diversity Phenotypic plasticity: One genotype produces 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.

Figure 7. 4 Plasticity of Growth Form in Ponderosa Pines

Concept 7. 1 Life History Diversity Phenotypic plasticity may result in a continuous range of sizes; or discrete types called 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.

Figure 7. 5 Phenotypic Plasticity in Spadefoot Toad Tadpoles

Concept 7. 1 Life History Diversity 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). Most plants and animals and many fungi and protists reproduce sexually.

Figure 7. 6 Life Cycle of a Coral

Concept 7. 1 Life History Diversity Isogamy: Gametes are equal in size. Organisms such as the green alga Chlamydomonas reinhardii have two mating types that produce isogametes. Anisogamy: Gametes of different sizes. Usually the egg is much larger and contains nutritional material. Most multicellular organism produce anisogametes.

Figure 7. 7 Isogamy and Anisogamy

Concept 7. 1 Life History Diversity Sexual reproduction has disadvantages: • An individual transmits only half of its genome to the next generation. • Population growth rate is only half that of asexually reproducing species. • Recombination and chromosome assortment during meiosis can break up favorable gene combinations.

Figure 7. 8 The Cost of Sex

Concept 7. 1 Life History Diversity Benefits of sexual reproduction: Recombination promotes genetic variation and increased ability of populations to respond to environmental challenges. A test of this idea using the nematode worm Caenorhabditis elegans was done by Morran et al. (2011).

Figure 7. 9 Benefits of Sex in a Challenging Environment

Concept 7. 1 Life History Diversity Complex life cycles have at least two stages that have different body forms and live in different habitats and eat different foods. Metamorphosis: Abrupt transition in form between the larval and juvenile stages. Most vertebrates have simple life cycles. Complex life cycles are common in insects, marine invertebrates, amphibians, and some fishes.

Figure 7. 10 The Pervasiveness of Complex Life Cycles

Concept 7. 1 Life History Diversity Some species have direct development —the fertilized egg develops into a juvenile without passing through a larval stage. Many parasites have evolved complex life cycles with specialized stages for each host.

Figure 1. 3 The Life Cycle of Ribeiroia

Concept 7. 1 Life History Diversity Many plants, algae, and protists also have complex life cycles. Plants and most algae have alternation of generations in which a multicellular diploid sporophyte alternates with a multicellular haploid gametophyte.

Figure 7. 11 Alternation of Generations in a Fern

Concept 7. 2 Reproductive patterns can be classified along several continua.

Concept 7. 2 Life History Continua Number of reproductive events during the organism’s lifetime: Semelparous species reproduce only once. Iteroparous species can reproduce multiple times.

Concept 7. 2 Life History Continua Semelparous species include annual plants. Agave—vegetative growth can last up to 25 years; also produces clones asexually. Giant Pacific octopus—female lays one clutch of eggs and broods them for six months, dying after they hatch.

Figure 7. 12 Agave: A Semelparous Plant?

Concept 7. 2 Life History Continua Iteroparous species include: • Trees such as pines and spruces • Most large mammals

Concept 7. 2 Life History Continua 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: Selection for high population growth rates; an advantage in newly disturbed habitats and uncrowded condition.

Concept 7. 2 Life History Continua K is the carrying capacity for a population. K-selection: Selection for lower growth rates in populations that are at or near K; an advantage in crowded conditions; efficient reproduction is favored.

Concept 7. 2 Life History Continua r-selected (“live fast, die young”): • Short life spans, rapid development, early maturation, low parental investment, high reproduction rates • Most insects, small vertebrates such as mice, weedy plant species

Concept 7. 2 Life History Continua 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 longlived plants such as oak and maple trees

Concept 7. 2 Life History Continua Most life histories are intermediate between these extremes. Braby (2002) compared three Australian butterfly species. The one in drier, less predictable habitats had more rselected characteristics. The two species found in more predictable wet forest habitats had Kselected characteristics.

Concept 7. 2 Life History Continua 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.

Concept 7. 2 Life History Continua Four habitat types possible: • Low stress, low disturbance • High stress, low disturbance • Low stress, high disturbance • High stress, high disturbance—not suitable for plant growth

Figure 7. 13 Grime’s Triangular Model

Concept 7. 2 Life History Continua Charnov proposed a scheme that removes the influence of size and time. For example, age of maturity is positively correlated with life span in many species. To remove effect of lifespan, divide a species’ average of maturity by its average life span to get a dimensionless ratio (c).

Figure 7. 14 A Dimensionless Life History Analysis

Concept 7. 2 Life History Continua Dimensionless ratios allow comparisons of very different life histories. c differs between ectothermic and endothermic animals: it takes fish and lizards longer to mature than mammals and birds.

Concept 7. 2 Life History Continua Charnov’s scheme may be most useful when comparing life histories across a range of taxonomy or size. Grime’s scheme may be best for comparing plant taxa. The r–K continuum is useful in relating life histories to population growth characteristics.

Concept 7. 3 There are trade-offs between life history traits.

Concept 7. 3 Trade-Offs Trade-offs: Organisms allocate limited energy or resources to one function at the expense of another. Size and number of offspring: • More investment in each individual offspring, fewer offspring can be produced. • Investments: Energy, resources, time and loss of chances for other activities such as foraging.

Concept 7. 3 Trade-Offs 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.

Figure 7. 15 Clutch Size and Survival

Concept 7. 3 Trade-Offs 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. In plants, seed size is negatively correlated with the number of seeds produced.

Figure 7. 16 Seed Size–Seed Number Trade-Offs in Plants

Concept 7. 3 Trade-Offs 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.

Figure 7. 17 Egg Size–Egg Number Trade-Off in Fence Lizards

Concept 7. 3 Trade-Offs Allocating resources to reproduction can decrease an individual’s growth rate, survival rate, or potential for future reproduction. Male fruit flies spend more time and energy courting unmated females than recently mated females.

Figure 7. 18 Trade-Offs between Reproduction and Survival (Part 2)

Concept 7. 3 Trade-Offs Similar trade-offs have been observed in mollusks, insects, mammals (including humans), fishes, amphibians, reptiles, and plants. By allocating resources to reproduction instead of growth, an individual will reproduce at a smaller size, with fewer offspring. This might decrease an individual’s potential for future reproduction.

Concept 7. 4 Organisms face different selection pressures at different life cycle stages.

Concept 7. 4 Life Cycle Evolution As selection pressures change, different morphologies and behaviors are adaptive at different life cycle stages.

Concept 7. 4 Life Cycle Evolution Small size has benefits and drawbacks. Small early life stages can be vulnerable to predation and competition for food. But small size can allow early stages to do things that adult stages cannot. Organisms have various mechanisms to protect the small life stages.

Concept 7. 4 Life Cycle Evolution Parental Investment: • Provisioning eggs or embryos—yolk and protective coverings for eggs, nutrient-rich endosperm in plant seeds • Parental care—invest time and energy to feed and protect offspring

Figure 7. 20 Kiwi Parental Investment

Concept 7. 4 Life Cycle Evolution Dispersal and Dormancy: • Small offspring are well-suited for dispersal • Dispersal can reduce competition among close relatives and allow colonization of new areas • Dispersal can allow escape from areas with diseases or high predation

Concept 7. 4 Life Cycle Evolution Dispersal ability has evolutionary significance. Hansen (1978) found that fossil marine snail species with swimming larvae had larger geographic ranges and tended to go extinct less than species that developed directly into crawling juveniles.

Figure 7. 21 Developmental Mode and Species Longevity

Concept 7. 4 Life Cycle Evolution Dormancy: State of suspended growth and development in which an organism can survive unfavorable conditions. Small seeds, spores, eggs, and embryos are best suited to dormancy—less metabolic energy is needed to stay alive. But some larger animals also enter dormancy.

Concept 7. 4 Life Cycle Evolution Complex life cycles may result from stage-specific selection pressures and help minimize the drawbacks of small, vulnerable early stages. Separate life history stages can evolve independently in response to size- and habitat-specific selection pressures.

Concept 7. 4 Life Cycle Evolution 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.

Concept 7. 4 Life Cycle Evolution In marine invertebrates, larvae are specialized for both feeding and dispersal in ocean currents. Many larvae have specialized ciliated bands for feeding, covering most of the body. They may also have spines, bristles, or other structures to deter predators.

Figure 7. 22 Specialized Defensive Structures in Marin Invertebrate Larvae

Concept 7. 4 Life Cycle Evolution Even in species with gradual morphological change, individuals may have different ecological roles depending on size and age—niche shift. Ecological niche: The physical and biological conditions that an organism needs to grow, survive, and reproduce.

Concept 7. 4 Life Cycle Evolution If the larval habitat is very favorable, metamorphosis may be delayed or eliminated. Some salamanders mature sexually while retaining larval morphology and habitat (paedomorphic).

Figure 7. 23 Paedomorphosis in Salamanders

A Case Study Revisited: Nemo Grows Up Sequential hemaphroditism: Changes 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.

Figure 7. 24 Sequential Hermaphroditism

Connections in Nature: Territoriality, Competition, and Life History “Settlement lotteries” also affect other species that compete for space. Trees in tropical rain forests compete for space and sunlight. Success of a seedling may depend on chance events, such as death of a nearby tree that creates a gap in the canopy.