1. Life History

2. CONCEPT 7.1 Life history patterns vary within and among species.
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.

3. Life history characteristics include: Age and size at sexual maturity
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

4. Concept 7.1
Life history patterns vary within and among species.

5. 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.

6. Figure 7.3 Life History Strategy
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7. 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.

8. 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.

9. Figure 7.4 Plasticity of Growth Form in Ponderosa Pines
Ponderosa pines in cool, moist climates allocate more biomass to leaves relative to sapwood than pines in warmer desert climates, resulting in different tree shapes.
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10. 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.

11. Figure 7.5 Phenotypic Plasticity in Spadefoot Toad Tadpoles
Carnivore tadpoles grow faster and metamorphose earlier. They are favored in ephemeral ponds that dry up quickly.
Omnivores grow more slowly and are favored in ponds that last longer; they metamorphose in more favorable conditions and have more chance of survival.
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12. 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.

13. Figure 7.6 Life Cycle of a Coral
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14. 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.

15. Figure 7.7 Isogamy and Anisogamy
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16. 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.

17. Figure 7.8 The Cost of Sex
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18. 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). 18

19. Figure 7.9 Benefits of Sex in a Challenging Environment
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20. 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.

21. Figure 7.10 The Pervasiveness of Complex Life Cycles
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22. 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.

23. Figure 1.3 The Life Cycle of Ribeiroia
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24. 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.

25. Figure 7.11 Alternation of Generations in a Fern
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26. Concept 7.2
Reproductive patterns can be classified along several continua.

27. 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.

28. 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.

29. Figure 7.12 Agave: A Semelparous Plant?
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30. Concept 7.2 Life History Continua
Iteroparous species include:
Trees such as pines and spruces
Most large mammals

31. 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.

32. 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.

33. 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

34. 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 long- lived plants such as oak and maple trees

35. 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 r- selected characteristics.
The two species found in more predictable wet forest habitats had K- selected characteristics.

36. 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.

37. 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

38. Figure 7.13 Grime’s Triangular Model
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 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.
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39. 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 age of maturity by its average life span to get a dimensionless ratio (c).

40. Figure 7.14 A Dimensionless Life History Analysis
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41. 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.

42. 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.

43. Concept 7.3
There are trade-offs between life history traits.

44. Size and number of offspring:
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.

45. 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.

46. Figure 7.15 Clutch Size and Survival
Experimental manipulation of clutch size in lesser black-backed gulls showed that in larger clutches, offspring have less chance of survival (Nager et al. 2000).
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47. Size of the propagule is a trade-off with the number produced.
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.

48. Figure 7.16 Seed Size–Seed Number Trade-Offs in Plants
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49. The size–number trade-off can also occur within species.
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.

50. Figure 7.17 Egg Size–Egg Number Trade-Off in Fence Lizards
Experiments by Sinervo (1990) on lizard eggs showed that smaller eggs developed faster and produced smaller hatchlings.
Small hatchlings grew faster, but were not able to sprint as fast to escape predators.
Selection may favor early hatching in northern populations because of shorter growing seasons.
Faster sprinting speed may be favored in southern populations where there are more predators.
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51. 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.

52. Figure 7.18 Trade-Offs between Reproduction and Survival (Part 2)
Farquhar (1981) showed that males kept with virgin females had a shorter life span than did males kept with inseminated females, showing a cost for sexual activity.
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53. 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.

54. Concept 7.4
Organisms face different selection pressures at different life cycle stages.

55. Concept 7.4 Life Cycle Evolution
As selection pressures change, different morphologies and behaviors are adaptive at different life cycle stages.

56. 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.

57. 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

58. Figure 7.20 Kiwi Parental Investment
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59. 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

60. 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.

61. Figure 7.21 Developmental Mode and Species Longevity
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62. 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.

63. 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.

64. 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.

65. 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.

66. Figure 7.22 Specialized Defensive Structures in Marin Invertebrate Larvae
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67. 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.
There should be an optimal time to undergo metamorphosis, or any niche shift, that maximizes survival.
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.

68. 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).
Mole salamanders have both aquatic paedomorphic adults and terrestrial metamorphic adults in the same population.
Frequency of paedomorphosis in these mixed populations depends on factors that influence survival and growth in the aquatic habitat.

69. Figure 7.23 Paedomorphosis in Salamanders
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70. 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.

71. Figure 7.24 Sequential Hermaphroditism
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72. 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.