SUMMARY

CONCEPT 7.1 Life history patterns vary within and among species.

7.1.1 Summarize the key stages that make up the life history of an organism.

Life histories are diverse, varying between individuals of the same species as well as between species.

The source of this variation may be genetic or environmental.

7.1.2 Explain how genetics and the environment act as controls on life history traits.

Life histories are under genetic control and subject to natural selection. Environmental variation can also impact life histories through phenotypic plasticity.

7.1.3 Compare the benefits and costs associated with sexual versus asexual reproduction.

Organisms may reproduce sexually or asexually. In many cases, the same organism can do both.

There are advantages and disadvantages to sexual reproduction. The high levels of genetic variation resulting from sexual reproduction may be beneficial in challenging environments.

7.1.4 Describe how additional complexity in a life cycle, such as larval and adult forms, may benefit a species.

Most organisms have complex life cycles with multiple stages that differ in size, morphology, or habitat.

CONCEPT 7.2 There are trade-offs between life history traits.

7.2.1 Illustrate how the number of offspring may affect the size of those offspring.

There is a trade-off between offspring size and number, such that organisms tend to produce large numbers of relatively small offspring or small numbers of relatively large offspring.

7.2.2 Explain how providing care to offspring may compromise other functions in adults.

Allocation of time and energy into caring for offspring lowers the ability of the adult to forage and may increase the risk of disease and predation on the adult.

7.2.3 Understand that allocating energy and resources to reproduction may affect parental growth, survival, and future reproduction.

An individual’s investment in current reproduction can result in a trade-off between reproduction and other life history traits, including survival, growth, and potential for future reproduction.

CONCEPT 7.3 Organisms face different selection pressures at different life cycle stages.

7.3.1 Contrast the benefits and costs associated with small size

in early life cycle stages.

The small sizes of early life cycle stages make them vulnerable to predation and food shortages.

Small life cycle stages are well suited to some important functions, such as dispersal and dormancy.

7.3.2 Explain how adaptations at specific stages in a complex life cycle may benefit the species.

Complex life cycles allow life histories the flexibility to respond to differences in selection pressures on different life cycle stages.

7.3.3 Compare the benefits of semelparity and iteroparity in the context of total lifetime reproduction of an organism.

Semelparous species reproduce only once in a lifetime, while iteroparous species reproduce multiple times.

CONCEPT 7.4 Life history patterns can be classified along several continua.

7.4.1 Evaluate the environmental conditions that would favor the persistence of r-selected and К-selected species.

Environments subject to frequent disturbances and low population size favor species exhibiting r-selected traits. Where the environment is stable with population sizes near the carrying capacity, К-selected traits are favored.

7.4.2 Describe the trade-offs in plant allocation described in Grime's competitive/stress/ruderal model.

Grime's triangular model categorizes plant life histories by the degree of competition, stress, and disturbance in the habitat type to which they are adapted.

7.4.3 Show how differences in species size or age can be accounted for in describing the allocation of energy and resources to reproduction and other life history stages.

Charnov's dimensionless life history analysis attempts to remove the effects of size and time in order to compare life histories across a broad taxonomic range.

REVIEW QUESTIONS

Many closely related animal species produce eggs of vastly different sizes. As we saw in Concept 7.3, one trade-off of producing larger eggs is that fewer eggs can be produced. Despite the apparent simplicity of this trade-off, it is still unclear why both strategies (many small eggs and few large eggs) are maintained in groups of closely related species. What are some other life history traits besides offspring number that might be correlated with egg size, and under what environmental conditions might those traits be advantageous? Can you think of any reasons why species that live in the same habitats continue to exhibit reproductive patterns that vary so widely?

2. Some animals exhibit both sexual and asexual reproduction, depending on the environmental conditions they experience. Rotifers are a classic example of this phenomenon. Females can produce diploid eggs by mitosis that hatch soon after release. In this manner, rotifer populations can double within hours. These same females, under other conditions, can produce haploid eggs that form males if unfertilized and form females if fertilized. What might be the reason for the maintenance in rotifers of both sexual and asexual reproduction?

3. The Nassau grouper is popular in Asia, where restaurantgoers can pick the grouper they want steamed for dinner from a selection of live fish swimming in tanks. Adult groupers can grow to large sizes (up to 3 feet long and 55 pounds), but those favored by restaurants are plate-sized juvenile and young adult fish. How would you expect the removal of these younger, smaller fish to affect the life history evolution of the remaining population? How might life history parameters such as age and size at reproduction and investment in growth versus reproduction evolve in response to fishing pressures?

HONE YOUR PROBLEM-SOLVING SKILLS

California sheephead (Semicossyphus pulcher) are predatory fish that are born as females and change sex to males when they become large enough to defend territories from other males.

Researchers measured the size at which sheephead became sexually mature (“size at maturation”) and the size at which they changed sex from female to male (“size at sex change”) for fish collected at Santa Catalina and San Nicolas Islands (Hamilton and Caselle 2015). The results are shown in the table. Little fishing occurred at these islands from 1940 to 1980. At Catalina, sheephead have been subjected to intensive fishing pressure from the early 1980s to the present. At San Nicolas, sheephead were subjected to intensive fishing from the early 1980s to 1998 but protected from fishing from 1999 on.

1. Do the data indicate that fishing by humans affects the sizes at which these fish reach sexual maturity and change sex? Explain.

2. Do size at maturation and size at sex change recover once fishing pressure is reduced? Explain which data you used to answer this question, and why.

		Catalina		San Nicholas
	Year	Size at Six at sex		Size at Size at sex


	maturation (mm)	change (mm)	maturation (mm)		change (mm)
1980	213	350	298		493
1998	198	256	211		294
2007	178	225	300		460

3. Fishing not only reduces the abundance of the targeted species, but also tends to remove larger individuals at higher rates than smaller individuals, leading to changes such as those shown in the table. Suppose larger sheephead produce more offspring than do smaller individuals (as is true for many species). Predict how protection from fishing will affect the average number of offspring produced per female and the abundance of the population over time.

LIST OF KEY TERMS

Allocation alternation of generations anisogamy competitive plants complex life cycle direct development Dispersal fitness isogamy iteroparous

K-selection life history life history strategy metamorphosis morphs paedomorphic phenotypic plasticity r-selection

ruderals

Semelparous

sequential hermaphroditism

stress-tolerant plants

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Source: Bowman W., Hacker S.. Ecology. 6th ed. — Oxford University Press,2023. — 744 p.. 2023

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