READING NOTES - Chapter 7
Life History
Nemo Grows Up: A Case Study
A rhinoceros produces one calf that develops in her womb for 16–18 months and can walk well several days after birth, but requires more than a year of care before it becomes fully independent
In popular media, we humans often depict other animals as having family lives similar to ours
For example, in the animated film Finding Nemo, clownfish live in families with a mother, a father, and several young offspring
The clownfish that live in an anemone interact according to a strict pecking order that is based on size
The largest fish in the anemone is a female
The next fish in the hierarchy, the second largest, is the breeding male
The remaining fish are sexually immature nonbreeders
If the female dies, as in Nemo’s story, the breeding male undergoes a growth spurt and changes sex to become a female, and the largest nonbreeder increases in size and becomes the new breeding male
The breeding male clownfish mates with the female and cares for the fertilized eggs until they hatch
The hatchling fish leave the anemone to live in the open ocean, away from the predator-infested reef
The young fish eventually return to the reef and develop into juveniles. Then they must find an anemone to inhabit
When a juvenile fish enters an anemone, the resident fish allow it to stay there only if there is room
If there is no room, the young fish is expelled and returns to the dangers of an exposed existence on the reef
This life cycle, with its expulsions, hierarchies, and sex changes, is certainly as colorful as the fish that live it
Introduction
Your personal history might consist of a series of details about the course of your life:
Your birth weight
When you started walking and talking
Your adult height
Other relevant information about your development
Similarly, an individual organism’s life history consists of major events related to its growth, development, reproduction, and survival
Life History Diversity
Studying variation in life history traits and analyzing the causes of that variation helps us to understand how life history traits interact with the environment and influence the potential growth rate of populations
In order to understand such analyses, it is helpful first to examine some of the broad life history patterns found within and among species
Individuals Within Species Differ In Their Life Histories
Some members of your social group reached developmental milestones such as puberty earlier or later than others
Different women may have different numbers of children with different age gaps between them
Despite this variation, it is possible to make some generalizations about life histories in Homo sapiens
The life history strategy of a species is the overall pattern in the timing and nature of life history events averaged across all the individuals in the species
The life history strategy is shaped by the way the organism divides its energy and resources between growth, reproduction, and survival, and the associated trade-offs with the allocation patterns
Genetic Differences
Much of life history analysis is concerned with explaining how and why life history patterns have evolved to their present states
Life histories are believed to be adapted to maximize fitness
The genetic contribution of an organism’s descendants to future generations, determined both by the reproductive rate of the parent and the survival rates of both parent and offspring
Thus, although life histories often serve organisms well in the environments in which they have evolved, they are optimal only in the sense of maximizing fitness subject to constraints
Environmental Differences
A single genotype may produce different phenotypes under different environmental conditions, a phenomenon known as phenotypic plasticity
Changes in life history traits often translate into changes in adult morphology
Allocation describes the relative amounts of energy or resources that an organism devotes to different functions
The result of allocation differences in ponderosa pines is that trees grown in different environments differ in adult shape and size
Phenotypic plasticity that responds to temperature variation often produces a continuous range of sizes
In other types of phenotypic plasticity, a single genotype produces discrete types, or morphs, with few or no intermediate forms
The more slowly growing omnivores are favored in ponds that persist longer, because they metamorphose in better condition and thus have better chances of survival as juvenile toads
Mode of Reproduction is a Basic Life History Trait
At the most basic level, evolutionary success is determined by successful reproduction
Despite this universal reality, organisms have evolved vastly different mechanisms for reproducing—from simple asexual splitting to complex mating rituals and intricate pollination systems
Asexual Reproduction
The first organisms to evolve on Earth reproduced asexually by binary fission (parental cell divides to produce two cells)
The sexual reproductive processes of meiosis, recombination, and fertilization arose later
Today, all prokaryotes and many protists reproduce asexually
While sexual reproduction is the norm in multicellular organisms, many can also reproduce asexually
Sexual Reproduction and Anisogamy
Most plants and animals reproduce sexually, as do many fungi and protists
Some protists, such as the green alga Chlamydomonas reinhardtii, have two different mating types, analogous to males and females except that their gametes are the same size
The production of equal-sized gametes is called isogamy
In most multicellular organisms, however, the two types of gametes are different sizes, a condition called anisogamy
Although sexual reproduction is widespread, it has some disadvantages
Because meiosis produces haploid gametes that contain half the genetic content of the parent, a sexually reproducing organism can transmit only half of its genetic material to each offspring, whereas asexual reproduction allows transmission of the entire genome
Another disadvantage of sex is that recombination and the independent distribution of chromosomes into gametes (during meiosis) can disrupt favorable gene combinations, potentially reducing offspring fitness
Finally, the growth rate of sexually reproducing populations is only half that of asexually reproducing ones, all else being equal
Sex has some clear benefits, including recombination, which promotes genetic variation and hence may increase the capacity of populations to evolve in response to environmental challenges such as drought or disease
The offspring of obligate selfers are very similar genetically to their parents, whereas the offspring of obligate outcrossers are more variable genetically; thus, these strains are well suited for testing the idea that sex is beneficial because it promotes increased levels of genetic variation
Life Cycles Are Often Complex
Complex life cycles can also lower competition between individuals of the same species, since species at different ages use of different resources
A complex life cycle is one in which there are at least two distinct stages that differ in their habitat, physiology, or morphology
In many cases, the transitions between stages in complex life cycles are abrupt
For example, many organisms undergo metamorphosis, an abrupt transition in form from the larval to the juvenile stage that is sometimes accompanied by a change in habitat
Some algae and all plants have complex life cycles in which a multicellular diploid sporophyte alternates with a multicellular haploid gametophyte
The sporophyte produces haploid spores that disperse and grow into gametophytes, and the gametophyte produces haploid gametes that combine in fertilization to form zygotes that grow into sporophytes
This type of life cycle, called alternation of generations, has been elaborated on in different plant and algal groups
Over the course of evolution, complex life cycles have been lost in some species that are members of groups in which such cycles are considered the ancestral condition
The resulting simple life cycles are sometimes referred to as direct development because development from fertilized egg to juvenile occurs within the egg prior to hatching and no free-living larval stage occurs
Trade-Offs
Organisms have limited amounts of energy and resources that can be invested in growth, reproduction, and defense
There is a Trade-Off Between Number and Size of Offspring
Many organisms show a trade-off between their investment in each individual offspring and the number of offspring they produce
Investment in offspring includes energy, resources, and time
In many cases, organisms that make a large investment in each offspring produce small numbers of large offspring, while organisms that make a small investment in each offspring produce large numbers of small offspring
Lack Clutch Size
The term “Lack clutch size” refers to the maximum number of offspring that a parent can successfully raise to maturity
Lack hypothesized that as a result of natural selection from the trade-off between numbers versus resource provisioning of young birds, the most productive clutch size would be found in natural populations
This hypothesis has generally been supported using experiments that manipulate the number of eggs in nests, and examining whether there are costs to unusually large clutch sizes
Trade-Offs in Organisms Without Parental Care
Parental care like that provided by birds and some other vertebrates is relatively rare
In organisms that do not provide parental care, resources invested in propagules (such as eggs, spores, or seeds) are the main measure of reproductive investment
In order to determine the consequences of egg size for offspring performance, Sinervo raised fence lizard eggs in the laboratory
He artificially reduced the size of some of the eggs by using a syringe to remove some yolk from them
To control for any possible effects of this method on egg development, he inserted a syringe into some other eggs but did not remove any yolk
Sinervo found that the reduced eggs developed faster than the unmanipulated eggs but produced smaller hatchlings
These small hatchlings grew faster than their larger siblings, but they were not able to sprint as fast to escape from predators
There Are Trade-Offs Between Current Reproduction and Other Life History Traits
As we’ve seen, when parents produce more offspring, their investment per offspring may decline
Such a decline can have various effects on the offspring, including reduced survival (as in lesser black-backed gulls) and reduced size (as in western fence lizards)
The allocation of resources to reproduction can also affect the parent by decreasing an individual’s growth rate, its survival rate, and its potential for future reproduction
Similarly, evidence for a trade-off between current reproduction and growth has been found in mollusks, insects, mammals (including humans), fishes, amphibians, and reptiles
Life Cycle Evolution
Each life cycle stage may have different habitat preferences, food preferences, and vulnerability to predation
These differences suggest that different morphologies and behaviors are adaptive at different life cycle stages
Differences in selection pressures over the course of the life cycle are responsible for some of the most distinctive patterns in the life histories of organisms
Small Size Has Benefits and Drawbacks
Small, early life cycle stages can be particularly vulnerable to predation because there are many predators that are big enough to consume them
These vulnerabilities are typically counterbalanced by behavioral, morphological, and physiological adaptations
Furthermore, in some organisms, small, mobile early stages can perform essential functions that are not possible for large adult stages
Parental Investment
In many organisms, the parents’ main investment in their offspring is the provisioning of the eggs or embryos
Animals add yolk to their eggs, which helps their offspring survive and grow through the small, vulnerable stages of life
Another pattern common among invertebrates is investment in energetically expensive egg coverings that protect the offspring during development
Plants provision the fertilized embryos in their seeds with endosperm, nutrient-rich material that sustains the developing embryo and often the young seedling
Dispersal and Dormancy
Although small offspring are vulnerable to many hazards, they are also well suited for several important functions, including dispersal and dormancy
Dispersal—the movement of organisms or propagules from their birthplace—is a key feature in the life history of all organisms
Even in organisms such as plants, fungi, and many marine invertebrates that are sessile or move very little as adults, the life cycle typically includes a stage in which dispersal occurs
Small size also makes eggs and embryos well suited to dormancy, a state of suspended growth and development in which an organism can survive unfavorable conditions
Many seeds are capable of long periods of dormancy before germination, which in extreme cases can last up to thousands of years
Most bacteria and some protists and animals can also undergo various forms of dormancy
Complex Cycles May Result From Stage-Specific Selection Pressures
Organisms with complex life cycles have multiple life stages, each adapted to its habitat and habits
This flexibility may be one of the reasons that complex life cycles are so common in so many groups of organisms
Because separate life history stages can evolve independently in response to size- and habitat-specific selection pressures, complex life cycles can minimize the drawbacks of small, vulnerable early stages
Larval Function and Adaptation
Functional specialization of particular life stages is a defining feature of complex life cycles
Having multiple stages with largely independent morphological features can result in a pairing of particular functions with particular stages
Such a pairing can reduce some of the trade-offs that result from simultaneously optimizing multiple functions
Timing of Life Cycle Shifts
Most organisms with complex life cycles use different habitats and food resources at different life stages
Such shifts can occur abruptly, as in organisms that undergo metamorphosis, but they can also occur more gradually
In species in which an abrupt metamorphosis occurs at the transition between life cycle stages, the organism spends relatively little time in vulnerable stages that are intermediate between larva and adult
In theory, there should be an optimal time to undergo metamorphosis, or any niche shift, that maximizes survival over the course of the life cycle
Thus, we might expect a niche shift to occur when the organism reaches a size at which conditions are more favorable for its survival or growth in the adult habitat than in the larval habitat
These aquatic, gilled adults are referred to as paedomorphic, which means that they result from a delay of some developmental events (loss of gills, development of lungs) relative to sexual maturation
Both aquatic paedomorphic adults and terrestrial metamorphic adults can exist in the same population of mole salamanders
Some Species Reproduce Only Once, While Others Reproduce Multiple Times
An important life history trait influencing reproductive success is the number of reproductive events they have during their lifetime
Semelparous species (also known as monocarpic in plants) reproduce only once in a lifetime, whereas iteroparous species (also known as polycarpic in plants) have the capacity for multiple bouts of reproduction
Many plant species complete their life cycle in a single year or less
Known as annual plants, such species are semelparous:
After one season of growth, they reproduce once and die
Theoretically, semelparous organisms gain an advantage in total lifetime reproductive output due to the conditions affecting the trade-off between reproduction and survival
Iteroparous organisms engage in multiple bouts of reproduction over the course of a lifetime
Life History Continua
Ecologists have proposed several classification schemes for organizing patterns of life history traits in relation to the environment
Most of these schemes make broad generalizations about associated life history traits and attempt to place these associations along continua between two extremes that are shaped by the ecological conditions that influence mortality rates and resource availabilities
Live Fast and Die Young, or Slow and Steady Wins The Race?
One of the best-known schemes for classifying life history diversity was also one of the first proposed
In 1967, Robert MacArthur and Edward O. Wilson coined the terms r-selection and K-selection to describe two ends of a continuum of life history patterns
The term r-selection refers to selection for high population growth rates
This type of selection can occur in environments where population density is low
In contrast, K-selection refers to selection for slower rates of increase, which occurs in populations that are at or approaching K, the carrying capacity or stable population size for the environment in which they live
One way to think of the r–K continuum is as a spectrum of population growth rates, from fast to slow
Organisms at the r-selected end of the continuum are often small and have short life spans, rapid development, early maturation, low parental investment, and high rates of reproduction
Plant Life Histories Can Be Classified Based on Habitat Characteristics
The success of a plant species in a given habitat, he argued, is limited by two factors:
Stress
Disturbance
Grime defined stress broadly as any external abiotic factor that limits vegetative growth
If we consider that in a given habitat, stress and disturbance may each be either high or low, then there are four possible habitat types:
High stress–high disturbance
Low stress–high disturbance
Low stress–low disturbance
High stress–low disturbance
Grime developed a model for understanding the three plant life history patterns that correspond to these three habitat types:
Competitive (low stress–low disturbance)
ruderal (low stress–high disturbance)
Stress-tolerant (high stress–low disturbance)
Under conditions of low stress and low disturbance, competitive plants that are superior in their ability to acquire light, minerals, water, and space should have a selective advantage
Grime classified plants that are adapted to habitats with high levels of disturbance and low levels of stress as ruderals
The ruderal strategy generally includes short life spans, rapid growth rates, heavy investment in seed production, and seeds that can survive in the ground for long periods until conditions are right for rapid germination and growth
Although stressful conditions may vary widely across habitats, Grime identified several features of stress-tolerant plants, including but not limited to slow growth rates, evergreen foliage, slow rates of water and nutrient use, high investment into defense from herbivores, and an ability to respond effectively to temporarily favorable conditions
Life Histories Can Be Classified Independent of Size and Time
By removing the effects of variables such as size or (in our case) time, a dimensionless ratio allows ecologists to compare the life histories of very different organisms
Charnov and Berrigan compiled data for a wide range of bird, mammal, lizard, and fish species
To remove the effects of life span, they focused their analyses on the age of maturity:
Life span dimensionless ratio, which they denoted c
While this dimensionless approach has some advantages over classification schemes that incorporate time and size, it also has potential disadvantages
Indeed, an emphasis on constant or “invariant” dimensionless life history parameters has been questioned by Nee et al. (2005), who argue that life history parameters can appear to be invariant simply as an artifact of the mathematical methods used to estimate them