4.1 Life History Strategies and Reproduction in Organisms

Life History: Reproduction

Overview of Life History Strategies

Life history strategies encompass the major events in an organism's life, including survival, growth, and reproduction. These strategies are shaped by evolutionary pressures and environmental conditions, resulting in a diversity of reproductive adaptations among organisms.

• Life history traits include age at first reproduction, number of reproductive events, number and size of offspring, and parental care.

• Ultimate goal: maximize survival and reproductive success.

Modes of Reproduction

Sexual vs. Asexual Reproduction

Organisms exhibit a range of reproductive strategies, from asexual to sexual reproduction, each with distinct advantages and disadvantages.

• Asexual reproduction: Offspring are genetic clones of the parent. Methods include binary fission (e.g., Escherichia coli), parthenogenesis (e.g., some lizards), and budding (e.g., corals).

• Sexual reproduction: Involves meiosis and fertilization, resulting in genetic recombination and increased genetic variation (e.g., bees, Daphnia, corals).

• Some species (e.g., Daphnia, corals) can alternate between sexual and asexual reproduction depending on environmental conditions.

• Example: Queen bees can produce female workers via fertilized eggs (sexual) or male drones via unfertilized eggs (asexual).

Advantages and Disadvantages

• Sexual reproduction increases genetic variation, which is beneficial in changing environments but has a 'cost of males'—only half the population (females) can produce offspring, slowing population growth.

• Asexual reproduction allows for rapid population growth (all individuals can reproduce), but results in less genetic variation and potential accumulation of harmful mutations.

• Sexual reproduction is favored in variable environments; asexual reproduction is favored in stable, well-adapted environments.

Case Study: Coral Bleaching

• Clonal populations (as in asexually reproducing corals) may be vulnerable to environmental changes, such as bleaching events, due to low genetic diversity.

• Example: A stand of Acropora coral in Florida was decimated by bleaching because it was genetically uniform.

Variation in Reproductive Strategies

Number of Reproductive Events: Semelparity vs. Iteroparity

Organisms differ in how many times they reproduce during their lifetime.

• Semelparity: Reproduce once and then die (e.g., salmon, century plant).

• Iteroparity: Reproduce multiple times over their lifespan (e.g., deer, humans).

• Tradeoff: If adult mortality is high, semelparity is favored; if low, iteroparity is favored.

• Some species blur the line (e.g., century plant can clone itself, resembling iteroparity).

Age at First Reproduction

The age at which organisms first reproduce varies and is often correlated with lifespan.

• Longer-lived species tend to reproduce at older ages.

• Human activities (e.g., fishing) can select for earlier reproduction, as seen in Atlantic cod, where fishing pressure led to a decrease in age at sexual maturity.

• Smaller, younger individuals often produce fewer offspring than larger, older individuals (e.g., vermillion rockfish).

Number and Size of Offspring

There is a tradeoff between the number of offspring produced and the investment in each offspring.

• Producing many small offspring increases the chance that some survive, but individual survival rates are lower (e.g., yellow tang fish).

• Producing fewer, larger offspring allows for greater parental investment and higher survival rates (e.g., bicolor damselfish, elephants).

• Graphical data (e.g., in seagulls) show that as clutch size increases, the percentage of chicks surviving to fledging decreases.

Parental Care

Parental care increases offspring survival but requires significant energy investment.

• Examples: Elephants (long gestation and care period), bicolor damselfish (egg care), rockfish (live birth).

• Species with parental care tend to produce fewer offspring with higher survival rates.

Sexual Selection and Reproductive Investment

Male vs. Female Reproductive Strategies

• Females typically invest more in reproduction (larger gametes, gestation, provisioning), making them choosier in mate selection.

• Male reproductive success is often limited by access to mates, leading to the evolution of traits that increase mating success.

Types of Sexual Selection

• Intra-sexual selection: Competition within one sex (usually males) for access to mates. Leads to size dimorphism (e.g., larger males in salmon, bighorn sheep, elephant seals, humans).

• Inter-sexual selection: Mate choice, typically by females, favoring traits that attract the opposite sex (e.g., elaborate plumage in birds, ornate fins in guppies, large horns in sheep).

Exceptions and Hermaphroditism

• In species where males invest equally in offspring (e.g., seahorses, penguins), female choosiness may be reduced.

• Hermaphroditism: Individuals possess both male and female reproductive organs (common in plants and some animals). Can be simultaneous or sequential.

• Sequential hermaphrodites: Change sex during their lifetime. Protandry: male to female (e.g., some groupers). Protogyny: female to male (e.g., wrasses).

Size Advantage Hypothesis

The size advantage hypothesis explains why sex change occurs in some species:

• If larger size increases reproductive success more in one sex, individuals may change sex to maximize fitness.

• Example: In some species, larger males have higher fertility (favoring protogyny); in others, larger females do (favoring protandry).

Life History Classification: r- and K-Selection

r-Selected vs. K-Selected Species

Life history traits can be classified along a continuum from r-selection to K-selection, based on population growth strategies.

• r-selected species: Maximize population growth rate (r). Traits include rapid development, early reproduction, small body size, semelparity, short lifespan, and low competitive ability. Common in variable environments.

• K-selected species: Maximize competitive ability near carrying capacity (K). Traits include slower development, later reproduction, larger body size, iteroparity, long lifespan, and high competitive ability. Common in stable environments.

• Most organisms exhibit a mix of r- and K-selected traits; these strategies exist on a continuum.

Comparison Table: r-Selected vs. K-Selected Species

Trait	r-Selected Species	K-Selected Species
Development Rate	Rapid	Slow
Reproductive Timing	Early	Late
Body Size at Reproduction	Small	Large
Number of Reproductive Events	Often Semelparous	Often Iteroparous
Lifespan	Short	Long
Competitive Ability	Low	High
Environmental Stability	Variable	Stable

Summary of Major Points

• Life history strategies are diverse and shaped by evolutionary tradeoffs between survival, growth, and reproduction.

• There is no universally superior strategy; the optimal strategy depends on environmental conditions and species-specific factors.

• Tradeoffs are inherent: investing in one trait often reduces investment in another.

Additional info: The concepts of r- and K-selection are foundational in ecology and population biology. The equations for population growth rate (r) and carrying capacity (K) are:

• Exponential growth: $\frac{dN}{dt} = rN$

• Logistic growth: $\frac{dN}{dt} = rN \left(1 - \frac{N}{K}\right)$