AP Syllabus focus:
‘Organisms use various reproductive strategies, sometimes switching between asexual and sexual reproduction as energy availability changes.’
Reproduction is energetically expensive, so organisms evolve strategies that match when and how they reproduce to energy supply. These strategies influence survival, number of offspring, and genetic diversity, and can shift when resources fluctuate.
Core idea: energy as a limiting resource for reproduction
Energy captured or consumed must be allocated among maintenance, growth, storage, and reproduction. When energy is limited, organisms face trade-offs: investing more in reproduction usually reduces investment in survival or future reproduction.
Life history strategy: An organism’s evolved pattern of allocating energy to growth, survival, and reproduction across its lifetime (e.g., timing of maturity, number/size of offspring, parental care).
Reproductive output is therefore shaped by:
Energy availability (food, light for photosynthesizers, nutrient supply)
Energetic costs of producing gametes, mating, pregnancy/seed production, and parental care
Environmental predictability (stable vs highly variable resources)
Energy-driven trade-offs in reproductive strategies
Quantity vs quality of offspring
With abundant energy, some organisms increase fecundity (number of offspring). Under energy limitation, selection may favor fewer offspring with higher survival probability.
High-energy conditions may support:
More eggs/seeds/young
Longer breeding seasons or more frequent reproductive events
Low-energy conditions may favor:
Fewer, larger offspring (greater energy per offspring)
Delayed reproduction until energy stores recover
Timing: when to reproduce
Reproduction often occurs when energy intake or stored reserves can support it.
Seasonal breeding aligns reproduction with predictable peaks in food or sunlight
Capital breeding uses stored energy (e.g., lipids) to reproduce when feeding is difficult
Income breeding relies on energy acquired during the reproductive period, so reproduction tracks current resource intake
Parental investment and care
Parental care increases offspring survival but raises energetic costs for parents.
When energy is plentiful, extended care (feeding, guarding, brooding) may be possible
When energy is scarce, organisms may reduce care, abandon broods, or skip breeding to preserve parental survival and future reproductive potential
Switching between asexual and sexual reproduction
A key syllabus emphasis is that some organisms switch reproductive mode as energy availability changes.
This diagram summarizes the cyclic parthenogenetic life cycle in Daphnia, highlighting transitions between a parthenogenetic (asexual) phase and a sexual phase. It also shows how sexual reproduction produces protected, dormant eggs (ephippia) that can survive unfavorable conditions and hatch when conditions improve. Use it to connect “environmental stress/low resources” with a switch toward sexual reproduction and resistant life stages. Source
Why asexual reproduction can be favored when energy is high
Asexual reproduction can be energetically efficient because it avoids costs of mate-finding and can rapidly increase numbers.
Advantages under high energy/resource availability:
Fast population increase when conditions support growth
Efficient conversion of energy into offspring
Preservation of successful genotypes in stable, favorable conditions
Common forms include budding, fragmentation, and many types of vegetative reproduction in plants
Why sexual reproduction can be favored when energy is low or conditions worsen
Sexual reproduction often costs more energy (gamete production, mating behaviors, meiosis), but it increases genetic variation, which can improve the chance that some offspring survive stressful or changing environments.
Benefits when energy is limited or conditions are unpredictable:
Increased variation may help offspring tolerate scarcity, disease, or shifting abiotic factors
Recombination can reduce the buildup of harmful mutations across generations
In many species, stress signals linked to low resources can trigger increased sexual reproduction or formation of resistant life stages (e.g., durable eggs or seeds)
How organisms “decide” to switch
Switching is regulated by physiological and environmental cues that correlate with energy status.
This figure depicts the annual aphid life cycle, contrasting repeated parthenogenetic generations (rapid clonal reproduction) with a seasonal shift to sexual reproduction. The labeled cycle links environmental conditions (e.g., seasonal change) to the production of sexual morphs and eggs that persist through harsh periods. It provides a concrete model for how external cues can reroute development and reproductive mode. Source
Internal cues:
Low energy reserves (reduced stored carbohydrates/fats)
Hormonal changes that suppress or activate reproductive pathways
External cues:
Reduced food availability, shorter day length, or nutrient limitation
Increased population density that lowers per-capita resource access
Ecological and evolutionary consequences
Energy-linked reproductive strategies influence:
Population persistence: skipping reproduction during shortages can prevent adult mortality and enable future recovery
Genetic diversity: increased sexual reproduction during stressful periods can raise variation in the next generation
Fitness: strategies that match reproduction to energy availability tend to yield higher lifetime reproductive success than strategies that reproduce at energetically unsustainable times
FAQ
Many animals use endocrine signals that integrate nutrition with reproduction (e.g., hormones reflecting fat stores and short-term feeding).
Low energy can suppress gonad-stimulating pathways, reducing ovulation/spermatogenesis and mating behaviour, while restored energy can reactivate them.
Not always.
Capital breeding helps when feeding is impossible during reproduction, but requires reliably accumulating stores beforehand.
Income breeding can be advantageous if small, frequent feeding opportunities occur during reproduction, even if overall resources fluctuate.
Costs can include producing specialised gametes, meiosis, finding mates, courtship displays, competition, and increased predation risk during mating.
These costs reduce the fraction of energy that can be converted directly into offspring numbers.
Plants may shift allocation among seed number, seed size, and protective structures (e.g., endosperm/seed coat), depending on resource supply.
Animals often trade off egg/offspring size with clutch/litter size, especially when parental provisioning is energy-limited.
Switching may be constrained by genetics, developmental timing, or absence of mates.
In some environments, maintaining a well-adapted genotype asexually can still outperform sexual reproduction if the stress is brief or the population is already highly suited to local conditions.
Practice Questions
Explain one way that low energy availability can change reproductive strategy in an organism. (2 marks)
States a valid change linked to low energy (e.g., fewer offspring, delayed breeding, reduced parental care, switch to sexual reproduction for variation) (1)
Explains it in terms of energy trade-off or improved survival/offspring survival under scarcity (1)
Some organisms can reproduce asexually when resources are abundant but increase sexual reproduction when resources become limited. Describe how energy availability can favour each mode of reproduction and explain one advantage of switching for long-term reproductive success. (5 marks)
Asexual favoured when energy/resources abundant because it is faster and/or avoids costs of finding mates/courtship (1)
Links high energy to rapid production of many offspring/fast population increase (1)
Sexual favoured when conditions worsen/energy limited because it increases genetic variation via recombination (1)
Explains why variation can improve survival in changing/stressful conditions (1)
States one advantage of switching for long-term success (e.g., maximise reproduction in good years but maintain adaptable offspring in bad years; reduces extinction risk) (1)
