Rucete ✏ Campbell Biology In a Nutshell
Unit 8 ECOLOGY — Concept 53.4 Life History Traits Are Products of Natural Selection
Life history traits, such as age at first reproduction, reproductive frequency, and number of offspring, are shaped by natural selection. These traits reflect trade-offs between survival and reproduction, and they vary widely among species depending on environmental pressures.
1. Key Life History Traits
- Age at first reproduction
- Frequency of reproduction
- Number of offspring per episode
- Examples:
- Loggerhead turtles: reproduce at ~30 years, lay multiple clutches
- Coho salmon: reproduce once at 3–4 years, then die
2. Semelparity vs Iteroparity
- Semelparity ("one-shot" reproduction):
- Reproduce once, then die
- Seen in unpredictable environments (e.g., agave, coho salmon)
- Many offspring produced at once
- Iteroparity (repeated reproduction):
- Multiple reproductive events over time
- Seen in loggerhead turtles, oaks, horses
- Favored when offspring survival is likely and parental care is beneficial
3. Offspring Number and Parental Investment
- Trade-off: More offspring → less care per individual
- Plants in disturbed habitats (e.g., dandelions): many small seeds
- High offspring mortality favors quantity over quality
- Fewer offspring → more care per offspring
- Brazil nut tree: large nutrient-rich seeds
- Primates: 1–2 young, high parental investment
- Eurasian kestrel study: larger broods → lower parental survival, highlighting cost of reproduction
4. r-Selection vs K-Selection
- r-selection:
- Favors high reproductive rates
- Typical in uncrowded, variable environments (e.g., weeds)
- Maximizes population growth rate (r)
- K-selection:
- Favors competitive ability and efficiency
- Common in stable, crowded habitats (e.g., forests)
- Maximizes survival near carrying capacity (K)
- These are idealized extremes; most species fall on a spectrum between them
In a Nutshell
Life history traits evolve as adaptations to the environment, shaped by trade-offs between reproduction and survival. Some species reproduce once with many offspring, while others reproduce repeatedly with fewer, well-cared-for young. These patterns—r- and K-selection—reflect evolutionary strategies for thriving in different ecological conditions.