Natural selection shapes life history patterns, favoring strategies that maximize fitness. Organisms evolve to balance growth, maintenance, and reproduction based on their environment. Trade-offs between competing demands are crucial as resources are limited.

Reproductive strategies vary widely, from semelparity (reproducing once) to iteroparity (reproducing multiple times). The optimal strategy depends on factors like adult survival probability, offspring survival, and resource availability. Balancing fecundity and parental care is key to reproductive success.

Life Histories and Natural Selection

Natural selection in life history patterns

• Natural selection favors life history patterns and reproductive strategies that maximize an individual's fitness
  • Fitness is the ability to survive and reproduce, passing on genes to future generations (Darwin's finches, antibiotic-resistant bacteria)
• Selection pressures shape life history traits
  • Age at first reproduction (early maturation in high-mortality environments)
  • Number and size of offspring (many small offspring in unpredictable environments, few large offspring in stable environments)
  • Parental investment in offspring care (high investment in K-selected species, low investment in r-selected species)
  • Lifespan and age-specific mortality rates (longer lifespans in low-mortality environments, shorter lifespans in high-mortality environments)
• Organisms evolve life history strategies that optimize the allocation of resources between growth, maintenance, and reproduction
  • Trade-offs exist between these competing demands as resources are limited (energy allocated to reproduction reduces energy available for growth and maintenance)
• Environmental factors influence the evolution of life history patterns
  • Stable environments may favor longer lifespans and slower reproduction (elephants, sequoia trees)
  • Unpredictable or harsh environments may favor faster maturation and higher reproductive output (annual plants, insects)
  • Environmental selection pressure drives the evolution of specific life history traits

Semelparity vs iteroparity strategies

• Semelparity (also known as "big bang" reproduction)
  • Organisms reproduce only once in their lifetime, investing heavily in a single reproductive event
  • Examples include annual plants (sunflowers), Pacific salmon, and many insects (mayflies)
  • Advantages:
    1. Allows for a large investment in offspring when conditions are favorable
    2. Can be advantageous in unpredictable environments where adult survival is uncertain
• Iteroparity (also known as repeated reproduction)
  • Organisms reproduce multiple times throughout their lifetime
  • Examples include most vertebrates (humans, birds), perennial plants (oak trees), and many invertebrates (lobsters)
  • Advantages:
    1. Spreads the risk of reproductive failure over multiple attempts
    2. Allows for adaptive adjustments to changing environmental conditions
• The evolution of semelparity or iteroparity depends on factors such as adult survival probability, offspring survival probability, and resource availability and predictability (desert plants tend towards semelparity, tropical plants towards iteroparity)

Tradeoffs in reproductive success

• Fecundity refers to the number of offspring produced per reproductive event
  • High fecundity often correlates with lower parental investment and smaller offspring size (cod, dandelions)
  • Low fecundity often correlates with higher parental investment and larger offspring size (whales, coconut palms)
• Parental care encompasses behaviors that increase the survival and fitness of offspring
  • Examples include provisioning (bird parents bringing food to chicks), protection (bear mothers defending cubs), and teaching of skills (meerkats showing pups how to hunt scorpions)
  • Parental care is costly in terms of time, energy, and resources diverted from other activities
• Offspring survival depends on factors such as:
  • Size and developmental stage at birth or hatching (precocial vs altricial young)
  • Parental care and protection (higher survival in species with intensive parental care)
  • Environmental conditions and resource availability (higher mortality in times of scarcity or harsh conditions)
• Species exhibit different strategies along the fecundity-parental care continuum
  • r-selected species prioritize high fecundity and low parental care
    • Adapted to unpredictable or ephemeral environments
    • Examples: many insects (flies), annual plants (weeds), and small mammals (mice)
  • K-selected species prioritize low fecundity and high parental care
    • Adapted to stable, competitive environments
    • Examples: many large mammals (gorillas), birds (eagles), and long-lived plants (redwoods)
• The optimal balance between fecundity and parental care depends on the specific ecological context and evolutionary history of each species (island species often evolve towards K-selection compared to mainland relatives)

Life History Strategies and Evolutionary Fitness

• Life history strategies are sets of evolved traits that influence an organism's schedule of reproduction and survival
• These strategies are shaped by natural selection to maximize evolutionary fitness
• Reproductive success is a key component of evolutionary fitness, measured by the number of offspring that survive to reproduce
• Resource allocation plays a crucial role in life history strategies, as organisms must balance energy investment between growth, maintenance, and reproduction