45.2 Life Histories and Natural Selection
4 min read•june 14, 2024
Natural selection shapes patterns, favoring strategies that maximize . 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 (reproducing once) to (reproducing multiple times). The optimal strategy depends on factors like adult survival probability, offspring survival, and resource availability. Balancing and parental care is key to .
Life Histories and Natural Selection
Natural selection in life history patterns
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- 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
- 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)
- 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)
- drives the evolution of specific life history traits
Semelparity vs iteroparity strategies
- (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:
- Allows for a large investment in offspring when conditions are favorable
- Can be advantageous in unpredictable environments where adult survival is uncertain
- (also known as )
- Organisms reproduce multiple times throughout their lifetime
- Examples include most vertebrates (humans, birds), perennial plants (oak trees), and many invertebrates (lobsters)
- Advantages:
- Spreads the risk of reproductive failure over multiple attempts
- 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
- 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 ( vs 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)
- r-selected species prioritize high fecundity and low parental care
- 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 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
- Reproductive success is a key component of evolutionary fitness, measured by the number of offspring that survive to reproduce
- plays a crucial role in life history strategies, as organisms must balance energy investment between growth, maintenance, and reproduction
Key Terms to Review (25)
Altricial: Altricial refers to a developmental strategy in which offspring are born in a relatively immature state, requiring significant parental care and nurturing to survive and thrive. This strategy is commonly observed in many bird species, where hatchlings are born blind, featherless, and dependent on their parents for food and protection. Altricial young often develop rapidly during their early days and weeks of life, benefiting from the care provided by their parents, which can influence survival rates and reproductive strategies in different environments.
Bet-hedging: Bet-hedging is a strategy used by organisms to maximize their chances of survival and reproduction in unpredictable environments by diversifying their life history traits. This approach allows species to spread the risk across different strategies, increasing the likelihood that at least some individuals will thrive despite variable conditions. Bet-hedging is a response to the uncertainties of environmental factors such as food availability, predation pressure, and climate change, enabling species to adapt more effectively over time.
Big bang reproduction: Big bang reproduction is a reproductive strategy characterized by a single, large reproductive event where an organism produces a massive number of offspring in a short time frame, often in response to specific environmental cues. This approach maximizes the chances of offspring survival by saturating the environment with young, which can overwhelm predators and ensure that at least some survive to maturity. It reflects an evolutionary strategy shaped by natural selection, particularly in unpredictable or variable environments.
Energy budget: An energy budget is the balance of energy intake and expenditure in an organism. It determines how organisms allocate energy to growth, reproduction, and maintenance activities.
Environmental Selection Pressure: Environmental selection pressure refers to the external factors that influence the survival and reproductive success of organisms within a given environment. These pressures can include factors like predation, competition for resources, climate, and disease, which shape the traits that become favorable or unfavorable in populations over time. As organisms adapt to these pressures, they may evolve specific traits that enhance their fitness in a particular ecological context.
Evolutionary (Darwinian) fitness: Evolutionary (Darwinian) fitness is the measure of an organism's ability to survive and reproduce in its environment. It directly correlates with the number of viable offspring an individual contributes to the next generation.
Evolutionary fitness: Evolutionary fitness refers to an organism's ability to survive and reproduce in its environment, contributing its genetic material to the next generation. It encompasses not just survival, but also the effectiveness of an organism in passing on its genes, which often varies based on environmental pressures. This concept is crucial for understanding how certain traits become more common in a population over time as those traits enhance reproductive success.
Fecundity: Fecundity is the reproductive capacity of an individual or population, often measured by the number of eggs or offspring produced. It plays a critical role in understanding population dynamics and evolutionary fitness.
Fecundity: Fecundity refers to the reproductive capacity of an individual or population, often measured by the number of offspring produced over a specific period. It plays a crucial role in understanding population dynamics and how different life history strategies can affect the survival and growth of populations. High fecundity can lead to rapid population growth, but it also involves trade-offs with other life history traits such as longevity and parental investment.
Fitness: Fitness refers to the ability of an organism to survive and reproduce in its environment. It is not just about physical strength or speed but encompasses a range of factors that contribute to reproductive success, such as the organism's behavior, health, and adaptability to environmental changes.
Iteroparity: Iteroparity is a reproductive strategy where organisms reproduce multiple times throughout their lifespan. This strategy contrasts with semelparity, where organisms reproduce only once before dying.
Iteroparity: Iteroparity is a reproductive strategy characterized by the ability of an organism to reproduce multiple times throughout its life cycle. This strategy is often advantageous in stable environments where the likelihood of offspring survival increases with repeated breeding opportunities. It contrasts with semelparity, where organisms reproduce only once before death, highlighting how different life strategies can affect population dynamics and evolutionary success.
K-selection: K-selection refers to a reproductive strategy where organisms produce fewer offspring but invest more time and resources into each one, enhancing their chances of survival in stable environments. This strategy is associated with species that thrive in competitive conditions, where the focus is on quality rather than quantity of offspring. K-selected species often exhibit longer lifespans, delayed reproduction, and parental care, which increases the likelihood of offspring reaching maturity.
Life history: Life history describes the series of events and strategies an organism undergoes from birth to death, including growth, reproduction, and survival. It is shaped by evolutionary pressures to maximize fitness in a given environment.
Life history strategy: Life history strategy refers to the evolutionary patterns and adaptations organisms develop regarding their growth, reproduction, and survival. These strategies involve trade-offs that determine how organisms allocate resources over their lifetimes, influencing population dynamics and species interactions within ecosystems. Understanding these strategies helps in analyzing population demography and the role of natural selection in shaping the life cycles of various species.
Life history traits: Life history traits refer to the biological characteristics and strategies that organisms exhibit throughout their life cycle, influencing their reproductive success and survival. These traits include aspects such as age at first reproduction, number of offspring, frequency of reproduction, and lifespan. Understanding these traits is essential in the context of natural selection as they directly impact an organism's fitness in its environment.
Parental Investment: Parental investment refers to the time, energy, resources, and care that parents provide to their offspring to enhance their chances of survival and reproductive success. This concept plays a crucial role in shaping life histories, as the level and type of investment can influence not only the growth and development of young but also the evolutionary strategies that different species adopt in response to environmental pressures and reproductive opportunities.
Precocial: Precocial refers to a type of animal development where offspring are relatively mature and mobile from the moment of birth or hatching. This characteristic is particularly notable in certain bird species, where chicks are capable of walking, swimming, and foraging for food shortly after emerging from the egg, which significantly influences their survival strategies and parental care dynamics.
R-selection: r-selection refers to a reproductive strategy where organisms produce a large number of offspring, each with a low probability of survival to adulthood. This strategy is advantageous in unstable or unpredictable environments, allowing species to exploit available resources rapidly. Organisms that follow r-selection tend to prioritize quantity over quality in reproduction, which can lead to quick population growth during favorable conditions.
Repeated reproduction: Repeated reproduction refers to a reproductive strategy in which an organism produces multiple offspring throughout its lifespan, rather than investing all reproductive efforts in a single reproductive event. This strategy allows species to adapt to changing environments and maximize reproductive success over time, influencing their evolutionary fitness and life history traits.
Reproductive success: Reproductive success refers to the passing of genes to the next generation in a way that leads to an increase in offspring that survive to reproduce themselves. This concept is crucial in understanding how natural selection operates, as individuals that are more successful in reproducing pass on their genetic traits, influencing the evolutionary trajectory of a population over time.
Resource Allocation: Resource allocation refers to the distribution of resources among various competing needs, focusing on how organisms manage their energy, time, and materials to maximize reproductive success and survival. This concept is crucial in understanding life histories, as different strategies for resource allocation can lead to varying reproductive outputs, growth rates, and overall fitness in natural selection scenarios.
Semelparity: Semelparity is a reproductive strategy where an organism reproduces only once in its lifetime, often in a single, large reproductive event. After reproduction, the organism typically dies, investing all its energy into the single reproductive effort.
Semelparity: Semelparity is a reproductive strategy where an organism reproduces only once in its lifetime, often producing a large number of offspring at that single event. This strategy is typically seen in species that face high mortality rates or environmental unpredictability, allowing them to invest all their resources into a single reproductive effort to maximize their chances of offspring survival. Semelparity contrasts with iteroparity, where organisms reproduce multiple times throughout their lives.
Trade-off analysis: Trade-off analysis refers to the evaluation of the benefits and costs associated with different life history strategies in organisms, particularly in the context of natural selection. It involves understanding how organisms allocate resources to growth, reproduction, and survival, recognizing that increasing one may often come at the expense of another. This analysis helps explain variations in life history traits across species, as well as their adaptive significance in response to environmental pressures.