March 20
Nemo Grows Up: A Case Study
Interspecies Relationships: In the film Finding Nemo, the family structure is human-like. In reality, typically to clownfish spend their entire adult lives within a single sea anemone. Despite living together, they are usually unrelated.
Social Hierarchy and Sex Determination: - The largest fish in the anemone is the female. - The second largest fish is the breeding male. - All remaining fish in the group are immature nonbreeders. - Sequential Hermaphroditism: This refers to changes in sex during the course of a life cycle. All clownfish are born male. If the dominant female dies, the breeding male undergoes a sex change to become female, and the next largest fish becomes the new breeding male.
Mechanics of Sequential Hermaphroditism: - The timing of sex change is intended to take advantage of the high reproductive potential of different sexes at different sizes. - Larger fish are capable of producing more eggs. - Smaller fish can produce sperm, which are characterized as "Cheap Sperms" because they are smaller and require fewer resources to produce compared to "Costly Eggs."
Maintaining Size Hierarchy: - Hierarchy is maintained by the regulation of growth rates (Buston 2003). - If two fish become too similar in size, a fight occurs, and one fish is expelled from the sea anemone. - Experimental Evidence: Removal of the breeding male resulted in the growth of the next largest male, but this growth was limited—the fish grew only until it could function as the breeding male, purposefully avoiding becoming large enough to threaten the female.
Connections in Nature: - Territoriality and Competition: Clownfish are completely dependent on the sea anemone for protection. Conflict leads to expulsion and likely death before reproduction; hence, there is strong selection pressure to avoid conflict. - Evolutionary Drivers: Individuals that avoid growing to a size that triggers conflict are more likely to survive. Staying in an anemone and "biding time" offers a higher chance of reproductive success than seeking a new anemone. - Settlement Lotteries: This term describes competition for space in crowded habitats where resources like sea anemones are scarce. Juveniles must find an anemone with space to enter the hierarchy. This "lottery" dynamic (where only a few "win" access) is crucial for maintaining species diversity in competitive environments.
Life History and Reproductive Diversity
C. elegans (Nematodes): - Populations consist of males and hermaphrodites. - Hermaphrodites reproduce via self-fertilization ("selfing") or mating with males ("outcrossing"). - Pathogen Experiment: When exposed to a bacterial pathogen, outcrossing rates in wild-type populations increased. Typical outcrossing rates in wild-types range from to . - Genetic Manipulations: Laboratory strains were created as either obligate outcrossers or obligate selfers. - Survival Results: Obligate selfers were consistently driven to extinction by pathogens. Wild-type and obligate outcrossers persisted because their offspring possessed greater genetic variation, demonstrating the benefits of sex in challenging environments.
Coral Life Cycles: - Multicellular organisms like corals use both sexual and asexual reproduction. - Reef-forming colonies grow through asexual reproduction before producing eggs and sperm. The sexually produced offspring then establish new colonies.
Parthenogenesis: - Defined as "Virgin Birth," this is a rare method of asexual reproduction where a female produces an embryo without sperm fertilization (e.g., the Asian water dragon). - It is rare in vertebrates. Factors influencing this include time, energy, presence of males, and food availability.
Darwinian Fitness and Mating Strategies
Darwinian Fitness: Defined as the number of offspring an organism leaves behind which themselves survive to reproduce. If an organism is a physically superior specimen but its young do not survive, its fitness is mathematically zero.
Alternative Mating Strategies: - Dung Beetles: Large "guarding" males fight for female access, while small "sneaking" males use agility and deception to mate. - Coho Salmon: Exhibit dimorphic males: - Hooknose male: Large, competitive fighters. - Jack male (Sneaker): Smaller males that use a sneaking strategy. - Biological Perspective: There are no moral judgments in these strategies; they are simply biological adaptations for reproduction.
Life History Continua: r-selection vs. K-selection
r-selection (Selection for high population growth): - "r" stands for reproduction. - Advantaged in newly disturbed or uncrowded, low-quality, less competitive habitats. - Characteristics of r-selected species ("live fast, die young"): Short life spans, rapid development, early maturation, low parental investment, and high reproduction rates (e.g., insects, mice like the Mus musculus, weedy plants). - Mouse Statistics: Estrous cycle of to , reproduces , gestation of , litter size of to , and sexual maturity at to .
K-selection (Selection for lower growth rates): - Occurs in populations near "K," the Carrying Capacity. - Carrying Capacity (K): The maximum population size an environment can sustain given food, habitat, water, and resources. - Favored in crowded conditions where efficient reproduction is prioritized. - Characteristics of K-selected species ("slow and steady"): Long-lived, slow development, late maturation, high investment in offspring, and low reproduction rates (e.g., large mammals like elephants, reptiles, oak and maple trees). - Elephant Statistics: Estrous cycle of to , reproduces every to , gestation of to , per litter, raised for , and sexual maturity at .
Continuum Examples (Approximation of offspring per year): - Oyster: (r-extreme). - Fish (Tuna): . - Frog: . - Hare: . - Large Cat (Puma): . - Chimpanzee: (K-extreme). - Note: Most life histories exist between these two extremes.
Complex Life Cycles
Structure: Includes at least two stages with distinct body forms, habitats, and diets.
Metamorphosis: An abrupt transition between a larval stage and a juvenile stage.
Commonality: Frequent in insects, marine invertebrates, amphibians, and some fishes.
Examples: - Ant lion (Euroleon nostras): Larva is soil-dwelling. - Sea urchin (Paracentrotus lividus): Develops from a free-swimming larval stage into an adult.
Trade-offs in Reproduction
Size vs. Number of Offspring: The larger the investment in an individual offspring (energy, resources, time), the fewer offspring can be produced.
Clutch Size and Survival: Experiments on lesser black-backed gulls (Nager et al. 2000) showed that larger clutches (achieved by giving birds extra eggs) resulted in lower offspring survival due to reduced resources like food and parental care per chick.
Propagule Investment: In species without parental care, resources are invested in eggs, spores, or seeds (propagules). - In plants, seed size is negatively correlated with seed quantity.
Reproduction vs. Growth: In Douglas fir trees, the thickness of annual growth rings (indicating growth rate) declines when the tree produces many cones.
Western Fence Lizards (Sceloporus occidentalis) Case Study: - Different subspecies include S. o. becki (Island), S. o. biseriatus (San Joaquin), S. o. bocourtii (Coast Range), S. o. longipes (Great Basin), S. o. occidentalis (Northwestern), and S. o. taylori (Sierra). - Northern Populations: Have larger clutch sizes but smaller eggs. - Southern Populations: Have smaller clutch sizes but larger eggs. - Selection Drivers: - North: Shorter growing seasons and long winters favor early hatching (small eggs develop faster). - South: Higher predator density favors larger hatchlings with faster sprinting speeds.
Current vs. Future Reproduction: Resource allocation to current reproduction can decrease growth, survival, or future reproductive potential. Ricklefs (1977) noted a trade-off between annual fecundity and annual survivorship in birds—this principle also applies to humans.