ZOO*3700 Lecture 04 - Life History I

Overview

Alex Smith uses invertebrate examples to explain life-history evolution: natural selection shapes how organisms allocate finite time and energy among growth, survival, and reproduction. Cricket wing morphs, parasitic flatworms, echinoderm larvae, caddisflies, and Daphnia illustrate trade-offs, environmental effects, body-size patterns, and reproductive strategies, including alternating sexual and asexual reproduction.

Key takeaways

Life-history evolution is fundamentally about allocating a finite resource budget: greater reproductive investment can reduce resources for growth, maintenance, or energy storage.
Cricket wing polymorphism makes the dispersal–reproduction trade-off visible: long-winged females invest more in flight muscles, while short-winged females have comparatively larger ovaries.
Environmental pressures help shape life-history strategies: intense competition is associated with slower development and later reproduction, while high predation risk can favor rapid reproduction and larger litters.
Body size alone can misrepresent an organism’s ecological experience: a large marine adult may spend much of its life as a larva in a low-Reynolds-number, viscous environment.
Caddisfly experiments show that extra larval investment in rebuilding silk retreats can reduce adult dry thoracic mass, linking developmental costs to later traits.
Facultative parthenogenesis in Daphnia combines rapid asexual reproduction in favorable conditions with sexual ephippia production that helps populations persist through adverse conditions.

Chapters

0:00 Course Context: Case Studies, Plankton, and Life-History Evolution

Alex Smith explains that case-study answer keys are not released; students should compare reasoning with classmates before seeking one-on-one help.
The lecture connects last week’s plankton topics—vertical migration, transparency, bioluminescence, and dimethyl sulfide (DMS)—to life-history evolution.
The week’s themes are body size, lifespan, resource trade-offs, and invertebrate reproductive strategies.

5:12 Life-History Theory and the Finite Resource Budget

Life history is the pattern of growth, survival, and reproductive events across an organism’s lifespan.
Life-history theory predicts that ecology shapes correlations—and sometimes direct trade-offs—among traits affecting growth, survival, and reproduction.
Using Reznick’s resource-allocation framing, an organism divides a finite budget among reproduction, growth, maintenance, and energy storage.
Increasing investment in reproduction can leave fewer resources for growth, maintenance, or fat storage.

10:53 Cricket Wing Morphs Show Growth–Reproduction Trade-Offs

Long-winged and short-winged crickets are alternative morphs within the same species, not separate species.
Long-winged females invest more in flight muscles and dispersal capacity, while their ovaries are comparatively smaller.
Short-winged females have comparatively smaller flight muscles and larger ovaries, illustrating an allocation trade-off between dispersal-related investment and egg production.
The comparison is relative: neither morph lacks muscles or ovaries, but investment differs between strategies.

14:27 Slow and Fast Life Histories Reflect Ecological Pressures

Intense competition is associated with slower strategies: longer development, later maturity, smaller litters, larger body size, longer lifespan, and repeated reproduction.
High external mortality, such as predation, favors faster strategies: rapid development and reproduction, larger litters, and lower investment per offspring.
These patterns form a continuum rather than a strict two-category division; the Lord of the Rings comparison is offered as an optional analogy.
Life-history theory helps explain variation in body size, age at maturity, offspring number, and lifespan.

18:40 Body Size Links Host Biology, Parasites, and Larval Investment

Body size is useful across ecology and zoology because it is noticeable, relatively easy to measure, and connected to movement and environmental scale.
Parasitic flatworms generally become larger with host size; host diet, gut size, habitat, and metabolic state can also influence parasite size.
Endothermic hosts can provide a more consistent growth environment than ectothermic hosts; the observed pattern was described mainly as a change in intercept rather than slope.
Echinoderm larvae illustrate a size-related shift from planktotrophy, with larvae feeding in the plankton, toward lecithotrophy, with greater yolk or brooding investment.

30:12 Lifespan and Caddisfly Larvae Reveal Developmental Trade-Offs

Invertebrate lifespans range from hours or days to hundreds of years, making survival and age at maturity important life-history traits.
Caddisfly larvae build protective silk retreats incorporating substrate; experimentally forcing larvae to rebuild increases their metabolic investment.
The rebuilding treatment reduced adult dry thoracic mass, showing how larval resource expenditure can constrain later adult traits.
Because eggs generally require more resources than sperm, the lecture predicts that comparable larval costs could also reduce female reproductive investment.

36:29 Finite Time and Energy Constrain Reproductive Investment

Both resources and lifespan are finite, and biological processes consume time as well as energy.
As energy allocated to reproduction rises, energy available for growth tends to fall; producing more offspring also generally reduces investment per offspring.
Ecological roles can modify the pattern: carers and provisioners may face different effects on long-term fecundity.
Life-history traits reflect both intrinsic factors, such as physiology and development, and extrinsic factors, including ecology and age-specific mortality.

39:53 Marine Body-Size Data and the Reynolds-Number Scale

A large marine-organism body-size database compiles measurements such as length, width, and diameter across many taxa.
A rough size threshold separates organisms dominated by viscous forces at low Reynolds numbers from those more affected by inertial forces at larger Reynolds numbers.
Adult-size records can obscure early-life ecology: a large adult may spend much of its lifespan as a small larva in a different physical regime.
Logarithmic axes help display biological measurements spanning large size ranges; colonial sponges, cnidarians, and bryozoans also raise questions about colony versus individual size.

45:30 Asexual Reproduction: Parthenogenesis, Budding, and Fission

Invertebrate asexual reproduction includes parthenogenesis, which produces diploid eggs, and budding, as in Hydra.
Parthenogenesis can be obligate, as in some bdelloid rotifers where males have not been observed, or facultative, as in some Daphnia.
Fission occurs in soft-bodied animals such as flatworms and sponges; subdivided planarians can regenerate into multiple animals.
Colonial organisms complicate the definition of an individual, so bryozoans and similar taxa are often better understood as modular colonies than as isolated zooids.

48:31 Daphnia Alternate Asexual Growth with Sexual Resting Eggs

Under favorable conditions, Daphnia commonly reproduce asexually, with diploid eggs developing directly into daughters.
When conditions worsen—for example, with changes in temperature, light, productivity, or food availability—investment in males and sexual reproduction increases.
Sexually produced eggs form ephippia, which can persist or disperse via water, wind, or attachment to animals, helping Daphnia endure unfavorable conditions.
The lecture closes with course logistics: pre-Thanksgiving labs, including Dairy Bush sampling, move back one week; students should continue their R assignment and case-study preparation.

Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, Alex Smith.