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Lecture 6

Biology 2483A Lecture Notes - Lecture 6: Polyphenism, Allometry, Semelparity And Iteroparity


Department
Biology
Course Code
BIOL 2483A
Professor
Lisa Henry
Lecture
6

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Lecture 6: Life History
Life history: record of an organism’s events relating to its growth, development,
reproduction, and survival
Life history characteristics include:
Age and size at sexual maturity
Amount and timing of reproduction
Survival and mortality rates
Case study: Nemo Grows Up
In real life, 2-6 clownfish spend their entire adult lives within one sea anemone,
but are not usually related
The largest fish is a female and the next largest is the breeding male
The remaining fish are immature nonbreeders
There is a strict pecking order in the group that is based on body size
If the female dies, the breeding male becomes a female and the next largest fish
becomes the breeding male
Hatchlings move out of the anemone, and juveniles must find a new anemone to
inhabit
Why do clownfish maintain the hierarchy?
- They are completely dependent on protection by the sea anemone  they are
easy prey outside the anemone (they can’t swim that easily)
- Conflicts result in expulsion and death, probably without having reproduced
There is a strong selection pressure to avoid conflict  sea anemones are a scarce
resource for clownfish
Growth regulation mechanisms have evolved because individuals that avoid
growing to a size that necessitates conflict are more likely to survive and
reproduce
Life History Diversity
Individuals within a species show variation in life history traits due to genetic
variation or environmental conditions
Life history strategy: is the overall pattern in average timing and nature of life
history events of a species
- Due to genetic variation and environment they experience

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Natural selection favors individuals whose life history traits result in their having
a better change of surviving and reproducing
Ideal or optimal life histories maximize fitness  genetic contribution to future
generations
Changes in life history traits can cause change in adult morphology
- Ex. trees grown in deserts have much different trunk diameters from their
height
Phenotypic plasticity: one genotype may produce different phenotypes under
different environmental conditions
- Ex. growth and development may be faster in higher temperatures
Phenotypic plasticity may result in a continuous range of sizes, or discrete types
called morphs
Polyphenism: a single genotype produces several distinct morphs
- Spadefoot toad tadpoles have small omnivore morphs and larger carnivore
morphs
- Carnivore tadpoles grow faster and metamorphose earlier  they are favored in
ephemeral ponds that dry up quickly
- Omnivores grow more slowly and are favored in ponds that last longer  they
metamorphose in more favorable conditions and have more chance of survival
- Omnivore morph feed on bottom of pond on detritus and carnivore feed in
water on shrimp  two distinct phenotypes and nothing in between
- Different body morphology results from different growth rates of body parts
in both the Ponderosa pines and spadefoot toads
Allometry: different body parts grow at different rates, resulting in differences in
shape or proportion
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Modes of Reproduction
1) Asexual reproduction
Simple cell division (binary fission)
Used in all prokaryotes and many protists
Some multicellular organisms reproduce both sexually and asexually (ex. corals)
- All segments of coral’s life cycle are genetically related to the polyp
- Different parts are asexual vs. sexual
2) Sexual reproduction
Recombination promotes genetic variation and increased ability to respond to
environmental challenges
The Cost of Sex
3) Isogamy
Gametes are equal in size
Ex. the green algae Chlamydomonas reinhardii
4) Anisogramy
Gametes of different sizes
Usually the egg is much larger and contains nutritional material
Most multicellular organisms produce anisogametes
5) Direct development
In some species where the fertilized egg develops into a juvenile without passing
through a larval stage
Life Cycles
Most vertebrates have simple life cycles without abrupt transitions
Complex life cycles are common in insects, marine invertebrates, amphibians, and
in some plants, algae, protists and fishes
Complex life cycle: have at least two stages, with different body forms and that
live in different habitats
Metamorphosis: abrupt transition in form between the larval and juvenile stages
Classification of Life History Strategies
Based on number of reproductive events per lifetime:
Semelparous: species that reproduce only once
- Annual plants
- Agave: vegetative growth can last up to 25 years (but also produces cones
asexually)
Iteroparous: species that can reproduce multiple times
- Trees such as pines and spruces
- Most large mammals
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