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Lecture 6

Biology 2483A Lecture Notes - Lecture 6: Limiting Factor, Backcrossing, Intraspecific Competition

Course Code
BIOL 2483A
Hugh Henry

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Ecology Lecture 6:
Life History
Life History definition: An organisms life history is a record of 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.
Nemo Grows Up: A Case Study
In real life, two to six clownfish spend their entire
adult lives within one sea anemone, but are not
usually related.
The largest fish is a female; the next largest is the
breeding male. The remaining fish are immature
There is a strict pecking order in the group, 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 the clownfish maintain the hierarchy?
They are completely dependant on protection by the sea anemone. They are
easy prey outside of the anemone
Conflicts result in expulsion and death, probably without having reproduced
So 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
Life History Diversity
Individuals within a species show variation in life history traits due to
genetic variation or e nvironmental conditions
The life history strategy of a species is the overall pattern in average timing
and nature of life history events
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Some life history traits are determined genetically
Natural selection favours individuals whose life history traits result in their
having a better chance of surviving and reproducing
Ideal or optimal life histories maximize fitness (genetic contribution to future
But none are perfect, all organisms face constraints and ecological tradeoffs
Phenotypic plasticity: One genotype may produce different phenotypes under
different environmental conditions
Ex. Growth and development may be faster in higher temperatures
Changes in life history traits can cause change in adult morphology
Morphs: phenotypic plasticity can result in a discrete range of sizes
It can also result in a continuous range of sizes
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Polyphenism: a single genotype produces several distinct morphs
Ex. Spadefoot toad tadpoles have small omnivore morphs and larger
carnivore morphs
Carnivore tadpoles grow faster and metamorphose earlier, they are favoured
in ephemeral ponds that dry up quickly
Omnivores grow more slowly and are favoured in ponds that last longer, the
metamorphose in a more favourable condition and have more chance of
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 and proportion
Modes of reproduction:
Asexual reproduction: Simple cell division (binary fission)
All prokaryotes and many protists
Some multicellular organisms reproduce both sexually and asexually
Ex. Coral
Sexual reproduction:
Benefits: Recombination promotes genetic
variation and increased ability to respond to
environmental challenges
Disadvantages: An individual transmits only
half of its genome to the next generation;
population growth rate is slower
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