BIO120H1 Chapter Notes - Chapter 7: Enteroctopus Dofleini, Guppy, Invertebrate

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5 Feb 2013
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Chapter 7: Life History
NEMO GROWS UP: A CASE STUDY
The offspring produced by different organisms vary tremendously
Nemo’s father would have changed sex and become a female after Nemo loses his mother to a
predator
Clownfish spend their entire adult lives within a single sea anemone
In what appears to be a mutually beneficial relationship, the anemone protects the clownfish by
stinging their predators
The clownfish, in turn, may help the anemone by eating its parasites or driving away its predators
Two to six clownfish typically inhabit a single anemone
o The largest fish in the anemone is a female
o The second largest, is the breeding male
o The remaining fish are sexually immature nonbreeders
o If the female dies, as in Nemo’s story, the breeding male undergoes a growth spurt and
changes sex to become a female, and the largest nonbreeder increases in size and becomes
the new breeding male.
The hatchling fish leave the anemone to live in the open ocean, away from the predator-infested
reef
When a juvenile fish enters an anemone, the resident fish allow it to stay there only if there is
room. If there is no room, the young fish is expelled and returns to the dangers of an exposed
existence on the reef
Why do clownfish engage in these complicated machinations just to produce more clownfish?
These solutions are often well suited for meeting the challenges and constraints of the
environment where a species lives
INTRODUCTION
Human history is a record of past events
An individual organism’s life history is a record of major events related to its growth,
development, reproduction, and survival
The timing and nature of life history traits, and therefore the life history itself, are products of
adaptation to the environment in which the organism lives
Biologists analyze life history patterns in order to understand the trade-offs, constraints, and
selection pressures imposed on different stages of an organism’s life cycle
Concept 7.1 Life history Patterns Vary Within and Among Species
LIFE HISTORY PATTERNS VARY WITHIN AND AMONG SPECIES
The study of life histories is concerned with categorizing variation in life history traits and
analyzing the causes of that variation
INDIVIDUALS WITHIN SPECIES DIFFER IN THEIR LIFE HISTORIES
Individual differences in life history traits are ubiquitous
Despite this variation, it is possible to make some generalizations about life histories in Homo
sapiens:
- For example. Women typically have one baby at a time, reproduction usually occurs
between the ages of 15 and 45, and so on
Life History Strategy of a species is the overall pattern in the timing and nature of life history
events averaged across all the individuals in the species
- Is shaped by the way the organism divides its energy and resources between growth,
reproduction, and survival
- Within a species, individuals often differ in how they divide their energy and resources
among these activities. Such differences may result from genetic variation, from differences
in environmental conditions, or from a combination of both
- the life history strategy is determined by effects of natural selection, not the choices of
the individual organism
Genetic Differences Some life history variation within species is determined genetically
- Heritable variation in life history traits is the raw material on which natural selection acts
- Selection favors individuals whose life history traits result in their having a better chance of
surviving and reproducing than do individuals with other life history traits
Geologists sometimes describe life histories as optimal in that they are adapted to maximize
fitness
Fitness the genetic contribution of an organism’s descendants to future generations
No organism has a perfect life history-that is, one that results in the unlimited production of
descendants
All organisms face constraints that prevent the evolution of a perfect life history
Although life histories often serve organisms well in the environments in which they have evolved,
they are optimal only in the sense of maximizing fitness subject to constraints
Environmental Differences A single genotype may produce different phenotypes under different
environmental conditions, a phenomenon known as phenotypic
Plasticity
- Ex. Most plants and animals grow at different rates depending on temperature
Changes in life history traits often translate into changes in adult morphology
- Ponderosa pine trees in cool, moist climates allocate more re-sources to leaf production
than do trees in desert climates
- Desert trees are shorter than those grown in cooler climates, but for a given height,
they have thicker trunks
- They also have lower photosynthetic rates and consume less CO2 because they have
fewer leaves
Allocation describes the relative amounts of energy or resources that an organism devotes to
different functions
The result of allocation differences in ponderosa pines is that trees grown in different
environments differ in their adult shape and size
Phenotypic plasticity that responds to temperature variation often produces a continuous range of
sizes
In other types of phenotypic plasticity, a single genotype produces discrete types, or morphs,
with few or no intermediate forms
The differing body shapes of omnivores and carnivores result from differences in the relative
growth rates of different body parts: carnivores have bigger mouths and stronger jaw muscles
because of accelerated growth in those areas
Field studies show that the proportion of omnivore and carnivore morphs is affected by food
supply
The more slowly growing omnivores are favored in ponds that persist longer because they
metamorphose in better condition and thus have better chances of survival as juvenile toads
changes in the environment affect the relative growth rates of different body parts
In the pines. The relative growth rates of leaves and sapwood determine body shape, while in the
toads, the relative growth rates of the jaw and the rest of the body determine whether the
tadpole is a carnivore or an omnivore
- These patterns are examples of allometry, or differential growth of body parts that results
in a change in shape or proportion with size
- Allometry is a very common mechanism of variation within and among species
Adaptation must be demonstrated rather than assumed