Oct 3 2013
Sequence of live events in an organism. (growth/development/reproduction/death) We are
interested in all of these factors. For a human, you go through a period of growth and then you
reproduce. On average, you live to 80. Our perception of life history is biased based on our
experiences. However, many organisms experience very different life histories.
Life history characteristics include: age and size at sexual maturity, amount and timing of
reproduction, survival and mortality rates
Clown Fish Case Study-FINDING NEMO
Clown Fish hang in sea anemones for protection. Anemones sting the predators of clownfish,
but not the clownfish themselves. The fish also benefit the anemone by eating its parasites or
driving its predators away.
Usually 2-6 will hang out in one sea anemone for their entire adult lives. However, it is unlikely
that they are related.
Largest clownfish is female. Next largest is the breeding male. All other fish are immature non
breeders. They are simply biting their time waiting for the others to die off so that they can take
their place. (based on body size) In finding Nemo, his mom died. If this actually happened, his
dad would have turned into a female. If female died, breeding male would take her place and
the next fish in line will become the breeding male.
Hatchlings move out of the anemone, and
Figure 7.23 Clownfish Size Hierarchies
juveniles must find a new anemone to inhabit.
The reef is dangerous for them. They grow in
open water and then come back to the reef to
find open spots in anemones. When a juvenile
enters an anemone, they are only able to stay if
there is room.
Why do the Clownfish Maintain the
Clown fish are dependent on survival of being
around anemone. (colorful and slow) They are
easy prey outside of the anemone. If you are small and you try to change things, you get pushed
out of the anemone and get eaten. You see through natural selection that aggressive clownfish
do not end up reproducing. Therefore, there is strong selection pressure to avoid conflict. Sea anemones are a scarce
resource for clownfish.
Clown fish have growth regulation. They are able to avoid conflict by making sure they do not
grow too big at any one time. (allows them to ensure they live to an age in which they can
Life History Diversity
Within a species there is a lot of diversity. There are genetic differences, differences in their
environment and differences in their life history. Life history is a record of major events related
to its growth, development, reproduction, and survival.
Individuals within a species show variation in life history traits due to genetic variation or
The life history strategy of a
species is the overall pattern in
average timing and nature of life
history events. Life decisions are
made but these are genetically
decided. (How many offspring
and how big are they going to
be? Are you going to care for
offspring? How long to live? )
Natural selection favours
individuals whose life history
traits result in their having a
better chance of surviving and
reproducing. An organism
predisposed to do certain things
may or may not be selected for.
Optimal Life History
Most organisms you would think would converge on a certain optimal life history to maximize
their fitness (genetic contribution to future generations). However, it is not perfect. This comes
down to ecological tradeoffs.
A certain life strategy may be favoured under some conditions and not favoured under other
conditions. You can only allocate resources in one direction at any one time.
This is why there is environmental plasticity. There is a certain amount of plasticity in any
organism where they optimize how they grow/reproduce in respond to the environment Environmental conditions can produce different phenotypes from the same genotype. One
example is growth and development. You tend to grow faster at higher temperatures.
Individuals of a pine tree species are separated into 2 populations based on the environments in
which they grow. (Ones that grow in cool moist and ones that grow in desert.) For any given
height, you have a thinner diameter trunk for cool areas and a larger diameter trunk for desert
trees. This is due to species physiology. Dry environment trees need to conduct water so the
wide trunks allow them to have a lot of root/vessel elements so they can draw up water.
Changes in life history traits can cause
change in adult morphology.
Phenotypic plasticity may result in a continuous range of sizes; or discrete types called morphs
(ALL OR NOTHING) There is nothing in between. Depending on the environment, you assume
one of two phenotypes (another form of environmental plasticity)
You can call it a polyphenism in this case because you have one genotype that can produce
several distinct morphs.
An example of this is toad tadpoles. We have omnivore morphs and carnivore morphs.
(both from same genotype)What they develop into depends on the food they consume
in early development. Depending on the pond they are in, they can become omnivore
morphs and feed on detritus and junk at the bottom of the ponds, or they can become
carnivores (large jaws) that feed on fairy shrimp in the water column. Carnivore tadpoles
tend to grow very fast and they morph early. This is because they live in semi permanent
ponds (ephemeral ponds). It is to your advantage to get out of the pond as quickly as
you can so you do not dry up and die before you gain enough energy to turn to frogs and
get out. Omnivores grow slowly because they dwell in long lasting ponds. When they
eventually turn into toads, they are much better conditioned that toads from carnivore
tadpoles (better survival)
Different body morphs result from different growth rates of body parts in both the Ponderosa
pines and spadefoot toads. This is known as allometric growth. Allometric growth is when growth of certain body parts is favoured over others. Allometry is when 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 do
Some multicellular organisms reproduce sexually and asexuallly (coral)
Life Cycle of a Coral
Coral starts with larvae that forms pollup. Then it forms asexual units on top. This is why coral
takes so long to develop (laying down carbonate for asexual growth from a single pollup) There
comes a time when they undergo sexual reproduction. They give off a sperm and an egg
producing larvae sexually which then forms pollups. (life cycle)
High genetic variation so you can respond to environmental changes. Asexual reproduction gives
you many identical units. If the environment changes, you are not well adapted, everything dies
You are only passing on half of your genetic information to your offspring. You get lower
population growth rates. Half of the offspring produced by the sexual females are male and
must pair with a female to produce offspring. Therefore, asexual individuals increase in number
more rapidly, and after 7-8 generations almost 100% will be asexual. The Cost of Sex
Gametes of different sizes is common. Eggs are large due to their nutritional value. Most
multicellular organisms have gametes of different size. This is known as anisogamy.
We can also have identical gametes (isogamy)Green algae does this!
Complex Life Cycles
We see a relatively simple life cycle for most organisms (no major body changes or abrupt
transitions) However, for many organisms like amphibians and
insects/plants/algae/protists/fishes and marine invertebrates we get complex life cycles.
The small early stages of many animal life cycles look and behave completely differently from
adult stages. They frequently eat different foods and prefer different habitats.
Complex Life Cycles: There are at least 2 different stages with 2 different body forms that often
live in 2 very different habitats. They often go through metamorphism which is an abrupt
transition in between the larval and juvenile stages. Many organisms in the ocean like plankton
float around in larvae change and then after metamorphosis they attach somewhere and
become a much larger/different organism.
Complex life cycles and metamorphosis result when offspring and parents are subjected to very
different selection pressures.
In terms of our cells, we have direct development. Fertilizing develops into a juvenile without
passing through a larval stage. Juvenile is mini version of an adult. (laying eggs which directly
hatch into a juvenile)
Classifying Life History Strategies
How many reproductive events will happen within a lifetime? Semelparous: Species reproduce only once. These species include annual plants (complete life
cycle within one year or less-After they germinate from a seed, they reproduce once and die)
and Agave. Agave grows a vegetative leaf for 25 years, then they flower and die. However, at
the same time Agave is reproducing sexually it also reproduces asexual clones on the ground.
(same genetic info as plant-flowers produce clumps of seeds that drop off and root around the
parent plant) The tree lives on through these clones. In this sense, they are not semelparous at
Iteroparous: Species reproduce multiple times. These species include pine trees and spruces
which must put seeds out every year. Large mammals also typically have multiple reproduction
events throughout their life.
R Selection and K Selection
These strategies describe the two ends of a reproductive system continuum
R: Intrinsic rate of increase of a population
R Selected: For high population growth rates; an advantage in newly disturbed habitats and
uncrowded conditions. You start out with a population, there is a lot of open space and very few
individuals. There is a premium on reproducing quickly so the population can grow. R selected
species can be considered "live fast die young" They have short life spans, rapid development,
early maturation, low parental investment, and high reproductive rates. Most insects and small
invertebrates (mice & weedy plants) are R selected
K: Carrying capacity of a population
K Selected: For slower growth rates in populations that are at or near K; this is an advantage in
crowded conditions; efficient reproduction is favoured. When population is in growth (r phase)
You will actually reach a point when all of the space is taken up and the population stabilizes ( k-
carrying capacity) At K the population has reached stable equilibrium value and no more
expansion can occur. K species must be able to