Ecology Lecture No.7: Life History
Tuesday October 2 , 2012
-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, and survival and mortality rates.
Clownfish 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 nonbreeders. 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.
-Experiments with clownfish show that hierarchy is maintained by regulating growth rates. If two fish
become similar in size, a fight results and one is expelled from the anemone. They are completely
dependent on protection by the sea anemone. They are easy prey outside the anemone. Conflicts result
in expulsion and death, probably without having reproduced.
-So there is 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. The life history strategy of a species is the overall pattern in average timing
and nature of life history events. It is shaped by the way the organism divides its time and energy
between growth, reproduction, and survival (e.g. the metamorphosis of a frog).
-Some life history traits are determined genetically. Natural selection favors 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 generations). But none are perfect; all
organisms face constraints and ecological trade-offs.
-Phenotypic plasticity is where one genotype may produce different phenotypes under different
environmental conditions. For example, 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:
-Polyphenism describes a single genotype that 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.
-Different body morphology results from different growth rates of body parts in both the Ponderosa
pines and spadefoot toads. Allometry describes different body parts grow at different rates, resulting in
differences in shape or proportion.
Modes Of Reproduction:
Asexual reproduction is also known as simple cell division (binary fission) — all prokaryotes and many
protists. Some multicellular organisms like coral reproduce both sexually and asexually.
The Cost Of Sexual Reproduction:
-The benefits of sexual reproduction include recombination as it promotes genetic variation and
increased ability to respond to environmental challenges. Disadvantages include how an individual
transmits only half of its genome to the next generation; population growth rate is also slower.
Isogamy & Anisogamy:
-Isogamy refers to gametes that are equal in size (e.g. the green alga Chlamydomonas reinhardii)
Anisogamy refers to gametes of different sizes. In humans, the egg is usually much larger and contains
nutritional material. Most multicellular organisms produce anisogametes.
Development Of Species:
-Some species have direct development where the fertilized egg develops into a juvenile without
passing through a larval stage. Complex life cycles have at least two stages, with different body forms
and that live in different habitats. Metamorphosis is the abrupt transition in form between the larval
and juvenile stages.
Life History Continua:
-Classification schemes for reproductive patterns place the patterns on continua with extremes at each
end. The number of reproductive events per lifetime forms to categories under which species fall.
Semelparous species reproduce only once (e.g. annual plants). Iteroparous species can reproduce
multiple times (e.g. trees and most large mammals). Reproductive Strategies:
-r-selection and K-selection describe two ends of a reproductive strategy continuum. r is the intrinsic
rate of increase of a population. For high population growth rates there is an advantage to being an r-
selected species in newly disturbed habitats and uncrowded conditions. r-selected species have short
life spans, rapid development, early maturation, low parental investment, and high reproduction rates.
Most insects, small vertebrates such as mice, and weedy plant species are r-selected.
-K is the carrying capacity for a populatio