Biology 12

14.4 Life History

492 MHR • Unit 5 Population Dynamics

As described earlier, populations vary with

respect to the factors that regulate their size. They

also vary with what are called the life history

features of their members. Life historyrefers to the

schedule of certain important events that occur

during the life of an organism. Important features

of an individual’s life history include the age at

which it reaches sexual maturity, how often it

reproduces, and how many offspring it has each

time. Life history also includes features related to

the life span of the organism.

Like most of its traits, the life history features

of an organism are genetically controlled and

are the result of natural selection operating over

evolutionary time. That is, these features have

been selected to maximize evolutionary fitness

(the number of offspring that survive to reproduce)

under certain environmental conditions. Because

the members of a population all live in the same

environment, they share similar life history

features. Ecologists therefore tend to refer to

the particular survival and reproductive patterns

shown by the individuals in a population as the

life history typical of that population.

Survivorship

As previously mentioned, one life history feature

that varies among populations is the average life

span of individuals. Ecologists often compare

different populations in this respect by using a

standard measure, called survivorship. Survivorship

is the proportion (percentage) of individuals in the

population that typically live to a given age. The

most accurate way to determine survivorship is to

monitor a large group of individuals all born at the

same time (a group like this is termed a cohort)

throughout their lives, recording the age

of death for each organism until the last one dies.

This information is then usually graphed as shown

in Figure 14.29.

Figure 14.29The three general patterns of survivorship

are known as Types I, II, and III. Note that the number of

individuals in the population is often standardized to 1000,

regardless of the true size of the cohort. Which type of

survivorship do you think is characteristic of human

populations?

After studying many different populations,

ecologists established that there were three general

patterns of survivorship (as Figure 14.29 illustrates).

In populations that have a Type III survivorship

pattern, most individuals die at a young age

(probably soon after birth or hatching) and very

few survive long enough to reproduce. In contrast,

populations with a Type I survivorship pattern

have high juvenile survival. Most individuals in

II

III

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10

100

1000

100

Number of survivors

Percent of life span

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EXPECTATIONS

Describe the features of an organism’s life history, and discuss the evolution

of the life history typical of a particular population.

Distinguish between three general patterns of survivorship.

Discuss the factors that relate to the fecundity of females in different

populations.

Describe what is meant by the age structure of a population and what

produces it.

Differentiate between equilibrial and opportunistic life histories, and

describe the type of selection, ror K, that tends to produce each of these

life history patterns.