Biology 12

484 MHR • Unit 5 Population Dynamics

Figure 14.22When two species of paramecia (Paramecium

aureliaand P. caudatum) are grown in separate cultures in a

laboratory, their populations follow a logistic curve to reach

their own carrying capacities. This is shown by the solid

lines. However, when they are grown together, P. caudatum

goes extinct in the mixed culture, shown by the dashed line

in (B), since P. aureliais a much better competitor.

What this means is that differences between

species can be increased as a result of natural

selection, with interspecific competition as the

driving force of evolutionary change. In each

competing species, the individuals that are most

different from their competitors will be best able to

avoid competitive interactions and will therefore

obtain the most resources. For example, if two

species of birds compete for seeds of roughly equal

sizes, those individuals of both species that can eat

larger or smaller seeds will be able to find more food.

They will, therefore, be more likely to survive and

reproduce than will members of their own species

that cannot avoid interspecific competition. As a

result, their alleles — coding for the characteristics

(such as beaks that can handle different seeds) that

distinguish them from their competitors — will

increase in frequency in subsequent generations. In

this way, natural selection can produce increased

divergence between competing species. In Figure

14.23, natural selection may take two different

routes to lead to reduced competition. The total

range of prey sizes taken by the two species may

increase, with one species extending its preferences

to smaller items than were taken previously, while

the other may include larger foods than were eaten

before. Alternatively, the total range of prey eaten

may remain the same, but the niche of each species

may shrink; one species may become a specialist

on small prey while the other takes mainly large

prey. Over time, this can increase the diversity

of species living in a community.

Paramecium caudatum

200

100

grown alone

mixed

culture

Number of paramecia

(per mL)

Paramecium aurelia

800

400

grown alone

mixed

culture

Time

Time

Number of paramecia

(per mL)

0

0

B

A

Figure 14.23Within a species, individuals vary in many of

their features. As a result, they use a range of whatever

resources are necessary for their survival and reproduction.

In this example, the members of two species, A and B, vary

with respect to the size of their prey, but some individuals

of both species eat intermediate-sized prey. This overlap

produces interspecific competition, which may lead to

evolutionary changes in both species.

Number ofindividuals

Number ofindividuals

Prey size

or

species A species B

Prey size Prey size