Community Ecology Processes, Models, and Applications

associated recombination during meiosis. Alterna-
tively, mutations, insertions or deletions can change
the genetic make-up of individuals. The majority of
mutations and new genetic combinations are be-
lieved to be deleterious, either because they render
an enzyme or promoter region non-functional or
because they break up co-adapted gene complexes.
Natural selection will quickly purge such variation
from the population. But even without selection a
significant proportion of genetic variation is lost
each generation through the random effect of ge-
netic drift. Especially in small populations, random
drift can quickly reduce genetic variation. Only if
genetic variation has a clear adaptive value will it
be maintained in the population, because the fitness
benefits associated with genetic diversity in a pop-
ulation will decrease extinction risk.
Two principal mechanisms of reducing extinc-
tion risk have been put forward. First, the tangled
bank hypothesis proposes that individuals with
different genotypes may each use a slightly differ-
ent niche and therefore together are able to extract
more food from their environment than genetically
identical individuals (Bell 1991; Barrettet al. 2005).
In addition, different genotypes may compete less
because they explore different microniches (Anto-
novics 1978). The second hypothesis to explain the
advantage of genetic diversity is the Red Queen
hypothesis, which argues that genetic diversity re-
duces the risk of infection or attack by natural ene-
mies (Jaenike 1978). Empirical studies have
provided ample support for the benefits of genetic
diversity within populations, including positive
correlations with increased resistance to distur-
bance by grazing (Hughes and Stachowicz 2004),
increased productivity and foraging rate (Mattila
and Seeley 2007) and enhanced settling success
(Gamfeldtet al. 2005).
However, few studies have actually determined
the underlying mechanism of fitness enhance-
ments. The tangled bank hypothesis assumes that
different genotypes show positive complementari-
ty, because microniche differentiation causes them
to compete less with other genotypes than with
their own genotype. In a study by Reuschet al.
(2005) on a coastal community dominated by the
seagrass speciesZostera marina, experimental plots
with a high genotypic diversity produced a higher

shoot number and more biomass than genetically

impoverished communities or monocultures of

genotypes (Fig. 11.1). They were able to attribute

the effect of genotypic diversity to positive interac-

tions between genotypes, particularly facilitation.

Some genotypes that performed poorly in monocul-

ture had a proportionally strongly reduced mortal-

ity in genotypic mixtures (Reuschet al. 2005). Other

studies, specifically testing the advantage of genetic

complementarity using clonal diversity, also found

effects. Semlitschet al. (1997) found significant dif-

ferences in the life history traits among different

clonal groups inRana esculentaand an increased

proportion of metamorphosed larvae in clonal mix-

tures of frogs compared with a clone reared alone.

In another study, genetically diverse groups of

clones were demonstrated to be better invaders

than genetically uniform groups of invaders. The

better performance of the genetically diverse group

of invaders was attributed to competitive release

experienced by individuals in genetically diverse

populations (Tagget al. 2005).

Experimental evidence for improved population

performance by reduced infection risk in genetical-

ly diverse populations is rare. Schmid (1994) found

evidence that genetic diversity can influence mil-

dew infection levels inSolidago altissima. Infection

rate affected individual performance with

increased height and biomass of less infected

plants, but mean plant performance per plot was

not correlated with genetic diversity levels. The few

studies so far indeed provide evidence for genetic

complementarity as a significant mechanism main-

taining genetic variation.

June

One genotype

70

60

40

20

Three genotypes

Leaf shoots (0.25 m

2 )

Six genotypes

Unmanipulated

**

*

*

May July August September

Figure 11.1Comparison of mean leaf shoot density (SE)

ofZostera marinaamong one-, three- and six-genotype

treatments after climate perturbation, and natural shoot

density at the experimental site. From Reuschet al.

(2005). Copyright 2005 National Academy of Sciences,

USA.

EVOLUTIONARY PROCESSES IN COMMUNITY ECOLOGY 153