Life History Characteristics
63
dictable habitats in conjunction with uniform spatial distribution of resources. In contrast, iteroparity - that is the distribution of the reproductive
effort over time - is the appropriate strategy against either reproductive failure in unfavourable periods (Callow 1978) or heterogeneous spatial distribution of resources (Blower 1969). Distribution of reproduction over time
results from distribution of the reproductive effort among the different age
classes, from differing time spans to maturity, or even from life cycle timing
leading to overlapping age distribution.
As to sexuality, Norton and Palmer (1991) argued that parthenogenetic
species are broadly adapted. This stems from the fact that among parthenogenetic species, selection operates on clonal grounds and is usually associated with considerable genetic variation. By contrast, selection in sexual populations, acts on individual loci or linkage groups, so, the chance of broadly
adapted whole genomes evolving is greater in parthenogenetics. Norton and
Palmer (1991) concluded that broadly adapted thelytokous populations are
at an advantage in abiotically unstable habitats where changes in local environmental conditions occur continuously. Obviously, the authors linked the
non-stability of habitats favouring parthenogenesis to abiotic unpredictability over time rather than to spatial heterogeneity. The authors also
studied reproductive tactics of oribatids, considering biotic rather than
abiotic unpredictability to be more important for sexuality. Within a coevolutionary context they argued that in complex communities - inducing higher
biotic unpredictability - plastic genomes are a prerequisite for the survival
of populations facing sexual parasites, sexual predators or sexual competitors.
In the light of the above assumption, the situation remains controversial in
Mediterranean regions characterised by a highly heterogeneous distribution
of resources coupled with predictable seasonal and interannual oscillations
in climatic variables. Thelytoky and sexual reproduction, semelparity and
iteroparity, as well as precocity and differing time spans to maturity seem to
be strategies employed with equal probability.
5.1.3.1
Thelytoky and Sexual Reproduction
At first glance, taking into account life history traits such as shorter generation times, relatively low fecundity, and the need for recolonisation of
micro sites after anthropogenic disasters, the contribution of parthenogenetic species might be important for Mediterranean regions. Nevertheless, as
was argued in the previous section, sexuality and parthenogenesis are the
by-product of trade-offs among abiotic and biotic predictabilities over
time. Sexuality is preferred in abiotically stable environments favouring the
establishment of complex communities, whilst parthenogenetics is favoured in abiotic ally unstable habitats.
63
dictable habitats in conjunction with uniform spatial distribution of resources. In contrast, iteroparity - that is the distribution of the reproductive
effort over time - is the appropriate strategy against either reproductive failure in unfavourable periods (Callow 1978) or heterogeneous spatial distribution of resources (Blower 1969). Distribution of reproduction over time
results from distribution of the reproductive effort among the different age
classes, from differing time spans to maturity, or even from life cycle timing
leading to overlapping age distribution.
As to sexuality, Norton and Palmer (1991) argued that parthenogenetic
species are broadly adapted. This stems from the fact that among parthenogenetic species, selection operates on clonal grounds and is usually associated with considerable genetic variation. By contrast, selection in sexual populations, acts on individual loci or linkage groups, so, the chance of broadly
adapted whole genomes evolving is greater in parthenogenetics. Norton and
Palmer (1991) concluded that broadly adapted thelytokous populations are
at an advantage in abiotically unstable habitats where changes in local environmental conditions occur continuously. Obviously, the authors linked the
non-stability of habitats favouring parthenogenesis to abiotic unpredictability over time rather than to spatial heterogeneity. The authors also
studied reproductive tactics of oribatids, considering biotic rather than
abiotic unpredictability to be more important for sexuality. Within a coevolutionary context they argued that in complex communities - inducing higher
biotic unpredictability - plastic genomes are a prerequisite for the survival
of populations facing sexual parasites, sexual predators or sexual competitors.
In the light of the above assumption, the situation remains controversial in
Mediterranean regions characterised by a highly heterogeneous distribution
of resources coupled with predictable seasonal and interannual oscillations
in climatic variables. Thelytoky and sexual reproduction, semelparity and
iteroparity, as well as precocity and differing time spans to maturity seem to
be strategies employed with equal probability.
5.1.3.1
Thelytoky and Sexual Reproduction
At first glance, taking into account life history traits such as shorter generation times, relatively low fecundity, and the need for recolonisation of
micro sites after anthropogenic disasters, the contribution of parthenogenetic species might be important for Mediterranean regions. Nevertheless, as
was argued in the previous section, sexuality and parthenogenesis are the
by-product of trade-offs among abiotic and biotic predictabilities over
time. Sexuality is preferred in abiotically stable environments favouring the
establishment of complex communities, whilst parthenogenetics is favoured in abiotic ally unstable habitats.