Fig. 11.5. Conceptualisation of how the external factors steadily change the species composition. The possible shifts in species composition are determined by the gene pool, which is steadily changing due to mutations and new sexual recombinations of genes. The development is, however, more complex. This is indicated by (1) arrows from "structure" to "external factors" and "selection" to account for the possibility that the species are able to modify their own environment (see below) and thereby their own selection pressure; (2) an arrow from "structure" to "gene pool" to account for the possibilities that the species can to a certain extent change their own gene pool.

reactiOns, including an OrganisatiOn Of the netwOrk. There is, hOwever, One change that seems tO keep OngOing cOntinuOusly and tO have a directiOnal cOmpOnent. This is the change referred tO as evOlutiOn that is defined as the change in the prOperties Of pOpulatiOns Of Organisms Over time. The pOpulatiOn is the unit Of evOlutiOn, althOugh genes, individuals and species alsO play a rOle.

Evolution is closely related to the genetic pool. It is the result of the relations between the dynamics Of the external factOrs and the dynamics Of the genetic pOOl. The external factOrs steadily change the cOnditiOns fOr survival, and the genetic pOOl steadily cOmes up with new sOlutiOns tO the prOblem Of survival.

Darwin's theory assumes that populations consist of individuals that

1. On average prOduce mOre Offspring than is needed tO replace them upOn their death— this is the prOperty Of high reprOductiOn. An enOrmOus amOunt Of genetic variatiOn is prOduced in every generatiOn, but Only a few individuals Of the generatiOn will survive tO prOduce the next generatiOn.

2. Have Offspring which resemble their parents mOre than they resemble randOmly chOsen individuals in the pOpulatiOn—this is the prOperty Of inheritance.

3. Vary in heritable traits influencing reproduction and survival (i.e. fitness)—this is the prOperty Of variatiOn. All the individuals Of a pOpulatiOn differ genetically frOm each Other.

Only a (minOr) part Of the individuals fOrming a pOpulatiOn survives and reprOduces. These survivOrs are nOt a randOm sample Of the pOpulatiOn, but they are characterised by the pOssessiOn Of certain prOperties/attributes that favOur survival under the prevailing environmental conditions. The survivors have the properties that make them particularly well adapted fOr the envirOnment. The mOre survivOrs, the mOre exergy the system has— the survival is measured by the distance frOm thermOdynamic equilibrium. There are, Of cOurse, nO agents invOlved in the selectiOn, but the individuals withOut the best fitness tO the prevailing envirOnmental cOnditiOns will be eliminated. The Organisms with the right cOmbinatiOn Of prOperties adapted tO the prevailing envirOnmental cOnditiOns are the organisms that will be able to contribute most to the exergy of the system through survival.

All the above-mentioned three properties are parts of the presentation in Fig. 11.5. High reprOductiOn is needed tO get a change in the species cOmpOsitiOn caused by changes in external factors. Variability is represented in the short- and long-term changes in the genetic pool, and inheritance is needed to see an effect of the fitness test in the long run.

WithOut inheritance every new generatiOn wOuld start frOm the same pOint and it wOuld not be possible to maintain the result of the fitness test. Evolution is able to continue from the already Obtained results.

The species are cOntinuOusly tested against the prevailing cOnditiOns (external as well as internal factOrs) and the better they are fitted, the better they are able tO maintain and even increase their biomass. The specific rate of population growth may even be used as a measure for the fitness (see e.g. Stenseth, 1986). But the property of fitness must of course be inheritable tO have an effect On the species cOmpOsitiOn and the ecOlOgical structure Of the ecOsystem in the lOng run. Natural selectiOn has been criticised fOr being a tautOlOgy: fitness is measured by survival, and survival Of the fittest therefOre means the survival Of the survivors. However, the entire Darwinian theory including the above-mentioned three assumptions cannot be conceived as a tautology, but may be interpreted as follows: the species offer different solutions to survival under given prevailing conditions, and the species that have the best combinations of properties to match the conditions have also the highest probability of survival and growth. The formulation by Ulanowicz (1986) may also be applied: those populations are the fittest that best enhance the autocatalytic behaviour of the matter-energy loops in which they participate.

Anthropogenic changes in external factors (i.e. anthropogenic pollution) have created new problems, because new genes fitted to these changes do not develop overnight, while most natural changes have occurred many times previously and the genetic pool is therefore prepared and fitted to meet the natural changes. The spectrum of genes is sooner or later able to meet most natural changes, but not all of the man-made changes, because they are new and untested in the ecosystem.

Evolution moves toward increasing complexity in the long run; see Fig. 11.6. The fossil records have shown a steady increase of species diversity. There may be destructive forces, for instance anthropogenic pollution or natural catastrophes (for a shorter time), but the probability that

1. new and better genes are developed;

2. new ecological niches are utilised will increase with time. The probability will even (again excluding the short time perspective) increase faster and faster, as the probability is roughly proportional to

Fig. 11.6. Changes in species diversity over geological time. Redrawn from Raup and Sepkowski (1982).

the amount of genetic material where mutations and new sexual recombinations can be developed.

It is equally important to note that a biological structure is more than an active nonlinear system. In the course of its evolution, the biological structure is continuously changed in such a way that its structural map itself is modified.

The overall structure thus becomes a representation of all the information received. Biological structure represents through its complexity a synthesis of the information with which it has been in communication (Schoffeniels, 1976). Evolution is maybe the most discussed topic in biology and ecology and millions of pages have been written about evolution and its ecological implications.

Today the basic facts of evolution are taken for granted and interest has shifted to more subtle classes of fitness/selection, i.e. towards an understanding of the complexity of the evolutionary processes. One of these classes concerns traits that influence not only the fitness of the individuals possessing them, but also the entire population. These traits overtly include social behaviours, such as aggression or cooperation, and activities that through some modification of the biotic and abiotic environment feed back to affect the population at large, for instance pollution and resource depletion.

The more species are on Earth, the more possibilities the ecosphere offers to utilise the available resources and ecological niches and the more possibilities the ecosphere has to adapt to new, expected or unexpected, conditions. The number of species should therefore be an indirect measure of the utilisation of the available resources to move away from thermodynamic equilibrium, or expressed differently, as the width of ecological information. The same considerations are applied when the survival of species is discussed. Endangered species often have very little diversity in the gene pool (Lewin, 1994).

The depth of ecological information may be expressed by using the weighting or conversion factors in Table 5.1 in Chapter 5 for the most developed organism at a given time. We do not know the exact number of species today: a rough estimation is 106-107, and we do not have good estimations of the number of species during evolution at all. It is therefore proposed to use the number of marine families (Raup and Sepkowski, 1982; see Fig. 11.6) as a proper relative measure of the ecological information width. We do know approximately from fossil records when each species emerged. Consequently, we could get a first relative estimation of an evolution index by multiplication of these possible expressions for the width and the depth of biological information that should be closely related to the relative evolution of the exergy of the biosphere (J0rgensen, 2000b).

Fig. 11.7 shows the development over geological time from 55o Ma to today of the proposed above evolution index = (number of marine families) X (weighting factor for the most developed organism at a given time). The applied weighting factors are shown in Table 11.3. Note in Figs. 11.6 and 11.7 the decrease in evolution index about 220 Ma ago and 65 Ma ago due to extinction of a high number of species (the dinosaurs—65 Ma ago, probably by a catastrophic event). The overall trend is (in spite of some fluctuations due to catastrophic events) toward an increase of the evolution index. When the index is decreasing it is due to a sudden and/or major change of the environmental conditions, because the forcing functions of the ecosphere are changed stochastically. The index will,

Evolution Index

Was this article helpful?

0 0

Post a comment