## I.11 Biological evolution has most of the attributes of a random process

We can differentiate processes with random (stochastic) behaviour and nonrandom (deterministic) behaviour. If we know the momentary state of a system with deterministic behaviour and if we also know how the signals will gradually arrive at its inputs (for systems with a memory, also what signals came to its inputs in the past) we can predict the future behaviour of this system with complete certainty. This is not true of a system with stochastic behaviour, as such a system always contains at least one element whose behaviour can also be determined or affected not only by regular input signals but also by chance. If combination of signals 1 appears at the input of an element with deterministic behaviour present in state 1, the element will pass to state A in 100% of cases. In contrast, an element with stochastic behaviour in the same situation can pass to at least two different states, A andB. The probabilities of passage to these two states can differ substantially; the element can pass to state A with 99.99% probability and to state B with a probability of 0.01%. In such a case, the behaviour will approach deterministic behaviour and we can estimate it quite precisely on the basis of our knowledge of the state of the system and the input signals. In contrast, when an element passes to both of the possible states with a probability of 50%, it behaves quite chaotically and we cannot predict its behaviour or the behaviour of the system, of which it is a part, in advance. The relationship between unpredictable and deterministic behaviour and especially the related relationship between randomness and necessity are undoubtedly very interesting philosophical problems. However, simultaneously, this problem lies well outside the framework of evolutionary biology, so it will not be further considered here.

It is quite clear that the process of evolutionary biology is a stochastic process. Living organisms and their environment, i.e. the system in which the process occurs, contain an enormous number of elements whose behaviour is determined to a greater or lesser degree by chance.

If an advantageous new mutation appears in the DNA of an individual, that brings its bearer increased resistance to a certain disease, we could theoretically expect that the descendants of this mutant will gradually become predominant in the population, i.e. that the particular mutation will become fixed. In actual fact, this need not occur. The resistant mutant could be killed while still young by a falling tree, could be eaten by a predator, or the particular population need not necessarily encounter the relevant disease for a long time. Thus, the potentially advantageous mutation will most probably be eliminated by genetic drift (see Chap. IV).

As will be shown in Chap. III, mutations themselves occur more or less by chance, so that the character and order in time of their occurrence cannot be in any way predicted. On the other hand, it must be admitted that some micro-evolutionary processes are highly deterministic, such as natural selection, molecular drive, meiotic drive. This means that, in short experiments, we are frequently capable of predicting the course of the relevant micro-evolutionary changes on the basis of the properties of the individual organisms. When several islands in the Greater Antilles were experimentally colonized by populations of small lizards of the Anolis genus, it could be observed that genetic fixation of very similar morphological changes occurred in all the populations after several years (Losos, Warheit, & Schoener 1997). In contrast, as a consequence of participation in the random process of mutagenesis, the results of repeated laboratory experiments, in which the microevolution of bacterial populations is modelled under strictly controlled conditions of continuous cultivation, are frequently quite different (Lenski & Travisano 1994; Lenski et al. 1998).

Biological evolution as a whole, i.e. encompassing both microevolutionary and macroevolutionary processes, is a very long-term process. Simultaneously, organisms are systems with a memory, whose future development depends on the development that they underwent in the past. The effects of random processes also logically accumulate andare amplified. If it were possible to perform a long-term evolutionary experiment consisting, for example, in colonizing completely identical islands with completely identical species of organisms and subsequent long-term observation of the evolution of these species on the individual islands, it is quite certain that the flora and fauna of the individual islands would gradually become different through the effect of random processes.

Evolutionary processes are governed by their own laws. As we gradually come to understand these laws, we are increasingly capable of predicting the course of evolutionary processes. As the element of chance plays a role in evolutionary processes in many respects, it is quite impossible that we would be able, sometime in the future, to specifically (and thus not statistically) predict the course of biological evolution of some species of organism. The influence of chance similarly excludes the possibility that evolution could occur in exactly the same way at two places in the universe, i.e. that the same kinds of organisms could develop on two different planets.