Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Biologists study a great many evolutionary processes on the models of an adaptive landscape.The American biologist Sewall Wright introduced the model of the adaptive landscape into evolutionary biology at the beginning of the nineteen thirties. In actual fact, this consists of two quite different models, between which not even their author always sufficiently differentiated. This is an abstract model of either the environment and the individual organisms or the environment and populations. In the former case, the evolution of organisms in the environment can be conceived, e.g., as a three-dimensional topographical map, in which the x and y coordinates correspond to two qualities of a hypothetical organism (e.g. body weight and maximum speed of movement); in the second case, the frequency of the alleles are on the x and y axes at two different loci and the average biological fitness of the given population is on the z axis. Let us now return to the first model of an adaptive landscape. The shape of the surface of an adaptive landscape, the system of hills and valleys, is given independently of the properties of the organisms and determines the distribution of future niches in the particular environment. The projection of the points onto the surface of the adaptive landscape, i.e. coordinate z, determines the fitness of each individual organism (characterized in our model by properties X and Y). It is apparent that various combinations of X and Y are variously advantageous from the standpoint of natural selection. Mutations, i.e. a change in coordinate x or y, move the organism from one place to another, also shifting their projections onto the surface of the adaptive landscape. Only mutations that shift the projection of the organism uphill in the plane of the adaptive landscape, to places with larger coordinate z, can be fixed by natural selection. It is apparent that, under suitable circumstances, in time, organisms climb up to the peaks of the individual hills.

see Speciation by coevolutionary lift

Changes in time in the character of cladogenesis within a certain phylogenetic line are a striking and simultaneously difficult-to-explain macroevolutionary phenomenon. The paleontological record contains a number of phylogenetic lines that first continued in a state of evolutionary calm for a longer time after their formation, underwent speciation with only minimal frequency and their members also accumulated only a minimum of anagenetic changes. This was followed by turbulent evolutionary activity with splitting off and mutual phenotype diversification of a large number of species in a short period of time. On the phylogenetic tree, this phase appears as sudden radiation of a large number of phylogenetic lines from practically a single point. The period of radiation is again followed by a period of evolutionary calm, with a minimum of new branches, and evolution occurs only within the formerly split-off branches and tends to have the character of adjustment of the adaptation to the conditions of the environment rather than the creation of new diversity. In this period, specialized forms emerge in the line, frequently with large body dimensions that readily become extinct during the next mass extinction event. The last phase in the existence of a phylogenetic line from the viewpoint of its cladogenesis consists in evolutionary stagnation and decline – the species undergo speciation more slowly than extinction and thus the entire line gradually dies out. In some cases, rejuvenation can occur in any phase of the phylogenesis of the line, i.e. new radiation occurs in one of its branches and lines formed during this event then gradually replace most of the lines formed during the previous radiation (Fig. XXVI.2).

            The described process was sometimes interpreted in the past as maturing and ageing of the phylogenetic line. However, it is very probable that this is actually only a superficial analogy with the life cycle of a biological individual. In the life of a biological individual, i.e. in the section defined on the one hand by its birth and on the other hand by the physical destruction of its body, there actually do exist physiological processes determining the alternation of developmental phases, during which the viability of the individual and its ability to reproduce first increase and then decrease, with a proportional change in the probability of its death. However, this is not the case in the life of a phylogenetic line. Only individual species are formed and become extinct (and thus it is, at the very least, theoretically quite possible that the individual species can age, see XXVI.5), but the phylogenetic line encompassing all the species of organisms on the Earth lasts without interruption from the moment of emergence of life on this planet and only gradually branches over time. The individual lines (phylogenetic sublines) do, of course, disappear through extinction; however, the newly formed branches have, at the moment of their formation, the same age as all the branches existing on the Earth at the given moment. The individual taxa, monophyletic sections of the phylogenetic line defined on the phylogenetic tree by a taxonomist on the basis of an important trait, i.e. on the basis of the attained level of anagenesis, can, of course, emerge and disappear. The evolutionary emergence of a new taxon corresponds not to the formation of a new individual, but only to the formation of the relevant new anagenetic traits in a species within a long-existing phylogenetic line. Simultaneously, various taxa are delimited on the basis of very diverse traits. There is absolutely no reason to assume that the formation of basically an arbitrary trait would automatically initiate the process of ageing of the relevant phylogenetic subline. However, the anagenetic traits themselves, or rather the complex of anagenetic traits that characterize a new taxon, can age, e.g., can freeze, see below.

            It is highly probable that the “life cycle” of a taxon is not a result of its ageing and real changes in the traits of the relevant species. It is more probably a manifestation of mathematical laws following from the existence of fluctuations in speciation rates within a single phylogenetic line and also a consequence of the manner in which we obtain input data. A taxon that did not undergo at least one period of radiation has a very low chance of surviving in nature long enough for us to be able to observe it in the paleontological record and thus of being included in our considerations. There is low probability that a taxon will be subject to constant radiation; radiation is apparently always caused by some specific, relatively rare anagenetic change (see XXVI.2.1). The probability of radiation of a certain line is simultaneously apparently negatively affected by the overall biodiversity in the environment (see, e.g., the king of the hill effect, XXII.5.5, and the discussion of the model of the ecological network, XXII.7). As a consequence, a global balance is apparently maintained between the rate of speciation and the rate of extinction. However, this means that a taxon that does not undergo radiation in time becomes extinct. Obviously, once a taxon has become extinct, it can no longer undergo radiation and can thus not return to the paleontological record.

An alternative explanation is provided by the theory of frozen plasticity, according to which all evolutionary lines gradually age (and thus increasingly old lines emerge) {15429}. Old lines have reduced probability of evolutionary thawing and formation of new fundamental anagenetic features – and thus important evolutionary radiation, as more and more traits that were originally capable of thawing following peripatric speciation become permanently frozen. This phenomenon could explain why, for example, the great majority of disparities, the great majority of animal strains evolved in the Cambrian and, since that time, the number of basic body plans has tended to decrease on the Earth {12783}.

clinal variability

see Mutations at the level of the entire chromosome set

see Selection Intraspecific and  interspecific

Parasites can be divided into ectoparasites and endoparasites. Ectoparasites live on the surface of the bodies of the host or only in their “dwellings” (nidocolous parasites). In contrast, endoparasites live directly in the organs or tissues of the host species. A conspicuous anagenetic tendency is sometimes observed for endoparasites, i.e. a trend towards simplification of the body structure and individual physiological functions. This trend sometimes even goes so far that an originally multicellular organism becomes a more or less unicellular organism. A typical example is Myxozoa, a group of organisms parasitizing mostly on fish, in which the methods of molecular taxonomy have recently shown that these are not protozoa, but extremely reduced metazoa (Smothers et al. 1994; Siddall et al. 1995). A great many kinds of parasitic crustaceans have comparably greatly reduced body structure. Viruses are located at the other end of the scale of organismal complexity and they could well provide us with similar surprises in the future. It cannot be excluded that it will be found in time that some viruses are actually extremely reduced bacteria. (Well, probably not, but it would be nice, wouldn’t it?)

The reason for the simplification of the body structure of endoparasites is primarily the fact that a great many physiological functions related, for example, to maintenance of homeostasis, are left to the host organism by the parasite. The internal environment of the organism is relatively stable and rich in some easily processed nutrients. Consequently, a great many functions that are vital for freely living organisms are superfluous for a parasite and it can gradually get rid of them during evolution.

            It is even evolutionarily advantageous for a parasite if it manages to simplify its body structure as much as possible. The more differentiated tissues and organs its body contains, the greater the number of kinds of proteins that it must synthesize and the more easily can the immune system of the host recognize its presence and foreign nature.

            The specific cause of the reduction of the nervous system lies in the extreme predictability and stability of the environment in which the parasite infrapopulation lives. As a result of this predictability, the body of a host is sometimes designated as a third type of environment (in addition to aquatic and terrestrial environments) (Sukhdeo 2000). While every individual of nonparasitic species lives under quite unique conditions, parasites in the bodies of organisms are always in exactly the same, although highly heterogeneous environment. The distribution of the individual organs in the body of the host is almost identical in all individuals of a particular species. Consequently, while the nervous system and learning mechanisms are necessarily of great importance in controlling behavior in nonparasitic organisms, in parasitic organisms, these functions can be taken over by inborn fixed patterns of behavior, initiated without regard to stimuli from the environment (see XVI.2).

            In the past, the existence of predominating trends towards reducing complexity in parasitic organisms was considered to be a quite obvious fact (Poulin 1995b). As was later properly emphasized by a great many authors, the reason for general acceptance of this fact was an elementary methodical mistake. Instead of comparing the complexity of parasites in a certain phylogenetic branch with the complexity of nonparasitic organisms of the sister phylogenetic branch, the complexity of parasites was frequently compared with that of their hosts. It is obvious that, when a cow is compared to its tapeworm, the cow must appear more complex. However, when the complexity of parasitic and nonparasitic flatworms were compared, the result was far less obvious.

At the present time, the opinion pendulum has swung to the other extreme. To the contrary, especially amongst parasitologists and cladists, there is tendency towards throwing into doubt the trend associated with simplification of parasites (Brooks & McLennan 1993). Parastiologists have a quite understandable tendency to “favor” the objects of their study, so that they quite willingly accept the opinion that Sacculina barnacles, a crustacean that is most reminiscent of the mycellium of honey funguses, are, in fact, just as complicated as river crayfish; while it is true that they are lacking some organs, they quite certainly have a large number of evolutionarily new features, such as various types of receptors, at a microscopic or ultra-microscopic level. Support for this opinion of parasitologists by cladists then follows from their programmed lack of interest in similarity, and thus in the appearance of studied organisms. Cladists attempt to reconstruct the cladogenesis of organisms entirely on the basis of their mutual relatedness, where they estimate this relatedness wholly on the basis of sharing of new evolutionary features, synapomorphies (see XXIII.6). A new evolutionary feature could consist in both the acquisition and the loss of a certain body structure. Simultaneously, the complexity of the particular structures is of no importance in the formation of evolutionary hypotheses on the relatedness of organisms. If the particular evolutionary change occurred in one stage, then the loss of a complex structure, such as an eye, is equally important as the one-stage loss or acquisition of any other trait. Thus, if cladists attempt to compare the character of the evolution of nonparasitic and parasitic organisms, they first count the number of new evolutionary features that arose independently in a certain time interval. In this comparison, there understandably need not be any difference between parasites and nonparasitic species. When a cladist goes into greater detail and compares the number of new evolutionary features in both organisms, characterized by the loss of an organ and acquisition of a new organ, they do not in any way differentiate between the complexity of the lost and newly acquired organs. Simply on the basis of mechanical comparison of the matrices of evolutionary changes, they then come to the conclusion that tapeworms are an extraordinarily rapidly and progressively developing group. While they have lost a few organs, such as eyes and a digestive system, they have, on the other hand, acquired a large number of organs (mostly various types of chemoreceptors). Thus, only 6% of evolutionary changes here corresponded to the loss of organs (Brooks & McLennan 1993).

            Because of the difficulty of understanding the concept of complexity (see I.3), it will still long not be possible to exactly decide to what degree and whether trends towards a decrease in complexity predominate overall in parasites. It is, however, obvious that parasites provide us with a great many examples of secondary reduction of organs that have lost their original function in their new environment. In any case, the character of anagenesis of parasitic animals, i.e. at the very least the occasional trend towards simplified body structure, and thus towards reduction of the overall complexity of the organism, indicates that biological evolution need not always be connected with an increase in the complexity of biological systems. As was already pointed out in Sect. I.2, an increase in the complexity accompanies biological evolution; however, in a number of cases this parameter is quite independent of biological evolution. The only trait that differentiates biological evolution from other types of evolution is thus the formation of adaptive traits.

see Mutations at the level of the entire chromosome set

Presumed negative impact of evolution theory on people’s ethical attitudes

Warning coloration

Some authors are of the opinion that, under certain circumstances, the female can, to the contrary, create conditions for the most effective intergametic competition. It is known that the females of a great many animal species repeatedly copulate with a large number of different males. The arena hypothesis assumes that the purpose of this behavior consists in accumulation of sperm samples from a large number of males, thus creating conditions for effective competition between sperm derived from various individuals (Birkhead, Moller, & Sutherland 1993; Telford & Jennions 1998; Zeh & Zeh 1996). However, this hypothesis assumes that there is a positive correlation between the biological quality of sperm and the biological quality of the multicellular organism.

            The assumption that the biological quality of an individual is highly correlated with the competition potential of his sperm is rather bold; however, there is no reason why this should be rejected a priori. While organisms have created some mechanisms that prevent the emergence of competition between the sperm derived from a single individual (see, e.g., the above-mentioned inactivity of their genes), on the other hand the properties of the sperm are quite certainly determined by the quality of the genome of the male. Consequently, it can be expected that the quality of the sperm produced and thus its chance of success in the arena could depend on the biological quality of the males. Even if this were not true, it is still possible that sperm derived from various males compete in the arena independent of their quality but in dependence on their number. The situation is much more favorable in this case, as it can be expected almost with certainty that there exists a correlation between the quality of the male and the number of sperm that he can afford to produce.

            It has so far been verified only in a very few cases that promiscuity amongst females increases the fitness of the progeny. For example, it has been observed in the common yellow-toothed cavy (Galea musteloides) that females that were allowed to copulate with four males raised a larger number of healthy progeny that females that could copulate with only a single male; simultaneously, the size of the litter did not differ (Keil & Sachser 1998) (Fig. XIV.5)._s

Coevolution of a predator and its prey, parasite and its host or two species competing for utilization of a certain resource or having a common predator, and even the coevolution of two mutualistic species very frequently has the character of a sort of “arms race”. A certain new evolutionary feature emerges in one of the species, increasing the fitness of its bearers at the expense of members of another species, and thus it gradually spreads in the population of this species (Fig. XVIII.1). Such a new evolutionary feature could, for example, consist in stronger jaw muscles, enabling a predator fish to crack the shell of a certain kind of snail and include it in its diet. This would create a selection pressure on the snail to create a stronger shell, or to synthesize a poisonous or terrible-tasting substance to defend itself against attacks by the fish. The emergence of such an adaptation in the snails would produce a selection pressure on the predatory fish which, sooner or later, would lead to the creation of a suitable counter-strategy, enabling it to somehow overcome the effect of the new evolutionary trait in the snail – for example, through the formation of even stronger jaw muscles or of sharper horny jaws, or the emergence of some suitable pattern of behavior that would enable it to break even stronger shells. A new round in the arms race can be started by either the “attacker” or the “defender” and the participants in this race need not be only a pair of species, but can even consist in large groups of mutually interacting species.

            A great many paleontologists assume that some rapid changes in fauna that occurred in certain short periods in the history of the Earth, with the simultaneous participation of an enormous number of species, emerged as a product of just such arms races. A certain new evolutionary feature emerged in one species, creating a selection pressure on a great many species in the environment; they had to somehow react adaptively to this pressure, other species had to react to their reactions, so that the wave of anagenetic changes could finally affect a great portion of the species in the particular ecosystem in a very short period of time (Morris 1995).

            Arms races can also occur at an intraspecific level and this can happen on a relatively short time scale in microevolutionary processes. The members of the male or female sex can increase their fitness at the expense of the members of the other sex. Under normal conditions, the speed of evolution of males and females is comparable, so that it cannot be observed that the members of one sex would gain an apparent advantage over the members of the other sex. However, by suitably arranging an experiment, we can ensure, for example, that the females will not be able to respond to the evolutionary drives of the males. The experiment is performed in that the males are allowed to interact ecologically, ethologically and sexually with the females of a certain strain for a number of successive generations, where only the males of the obtained progeny are allowed to survive. The females are replaced in the experimental population in each generation from the stock population (and are thus completely “naive” from the standpoint of interaction with the particular selected line of males). The results of such an experiment performed on drosophila indicate that the males gained a considerable advantage over the females after 41 generations (Rice 1996). They achieved copulation with the females of the particular strain much faster; following copulation, the time before the females began to copulate with some other male was prolonged and, in case of multiple copulation with their own and foreign males, only a small percentage of the eggs were fertilized by the foreign males. The females lost out in the evolutionary battle, not only in the limited ability to chose, from their point of view, the best father for their progeny, but also in shortening of their lifetimes following copulation with the test males (but not with control males) (Fig. XVIII.2).

            According to some authors, the coevolutionary battle between the sexes in sexually reproducing species is the main reason for the genetic divergence of allopatric populations and is thus indirectly an important motor for allopatric speciation (Martin & Hosken 2003).

see  Speciation by coevolutionary lift