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

see Selection for heterozygotes

However, parasites exploit their environment, which is generally very rich in sources of nutrients, only temporarily, sometimes even transitorily, in terms of their lifetimes. The parasite generally dies with the death of the attacked individual and, in the vast majority of cases, its death is accompanied by the extinction of the relevant infrapopulation of parasites (a population of parasites living in/on one host). While, for organisms living independently, the critical parameters determining fitness are generally the rate of reproduction or the economy of use of food sources, for most parasites, the ability to be transmitted from an infected host to an uninfected host is of key importance. A parasite has only a very limited ability to affect its fitness through reproducing within a single host. From the standpoint of fitness, the number of progeny is of key importance, but only if they manage to get into other individuals in the host population. Consequently, most adaptive traits that are encountered amongst parasites are in some way related to the transmission of the parasite in the host population.

            The effectiveness of transmission is characterized by the basic reproductive rate R₀, i.e. the number of individuals of the host species that are infected, on an average, by one already infected host in the population that has not yet been affected by the parasite, i.e. in a population of uninfected and not-immune individuals (see also XVII.5). In real populations there are, of course, individuals that have already been infected and immune individuals, so that the actual number of individuals infected from a single source, i.e. the actual reproductive rate, R, is usually much smaller. Its value can be calculated from the equation

where q is the fraction of susceptible individuals, i.e. not yet infected and not-immune individuals in the host population. For endemically occurring parasites, the actual reproductive rate is equal to unity in the long term, so that the number of infected individuals in the population remains constant over the long term. The actual reproductive rate is a very important parameter from the epidemiological standpoint. From the standpoint of study of microevolution in the parasite population, however, rate R₀ is of key importance. 

Optimization of the reproduction of the parasite, usually, although not necessarily, accompanied by optimization of pathogenicity and virulence, is a typical evolutionary adaptation permitting influencing of the effectiveness of transmission and thus maximizing of R₀.  The ability to form resistant latent stages is also common; these can survive in the infectious state in nature until they manage to enter a suitable host. This aspect also encompasses the frequently very complicated life cycles of parasites, which often include a number of intermediate host organisms, through which the parasite gets from one host to another. Other adaptive traits are related to the ability of parasites to adaptively change the traits and behavior of the host organism to assist in its reproduction and dissemination.

see History of evolutionism – classical Darwinist period

see clinal variabilit

The probability of superinfection and thus the optimal rate of multiplication of the infrapopulation and optimum parasite virulence do not depend only on the size of the parasite population at the given site, but also on the biodemographic parameters (life history) of the host population. If the host population is short-lived (for example small rodents) or if, for example, the host population at the given time exhibits a higher death rate for reasons not associated with the actual parasitosis, it is advantageous for the parasite to multiply more rapidly in the hosts and thus to damage them more than if the hosts were long-lived (Ebert & Herre 1996; Restif, Hochberg, & Koella 2002). This again contributes to an increase in the virulence of all parasites at times of famine, epidemics or military battles.

Similarly, greater mobility of individuals in the host population leads to an increase in parasite virulence. If the members of the given population exhibit low mobility and motility, infection is transferred especially between close neighbors. From the viewpoint of the rate of spreading, this is advantageous for those parasites that do not damage their hosts much (Haraguchi & Sasaki 2000). In contrast, if individuals of the host population move over large distances in their area of occurrence, low virulence does not constitute any advantage for the parasites.

More virulent parasite populations survive in more numerous host populations compared to small populations that are present, for example, at the edges of the areas of occurrence of a particular species,. Similarly, more virulent strains of parasites are usually at an advantage in growing host populations; while parasites with lower virulence are at an advantage in populations with constant or even decreasing size. This is a result of the fact that, in a growing population, there is a high probability that the infectious stage of the parasite will encounter an uninfected and unimmunized host, while this probability is lower in a stagnant or decreasing population and thus it is preferable for the parasites in a shrinking population to harm their hosts less (Knolle 1989; Combes 2001; Ebert 2000). In tropical areas, mosquitoes, acting as vectors for malaria (i.e. hosts whose role in the life cycle of the parasite is to provide mobility for immobile parasites), are active throughout the year and thus infection is transmitted throughout the year. This is perhaps why Plasmodium species causing malaria (Plasmodium falciparum) are more virulent in these areas than in subtropical or temperate regions. In areas where the frequency of transmission of a disease is decreased in some seasons of the year, for example because of the absence of insect vectors, species that are capable of forming latent stages in the host or species that have a long incubation time (Plasmodium ovale, P. vivax) are at an advantage. A similar phenomenon can be observed for Leishmania (Combes 2001).

Biodiversity denotes the mutual diversity of living systems. It has two components, the diversity (biodiversity), i.e. number of species, and disparity, i.e. the diversity (dissimilarity) of various forms of life.

Biological evolution is long-term, spontaneously occurring process, during which living systems are formed or were formed singly from nonliving systems, and these living systems then develop and mutually diversify.

see Extinction predispositio

A temporary, frequently very drastic reduction in the size of the population, followed by a return in the number of individuals in the population or species to the original size, is apparently important in evolution. This process and the evolutionary and genetic processes that accompany it are called the bottle-neck effect. The bottle-neck effect occurs, for example, when the size of the population is radically reduced by a certain biotic or abiotic factor whose action is only temporary. The same effect occurs when a new location is colonized by a small group of individuals of a certain species, in the extreme case one fertilized or even parthenogenetic female. Such an event can lead to accelerated evolution through accelerated anagenesis, as the gene pool of the founding population can differ drastically from the gene pool of the initial population (see XXVI.5).The direction of evolution is determined not only by the selection pressures, to which the population is exposed, but also by the structure of the gene pool of the given population. As the gene pools of the original and founding populations differ, the course of their evolution can also differ. This founder effect (Mayr 1963)has a number of genetic consequences and can even lead to the formation of a new species. In addition, as was discussed in Section VII.3, selection pressure does not act as strongly in small populations as in large populations, so that mutants that would be rapidly eliminated by natural selection can survive here.The bottle-neck effectthus allows evolution to overcome shallow valleys in the adaptive landscape (Wright 1931; Wright 1982).

At first glance it might seem that the bottle-neck effect would lead to the same decrease in polymorphism as a reduction in the size of the population. However, theoretical models of this phenomenon indicate that this need not be true, that the bottle-neck effect frequently does not lead to a substantial reduction in polymorphism. This is a result of the fact that the reduction in the size of the population is followed by its re-expansion, during which no alleles basically become fixed. The population expands into free ecological space and is thus not exposed to almost any intraspecies competition, so that the bearers of all the alleles transfer their genes to their progeny. If an allele “survives” the process of reduction of the population, in the following period of exponential increase it will apparently not be eliminated, even in the period when the number of individuals in the expanding population is still very small compared with the size of the original population. It is, however, apparent that a temporary reduction in the size of the population always leads to the loss of most rare alleles, i.e. alleles that were present in such low frequency in the original population that they disappeared from the gene pool at the instant when the size of the population was reduced. In this case, however, these are mostly neutral or almost neutral mutations that are maintained in the population by mutation pressure, i.e. as a consequence of constant formation of the same alleles through mutations. Thus passage through the bottle neck affects primarily type 1 polymorphism that is not of such fundamental importance in evolutionary and ecological processes as type 2 polymorphism (see VIII.3).

Study of the genetic polymorphism of a certain population yields information on whether it passed through a bottle-neck in the past (Emerson, Paradis, & Thebaud 2001). If a certain DNA segment is sequenced for a greater number of representatives of the relevant population and the mutual similarity of all pairs of sequences is compared, three fundamentally different results can be obtained. If the population was stable for a prolonged time and was relatively large, it contains a large amount of polymorphism, where some pairs of alleles differ in many positions, while other pairs differ just in few positions. A histogram of the number of mutations in which a pair of sequences differs is very irregular – it does not have one clear maximum. If the population was limited in size for a long period of time, the overall polymorphism, both in the numbers of alleles and in the numbers of differences between the alleles, is substantially smaller but the histogram of positions in which the individual alleles differ from one another is also irregular. If the population passed through a bottle neck and later substantially increased in size, the amount of polymorphism is also large, while the histogram of the number of different positions has a regular bell shape (Vonhaeseler, Sajantila, & Paabo 1996)(Fig. V.6).

It is known for a great many species of animals living in groups that, at the moment of a sudden change in the social structure of the group and immediately after this, especially in situations where there is a change in the alpha male in the group, massive infant mortality occurs. In some cases, the progeny are killed by the new head of the harem, but in other cases it seems that their mothers kill or abandon the offspring. The Bruce effect is the best-known example of such a phenomenon; this is encountered in mice and other species of rodents (Bruce 1959). If we remove the original male from a pregnant mouse and replace it by a foreign male, the female usually very rapidly aborts.

            These and related phenomena apparently accompany intersexual competition. From the viewpoint of the male, it is extremely important to be the biological father of the greatest possible number of offspring in the population. Any behavior that contributes to this goal is selectionally advantageous and will apparently become fixed during evolution. The male can best shorten the time until the individual females become pregnant with him by removing all foreign offspring as soon as possible. From the viewpoint of the female, such an approach is, of course, extremely disadvantageous, because she has already invested a great deal in the offspring. However, she mostly has no effective defense against this male strategy. To begin with, the females of a great many species are weaker than the males and, in addition, an offspring can be killed very quickly, while defense of an offspring against killing requires constant alertness and care. This asymmetry means that the females generally lose the battle for the young in advance, so that it is fundamentally more evolutionarily advantageous for her to cooperate with the male and to rapidly get rid of her own young or embryos.

            It should be stated that there are a number of other explanations for confirmation of the Bruce effect, including unilateral, pheromone-mediated manipulation on the part of the male or hidden (post-copulation) female choice – preferential conception with the winning – and thus genetically better partner (Storey 1994).