La Mettrie
see pre-Darwinist period
Lamarck
see History of evolutionism - pre-Darwinist period
Lamarckian microevolution in organisms without Weismann barrier
The absence of the Weismann barrier in plants and thus the possibility of intra-individual selection of cell lines secondarily facilitate a certain type of Lamarckian microevolution, specifically for effective and simultaneously hereditary adaptation of the individual to the local conditions of the habitat. Simultaneously, this ability can be extremely important for immobile plants that cannot move from an unsuitable habitat to a more suitable habitat, as can animals. The experience of plant breeders and physiologists shows that the body of a plant is frequently a genetic mosaic and that the individual cells from which its tissues and organs are formed differ in their genetic information. For example, it is known that at least 5000 of all 8800 plant varieties grown in Europe in 1899 were originally obtained as somatic mutants (Whitham & Slobodchikoff 1981). Simultaneously, the genetic diversity of plant cells is partly derived from classical mutations, partly from various types of directed mutations, frequently connected with transposon activity, and a substantial part apparently arises as a consequence of mitotic recombination. In mitotic recombination, DNA sections on pairs of homologous chromosomes, in two chromosome sets that the organism has available, change places. Thus, cells with new genotypes are not actually formed, but only with new pairs of recombined haplotypes. Nonetheless, these cells can differ in their biological properties. This difference is caused by the “position effect” (Goldschmidt 1946) (Fig. XII.5). As gene expression is generally controlled by regulation elements occurring in the DNA in the vicinity of the actual gene, it can readily occur that an allele that was highly expressed on the original chromosome ceases to be expressed following transfer to the homologous chromosome and rather the second allele begins to be expressed.
As soon as the individual cells in the plant differ genetically and in phenotype, natural selection can occur amongst them. Cells whose phenotype corresponds better to the local conditions of the habitat of the particular plant multiply and grow faster so that, finally, their progeny or their genealogical relatives and thus also genetically (or epigenetically) similar cells in the tissues of totipotent meristems predominate and lead to the formation of the generative organs of the particular individual (Pineda-Krch & Lehtila 2002) (see Fig. III.11). Similar intra-individual selection can, of course, occur even more readily at the level of the individual plant organs, e.g. branches. Thus, more effective adaptation to external conditions can occur in plants in this way and the individual adaptations can be transferred to the next generation both vegetatively and by sex cells (Flegr 2002).
Last Universal Common Ancestor
To the present day, 1-2 million species of living organisms and hundreds of thousands of extinct organisms have been described. However, most living species and even more extinct species have not yet been identified and probably never will be. Generally accepted estimates place the number of species now living on the Earth at several tens of millions. While the latest results suggest that, at the very least for insects, these estimates will probably be excessive (Novotný et al. 2002), it seems, on the other hand, that the biodiversity of parasitic organisms could be substantially underestimated.
All these species apparently evolved in the past from a common ancestor. It is not clear how many times life evolved on the Earth or how many times it was transported from the surrounding universe. In the light of the speed with which it appeared as soon as conditions became favourable for its existence, i.e. after the end of the period of massive bombarding of the surface of the Earth by cosmic projectiles and after a solid cooling crust was formed, it is quite possible that life evolves relatively easily and thus that this occurred repeatedly on the Earth. No surprising events can be completely excluded; nonetheless, it is highly probable that the representatives of only a single line have been preserved to the present day, either by accident or as a result of mutual competition. Available information, especially related to the uniformity of the basic molecular apparatus of organisms and the similarity of basic biochemical processes, such as replication of nucleic acid or proteosynthesis, unambiguously indicate that all the known species of organisms have a common ancestor. This common ancestor most probably lived on the Earth 3.8 billion years ago (Holland 1997). However, we have no proof that the traces indicating the existence of life found in rocks of this age were actually left by the ancestors of modern organisms, and not by the members of some other, extinct line. However, it is almost certain that life did not cease to exist on Earth from the most ancient times to the present day and that organisms have been present on the Earth for at least 3.5 billion years in such large numbers that they fundamentally affected the development of the atmosphere, hydrosphere and geosphere of the planet. What this life looked like and when, not the first, but the last common ancestor of all contemporary forms of life, i.e. the organism from which modern bacteria, archea (archeobacteria ) and eukaryotes, branched off at a certain moment and in a certain order, remains an interesting and still unresolved question. This hypothetical organism is designated by the abbreviation LUCA (Last Universal Common Ancestor) in the English literature. On the basis of current knowledge in comparative molecular biology, it rather seems that this was a quite advanced organism with a modern-type, fully developed proteosynthetic apparatus. According to some hypotheses, it could have lived on the Earth at a relatively late date, possibly 2 billion years ago (Schopf 2000).
Law of regression to the mean
Francis Galton, a cousin of Charles Darwin, studied the heritability of body height by the determination of the correlation between the mean value of a height of both parents and the mean height of their progeny. Galton and found two important facts. To begin with, he demonstrated that heritability in a given trait does actually exist, as tall parents actually tend to have tall children and short parents tend to have short children. Secondly, he formulated the law of regression to the mean. The further the mean height of the parents from the population mean, the greater was the probability that the height of their children would return back towards the population mean, rather than deviating even further from this mean than the deviation of the mean height of their parents. This return towards the mean can be explained by the existence of non-additive components of genetically determined variability. In a stabilized population, the population mean of the value of a quantitative characteristic should correspond to the optimal value of this trait from the standpoint of the biological fitness of its carriers. As individuals with larger or smaller values of the given trait are constantly removed from the population by normalized selection, a frequency of the individual alleles of the genes affecting the given trait is established in the gene pool of the population that leads to the optimal value of the given trait in the largest possible number of random combinations. If the heights of the mother and of the father deviate substantially from the mean, they most probably have some rare combination of the relevant alleles. This combination will disappear in their progeny either immediately or in the subsequent generations. If only genes with additive effect were relevant for the given trait, the progeny should not return to the population mean.
Galton could, of course, not correctly interpret the results of his study under the conditions at that time. He explained the existence of regression to the population mean as an indication that the characteristics of an individual are determined 50% by predispositions obtained from their parents, 25% by predispositions obtained from their grandparents, 12.5% from their great-grandparents, etc. If predispositions are interpreted as genes in the sense of cistrons, then this would be a quite erroneous explanation, as an individual obtains all his genes from his parents. However, if we realize that a predisposition can also be the effect of certain interactions of a combination of several alleles at various loci, a combination that an individual inherits from his predecessors but that can fall apart or, to the contrary, be formed with a certain probability in each generation, then this, at first glance erroneous explanation, can be basically correct. The probability that an individual inherited a certain combination of alleles from one of his great grandparents is less than the probability that he inherited it from his grandparents and this is again less than the probability that he would inherit it from one of his parents. See also the theory of frozen plasticity.
Lazarian species
– see Mass extinction, after the end of mass extinction events
Learned pattern of behavior
A further step in the evolution of mechanisms controlling the behavior organisms is the learned pattern of behavior, of which the conditioned reflex is a simple form. The neural systems of many kinds of animals are adapted so that, when a trigger stimulus for a specific unconditioned reflex is repeatedly accompanied or preceded by another stimulus, the organism will, after some time, also react by launching the particular behavior pattern in consequence of this other stimulus. The famous salivating Pavlov’s dogs are a textbook example of an experimentally produced conditioned reflex.
Learning
– seeCultural traits transmission of
Life cycle parameters
Life cycle parameters, for example the rate of maturation, length of life, number of progeny and the related biodemographic parameters of the population (life history characters), for example population size and rate of growth, can apparently easily change as a consequence of various mutations. With a certain degree of simplification, it can be stated that almost every mutation is manifested to a certain degree in the life-cycle parameters of its carrier. Simultaneously, the life-cycle parameters can very substantially affect the fitness of an individual. The level of investment into reproductive organs in plants or into reproductive conduct in animals can very readily reduce or increase the number of progeny left by a particular individual by an order of magnitude and thus proportionally change the probability that the mutation that caused the particular change is passed on to the next generation. The importance of the individual types of mutations for the fitness of an individual can be indirectly estimated on the basis of the degree of inheritability of the relevant phenotype differences (Houle 1992). Genetically determined traits that very strongly affect the fitness cannot survive for long in the population in the polymorphic state, as one or the other form of the particular trait can be eliminated relatively easily in the population through natural selection. This is manifested at the population level in that most of the intra-population variability in the particular trait or in the particular category of traits is of nongenetic nature or, if it is of genetic nature, it has very low inheritability, i.e. degree (or probability) with which it is transferred from parents to progeny. Numerous observations in nature and the laboratory have shown that life cycle parameters and biodemographic parameters in general have, on an average, much lower inheritability than morphological parameters (Mousseau & Roff 1987) (Fig. XII.11).
Life-dinner principle
From a general evolutionary standpoint, another phenomenon is very interesting and apparently determines to a substantial degree the result of the evolutionary battle between a parasite and its host. The two participants in the co-evolutionary process, here the parasite and its host, are not concerned to the same degree with the result of mutual interactions. While losing the battle with the host organism generally leads to death for the parasite (as an individual), for the host it generally leads only to a greater or smaller reduction in its fitness (Dawkins & Krebs 1979; Dawkins 1982). The fact that the host organism is sometimes killed by the parasite or is not able to reproduce and its fitness thus decreases to zero does not much affect the situation. This is not a typical situation as it is generally in the interest of the parasite not to kill its host and thus the relevant selection pressures to which the parasite is exposed mostly lead to a gradual reduction in its pathogenicity to a certain optimal level (see XIX.5).
The “life-dinner” principle is valid not only in the relationship between a parasite and its host, but also in a great many other inter-species and intra-species interactions. This principle was originally identified (and named) in systems of the predator and prey type. Put simply, rabbits run faster than foxes for the simple reason that they are running for their lives, while the fox is only concerned about its supper.
Lilliputian phenomenon
see Mass extinction, after the end of mass extinction events
Linkage disequilibrium
Equilibrium numbers of the individual genotypes corresponding to the numbers of the individual alleles will become balanced even if a genetic linkage exists between the studied loci; however, equilibrium will not be established immediately in this case, but rather gradually, where the rate of establishment of equilibrium is inversely proportionate to the strength of the genetic linkage. The population can be inthe linkage disequilibrium, i.e. the numbers of the individual genotypes need not correspond to their theoretical frequencies calculated on the basis of the relative numbers of the individual alleles, for several reasons. The degree of disequilibrium is most frequently expressed as the coefficient of linkage disequilibrium D which, for genes located in two different alleles, is calculated from the equation D= ¦n¦n- ¦r¦r, where ¦nand ¦nare the frequencies of the individual genotypes containing the unrecombined halotypes and ¦rand ¦rare the frequencies of individuals with genotypes containing recombined halotypes. Disequilibrium can, first of all, be caused by the fact that some genotypes are not viable and are continuously eliminated from the population by natural selection. In this case, zygotes with the individual genotypes are formed with the expected theoretical frequency, but some genotypes are frequently eliminated already during embryogenesis, prior to the birth of the individual. The second reason for the existence of linkage disequilibrium could be the existence of genetic linkage between the loci of interest. Each population has a unique history; at some point in the past, it was formed by splitting off from some other population or other populations. If a population was formed from more than one original population, the frequencies of the individual genotypes in the founding population will not apparently correspond to equilibrium values calculated on the basis of the frequencies of the individual alleles. Because of the existence of genetic linkage, the equilibrium frequencies of the individual genotypes will be established only gradually over a large number of generations
Linné
see History of evolutionism - pre-Darwinist period
Living fossils
The rates of evolution can be compared on the basis of the number of quantitative changes that occur in a certain phylogenetic line within a given time interval. As various types of organisms differ in the number of traits that can be identified, it makes sense to use this means of estimating rates of evolution only to compare rates for similar organisms. This method is most useful for monitoring the changes in the rates of evolution within a single phylogenetic line. When, for example, this method was employed to follow the rate of evolution in lungfish, i.e. a group of vertebrates whose members appeared in the paleontological record approximately 340 million years ago, it was found that this rate achieved a maximum of 2.5 changes per million years at a time 290 million years ago, but decreased to about one tenth of this value 250 million years ago (Westoll 1949). Over the past 200 million years, the morphology of lungfish has practically not changed, so that their modern representatives are mostly given as examples of “living fossils”. Understandably, the rate of evolution in an evolutionary line depends not only on the rate of changes occurring in a single species, but also on the number of species in the given line. As, at the present time, there are only six known species of lungfish, it is not very surprising that the rate of evolution measured in terms of the number of evolutionary changes is very low at the present time.
Logistic function
see selection r- and K
Lottery model of advantage of sexuality
The lottery model (sometimes also called the best man model) is based on the fact that the area of occurrence of any species is mostly heterogeneous to a greater or lesser degree (Williams 1975). Simultaneously, a more or less random sample of the offspring of various parents ends up in each microhabitat in each generation. The individuals compete together in the microhabitat and only the best of them or, to be more exact, those whose phenotype best corresponds to the properties of the particular microhabitat are successful in the competition and leave offspring. The progeny of asexually reproducing species would probably be very well adapted to some type of microhabitat. Those that end up in some other microhabitat would, however, most probably lose out in competition with the offspring of sexually reproducing parents. Because of their polymorphism, part of the progeny of sexually reproducing organisms would always have properties that would be especially suitable for any type of microhabitat.
LUCA
see Last Universal Common Ancestor
Lysenkoism
Soviet Lysenkoism was an unfortunate chapter in the biology of the 20th century. In connection with this subject, mention should be made of the aspect of jump transitions between two or more species. Lysenkians assumed that an organism is capable of reacting to some external stimuli in that it switches the ontogenetic system in such a way that, in a single jump, the progeny acquire the character of the members of a different biological species. Descriptions of observations in which poorly fertilized wheat suddenly began to form rye caryopses in its spikes or poorly fertilized rye began to form couch grass caryopses now sound like a students’ rag or April-fools joke.
Lysenkists completely discredited their learning, not only in that they physically liquidated their opponents in the interests of rapid dissemination of their ideas, or rather in the interests of fulfilling their ambitions of power, but also in that they extensively falsified their experimental data. Consequently, the results that were accumulated at the time of Lysenkoism are mostly worthless. The very idea of the transition of one species into another through switching of alternative ontogenetic programs cannot, however, be automatically rejected. However, contemporary knowledge related to the course and mechanisms of biological evolution exclude the possibility that such a phenomenon could have any importance in the evolution of organisms.
It is true that all organisms with sexual dimorphism and organisms whose life cycle includes a larval phase are capable of maintaining genes for two or more different, mutually exclusive ontogenetic programs in their genome. However, in these cases, natural selection constantly controls the functioning of the individual ontogenetic programs and greatly “penalizes” individuals that have a program damaged by mutations. However, if the population were to employ only a single program for a long time, it is probable that the genes for an alternative program would be gradually inactivated as a consequence of the accumulation of mutations. See also Frozen plasticity theory.