What can be judged by measuring fluctuating asymmetry?

{"title": "From the Chaos of Observations to Dynamic Typology: A Discussion Using the Example of Green Frog Populations. What Can Be Inferred by Measuring Fluctuational Asymmetry?", "summary": "The article is dedicated to the problems of studying fluctuating asymmetry in biology. The author discusses what can be inferred by measuring fluctuating asymmetry and how this method can be used to study the stability of development in organisms.", "body": "The renowned Russian biologist Dmitry Shabanov discusses what can be inferred by measuring fluctuating asymmetry. He begins with the idea of the famous Austrian biologist Ludwig von Bertalanffy that an organism is a dynamic expression of ontogeny, a process of self-formation. When studying the properties of organisms, it is useful to consider how they are determined and what their causes are.\n\nLudwig von Bertalanffy (1901–1972), the founder of general systems theory, and one of his ideas\n\nIt is good when we can study the variability of genetically identical organisms, clones. When such studies were successful, a rather wide intra-clonal variability was recorded. Each clone is characterized not by specific trait values, but by the probability distribution of these traits!\n\nThis last idea can be explained using the metaphor of the epigenetic landscape, attributed to the great English biologist C. H. Waddington.\n\nConrad Hal Waddington (1905–1975) and his epigenetic landscape metaphor, proposed in 1957\n\nOntogeny in this metaphor is likened to a ball rolling down a complex surface. Its shape can be judged by the probability distribution of developmental outcomes. Each genotype and each clone is characterized by its own surface shape.\n\nIt is good to study organisms that are easy to clone, for example, through vegetative propagation in many plants, parthenogenesis in silkworms, or perhaps by crossing semi-clones in green frogs. But what if cloning is impossible or difficult? Human geneticists are actively searching for natural human clones – identical twins. Twin analysis is based on their study, but twins are not enough to solve all problems…\n\nOne alternative to studying twins is that in organisms with bilateral symmetry, there are two 'clones' – the right and left halves of the body. They are generally symmetrical, but differences are observed in detail, manifestations of asymmetry. Some of these allow us to judge the stability/instability of development. A common classification of asymmetry types was proposed by the renowned American theoretical biologist Lee Van Valen.\n\nLee Van Valen (1935–2010) and his classification of asymmetry types, proposed in 1962\n\nTo understand which type of asymmetry we are dealing with in a given case, we need to examine the distribution of deviations from symmetry to the right and left. Some structures show a consistent deviation in a particular direction. For example, everyone knows that the human heart is on the left. In reality, the situation is more complex: humans have two hearts, right and left, and each consists of an atrium and a ventricle. The right heart pumps blood through the pulmonary circulation, and the left through the systemic circulation. Naturally, the left heart is normally larger than the right, and the entire paired structure is shifted to the left. This is functional asymmetry.\n\nMy right hand is significantly larger than my left. This is likely because, as a right-handed person, I use it more – and thus train it more intensively. Left-handed people often have a larger left hand. The situation of equal development of both structures is much rarer. This is an example of antisymmetry, where the symmetry of a structure is less common than the dominance of one side.\n\nThe situation with ear shape is more interesting. In most people, the ears are relatively similar, but in some, the right is larger, and in others, the left. Small differences in size are more common, large ones are rarer. It is quite likely that these differences are a manifestation of fluctuating asymmetry (FA). Thus, FA is a manifestation of random and undirected deviations from strict bilateral symmetry, a result of developmental instability (more precisely, imperfect stability). It can be viewed as macroscopic consequences of microscopic stochastic events.\n\nThe optimistic view of FA is that by finding traits that reflect it well, we get an excellent tool for studying developmental stability.\n\nImagine a vast birch forest. Each leaf of each birch is a bilaterally symmetrical organ. We collect small samples of leaves from different parts of this forest, measure their asymmetry, and find out where the problem spots are located in this forest: they are marked by an increase in FA. Soil contamination, waterlogging, and parasite infestation all cause similar changes in leaf symmetry. But as a method of mass analysis for answering the question 'Is everything normal?', FA assessment can be very useful.\n\nThere are a number of factors that can affect developmental stability, and through it, FA. Optimal developmental conditions, a clean environment, high individual fitness, and optimal genetic distance between parents reduce FA. Conversely, unfavorable conditions, pollution, individual maladaptation, and offspring resulting from inbreeding or distant hybridization increase it.\n\nIs everything clear? Now I will tell you about three works in which I participated to varying degrees.\n\nFirst example. For two consecutive years during field practice, students study the FA of fish. Last year, the work was done on perch. The results have already been published in Ukrainian, and I will now summarize their content. This year, work with a similar design was done on both perch and bream; last year's results were convincingly confirmed.\n\nWe caught a fairly large sample of small perch, constructed their size distribution, and confirmed that they belonged to three well-distinguishable size groups. We checked their age by the lines on their scales; we found that the three size groups corresponded to age groups. The smallest were perch that appeared in the year of the study, larger ones were from the previous year, and even larger ones were from the year before that. For all fish, we counted the number of rays in the pelvic and pectoral fins on the right and left sides, as well as the number of scales along the lateral line. We examined how the deviations from symmetry of these countable traits were distributed; we confirmed that they generally met the criteria for FA. We compared the level of FA characteristic of fish of different ages, and also (within each age group) for medium-sized, small, and large fish (i.e., those differing in growth rate).\n\nIt turned out that the fish from the year the study was conducted were the most asymmetrical; the FA in perch from 2012 was lower, and in 2013