Illustrated with the London Marathon
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The time distribution of biological phenomena – illustrated with the London marathon Miguel Franco Corresp. 1 1 School of Biological and Marine Sciences, University of Plymouth Corresponding Author: Miguel Franco Email address: [email protected] Background. The time distribution of biological phenomena (phenology) is a subject of wide interest, but a general statistical distribution to describe and quantify its essential properties is lacking. Existing distributions are limiting, if not entirely inappropriate, because their parameters do not in general correlate with biologically relevant attributes of the organism and the conditions under which they find themselves. Methods. A distribution function that allows quantification of three essential properties of a biological dynamic process occurring over a continuous timescale was derived from first principles. The distribution turned out to have three parameters with clear meanings and units: (i) a scaled rate of completion (dimensionless), (ii) a measure of temporal concentration of the process (units: time-1), and (iii) an overall measure of temporal delay (units: time). Its performance as an accurate description of the process was tested with completion data for the London Marathon employing non-linear regression. Results. The parameters of the distribution correlated with biological attributes of the runners (gender and age) and with the maximum temperature on the day of the race. These relationships mirrored known differences in morphology and physiology of participants and the deterioration of these biological attributes with age (senescence), as well as the known effects of hypo- and hyperthermia. Discussion. By relating the variation in parameter values to possible biological and environmental variables, the marathon example demonstrates the ability of the distribution to help identify possible triggers and drivers of the duration, shape and temporal shift of its temporal distribution. This more detailed account of the effect of biological and environmental variables would provide a deeper insight into the drivers of a wide variety of phenological phenomena of high current interest, such as the shifting patterns of leafing, flowering, growth, migration, etc. of many organisms worldwide. PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27175v1 | CC BY 4.0 Open Access | rec: 6 Sep 2018, publ: 6 Sep 2018 1 2 3 4 The time distribution of biological phenomena – illustrated with the 5 London marathon 6 (A biologically interpretable time distribution) 7 8 9 Miguel Franco 10 11 12 13 School of Biological and Marine Sciences, University of Plymouth, Drake Circus, 14 Plymouth PL4 8AA 15 16 17 18 19 [email protected] 20 ORCID: 0000-0002-7249-4981 21 Tel +44 (0)1752 584626 22 Fax +44 (0)1752 584605 PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27175v1 | CC BY 4.0 Open Access | rec: 6 Sep 2018, publ: 6 Sep 2018 23 Abstract 24 Background. The time distribution of biological phenomena (phenology) is a subject of wide 25 interest, but a general statistical distribution to describe and quantify its essential properties is 26 lacking. Existing distributions are limiting, if not entirely inappropriate, because their parameters 27 do not in general correlate with biologically relevant attributes of the organism and the 28 conditions under which they find themselves. 29 Methods. A distribution function that allows quantification of three essential properties of a 30 biological dynamic process occurring over a continuous timescale was derived from first 31 principles. The distribution turned out to have three parameters with clear meanings and units: (i) 32 a scaled rate of completion (dimensionless), (ii) a measure of temporal concentration of the 33 process (units: time-1), and (iii) an overall measure of temporal delay (units: time). Its 34 performance as an accurate description of the process was tested with completion data for the 35 London Marathon employing non-linear regression. 36 Results. The parameters of the distribution correlated with biological attributes of the runners 37 (gender and age) and with the maximum temperature on the day of the race. These relationships 38 mirrored known differences in morphology and physiology of participants and the deterioration 39 of these biological attributes with age (senescence), as well as the known effects of hypo- and 40 hyperthermia. 41 Discussion. By relating the variation in parameter values to possible biological and 42 environmental variables, the marathon example demonstrates the ability of the distribution to 43 help identify possible triggers and drivers of the duration, shape and temporal shift of its PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27175v1 | CC BY 4.0 Open Access | rec: 6 Sep 2018, publ: 6 Sep 2018 44 temporal distribution. This more detailed account of the effect of biological and environmental 45 variables would provide a deeper insight into the drivers of a wide variety of phenological 46 phenomena of high current interest, such as the shifting patterns of leafing, flowering, growth, 47 migration, etc. of many organisms worldwide. 48 Keywords: endurance sports, gender differences, phenology, senescence, time distribution 49 50 Introduction 51 The time course of biological phenomena, often measured as the time to completion of a 52 particular event (such as hatching, leafing, flowering, germination or the completion of an 53 athletics race) is of theoretical and practical interest (Berry et al. 1988; Edwards & Richardson 54 2004; El Helou et al. 2012; Johnson-Groh & Lee 2002). How biological phenomena unfold with 55 time is determined by the interaction of intrinsic biological features of the organism (genetics, 56 morphology, physiology) and environmental influences acting upon it (nutriments, conditions). 57 These interacting influences determine the triggering of the phenomenon, its rate of occurrence, 58 its duration, and generally the shape of the resulting statistical distribution, which is rarely 59 normal (e.g., Fig. 2 in El Helou et al. 2012). Both symmetrical (e.g., Gaussian) and asymmetrical 60 distributions (e.g., Richards function) are often employed to quantify these time courses. 61 However, although their statistical moments are useful in themselves, their parameter values 62 cannot be interpreted in a meaningful way, a fact recognized by Richards himself (Richards 63 1959). It would therefore be ideal to have a model whose parameters identify specific aspects of 64 the distribution that account, for example, for biological differences between organisms and the 65 environments under which they find themselves. If specific biological and/or environmental PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27175v1 | CC BY 4.0 Open Access | rec: 6 Sep 2018, publ: 6 Sep 2018 66 variables affect individual parameter values in predictable ways, the parameters would provide 67 useful insight into the possible biological mechanisms involved. In addition to a good statistical 68 fit, the most important aspect of the distribution must surely be its ability to account for these 69 effects, especially if aimed, beyond description and quantification, at a mechanistic 70 understanding of the process under study. 71 With these ideas in mind, the aims of this investigation were: (i) to develop a model of the time 72 course of biological phenomena from first principles, (ii) to obtain its essential statistical 73 properties, and (iii) to illustrate the insight that it provides on essential components of a 74 biological time course. Given the completeness of records for the London Marathon, I chose to 75 illustrate the usefulness of the model employing data from several instances of this athletics race. 76 It would seem, however, that the model is potentially applicable to a vast number of temporal 77 distributions (phenologies), perhaps including molecular and cellular processes too. 78 The model 79 The simplest time distribution is one that occurs at a constant rate. If hatching, invasion or 80 completion of a race occurred at a constant rate, the completion of events would naturally follow ln (1 ) 81 the exponential distribution, = 1 or = 1 (1 ) , where y/ymax is the 푥 ‒ 푟 푥 푦 푦푚푎푥 ‒ 푒 푦 푦푚푎푥 ‒ ‒ 푟 82 fraction of the final number of completed events (ymax) after x units of time, and r is their rate of 83 occurrence. Alternatively, the time course of completions from y=0 to y=ymax would be described 84 by: 85 = (1 (1 ) (1) 푥 푦 푦푚푎푥 ‒ ‒ 푟) PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27175v1 | CC BY 4.0 Open Access | rec: 6 Sep 2018, publ: 6 Sep 2018 86 If a biological time course does not follow this exponential distribution, the simplest conclusion 87 is that r is not constant. The question then becomes whether r changes in a systematic, 88 predictable fashion. The completion of events can be thought of as a probabilistic manifestation 89 of a phenomenon determined by a variety of attributes of the organism and the conditions under 90 which it finds itself. This probabilistic feature is appropriately described by the logit, the 91 logarithm of the odds, i.e., the ratio of “non-event” to “event” in a binomial process. The inverse 92 logit converts the logarithm of the odds into a probability (the probability of hatching, invading 93 or completing the race) making the inverse logit (the logistic function) a natural choice to 94 describe an expected monotonic change in the probability of the event occurring with time under 95 a given set of conditions. The general form of the logistic function (including a “position” or 1 96 time-delay parameter t, which would seem necessary for any biological process) is: ( ), 1 + ‒ 푐 푥 ‒ 푡 푒 97 where c and t are constants. 98 Applying this function to the rate of completion of events, r, results in 99= ( ) (2) 1 + 푟푚푎푥 ‒ 푐 푥 ‒ 푡 푟 푒 100 where rmax is the maximum biologically achievable proportional rate of completion under 101 particular conditions. Substitution of equation 2 into equation 1 yields 102= 1 (1 ( )) ( 1 + 푟푚푎푥 ) (3) ‒ 푐 푥 ‒ 푡 푥 푦 푦푚푎푥 ‒ ‒ 푒 103 This equation describes the essential features of the temporal dynamics of a biological process.