National environmental footprints and planetary boundaries: from methodology to policy implementation 59th LCA forum,Swiss Federal Institute of Technology,Zürich,June 12, 2015

The 59th LCA forum was held on 12 June, 2015 to discuss the situation with regard to national environmental footprints and their relation to planetary boundaries and to the global carrying capacity. This conference report presents the highlights of the LCA forum. Several approaches of how to quantify a safe operating space of the Earth were presented, such as the planetary boundary concept published by Rockström et al. (Nature 462:472–475, 2009) and the ecological footprint (Bastianoni et al. 2013). Several presenters showed how they transformed environmental planetary boundaries to national and per capita allowances. In a research project funded by the Swiss Federal Office for the Environment safe and unsafe areas were determined by combining the level of overshoot, the level of confidence in the information and the trend in the environmental load. The areas of climate change, biodiversity losses and nitrogen losses show a large overshoot on a global level but also from the point of view of Swiss consumption. Other organizations use the planetary boundary concept to identify companies which qualify for environmentally sustainable funds. Finally, life cycle impact assessment methods are being developed using the planetary boundary concept. The weighting step is based on the level of overshoot, which is close to “distance to target” approaches. It was discussed that the nine planetary boundaries face some consistency and operationalisation problems. For instance, land use changes cause biodiversity losses, which is a planetary boundary parameter in its own. Chemical pollution on the other hand is a general topic, for which a quantification approach has to be developed first (load as well as its planetary boundary). The discussion forum showed that individual countries and political entities like the European Union start monitoring their consumption based environmental footprint. Within this context, approaches and concepts are needed to define the environmentally safe operating space. The LCA forum showed that there is still basic research needed to reliably and consistently quantify relevant planetary boundaries (avoiding overlapping indicators) and to transfer these boundaries to per capita allowances.     

Tracking resource use relative to planetary boundaries in a steady-state framework: A case study of Canada and Spain

There is a growing understanding of the biophysical processes that regulate the stability of the Earth-system, yet human pressures on the planet continue to increase rapidly. Here, recent advances in defining Earth-system thresholds using the planetary boundaries framework are translated down to national and sub-national levels. A set of 10 indicators is developed in a biophysical accounting framework that links the sustainability of resource flows from the biosphere to final consumption. The indicator set includes three measures of physical stocks, three measures of aggregate resource consumption, and four indicators of sustainable scale. The four scale indicators are ratios of (i) cumulative carbon footprint relative to carbon budget, (ii) nutrient use relative to biogeochemical boundaries, (iii) blue water consumption relative to monthly basin-level availability, and (iv) land footprint relative to biocapacity. Taken together, the indicators measure how close high-consuming societies are to meeting the conditions of a “steady-state economy”, defined here as an economy with non-growing physical stocks and flows maintained within shares of planetary boundaries. The framework is applied over a 15-year period to the economies of Canada and Spain, along with two sub-national regions (Nova Scotia and Andalusia). Nova Scotia is the only study site experiencing stable or decreasing biophysical stocks and flows. None of the study sites are consuming resources within their shares of all four planetary boundaries. Overall, the set of indicators provides guidance for prioritizing which environmental pressures need to decline (and by how much) for societies to be more effective stewards of Earth-system stability.     

Reflecting the importance of human needs fulfilment in absolute sustainability assessments: Development of a sharing principle

Absolute environmental sustainability assessments (AESAs) evaluate whether the environmental impact of a product system is within its share of a safe operating space as determined by biophysical sustainability limits such as the planetary boundaries (PBs). The choice of sharing principle has significant influence on the result of an AESA, and any studies call for further research on how to share the safe operating space in an operational way that relates to the product's contribution to the welfare of the user. In this study, we develop the “Fulfilment of Human Needs” (FHN) principle as a sharing principle that operationalizes sufficientarianism (making sure everyone gets enough). The FHN principle is tested on two case studies (a food item and a textile) against four of the PBs: climate change, land-system change, water use, and nitrogen cycling. The operationalization of the FHN principle is slightly different between the PBs; the starting point for climate change is the average consumption pattern in countries classified as “most sustainable,” while for the other three PBs the status quo impact in the most sustainable countries is used. To operationalize the FHN principle on the product level, each consumption category is downscaled according to objective sources that determine the value delivered to the users. We demonstrate that, compared to other previously applied sharing principles, the FHN principle supports a stronger relation to the importance to the users of the delivered outcome.     

足迹家族研究综述

综合测度人类社会的可持续发展状态是生态经济学者追求的重要目标。足迹家族由生态足迹、碳足迹和水足迹等一系列足迹类指标整合而成,旨在为决策者系统评估与权衡人类活动的环境影响提供理论和技术支持。从理论探索、整合实践和分类比较等三方面对足迹家族的研究现状进行了综述;在此基础上围绕极具争议的足迹定义、计算方法和加权方式等问题,深入分析了阻碍当前研究进一步推进的关键性因素;指出未来应从建立足迹类型学、完善跨区域投入产出模型、细化产品和机构环境足迹标准等方面入手,推动实现足迹家族的量化整合;并首次提出了足迹家族与行星边界耦合的构想,以期为监测和预警人类活动的生态阈值、促进环境影响评价向可持续性评价转变提供科学依据。     

Maximizing affluence within the planetary boundaries

Purpose

Ordinary product LCA studies focus on measuring or minimizing environmental impact, but do not address if the product fits in a sustainable consumption pattern. This paper proposes a setup in which the planetary boundaries define the maximum impact, and the minimum requirements for a reasonable consumption level specify a lower impact level. Thus, a “safe operating space” remains.

Methods

We use an IO table for EU-27 and the consumption pattern of the Bulgarian population extrapolated to the EU level as driving climate impact. The EU’s policy targets are used as a planetary boundary for climate change.

Results

The 2020 target is shown to be able to accommodate the Bulgarian-style consumption, with room for a much higher GDP. The 2050 target, however, is too narrow, and a slightly smaller consumption pattern is needed to reach the target.

Conclusions

Although the approach is highly simplified and neglects many developments, the idea of using IO-tables and minimum consumption levels to backcast directions to be taken is expected to help policymakers. We acknowledge some important limitations of our approach, but accept these in the context of exploring future scenarios and how to get there, instead of predicting the future.     

Generation of an Industry-specific Physico-chemical Allocation Matrix. Application in the Dairy Industry and Implications for Systems Analysis (9 pp)

Background, Aims and Scope Allocation is required when quantifying environmental impacts of individual products from multi-product manufacturing plants. The International Organization for Standardization (ISO) recommends in ISO 14041 that allocation should reflect underlying physical relationships between inputs and outputs, or in the absence of such knowledge, allocation should reflect other relationships (e.g. economic value). Economic allocation is generally recommended if process specific information on the manufacturing process is lacking. In this paper, a physico-chemical allocation matrix, based on industry-specific data from the dairy industry, is developed and discussed as an alternative allocation method. Methods Operational data from 17 dairy manufacturing plants was used to develop an industry specific physico-chemical allocation matrix. Through an extensive process of substraction/substitution, it is possible to determine average resource use (e.g. electricity, thermal energy, water, etc) and wastewater emissions for individual dairy products within multi-product manufacturing plants. The average operational data for individual products were normalised to maintain industry confidentiality and then used as an industry specific allocation matrix. The quantity of raw milk required per product is based on the milk solids basis to account for dairy by-products that would otherwise be neglected. Results and Discussion Applying fixed type allocation methods (e.g. economic) for all input and outputs based on the quantity of product introduces order of magnitude sized deviations from physico-chemical allocation in some cases. The error associated with the quality of the whole of factory plant data or truncation error associated with setting system boundaries is insignificant in comparison. The profound effects of the results on systems analysis are discussed. The results raise concerns about using economic allocation as a default when allocating intra-industry sectoral flows (i.e. mass and process energy) in the absence of detailed technical information. It is recommended that economic allocation is better suited as a default for reflecting inter-industry sectoral flows. Conclusion The study highlights the importance of accurate causal allocation procedures that reflect industry-specific production methods. Generation of industry-specific allocation matrices is possible through a process of substitution/subtraction and optimisation. Allocation using such matrices overcomes the inherit bias of mass, process energy or price allocations for a multi-product manufacturing plant and gives a more realistic indication of resource use or emissions per product. The approach appears to be advantageous for resource use or emissions allocation if data is only available on a whole of factory basis for several plants with a similar level of technology. Recommendation and Perspective The industry specific allocation matrix approach will assist with allocation in multi-product LCAs where the level of technology in an industry is similar. The matrix will also benefit dairy manufacturing companies and help them more accurately allocate resources and impacts (i.e. costs) to different products within the one plant. It is recommended that similar physico-chemical allocation matrices be developed for other industry sectors with a view of ultimately coupling them with input-output analysis.     

Absolute Sustainability‐Based Life Cycle Assessment (ASLCA): A Benchmarking Approach to Operate Agri‐food Systems within the 2°C Global Carbon Budget

Given the increasing environmental impacts associated with global agri‐food systems, operating and developing these systems within the so‐called absolute environmental boundaries has become crucial, and hence the absolute environmental sustainability concept is particularly relevant. This study introduces an approach called absolute sustainability‐based life cycle assessment (ASLCA) that informs the climate impacts of an agri‐food system (on any economic level) in absolute terms. First, a global carbon budget was calculated that is sufficient to limit global warming to below 2°C. Next, a share of the carbon budget available to the global agri‐food sector was estimated, and then it was shared between agri‐food systems on multiple economic levels using four alternative methods. Third, the climate impacts of those systems were calculated using life cycle assessment methodology and were benchmarked against those carbon budget shares. This approach was used to assess a number of New Zealand agri‐food systems (agri‐food sector, horticulture industries and products) to investigate how these systems operated relative to their carbon budget shares. The results showed that, in 2013, the New Zealand agri‐food systems were within their carbon budget shares for one of the four methods, and illustrated the scale of change required for agri‐food systems to perform within their carbon budget shares. This method can potentially be extended to consider other environmental impacts with global boundaries; however, further development of the ASLCA is necessary to account for other environmental impacts whose boundaries are only meaningful when defined at a regional or local level.     

Life cycle assessment applying planetary and regional boundaries to the process level: a model case study

The International Journal of Life Cycle Assessment - The planetary boundaries framework contains regional boundaries in addition to global boundaries. Geographically resolved methods to assess...     

中国省域环境可持续性时空格局及其影响因素

人类活动已成为人类世背景下全球环境变化的主要驱动力,因此将其合理调控在地球环境边界之内是实现可持续发展的前提条件。作为全球尺度的环境边界,行星边界秉持"地球系统观",为统筹不同区域的环境可持续性评估提供了新视角。以行星边界为切入点,评估中国省域主要人类活动的环境可持续性状况,揭示其时空格局演变及社会经济影响因素。结果表明：（1）中国省域碳、氮、磷排放的不可持续性北部整体高于南部,分异程度随时间逐步拉大,不可持续省份数量的占比均已超出2/3;省域水、土地利用的可持续性南部整体高于北部,保持相对稳定,可持续省份数量的占比均已超出3/4。（2）中国省域环境可持续性大体受人口、经济、技术等因素的综合影响,且各类环境要素的可持续性之间存在一定的协同效应。其中,各类环境可持续性均受人口规模的正向驱动,同时氮、磷、水和土地的可持续性均受农业活动的负向驱动。（3）碳排放可持续性主要受能源消费强度的负向驱动,而磷排放可持续性同时受人口城镇化率的负向驱动,水可持续性受二产占比的负向驱动。基于行星边界的环境可持续性研究,可为区域合理界定和有效承担全球环境可持续性责任提供科学参考。     

A Dutch Approach to the European Directive on Integrated Pollution Prevention and Control: Using Life-Cycle Assessment for the Integrated Assessment of Technologies

In light of the European Directive on Integrated Pollution Prevention and Control (IPPC Directive), traditional environmental regulation can be improved using the framework of industrial ecology. The objective of the IPPC Directive is to achieve a high level of protection of the environment as a whole (Article 1) by applying the best available techniques (BAT). In essence, the IPPC Directive obliges member states of the European Union to include considerations such as resources, energy, waste, and multimedia emissions when permitting industrial installations. This is a marked contrast to traditional environmental regulation that focuses on individual media of an individual site. In order to take all considerations into account, an integrated assessment of technologies is needed, for which a standard method is currently lacking.
In this article, a systematic approach is introduced for the integrated assessment of IPPC technologies using life-cycle assessment (LCA), a form of environmental assessment that can be broadened to an overall assessment of environmental, economic, and social aspects. This systematic approach has proven to be successful for the environmental assessment of the described cases. It is suggested here that weighting can be omitted for the evaluation of IPPC technologies. Leaving the weighting step to competent authorities of member states and allowing them to consider local issues provides maximum opportunity for the subsidiarity and flexibility principles of the IPPC Directive.     

Optimality analysis of Th1/Th2 immune responses during microparasite-macroparasite co-infection, with epidemiological feedbacks

Individuals are typically co-infected by a diverse community of microparasites (e.g. viruses or protozoa) and macroparasites (e.g. helminths). Vertebrates respond to these parasites differently, typically mounting T helper type 1 (Th1) responses against microparasites and Th2 responses against macroparasites. These two responses may be antagonistic such that hosts face a 'decision' of how to allocate potentially limiting resources. Such decisions at the individual host level will influence parasite abundance at the population level which, in turn, will feed back upon the individual level. We take a first step towards a complete theoretical framework by placing an analysis of optimal immune responses under microparasite-macroparasite co-infection within an epidemiological framework. We show that the optimal immune allocation is quantitatively sensitive to the shape of the trade-off curve and qualitatively sensitive to life-history traits of the host, microparasite and macroparasite. This model represents an important first step in placing optimality models of the immune response to co-infection into an epidemiological framework. Ultimately, however, a more complete framework is needed to bring together the optimal strategy at the individual level and the population-level consequences of those responses, before we can truly understand the evolution of host immune responses under parasite co-infection.     

A Systems Approach to Sustainable Technical Product Design

Many existing methods for sustainable technical product design focus on environmental efficiency while lacking a framework for a holistic, sustainable design approach that includes combined social, technical, economic, and environmental aspects in the whole product life cycle, and that provides guidance on a technical product development level. This research proposes a framework for sustainable technical product design in the case of skis. We developed a ski under the Grown brand, benchmarked according to social, environmental, economic, and technical targets, following an initial sustainability assessment, and delivered the first environmental life cycle assessment (ELCA) and the first social life cycle assessment (SLCA) of skis. The framework applies a virtual development process as a combination of ELCA to calculate the environmental footprint as carbon equivalents of all materials and processes and a technical computer‐aided design (CAD) and computer‐aided engineering (CAE) simulation and virtual optimization using parameter studies for the nearly prototype‐free development of the benchmarked skis. The feedback loops between life cycle assessment (LCA) and virtual simulation led to the elimination of highly energy intensive materials, to the pioneering use of basalt fibers in skis, to the optimization of the use of natural materials using protective coatings from natural resins, and to the optimization of the production process. From an environmental perspective, a minimum 32% reduction in carbon equivalent emissions of materials in relation to other comparably performing skis has been achieved, as well as a pioneering step forward toward transparent communication of the environmental performance by the individual, comparable, and first published ski carbon footprint per volume unit.     

Adopting Lead-Free Electronics: Policy Differences and Knowledge Gaps

For more than a decade, the use of lead (Pb) in electronics has been controversial: Indeed, its toxic effects are well documented, whereas relatively little is known about proposed alternative materials. As the quantity of electronic and electrical waste (e‐waste) increases, legislative initiatives and corporate marketing strategies are driving a reduction in the use of some toxic substances in electronics. This article argues that the primacy of legislation over engineering and economics may result in selecting undesirable replacement materials for Pb because of overlooked knowledge gaps. These gaps include the need for: assessments of the effects of changes in policy on the flow of e‐waste across state and national boundaries; further reliability testing of alternative solder alloys; further toxicology and environmental impact studies for high environmental loading of the alternative solders (and their metal components); improved risk assessment methodologies that can capture complexities such as changes in waste management practices, in electronic product design, and in rate of product obsolescence; carefully executed allocation methods when evaluating the impact of raw material extraction; and in‐depth risk assessment of alternative end‐of‐life (EOL) options. The resulting environmental and human health consequences may be exacerbated by policy differences across political boundaries. To address this conundrum, legislation and policies dealing with Pb in electronics are first reviewed. A discussion of the current state of knowledge on alternative solder materials relative to product design, environmental performance, and risk assessment follows. Previous studies are reviewed, and consistent with their results, this analysis finds that there is great uncertainty in the trade‐offs between Pb‐based solders and proposed replacements. Bridging policy and knowledge gaps will require increased international cooperation on materials use, product market coverage, and e‐waste EOL management.     

A building sector related procedure to assess the relevance of the usage phase

Intention, Goal, Scope, Background  Life Cycle Assessments (LCA) are increasingly used in the building sector as a decision-support tool in the design phase of buildings to enable environmentally sound choices of materials and products. The practice in Europe of today regarding product choices is mainly based on cradle to gate LCAs and is quite commonly disregarding the usage phase. The main reason for this is the fragmented structure of the building sector in which the application of specific materials and products is unknown to the manufacturer. The environmental information supplied by the manufacturers to the designers of buildings only relates to the production phase, limited to typical cradle to gate data. A specific material or product choice can however have a considerable impact during the usage phase. Predicting the magnitude of an environmental load originating in the usage phase, as the result of a building product can in some cases be based on information on both product- and building level. To date there are no structured procedures for the inclusion of this information in LCA-studies, even-though it is desirable to include the full product life cycle when including environmental parameters in a product choice. Objective  A procedure for assessing the relevance and the possibility to include the usage phase in a structured way is proposed. Considerable effort has also been put into explaining the underlying obstacles of today’s practice in handling the connection between the choice of building products and its resulting impacts in the usage phase. Methods  The proposed procedure is primary based on experiences and findings from a comprehensive study on maintenance of floor coverings, together with an inventory of the state of the art regarding LCA in the building sector. Results and Discussion  The procedure is divided into two steps where the first step is a preliminary estimate of the relevance of the usage phase in a building product comparison. Based on this step, step two can be entered. Step two is a judgement of the possibility to quantify the potential environmental loads that can occur in the usage phase. For step one, four different types of sources of potential environmental loads have been found; emissions from the product to the indoor environment, emissions from the product to the outdoor environment, influence on the resource flow in a building, and finally the demands for maintenance, leading to other recurring loads. For step two, the focus in this article is on maintenance, for which a model structure is proposed as a base for the development of a model to estimate the environmental loads. The three other sources of environmental loads are handled more briefly. Conclusions  The usage phase should to a larger extend be regarded in a product choice situation, when LCA is used as a tool. First, the relevance of including the usage phase should be assessed. Second, the possibility to estimate the environmental loads should be considered. The reason for an exclusion of the usage phase should more clearly be explained, if it is due to lack of relevance or data/models. Recommendation and Outlook  The proposed procedure shall be regarded as a way to obtain preliminary estimates of the relevance and possibilities to include the usage phase in a product choice situation. Thereby, the handling of the usage phase by the suggested procedure is not a method for estimating the environmental loads but rather a procedure for an inventory of the relevance and possibility to estimate the environmental loads.     

Proposal of a method for allocation in building-related environmental LCA based on economic parameters

Application and development of the LCA methodology to the context of the building sector makes several building specific considerations necessary, as some key characteristics of products in the building sector differ considerably from those of other industrial sectors. The largest difference is that the service life of a building can stretch over centuries, rather than decades or years as seen for consumer products. The result of the long service life is that it is difficult to obtain accurate data and to make relevant assumptions about future conditions regarding, for example, recycling. These problems have implications on the issue of allocation in the building sector, in the way that several allocation procedures ascribe environmental loads to users of recycled or reused products and materials in the future which are unknown today. The long service life for buildings, building materials and building components, is associated with the introduced concept of a virtual parallel time perspective proposed here, which basically substitutes historical and future processes and values with current data. Further, the production and refining of raw material as a parallel to upgrading of recycled material, normally contains several intermediate products. A suggestion is given for how to determine the comparability of intermediate materials. The suggested method for allocation presented is based on three basic assumptions: (1) If environmental loads are to be allocated to a succeeding product life cycle, the studied actual life cycle has to take responsibility for upgrading of the residual material into secondary resources. (2) Material characteristics and design of products are important factors to estimate the recyclable amount of the material. Therefore, a design factor is suggested using information for inherent material properties combined with information of the product context at the building level. (3) The quality reduction between the materials in two following product life cycles is indicated as the ratio between the market value for the material in the products. The presented method can be a good alternative for handling the problem of open-loop recycling allocation in the context of the building sector if a consensus for the use of the fictive parallel time perspective and the use of the design factor can be established. This as the use of the time perspective and design factor is crucial to be able to deal with the problem of long service lives for buildings and building materials and the specific characteristics of the same building materials and components built into different building contexts.     

全球变化影响下的生态系统风险的相关理论及联系

全球变化深刻影响着全球生态系统,全球变化胁迫超过一定程度则会导致生态系统恢复力下降,极端事件频发,从而使生态系统服务功能退化甚至丧失.量化全球变化的风险,进而制定恰当的人为适应策略是目前应对全球变化的重要途径.全球变化可能降低生态系统的恢复力,从而导致生态系统脆弱性升高,引发生态系统退化风险.目前,相关研究多依托基于星...     

Allocation, plasticity and allometry in plants

Allocation is one of the central concepts in modern ecology, providing the basis for different strategies. Allocation in plants has been conceptualized as a proportional or ratio-driven process (‘partitioning’). In this view, a plant has a given amount of resources at any point in time and it allocates these resources to different structures. But many plant ecological processes are better understood in terms of growth and size than in terms of time. In an allometric perspective, allocation is seen as a size-dependent process: allometry is the quantitative relationship between growth and allocation. Therefore most questions of allocation should be posed allometrically, not as ratios or proportions. Plants evolve allometric patterns in response to numerous selection pressures and constraints, and these patterns explain many behaviours of plant populations.

In the allometric view, plasticity in allocation can be understood as a change in a plant's allometric trajectory in response to the environment. Some allocation patterns show relatively fixed allometric trajectories, varying in different environments primarily in the speed at which the trajectory is travelled, whereas other allocation patterns show great flexibility in their behaviour at a given size. Because plant growth is often indeterminate and its rate highly influenced by environmental conditions, ‘plasticity in size’ is not a meaningful concept. We need a new way to classify, describe and analyze plant allocation and plasticity because the concepts ‘trait’ and ‘plasticity’ are too broad. Three degrees of plasticity can be distinguished: (1) allometric growth (‘apparent plasticity’), (2) modular proliferation and local physiological adaptation, and (3) integrated plastic responses. Plasticity, which has evolved because it increases individual fitness, can be a disadvantage in plant production systems, where we want to optimize population, not individual, performance.     

Theoretical exploration for the combination of the ecological,energy, carbon,and water footprints: Overview of a footprint family

Over the past two decades, a continuously expanding list of footprint-style indicators has been introduced to the scientific community with the aim of raising public awareness of how humanity exerts pressures on the environment. A deeper understanding of the connections and interactions between different footprints is required in an attempt to support policy makers in the measurement and choice of environmental impact mitigation strategies. Combining a selection of footprints that address different aspects of environmental issues into an integrated system is, therefore, a natural step. This paper starts with the idea of developing a footprint family from which most important footprints can be compared and integrated. On the basis of literature review in related fields, the ecological, energy, carbon, and water footprints are employed as selected indicators to define a footprint family. A brief survey is presented to provide background information on each of the footprints with an emphasis on their main characteristics in a comparative sense; that is, the footprints differ in many aspects more than just the impacts they are addressed. This allows the four footprints to be complementarily used in assessing environmental impacts associated with natural resource use and waste discharge. We evaluate the performance of the footprint family in terms of data availability, coverage complementarity, methodological consistency, and policy relevance and propose solutions and suggestions for further improvement. The key conclusions are that the footprint family, which captures a broad spectrum of sustainability issues, is able to offer a more complete picture of environmental complexity for policy makers and, in particular, in national-level studies. The research provides new insights into the distinction between environmental impact assessment and sustainability evaluation, properly serving as a reference for multidisciplinary efforts in estimating planetary boundaries for global sustainability.     

Where biology meets; or how science advances: Presidential Address to the Linnean Society delivered at the Anniversary Meeting, 24th May 1985

Major advances in 'unrestricted' sciences like biology commonly occur when individual scientists (or techniques) cross conventional discipline boundaries; intra-discipline studies are essential for the consolidation and progress of the science, but are less likely to produce significant insights. 'Restricted' (or exact) sciences ignore variation, and are probably less sensitive to warping from specialization. This generalization is illustrated by recent controversies in evolutionary biology, particularly the neutralism debates of the 1970s, where over-rigid adherence to theoretical models and unjustified assumptions about the effects of gene action were made. The consequence of some of these is shown by considering genetic changes in house mouse (Mus domesticus) populations which were used to demonstrate apparent drift operating on neutral traits, whereas longitudinal studies of closed populations proved that strong natural selection may operate; a proper understanding of genetical forces requires a knowledge both of the history of particular populations and of environmental pressures varying in time and space.