Adaptation: responding to environmental challenges

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Since Jean-Baptiste de Lamarck and Charles Darwin, we know that living beings were not created once and for all by a divine force to constitute a harmonious nature. The harmony of nature is only a matter of the imagination and every living species must constantly respond to the challenges imposed by the environment, both biotic and abiotic. Life has only been able to proliferate and diversify on Earth by developing adaptive capacities. They allow it to adapt to an often hostile environment, in any case always changing, either seasonally or over the long term. These adaptations cover all their biological characteristics, from physiology to morphology and ethology. They concern all levels of life organization, from the individual level to the population level.

1. Adaptations at the individual level

Physiological adaptation commonly refers to changes at the individual level. An individual’s metabolism changes temporarily, by regulating gene expression, in response to external conditions. Such coping mechanisms are essential for life and reproduction. The selection pressure for their development has certainly been very strong. There are many examples of this in all types of living organisms.

In the animal kingdom, let us mention two of the most famous. In humans, exposure to sunlight, especially ultraviolet rays, leads to the synthesis of a black pigment, melanin, which stops these rays and protects the skin from their mutagenic effects (see Cellular impact of solar UV). In all terrestrial mammals, the coat varies in density according to the seasons, and allows adaptation to seasonal temperature differences.

Encyclopédie environnement - adaptation - stomates ouverts ou fermés - leaf showing open or closed stomata
Figure 1. Section of a leaf showing open or closed stomata. [Source: svtmontacourbevoie.wordpress.com]
In plants, fixed to the ground and therefore unable to protect themselves from adverse external situations, these physiological adaptation mechanisms are even more essential (see The fixed life of plants and its constraints). Among the most important environmental factors for plants, there is of course the sun and water. Sunlight allows photosynthesis to proceed smoothly. It is easy to observe, in nature or even in house plants, the phototropism that directs the branches towards sunlight. They grow faster on the bright side than on the shaded side. But in the presence of high temperatures, it is also vital to avoid water loss. This is the role of foliar stomata that control the gas exchanges associated with photosynthesis. Through these openings in the leaf surface, plants absorb carbon dioxide and emit oxygen. But stomata then promote the evaporation of water or evapotranspiration. When the water loss is too high, they close and evaporation is stopped.

For plants living in arid regions, even more drastic adaptations of their own morphology were required (see the example of cactoid plants in the text Inheritance or convergence?) or the example presented at the beginning of this article by Welwitschia mirabilis, an endemic plant in the Namib Desert in southern Africa. But this is a second level of adaptation: that of the populations.

2. Population-level adaptations

Hereditary changes promote the adaptation of a population to a constantly changing environment and lead, in the long term, to the evolution of species. The living world has thus acquired mechanisms that allow it to be more and more autonomous from the environment: for example, homeothermicThe characteristic of animal species (birds, mammals) whose internal environment (blood and lymph) maintains a constant temperature, regardless of the temperature of the external environment, within very wide limits. animals or the ability to store water. Our species has even acquired the ability to modify the environment itself. It was on the mechanism of these evolutionary adaptations that Lamarck and Darwin’s theories radically diverged. The first explained biological evolution – “transformationism” at the time – by a direct and inheritable effect of the environment on individuals (see Theory of Evolution: misunderstanding and resistance and Lamarck and Darwin: two divergent visions of the living world). Darwin explained it by selecting, from random genetic variation – the “descent with modifications” – individuals capable of leaving the most descendants in a given environment. His theory has been verified by more than a century of research. This “Darwinian process” promotes genotypesInformation carried by the genome of an organism, contained in each cell in the form of deoxyribonucleic acid (DNA). In the DNA molecule, it is the sequence of nucleotides that constitutes the genetic information. that are most adapted to the environment and results, over generations, in the modification of species.

Figure 2. Lactose tolerance is widespread in Northern Europe, some parts of Africa and the Middle East. This can be linked to the abundant consumption of fresh milk. The mutations that caused this tolerance in adults would have appeared independently depending on the region. The persistence of different mutations in geographically separated human populations is an example of a convergent evolution in humans. [Source: Adapted from ref. [1]]
In the text “Genetic polymorphism and selection“, Michel Veuille cites the case of mutations that have become advantageous in the environmental context created by agricultural and medical techniques. Mutations that have rapidly established themselves in populations: insecticide resistance genes in insects, antibiotics in bacteria, etc. He also gives the example of lactase in the human species. The gene encoding this enzyme, which digests lactose, is active in infants and inactive in adults. Since the practice of Neolithic breeding – about 10,000 years ago – variants of this gene (alleles) that allow the synthesis of lactase even in the adult state have become advantageous. They are found in most peoples at variable frequencies, but all the more so as livestock farming is more important (Figure 2) [1]. This example also illustrates the fact that, contrary to a widespread idea, evolution is still ongoing in our species. Moreover, we do not see why and how it could be otherwise. As for the mechanism of natural selection, which can be viewed as the “engine” of adaptation and evolution, it is often misunderstood. The text “Inheritance or convergence? The winding paths of species evolution” talks about it at some length and presents it in a simple and precise way.

3. Epigenetic memory

A third level of adaptation falls between the two previous levels. Like the first level, it is individual and plays only on the regulation of gene expression, without intervention on their structure. But this third level involves a transient hereditary transmission of these regulations. This allows descendants, over a few generations, to benefit from the adaptive response of their parents. This is referred to as epigenetic memory or transgenerational effect (see Epigenetics, the genome and its environment). This type of phenomenon has been known for a long time, but it has remained relatively exceptional. The first known cases mainly concerned paramecia, unicellular eukaryotesUnicellular or multicellular organisms whose cells have a nucleus and organelles (endoplasmic reticulum, Golgi apparatus, various plasters, mitochondria, etc.) delimited by membranes. Eukaryotes are, along with bacteria and archaea, one of the three groups of the living organism. . Since 2000, examples have multiplied and cover a wide range of characters. They have often been observed in response to environmental stresses, but also in the case of maternal behaviours in rats [2].

Encyclopedie environnement - interactions adaptation - Arabidopsisthalian
Figure 3. Experimentation on Arabidopsis thaliana seedlings. [Source: photo © CEA-Grenoble]
We will cite two very similar examples that link to the text “The genome between stability and variability“. These are experiments showing the existence of mechanisms for stimulating this variability by external agents. The first example concerns the drosophilaSmall fly also called the vinegar fly or fruit fly. Because of its ease of breeding, the fruit fly is a model species in genetic research. [3] ; the second, the simplest to describe, a plant: the thale cress (Arabidopsis thaliana) [4]. In the latter example, UV irradiations or the action of flagellin (a trigger for plant defence systems) stimulate homologous recombinationType of genetic recombination where nucleotide sequences are exchanged between identical or similar DNA molecules.. This stimulation is transmitted to the descendants over at least four generations. In this way, the repair of possible DNA damage created by stressors will be facilitated. But at the same time, genomic variations will also be increased. This phenomenon of increased genetic variability due to environmental stresses, already well known in bacteria since the 1980s, therefore seems generalizable to plants and animals, with transgenerational effect. More and more research confirms this [5].

About this epigenetic memory, some popularization authors and even some scientists speak of a return to lamarckismMovement related to Lamarck’s ideas. Often reduced to the idea of the transmission of acquired traits, although Jean-Baptiste Lamarck’s transformist theory is much broader than that. (or even to lyssenkoismMovement relating to Lyssenko’s ideas which led to the establishment in the USSR of a policy of genetic and agricultural control. Nowadays, Lyssenkism is regularly used metaphorically to denounce the manipulation or distortion of the scientific method to support a pre-determined conclusion, often linked to a social or political objective.!). This is inappropriate because epigenetic changes are reversible, they can only allow temporary adaptation to stress. Not based on DNA sequences changes, they do not modify the genetic structure of the lineage concerned and a process of speciationEvolutionary process leading to the emergence of new living species that individualize from populations belonging to an original species. is therefore excluded. However, although there are only few studies on this subject, we may imagine that this type of transgenerational memory is complementary to the Darwinian process. It will allow individuals under environmental stress to survive and reproduce by transmitting protection to their descendants. If the stress disappears, the response fades and the population returns to its previous life; if it persists, this response allows the population to maintain itself and gives it time to adapt through the Darwinian process. The fact that genome variation mechanisms are stimulated for a few generations will facilitate this adaptation by increasing genetic variability.

 


References and notes

Cover image. Welwitschia mirabilis, a unique plant of the Namib Desert tSource: Petr Kosina via Visual hunt (CC BY-NC 2.0)]

[1] Leonardi M, Gerbault P, Thomas MG & Burger J (2012) The evolution of lactase persistence in Europe. A synthesis of archaeological and genetic evidence. J. Int. Dairy J. 22:88-97

[2] Zhang TY et al (2013) Epigenetic mechanisms for the early environmental regulation of hippocampal glucocorticoid receptor gene expression in rodents and humans. Neuropsychopharmacology. 38) :111-123

[3] Laurençon A et al (1998) Genetic variations and their regulation: the fruit fly has a lot to teach us. Med/Sci. No. 11, vol. 14, Nov. 98. See also: Science in the Present, 2000. Editions Enclyclopædia Universalis, p.148-152

[4] Molinier J et al (2006) Trangenerational memory of stress in plants. Nature 442:1046-1049

[5] Boyko A, Kovalchuk I. (2011) Genome instability and epigenetic modification-heritable responses to environmental stress? Curr Opin Opin Plant Biol 14:260-266


The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: BREGLIANO Jean-Claude (April 15, 2019), Adaptation: responding to environmental challenges, Encyclopedia of the Environment, Accessed October 11, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/life/adaptation-responding-to-environmental-challenges/.

The articles in the Encyclopedia of the Environment are made available under the terms of the Creative Commons BY-NC-SA license, which authorizes reproduction subject to: citing the source, not making commercial use of them, sharing identical initial conditions, reproducing at each reuse or distribution the mention of this Creative Commons BY-NC-SA license.

适应:应对环境挑战

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  自从让·巴蒂斯特·拉马克和达尔文以来,我们知道生命不是由神为了组成一个和谐的自然而一次性就创造出来的。自然的和谐只是一种想象,每一个活着的物种都必须不断地应对环境带来的生物和非生物挑战。生命只有通过发展适应能力,才能在地球上增殖和多样化。这些适应能力促使生物适应一个经常自身不利的环境,而且这个环境经常变化,有季节性或长期性的变化。这些适应性涵盖了它们从生理学到形态学和人行为学的所有生物学特征,涉及生命组织的各个层次,从个体层面到种群层面。

1. 在个体层面上的适应

  生理适应通常是指个体层面上的变化,通过调节基因表达,一个个体通过新陈代谢的暂时变化对外界环境条件作出应对。这种应对机制对生命和繁殖至关重要。选择压力对于这些生命的生存发育确实是非常大的。在所有类型的生物体中都有许多这样的例子。

  在动物王国里,让我们提到两个最著名的例子。对于人类来说,暴露在阳光下,特别是紫外线下,会导致一种黑色色素——黑色素的合成,黑色素能阻止这些光线,保护皮肤免受其诱变效应(参阅太阳紫外线的细胞影响)。在所有的陆生哺乳动物中,皮毛的密度因季节而变化,从而适应季节性的温度差异。

环境百科全书-适应-叶片切片气孔
图1. 叶片切片图显示开放或关闭的气孔。[资料来源:svtmontacourbevoie.wordpress.com]

  对植物来说,由于固着在地面上,从而无法保护自己免受不利的外部环境的影响,所以这些生理适应机制更加重要(参阅“植物的固定寿命及其限制条件”)。在这些对植物最重要的环境因素中,当然太阳和水是最重要的。阳光可以使光合作用顺利进行。在自然环境下,甚至在室内,都很容易观察到植物的趋光性引导枝杈向阳光生长。有光一侧的植物比无光一侧的长得更快。但在高温情况下,避免水分流失也至关重要。叶面气孔控制与光合作用相关的气体交换的作用,通过叶片表面的这些开口,植物吸收二氧化碳并释放氧气。但气孔也会促进水的蒸发或蒸腾作用。当水分流失过多时,气孔会关闭,水分蒸发停止。

  对于生活在干旱地区的植物,它们甚至需要自身进行更显著的形态上适应(请参见文章“遗传还是趋同?”中的仙人掌状植物的例子)或者是本文开头的百岁兰(Welwitschia mirabilis)的例子,那是非洲南部纳米布沙漠特有的一种植物。但这是第二个层面的适应:即种群的适应。

2. 种群层面上的适应

  遗传的变化促进了种群对不断变化的环境的适应,而且从长远来看,导致了物种的进化。生物界由此获得了使自身更加主动适应环境的机制:例如,恒温动物(鸟类、哺乳动物的特征。在非常大的温度范围内,不管外部环境的温度如何变化,其体内环境(血液和淋巴)仍保持温度不变),还有储存水的能力。我们人类甚至已经获得了改变环境本身的能力。正是基于这些进化适应的机制,拉马克和达尔文的理论发生了根本上的分歧。第一个是通过环境对个体的直接和可遗传的效应来解释了生物进化——当时的“进化论”(参见“进化理论:误解和抵制”和“拉马克和达尔文:生物界的两种不同的观点”)。达尔文通过从随机的遗传变异—“有改变的后代”—中选择能够在特定环境中留下最多后代的个体来解释这一点。他的理论已经被一个多世纪的研究所证实。达尔文理论中的进化过程能促进最适应环境的基因型(生物体的基因组所携带的信息,以脱氧核糖核酸(DNA) 的形式包含在每个细胞中。在 DNA 分子中,构成遗传信息的是核苷酸的序列)的生存,在经过很多世代后,最终导致物种的改变。

环境百科全书-适应-乳糖酶基因变异
图2. 乳糖耐受性在北欧、非洲部分地区和中东地区广泛存在。这可能与大量食用鲜奶有关。在成人中导致这种耐受性的突变情况可能会单独出现,这取决于所在区域。在地理上分离的人类种群中,持续存在的不同突变是人类趋同进化的一个例子。[资料来源:改编自参考文献[1]] (图2 Peccentage of adult population which can drink milk.
能喝牛奶的成年人口百分比)

  在文章“遗传多态性和选择”中,米歇尔·维耶(Michel Veuille)引用了在农业和医学技术创造的环境中变得有利的突变案例。突变迅速在生物种群中确立,如:昆虫中的杀虫剂耐药性基因、细菌中的耐抗生素基因等。他还举出了人体中乳糖酶的例子。编码消化乳糖的酶的基因在婴儿中活跃,在成人中不活跃。自从新石器时代(大约一万年前)育种?以来,这个基因(等位基因)的变异对在成年状态下合成乳糖酶有利。这些变异在大多数人中发生的频率不同,但在畜牧业更加发达的地区频率更高(图 2)[1]。这个例子证明了一个事实,即物种的进化仍在进行,这与人们普遍的观点相反。而且我们也不认为还有其它可能的情况。至于自然选择的机制,它可以被看作是适应和进化的“引擎”,这一观点经常被误解。文章“遗传还是趋同?物种进化的曲折路径”有详细论述,并以一种简单而精确的方式阐述了这个观点。

3. 表观遗传学记忆

  第三个层面上的适应介于之前两个之间。与第一层一样,它是独立作用的, 只参与基因表达的调控,而不干预其结构。然而这一类的适应包含了调节作用的瞬时遗传传递。这个机制使得,即使经过几代之后,其后代仍能够从它们的祖先的适应性反应中获益。这被称为表观遗传记忆或隔代效应(参阅“表观遗传学: 基因组及其环境”)。这种现象早广为人知,但仍然相对特殊。已知的例子主要涉及草履虫,单细胞真核生物(单细胞或多细胞生物,它们具有一个细胞核和由细胞膜分隔的多个细胞器(内质网、高尔基体、各种质体、线粒体等)。与细菌和古细菌一起,真核生物也是三大类群生物之一。自 2000 年以来,这样的例子大量增加,并涵盖了广泛的生物学特征。表观遗传记忆不但经常在环境胁迫反应的研究中被观察到,而且在老鼠的母性行为中也有被发现[2]

环境百科全书-适应-拟南芥幼苗的实验
图3. 拟南芥幼苗的实验。[资料来源:照片© CEA-Grenoble]

  我们将引用与“平衡在稳定性和变异性之间的基因组”相关的两个非常相似的例子。这些实验表明,存在外部因素刺激这种可变性的机制。第一个案例是果蝇也被称为醋蝇。由于其易于繁殖,果蝇是遗传学研究中的一个模式物种[3];第二个案例是最容易描述的植物:拟南芥[4]。在后一个案例中,紫外线照射或鞭毛虫素(植物防御系统的触发器)的作用刺激了同源重组这是一种核苷酸序列在相同或相似的DNA分子之间交换的遗传重组类型。这种刺激被传递给后代,至少可以传四代,促进对逆境胁迫因素产生的DNA损伤的修复。但与此同时,基因组变异也会增加。这种由于逆境胁迫而导致的遗传变异增加的现象,自20世纪80年代以来就在细菌中广为人知,因此似乎也适用于植物和动物,具有跨代效应。越来越多的研究也证实了这一点[5]

  关于这种表观遗传记忆,一些科普作家,甚至一些科学家提出拉马克主义(与拉马克的思想相关的运动)又回来了。尽管让·巴蒂斯特·拉马克的进化理论涉及的范围要宽广的多,但其通常被简化为获得性遗传(甚至是李森科主义(lyssenkoism)!)。这是不恰当的,因为表观遗传变化是可逆的,它们只能暂时适应逆境,不是基于 DNA 序列的变化,不会改变相关谱系的遗传结构,因此因此,物种形成(一种演化过程,导致新的生物物种从原始物种的个体中分化出来)被排除在外。然而,尽管对这方面的研究很少,我们仍可以想象,这种跨代效应是达尔文过程的补充。这将允许物种个体在环境胁迫下通过向后代传递保护性状来促进生存和繁殖。如果胁迫消失,这种机制就会消失,物种种群会回到以前的生存状态;如果胁迫持续存在,这种机制会允许种群自我维持,并给它时间通过达尔文过程进行适应。基因组变异机制被刺激几代,这就会通过增加遗传变异性促进这种适应。

 


参考资料及说明

封面图片:百岁兰(Welwitschia mirabilis),是纳米布沙漠(Namib Desert)的一种独特植物。来源: Petr Kosina via Visual hunt (CC BY-NC 2.0)]

[1] Leonardi M, Gerbault P, Thomas MG & Burger J (2012) The evolution of lactase persistence in Europe. A synthesis of archaeological and genetic evidence. J. Int. Dairy J. 22:88-97

[2] Zhang TY et al (2013) Epigenetic mechanisms for the early environmental regulation of hippocampal glucocorticoid receptor gene expression in rodents and humans. Neuropsychopharmacology. 38) :111-123

[3] Laurençon A et al (1998) Genetic variations and their regulation: the fruit fly has a lot to teach us. Med/Sci. No. 11, vol. 14,Nov. 98. See also: Science in the Present, 2000. Editions Enclyclopædia Universalis, p.148-152

[4] Molinier J et al (2006) Trangenerational memory of stress in plants. Nature 442:1046-1049

[5] Boyko A, Kovalchuk I. (2011) Genome instability and epigenetic modification-heritable responses to environmental stress? Curr Opin Opin Plant Biol 14:260-266

 


The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: BREGLIANO Jean-Claude (March 14, 2024), 适应:应对环境挑战, Encyclopedia of the Environment, Accessed October 11, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/zh/vivant-zh/adaptation-responding-to-environmental-challenges/.

The articles in the Encyclopedia of the Environment are made available under the terms of the Creative Commons BY-NC-SA license, which authorizes reproduction subject to: citing the source, not making commercial use of them, sharing identical initial conditions, reproducing at each reuse or distribution the mention of this Creative Commons BY-NC-SA license.