事象のランダム (random) な変化と、変化に対する特異的選択 (selection) 指向性による、事象の高度化・複雑化・精緻化の普遍性について ー Part 1

"Randomness" is the key factor for the evolution of various phenomena.
Directions of the evolution are determined by some specific windows of selections for random changes.

わたしのnoteにおいては、最新の科学・経済・社会等の問題に関して、英語の記事を引用し、その英文が読み易いように加工し、「英語の勉強ツール」と「最新情報収集ツール」としてご利用頂くことをmain missionとさせて頂きます。勿論、私論を書かせて頂くこともしばしです。

今回は、様々な事象のランダムな変化と、それらの変化に対する特異的な選択指向性が、事象の変化の方向性を決め、結果として、現在の在り様を形成してきたのではないかという、かなり普遍的な仮説を支持する様な、いくつかの記事をシリーズで紹介していきたいと思います。

Randomness / From Wikipedia, the free encyclopedia
In common parlance (専門用語/pɑ́rləns), randomness is the apparent or actual lack of pattern or predictability in events. A random sequence [of events, symbols or steps] often has no order and does not follow an intelligible pattern or combination. Individual random events are, by definition, unpredictable, but if the probability distribution <確率分布：確率変数に対して、各々の値をとる確率を表したもの> is known, the frequency of different outcomes over repeated events (or "trials") is predictable. Randomness applies to concepts of chance, probability, and information entropy. The fields of mathematics, probability, and statistics use formal definitions of randomness. In <statistics>, a random variable <確率変数：ある試行によって得られるすべての結果を指す変数であり、実際に試行、観測を行うまで何の結果が得られるか分からないもの> is an assignment of a numerical value to each possible outcome of an event space. Random variables can appear in random sequences. A random process is a sequence of random variables whose outcomes do not follow a deterministic pattern, but follow an evolution described by probability distributions. These and other constructs are extremely useful in probability theory and the various applications of randomness.

<In biology>
The modern evolutionary synthesis ascribes [the observed diversity of life] to [random genetic mutations followed by natural selection]. The latter (selected mutations) retains some random mutations in the gene pool due to [the systematically improved chance for survival and reproduction] that those mutated genes confer on individuals who possess them. Several authors also claim that evolution (and sometimes development) requires a specific form of randomness, namely the introduction of qualitatively new behaviors. Instead of the choice of one possibility among several pre-given ones, this randomness corresponds to the formation of new possibilities. The characteristics of an organism arise to some extent deterministically (e.g., under the influence of genes and the environment), and to some extent randomly. For example, the density of freckles (そばかす/frékl) that appear on a person's skin is controlled by genes and exposure to light; whereas the exact location of individual freckles seems random. As far as behavior is concerned, randomness is important if an animal is to behave in a way that is unpredictable to others. For instance, insects in flight tend to move about with random changes in direction, making it difficult for pursuing predators to predict their trajectories.

Would humans evolve again if we rewound (巻き戻す、巻き直す/rewind/riwáind) time?

By James Horton and Tiffany Taylor / BBC Future / 10th July 2019

この記事では、時間を遡らせて、再度、生物の進化のプロセスを歩ませた場合、人類は同じような進化の道を辿るかどうかに関する考察です。
ここで、最初に「今回は、様々な事象のランダムな変化と、それらの変化に対する特異的な選択指向性が、事象の変化の方向性を決め、結果として、現在の在り様を形成してきたのではないかという、かなり普遍的な仮説を支持する様な、いくつかの記事をシリーズで紹介していきたいと思います。」と書きましたが、時間を遡った場合に、DNAのランダムな変化の多様性はあまり変わらないと思いますが、特異的な選択指向性 (偶然による自然淘汰の指向性) に少しでも変化があれば、人類の進化の道は異なっていたと考えられます。

Our species emerged as a result of a mind-bending (びっくりするような) number of random events and mutations, but it may also have been inevitable that humans, or something like us, would walk the Earth.

What would happen if the hands of time (時計の針) were turned back to an arbitrary (任意の/ɑ́rbətrèri) point in our evolutionary history and we restarted the clock? American palaeontologist (古生物学者) Stephen Jay Gould proposed this famous thought experiment in the late 1980s – and it still grips the imagination of evolutionary biologists today.

Gould reckoned that if time was rewound, then evolution would drive life down a completely different path and humans would never re-evolve. In fact, he felt humanity’s evolution was so rare that we could replay the tape of life a million times and we wouldn’t see anything like Homo sapiens arise again.

His reasoning was that chance events play a huge role in evolution. These include enormous mass extinction events – such as cataclysmic asteroid impacts and volcanic eruptions. But chance events also operate at the molecular scale. Genetic mutation, which forms the basis of evolutionary adaptation, is reliant on chance events.

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Put simply, evolution is the product of random mutation. A rare few mutations can improve an organism’s chance of survival in certain environments over others. The split from one species into two starts from such rare mutations that become common over time. But further random processes can still interfere, potentially leading to a loss of beneficial mutations and increasing harmful mutations over time. This inbuilt randomness ought to suggest you’d get different life forms if you replayed the tape of life.

Of course, in reality, it’s impossible to turn back the clock in this way. We’ll never know for sure just how likely it was to have arrived at this moment as we are. Fortunately, however, experimental evolutionary biologists do have the means to test some of Gould’s theories on a microscale with bacteria.

Humanity’s evolution has relied upon millions of random events and chance mutations, but natural selection also lends a guiding hand. Microorganisms divide and evolve very quickly. We can therefore freeze billions of identical cells in time and store them indefinitely. This allows us to take a subset of these cells, challenge them to grow in new environments and monitor their adaptive changes in real time. We can go from the “present” to the “future” and back again as many times as we like – essentially replaying the tape of life in a test tube.

Many bacterial evolution studies have found, perhaps surprisingly, that evolution often follows very predictable paths over the short term, with the same traits and genetic solutions frequently cropping up (現れる). Consider, for example, a long-term experiment, in which 12 independent populations of Escherichia coli founded by a single clone, have been continuously evolving since 1988. That’s over 65,000 generations – there have only been 7,500-10,000 generations since modern Homo sapiens appeared. All the evolving populations in this experiment show higher fitness, faster growth and larger cells than their ancestor. This suggests that organisms have some constraints on how they can evolve.

There are evolutionary forces that keep evolving organisms on the straight and narrow. Natural selection is the “guiding hand” of evolution, reigning in the chaos of random mutations and abetting (けしかける、唆す、ほう助する、教唆する) beneficial mutations. This means many genetic changes will fade from existence over time, with only the best enduring. This can also lead to the same solutions of survival being realised in completely unrelated species.

We find evidence for this in evolutionary history where species that are not closely related, but share similar environments, develop a similar trait. For example, extinct pterosaurs (翼竜 [よくりゅう]は、中生代に生息していた爬虫類の一目、翼竜目に属する動物の総称。初めて空を飛んだ脊椎動物である。) and birds both evolved wings as well as a distinct beak (くちばし), but not from a recent common ancestor. So essentially wings and beaks evolved twice, in parallel, because of evolutionary pressures.

Pterosaurs evolved wings and beaks that were very different from those we see in modern birds. But genetic architecture is also important. Not all genes are created equal: some have very important jobs compared to others. Genes are frequently organised into networks, that are comparable to circuits, complete with redundant switches and “master switches”. Mutations in “master switches” naturally result in much bigger changes, because of the knock-on effect felt by all genes under its control. This means that certain locations in the genome will contribute to evolution more frequently, or with a larger effect, than others – biasing (偏らせる/báiəs) evolutionary outcomes.

But what about the underlying physical laws – do they favour predictable evolution? At very large scales, it appears so. We know of many laws governing our universe that are certain. Gravity, for example – for which we owe our oceans, thick atmosphere and the nuclear fusion in the sun that showers us with energy – is a predictable force. Isaac Newton’s theories, based on large scale deterministic forces, can also be used to describe many systems on large scales. These describe the universe as perfectly predictable.

If Newton’s view was to remain perfectly true, the evolution of humans was inevitable. However, this comforting predictability was shattered by the discovery of the contradictory but fantastical world of quantum mechanics in the 20th Century. At the smallest scales of atoms and particles, true randomness is at play – meaning our world is unpredictable at the most fundamental level.

It is unlikely Homo sapiens would evolve again if we rewound time, but something similar would appear. This means that the broad “rules” for evolution would remain the same no matter how many times we replayed the tape. There would always be an evolutionary advantage for organisms that harvest solar power. There would always be opportunity for those that make use of the abundant gases in the atmosphere. And from these adaptations, we may predictably see the emergence of familiar ecosystems. But ultimately, randomness, which is built into many evolutionary processes, will remove our ability to “see into the future” with complete certainty.

There is a problem in astronomy that acts as a fitting analogy. In the 1700s, a mathematical institute offered a prize for solving the “three-body problem”, involving accurately describing the gravitational relationship and resultant orbits of the sun, Earth and moon.
The winner essentially proved that the problem couldn’t be solved exactly. Much like the chaos introduced by random mutations, a little bit of starting error would inevitably grow, meaning that you couldn’t easily determine where the three bodies would end up in the future. But as the dominant partner, the sun dictates the orbits of all three to an extent – allowing us to narrow the possible positions of the bodies to within a range.

This is much like the guiding hands of evolution, which tether (束縛する、拘束する/téðər) adapting organisms to familiar routes. We may not be entirely sure where we’d end up if we rewound time, but the paths available to evolving organisms are far from (～から懸け離れている、決して～ではない) limitless. And so maybe humans would never appear again, but it’s likely that whatever alien (性質の異なる、異質な/éiliən) world replaced ours, it would be a familiar place.

Stephen Jay Gouldは、進化のプロセスにおいて、「 if time was rewound, then evolution would drive life down a completely different path and humans would never re-evolve.」と主張し、また、「His reasoning was that chance events play a huge role in evolution.」即ち、生物の進化において、random mutationsがベースとなり、それらのmutationsをselectする「chance events」が進化の方向性において大きな役割を果たしていると主張しています。また、本文中には、「Humanity’s evolution has relied upon millions of random events and chance mutations, but natural selection also lends a guiding hand.」とある様に、「natural selection」というrandom mutationsに対する特異的な選択指向性が、進化の方向性を決めていると解釈できると思います。