🦋 自然选择：环境当考官🦋 Natural Selection: the environment sets the exam
上一片我们看到，基因开关决定细胞造什么蛋白质。可要是环境突然变了——比如树干被煤烟熏黑——谁能活下来？这一片跟 🦋 桦尺蛾进考场。Last leaf, gene switches decide what proteins a cell builds. But when the environment suddenly changes — say, tree trunks blackened by soot — who survives? This leaf joins the 🦋 peppered moths in the exam room.
第 1 步：变异是随机的，不看环境脸色Step 1: Variation is random — it ignores the environment
考试开始前，先认识考生。Before the exam starts, meet the candidates.
桦尺蛾天生有深有浅，深浅来自随机的基因变异。注意：不是蛾子“想”变黑就能变黑，变异只是碰巧发生。树皮浅时，白蛾几乎隐形；树皮深时，黑蛾才占便宜。藏得住，就活得久。Peppered moths come in dark and light, thanks to random gene variation. Note: a moth cannot “decide” to turn black — the variation just happens. On pale bark the white moth nearly vanishes; on dark bark the black moth has the edge. Blend in, live longer.
那工业时代的煤烟，对蛾子做了什么？So what did the soot of the industrial age do to the moths?
第 2 步：环境当考官——藏不住的先被吃Step 2: The environment grades — the obvious get eaten first
工厂煤烟把树干熏黑。鸟从上面看：深色树皮上黑蛾像穿了隐身衣，白蛾一眼被看见。于是黑蛾活下来、生更多，白蛾越来越少。一代又一代，黑蛾比例从一半升到九成以上——不是蛾子变黑了，是黑蛾被“选中”了。Soot from the factories blackened the trunks. From above, birds saw it clearly: on dark bark the black moths wore an invisibility cloak while white moths popped out. So black moths survived and bred more, and white moths dwindled. Generation after generation, black rose from half to over ninety percent — the moths did not turn black; black moths were selected.
那“用进废退”的说法，错在哪？So where does the old “use and disuse” story go wrong?
第 3 步：不是谁想变，就能变Step 3: Wanting to change is not how it works
老说法（拉马克）认为：长颈鹿拼命伸脖子，脖子就变长，还能传给下一代。达尔文看到的是：脖子本来就有长有短，够得到高叶子的才活下来、生更多孩子。长脖子就这样一代代传开——环境“挑”出了它，不是脖子“努力”变长的。The old story (Lamarck) said: a giraffe stretches hard, its neck grows, and the longer neck passes to its children. Darwin saw something else: necks were already long or short at birth; the ones reaching the high leaves survived and had more young. Long necks spread generation by generation — the environment “picked” them, the necks did not stretch their way there.
🎮 你来当实验员（1 分钟）🎮 Your turn: lab assistant (1 minute)
道理讲完了。拖一拖“树皮颜色”滑块，选一个预测，再跑 5 代——看黑白蛾比例怎么变。4 关全对就是自然选择课代表。Theory done. Drag the “bark colour” slider, lock in a prediction, then run 5 generations — and watch the black-and-white ratio move. Clear all 4 levels to become a natural-selection ace.
一句话记住它：变异随机发生，环境负责挑选；活下来的留下更多后代，性状比例就一代代改变——这就是自然选择。Remember it in one line: variation happens at random, the environment does the choosing; survivors leave more offspring, so the ratio drifts generation by generation — that is natural selection.
变异是随机的，不是谁想变就能变Variation is random — no one changes on purpose环境当考官：保护色对不上，就先被吃掉The environment grades: poor camouflage gets eaten first适应者留下更多后代，比例悄悄改变Survivors leave more offspring, and the ratio quietly shifts
内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.
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