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五个手指的进化

Five fingers of evolution

(背景音乐)
(Music)
演化五要素的手指譬喻
The Five Fingers of Evolution.
若想对生物有完整的认识
A thorough understanding of biology requires
就要先深入了解演化的歷程
a thorough understanding of the process of evolution.
大多人都对自然天择耳熟能详
Most people are familiar with the process of natural selection.
但天择只是 驱动演化歷程的五要素之一
However, this is just one of five processes that can result in evolution.
在探讨这五要素之前
Before we discuss all five of these processes,
我们应先明白演化的定义
we should define evolution.
演化指的是基因库裡随时间累积的变异
Evolution is simply change in the gene pool over time.
那甚麼是基因库? 基因又是甚麼?
But what is a gene pool? And for that matter, what is a gene?
在开始钻研遗传学之前
Before spending any more time on genetics,
我们先来说个故事
let us begin with a story.
想像有艘船翻覆了, 10 位生还者游到一座荒岛的岸边
Imagine that a boat capsizes, and 10 survivors swim to shore on a deserted island.
没有人来救他们, 他们形成一个繁衍数千年的聚落
They are never rescued, and they form a new population that exists for thousands of years.
奇怪的是,其中 5 人是红髮
Strangely enough, five of the survivors have red hair.
红髮的特徵须经 父母双方各得一个红髮基因
Red hair is created when a person inherits two copies of the red gene from their parents.
若仅有一个红髮基因,就不会表现红髮的特徵
If you only have one copy of the gene, you won’t have red hair.
為了让情况更简单些,我们假设
To make this easier, we will assume
不是红髮的另外 5 人完全没有红髮基因
that the five non-redheads are not carriers of the gene.
因此红髮基因的最初机率是是50%
The initial frequency of the red hair gene
也就是 20 个基因裡有10 个红髮基因
is therefore 50 percent, or 10 of 20 total genes.
这些基因就组成基因库
These genes are the gene pool.
20 个不同的基因就像一叠纸牌
The 20 different genes are like cards in a deck
世代交替时就会重新洗牌一次
that keep getting reshuffled with each new generation.
交配就是让基因重新洗牌的机制
Sex is simply a reshuffling of the genetic deck.
这些洗过的牌会传递到下一代
The cards are reshuffled and passed to the next generation;
洗牌后还是有一半的红髮基因
the deck remains the same, 50 percent red.
重新洗牌过的基因会传递到下一代
The genes are reshuffled and passed to the next generation;
基因库裡红髮的比率依旧维持50%
the gene pool remains the same, 50 percent red.
虽然人口数量可能会与日俱增
Even though the population may grow in size over time,
得到红髮基因的机会仍维持 50%
the frequency should stay at about 50 percent.
一旦机率有了变异,演化就此发生
If this frequency ever varies, then evolution has occurred.
演化指的是基因库裡随时间累积的变异
Evolution is simply change in the gene pool over time.
从纸牌的角度来看
Think about it in terms of the cards.
如果得到纸牌的机率有了变异
If the frequency of the cards in the deck ever changes,
演化就发生了
evolution has occurred.
有 5 个因素能够造成机率改变
There are five processes that can cause the frequency to change.
我们运用 5 根手指以便於记忆这 5 项要素
To remember these processes, we will use the fingers on your hands,
从小指开始数到大拇指
starting from the little finger and moving to the thumb.
小指代表可能缩小的族群
The little finger should remind you that the population can shrink.
一旦族群缩小,演化就可能发生
If the population shrinks, then chance can take over.
假如一场瘟疫中只有 4 人倖存
For example, if only four individuals survive an epidemic,
他们的基因就会成為新基因库
then their genes will represent the new gene pool.
接着是无名指 (婚戒指)
The next finger is the ring finger.
无名指会提醒你择偶, 因为婚戒表示成双成对
This finger should remind you of mating, because a ring represents a couple.
若个人依据外表或地缘来择偶
If individuals choose a mate based on their appearance or location,
基因库的机率就可能改变
the frequency may change.
若红发个体仅和红发个体婚配
If redheaded individuals only mate with redheaded individuals,
他们自然形成新群落
they could eventually form a new population.
若无人愿与红发个体婚配
If no one ever mates with redheaded individuals,
红发基因数会减少
these genes could decrease.
再来是中指
The next finger is the middle finger.
中指 (middle finger) 里的 "M" 会让人联想到突变 (Mutation) 的 "M"
The M in the middle finger should remind you of the M in the word "mutation."
若突变产生新基因, 基因库的机率就会受影响
If a new gene is added through mutation, it can affect the frequency.
想像一个新的基因变异创造出新发色
Imagine a gene mutation creates a new color of hair.
基因库里的遗传机率会随之变化
This would obviously change the frequency in the gene pool.
食指会让你想起动作或移动
The pointer finger should remind you of movement.
假如新个体流入或迁入区域内
If new individuals flow into an area, or immigrate,
此区的基因库也会变动
the frequency will change.
如果个体自某区外流或者移出
If individuals flow out of an area, or emigrate,
基因库同样也会变动
then the frequency will change.
科学上称之为基因流动
In science, we refer to this movement as gene flow.
这 4 只手指代表的变因都能引发演化
All four of the processes represented by our fingers can cause evolution.
族群规模小,选择性交配
Small population size, non-random mating,
基因突变和流动
mutations and gene flow.
不过这些都不会导致演化适应
However, none of them lead to adaptation.
天择是唯一的途径
Natural selection is the only process
能让生物更适应所处环境
that creates organisms better adapted to their local environment.
我们用大拇指来记住这个要素
I use the thumb to remember this process.
大自然会对最适应环境的特征, 竖起大拇指说讚
Nature votes thumbs up for adaptations that will do well in their environment,
然后拇指朝下,淘汰无法适应环境的特征
and thumbs down to adaptations that will do poorly.
个体中无法适应环境的基因
The genes for individuals that are not adapted for their environment
会逐渐被适应性较佳的基因淘汰
will gradually be replaced by those that are better adapted.
红发只是众多性状的其中之一
Red hair is an example of one of these adaptations.
在较北方的气候区,红发人较具优势
Red hair is an advantage in the northern climates,
因为他们白皙的皮肤能吸收较多光线
because the fair skin allowed ancestors to absorb more light
进而合成较多维生素D,真是讚!
and synthesize more vitamin D. Thumbs up!
但是同样的红发在南方却是劣势
However, this was a disadvantage in the more southern climates,
因为过强的UV射线会导致癌症和降低受孕
where increased UV radiation led to cancer and decreased fertility.
大扣分!
Thumbs down!
其实就连拇指本身, 也是适应环和天择的产物
Even the thumb itself is an adaptation formed through the process of natural selection.
以上所讨论的都是微观演化
The evolution that we have described is referred to as microevolution,
因为它牵涉到的都是小改变
because it refers to a small change.
不过这样的演化可能
However, this form of evolution may
最终会导向巨观演化,形成新物种
eventually lead to macroevolution, or speciation.
地球上所有生物都源于一个共同的祖先
Every organism on the planet shares ancestry with a single common ancestor.
所有地球上现存的生物都经由演化
All living organisms on the planet are connected back in time
紧密相连,环环相扣
through the process of evolution.
看看你的双手
Take a look at your own hand.
这神奇杰作就是历经演化 5 要素的产物
It’s an engineering masterpiece that was created by the five processes I just described,
演化历程长达数百万年之久
over millions and millions of years.
你能凭记忆说出引发演化的 5 个因素吗?
Can you recall the five main causes of evolution from memory?
不能的话,再回顾一次忘记的片段
If you can’t, hit rewind and watch that part again.
若你成功的话,和附近的人用力击掌吧!
But if you can, give yourself or your neighbor a big five-fingered high five.

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https://www.youtube.com/watch?v=5NdMnlt2keE

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