演化五要素的手指譬喻
The Five Fingers of Evolution.
若想對生物有完整的認識
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.
大多人都對自然天擇耳熟能詳
Most people are familiar with the process of natural selection.
但天擇只是 驅動演化歷程的五要素之一
However, this is just one of five processes that can result in evolution.
在探討這五要素之前
Before we discuss all five of these processes,
我們應先明白演化的定義
we should define evolution.
演化指的是基因庫裡隨時間累積的變異
Evolution is simply change in the gene pool over time.
那甚麼是基因庫? 基因又是甚麼?
But what is a gene pool? And for that matter, what is a gene?
在開始鑽研遺傳學之前
Before spending any more time on genetics,
我們先來說個故事
let us begin with a story.
想像有艘船翻覆了， 10 位生還者遊到一座荒島的岸邊
Imagine that a boat capsizes, and 10 survivors swim to shore on a deserted island.
沒有人來救他們， 他們形成一個繁衍數千年的聚落
They are never rescued, and they form a new population that exists for thousands of years.
奇怪的是，其中 5 人是紅髮
Strangely enough, five of the survivors have red hair.
紅髮的特徵須經 父母雙方各得一個紅髮基因
Red hair is created when a person inherits two copies of the red gene from their parents.
若僅有一個紅髮基因，就不會表現紅髮的特徵
If you only have one copy of the gene, you won’t have red hair.
為了讓情況更簡單些，我們假設
To make this easier, we will assume
不是紅髮的另外 5 人完全沒有紅髮基因
that the five non-redheads are not carriers of the gene.
因此紅髮基因的最初機率是是50%
The initial frequency of the red hair gene
也就是 20 個基因裡有10 個紅髮基因
is therefore 50 percent, or 10 of 20 total genes.
這些基因就組成基因庫
These genes are the gene pool.
20 個不同的基因就像一疊紙牌
The 20 different genes are like cards in a deck
世代交替時就會重新洗牌一次
that keep getting reshuffled with each new generation.
交配就是讓基因重新洗牌的機制
Sex is simply a reshuffling of the genetic deck.
這些洗過的牌會傳遞到下一代
The cards are reshuffled and passed to the next generation;
洗牌後還是有一半的紅髮基因
the deck remains the same, 50 percent red.
重新洗牌過的基因會傳遞到下一代
The genes are reshuffled and passed to the next generation;
基因庫裡紅髮的比率依舊維持50%
the gene pool remains the same, 50 percent red.
雖然人口數量可能會與日俱增
Even though the population may grow in size over time,
得到紅髮基因的機會仍維持 50%
the frequency should stay at about 50 percent.
一旦機率有了變異，演化就此發生
If this frequency ever varies, then evolution has occurred.
演化指的是基因庫裡隨時間累積的變異
Evolution is simply change in the gene pool over time.
從紙牌的角度來看
Think about it in terms of the cards.
如果得到紙牌的機率有了變異
If the frequency of the cards in the deck ever changes,
演化就發生了
evolution has occurred.
有 5 個因素能夠造成機率改變
There are five processes that can cause the frequency to change.
我們運用 5 根手指以便於記憶這 5 項要素
To remember these processes, we will use the fingers on your hands,
從小指開始數到大拇指
starting from the little finger and moving to the thumb.
小指代表可能縮小的族群
The little finger should remind you that the population can shrink.
一旦族群縮小，演化就可能發生
If the population shrinks, then chance can take over.
假如一場瘟疫中只有 4 人倖存
For example, if only four individuals survive an epidemic,
他們的基因就會成為新基因庫
then their genes will represent the new gene pool.
接著是無名指 (婚戒指)
The next finger is the ring finger.
無名指會提醒你擇偶， 因為婚戒表示成雙成對
This finger should remind you of mating, because a ring represents a couple.
若個人依據外表或地緣來擇偶
If individuals choose a mate based on their appearance or location,
基因庫的機率就可能改變
the frequency may change.
若紅髮個體僅和紅髮個體婚配
If redheaded individuals only mate with redheaded individuals,
他們自然形成新群落
they could eventually form a new population.
若無人願與紅髮個體婚配
If no one ever mates with redheaded individuals,
紅髮基因數會減少
these genes could decrease.
再來是中指
The next finger is the middle finger.
中指 (middle finger) 裡的 "M" 會讓人聯想到突變 (Mutation) 的 "M"
The M in the middle finger should remind you of the M in the word "mutation."
若突變產生新基因, 基因庫的機率就會受影響
If a new gene is added through mutation, it can affect the frequency.
想像一個新的基因變異創造出新發色
Imagine a gene mutation creates a new color of hair.
基因庫裡的遺傳機率會隨之變化
This would obviously change the frequency in the gene pool.
食指會讓你想起動作或移動
The pointer finger should remind you of movement.
假如新個體流入或遷入區域內
If new individuals flow into an area, or immigrate,
此區的基因庫也會變動
the frequency will change.
如果個體自某區外流或者移出
If individuals flow out of an area, or emigrate,
基因庫同樣也會變動
then the frequency will change.
科學上稱之為基因流動
In science, we refer to this movement as gene flow.
這 4 只手指代表的變因都能引發演化
All four of the processes represented by our fingers can cause evolution.
族群規模小，選擇性交配
Small population size, non-random mating,
基因突變和流動
mutations and gene flow.
不過這些都不會導致演化適應
However, none of them lead to adaptation.
天擇是唯一的途徑
Natural selection is the only process
能讓生物更適應所處環境
that creates organisms better adapted to their local environment.
我們用大拇指來記住這個要素
I use the thumb to remember this process.
大自然會對最適應環境的特徵， 豎起大拇指說讚
Nature votes thumbs up for adaptations that will do well in their environment,
然後拇指朝下，淘汰無法適應環境的特徵
and thumbs down to adaptations that will do poorly.
個體中無法適應環境的基因
The genes for individuals that are not adapted for their environment
會逐漸被適應性較佳的基因淘汰
will gradually be replaced by those that are better adapted.
紅髮只是眾多性狀的其中之一
Red hair is an example of one of these adaptations.
在較北方的氣候區，紅髮人較具優勢
Red hair is an advantage in the northern climates,
因為他們白皙的皮膚能吸收較多光線
because the fair skin allowed ancestors to absorb more light
進而合成較多維生素D，真是讚!
and synthesize more vitamin D. Thumbs up!
但是同樣的紅髮在南方卻是劣勢
However, this was a disadvantage in the more southern climates,
因為過強的UV射線會導致癌症和降低受孕
where increased UV radiation led to cancer and decreased fertility.
大扣分!
Thumbs down!
其實就連拇指本身， 也是適應環和天擇的產物
Even the thumb itself is an adaptation formed through the process of natural selection.
以上所討論的都是微觀演化
The evolution that we have described is referred to as microevolution,
因為它牽涉到的都是小改變
because it refers to a small change.
不過這樣的演化可能
However, this form of evolution may
最終會導向巨觀演化，形成新物種
eventually lead to macroevolution, or speciation.
地球上所有生物都源於一個共同的祖先
Every organism on the planet shares ancestry with a single common ancestor.
所有地球上現存的生物都經由演化
All living organisms on the planet are connected back in time
緊密相連，環環相扣
through the process of evolution.
看看你的雙手
Take a look at your own hand.
這神奇傑作就是歷經演化 5 要素的產物
It’s an engineering masterpiece that was created by the five processes I just described,
演化歷程長達數百萬年之久
over millions and millions of years.
你能憑記憶說出引發演化的 5 個因素嗎?
Can you recall the five main causes of evolution from memory?
不能的話，再回顧一次忘記的片段
If you can’t, hit rewind and watch that part again.
若你成功的話，和附近的人用力擊掌吧!
But if you can, give yourself or your neighbor a big five-fingered high five.