The Five Fingers of Evolution.
五个手指的进化。
A thorough understanding of biology requires a thorough understanding of the process of evolution.
若想对生物有完整的认识，就要先深入了解进化的历程。
Most people are familiar with the process of natural selection.
大多人都对天择耳熟能详。
However, this is just one of five processes that can result in evolution.
但天择只是驱动进化历程的五要素之一。
Before we discuss all five of these processes, we should define evolution.
在探讨这五要素之前，我们应先明白进化的定义。
Evolution is simply change in the gene pool over time.
进化指的是基因库里随时间累积的变异。
But what is a gene pool? And for that matter, what is a gene?
那什么是基因库? 基因又是甚么?
Before spending any more time on genetics, let us begin with a story.
在开始钻研遗传学之前，我们先来说个故事。
Imagine that a boat capsizes, and 10 survivors swim to shore on a deserted island.
想象有艘船翻覆了，10位生还者游到一座荒岛的岸边。
They are never rescued, and they form a new population that exists for thousands of years.
没有人来救他们，他们形成一个繁衍数千年的新种群。
Strangely enough, five of the survivors have red hair.
奇怪的是，其中5个人是红头发。
Red hair is created when a person inherits two copies of the red gene from their parents.
红头发的特征须经父母双方各得一个红发基因。
If you only have one copy of the gene, you won't have red hair.
若仅有一个红发基因，就不会表现红发的特征。
To make this easier, we will assume that the five non-redheads are not carriers of the gene.
为了让情况更简单些，我们假设不是红发的另外5人完全没有红发基因。
The initial frequency of the red-hair gene is therefore 50 percent, or 10 of 20 total genes.
因此红发基因的最初机率是是50%，也就是20个基因里有10个红发基因。
These genes are the gene pool.
这些基因就组成基因库。
The 20 different genes are like cards in a deck that keep getting reshuffled with each new generation.
20个不同的基因就像一叠纸牌，世代交替时就会重新洗牌一次。
Sex is simply a reshuffling of the genetic deck.
交配就是让基因重新洗牌的机制。
The cards are reshuffled and passed to the next generation; the deck remains the same, 50 percent red.
这些洗过的牌会传递到下一代，洗牌后还是有一半的红发基因。
The genes are reshuffled and passed to the next generation; the gene pool remains the same, 50 percent red.
重新洗牌过的基因会传递到下一代，基因库里红发的比率依旧维持50%。
Even though the population may grow in size over time, the frequency should stay at about 50 percent.
虽然人口数量可能会与日俱增，得到红发基因的机会仍维持50%。
If this frequency ever varies, then evolution has occurred.
一旦这一机率有了变异，进化就此发生。
Evolution is simply change in the gene pool over time. Think about it in terms of the cards.
进化指的是基因库里随时间累积的变异。
If the frequency of the cards in the deck ever changes, evolution has occurred.
从纸牌的角度来看，如果得到纸牌的机率有了变异，进化就发生了。
There are five processes that can cause the frequency to change.
有5个因素能够造成机率的改变。

To remember these processes, we will use the fingers on your hands,
我们运用5根手指以便于记忆这5项要素，
starting from the little finger and moving to the thumb.
从小拇指开始数到大拇指。
The little finger should remind you that the population can shrink.
小指代表可能缩小的族群。
If the population shrinks, then chance can take over.
一旦族群缩小，进化就可能发生。
For example, if only four individuals survive an epidemic, then their genes will represent the new gene pool.
假如一场瘟疫中只有4人幸存，他们的基因就会成为新基因库。
The next finger is the ring finger. This finger should remind you of mating, because a ring represents a couple.
接着是无名指。无名指会提醒你择偶，因为婚戒表示成双成对。
If individuals choose a mate based on their appearance or location, the frequency may change.
若个人依据外表或地缘来择偶，基因库的机率就可能改变。
If redheaded individuals only mate with redheaded individuals, they could eventually form a new population.
若红发个体仅和红发个体婚配，他们自然形成新群落。
If no one ever mates with redheaded individuals, these genes could decrease. The next finger is the middle finger.
若无人愿与红发个体婚配，则红发基因数会减少。
The M in the middle finger should remind you of the M in the word 'mutation.'
再来是中指。中指里的M会让人联想到突变的M。
If a new gene is added through mutation, it can affect the frequency.
若突变产生新基因，基因库的机率就会受影响。
Imagine a gene mutation creates a new color of hair. This would obviously change the frequency in the gene pool.
想象一个新的基因变异创造出新发色，那么基因库里的遗传机率会随之变化。
The pointer finger should remind you of movement.
食指会让你想起动作或移动。
If new individuals flow into an area, or immigrate, the frequency will change.
假如新个体流入或迁入区域内，此区域的基因库也会变动。
If individuals flow out of an area, or emigrate, then the frequency will change.
如果个体自某区外流或者移出，基因库同样也会变动。
In science, we refer to this movement as gene flow.
科学上称之为基因流动。
All four of the processes represented by our fingers can cause evolution.
这4只手指代表的变因都能引发进化。
Small population size, non-random mating, mutations and gene flow. However, none of them lead to adaptation.
族群规模小，选择性交配，基因突变和流动。不过这些都不会导致进化适应。
Natural selection is the only process that creates organisms better adapted to their local environment.
天择是唯一的途径，能让生物更适应所处环境。
I use the thumb to remember this process.
我们用大拇指来记住这个要素。
Nature votes thumbs up for adaptations that will do well in their environment,
大自然会对最适应环境的特征竖起大拇指，
and thumbs down to adaptations that will do poorly.
然后拇指朝下，淘汰无法适应环境的特征。
The genes for individuals that are not adapted for their environment will gradually be replaced by those that are better adapted.
个体中无法适应环境的基因，会逐渐被适应性较佳的基因淘汰。
Red hair is an example of one of these adaptations. Red hair is an advantage in the northern climates,
红发只是众多性状的其中之一。在较北方的气候区，红发人较具优势，
because the fair skin allowed ancestors to absorb more light and synthesize more vitamin D. Thumbs up!
因为他们白皙的皮肤能吸收较多光线，进而合成较多维生素D。真是赞!
However, this was a disadvantage in the more southern climates,
但是同样的红发在南方却是劣势，
where increased UV radiation led to cancer and decreased fertility. Thumbs down!
因为过强的UV射线会导致癌症和降低受孕。太糟了!
Even the thumb itself is an adaptation formed through the process of natural selection.
其实就连拇指本身也是适应天择的产物。
The evolution that we have described is referred to as microevolution, because it refers to a small change.
以上所讨论的都是微观进化，因为它牵涉到的都是小改变。
However, this form of evolution may eventually lead to macroevolution, or speciation.
不过这样的进化可能最终会导向巨观进化，形成新物种。
Every organism on the planet shares ancestry with a single common ancestor.
地球上所有生物都源于一个共同的祖先。
All living organisms on the planet are connected back in time through the process of evolution.
所有地球上现存的生物都经由进化紧密相连。
Take a look at your own hand.
看看你的双手。
It's an engineering masterpiece that was created by the five processes I just described, over millions and millions of years.
这神奇杰作就是历经我刚刚说的进化5要素的产物，进化历程长达数百万年之久。
Can you recall the five main causes of evolution from memory?
你能凭记忆说出引发进化的5个因素吗?
If you can't, hit rewind and watch that part again.
不能的话，再倒回去看一下这一部分。
But if you can, give yourself or your neighbor a big five-fingered high five.
但是如果你可以的话，那就和附近的人用力击掌吧!