Q-Eng

TED-Ed 006・・Five fingers of evolution（進化の５本指）　　　　　代表頁に戻る

05分・・170wpm


(Music)
The Five Fingers of Evolution. A thorough understanding of biology requires a thorough understanding of the process of evolution. Most people are familiar with the process of natural selection. However, this is just one of five processes that can result in evolution. Before we discuss all five of these processes, we should define evolution. Evolution is simply change in the gene pool over time. But what is a gene pool? And for that matter, what is a gene? Before spending any more time on genetics, let us begin with a story.

Imagine that a boat capsizes, and 10 survivors swim to shore on a deserted island. They are never rescued, and they form a new population that exists for thousands of years. Strangely enough, five of the survivors have red hair. Red hair is created when a person inherits two copies of the red gene from their parents. If you only have one copy of the gene, you won't have red hair. To make this easier, we will assume that the five non-redheads are not carriers of the gene. The initial frequency of the red hair gene is therefore 50 percent, or 10 of 20 total genes. These genes are the gene pool. The 20 different genes are like cards in a deck that keep getting reshuffled with each new generation. Sex is simply a reshuffling of the genetic deck. The cards are reshuffled and passed to the next generation; the deck remains the same, 50 percent red. The genes are reshuffled and passed to the next generation; the gene pool remains the same, 50 percent red. Even though the population may grow in size over time, the frequency should stay at about 50 percent. If this frequency ever varies, then evolution has occurred. Evolution is simply change in the gene pool over time. Think about it in terms of the cards. If the frequency of the cards in the deck ever changes, evolution has occurred. There are five processes that can cause the frequency to change.

To remember these processes, we will use the fingers on your hands, starting from the little finger and moving to the thumb. The little finger should remind you that the population can shrink. If the population shrinks, then chance can take over. For example, if only four individuals survive an epidemic, then their genes will represent the new gene pool. The next finger is the ring finger. This finger should remind you of mating, because a ring represents a couple. If individuals choose a mate based on their appearance or location, the frequency may change. If redheaded individuals only mate with redheaded individuals, they could eventually form a new population. If no one ever mates with redheaded individuals, these genes could decrease. The next finger is the middle finger. The M in the middle finger should remind you of the M in the word "mutation." If a new gene is added through mutation, it can affect the frequency. Imagine a gene mutation creates a new color of hair. This would obviously change the frequency in the gene pool. The pointer finger should remind you of movement. If new individuals flow into an area, or immigrate, the frequency will change. If individuals flow out of an area, or emigrate, then the frequency will change. In science, we refer to this movement as gene flow.

All four of the processes represented by our fingers can cause evolution. Small population size, non-random mating, mutations and gene flow. However, none of them lead to adaptation. Natural selection is the only process that creates organisms better adapted to their local environment. I use the thumb to remember this process. Nature votes thumbs up for adaptations that will do well in their environment, and thumbs down to adaptations that will do poorly. The genes for individuals that are not adapted for their environment will gradually be replaced by those that are better adapted. Red hair is an example of one of these adaptations. Red hair is an advantage in the northern climates, because the fair skin allowed ancestors to absorb more light and synthesize more vitamin D. Thumbs up! However, this was a disadvantage in the more southern climates, where increased UV radiation led to cancer and decreased fertility. Thumbs down! Even the thumb itself is an adaptation formed through the process of natural selection. The evolution that we have described is referred to as microevolution, because it refers to a small change. However, this form of evolution may eventually lead to macroevolution, or speciation. Every organism on the planet shares ancestry with a single common ancestor. All living organisms on the planet are connected back in time through the process of evolution.

Take a look at your own hand. It's an engineering masterpiece that was created by the five processes I just described, over millions and millions of years. Can you recall the five main causes of evolution from memory? If you can't, hit rewind and watch that part again. But if you can, give yourself or your neighbor a big five-fingered high five.