Based on the Hotter-Colder game and the gamble that I showed you, I hope that the distinction between pure randomness and biased randomness is clear to you, but I’ll give you the definitions again. Pure randomness means that each possible outcome is equally likely, and biased randomness means some different possible outcomes are not equally likely.

Now I’ll tell you a story to demonstrate how the theory of natural selection is based on biased randomness, not pure randomness. Even though I made it up, this story might be true. The numbers I chose probably aren’t exactly right, but my story might explain how giraffes evolved long necks. (There’s no way to know, because no humans were there to observe and record the process when it happened.)

So, suppose that, long, long ago, there was a population of vegetarians that couldn’t climb trees. I’ll call these creatures “giraffes.” For a long time, giraffes ate grass, but one year, due to some kind of climate change, all the grass died. So then the giraffes had to eat leaves to survive.

Now, giraffes had a variety of neck lengths. Giraffes with necks that were too short to reach the lowest leaves on the trees starved, so they didn’t reproduce. Giraffes with necks that were just long enough to reach the lowest leaves had to compete for those leaves with the giraffes that had longer necks, so they had a relatively low environmental fitness value, and there were fewer of them in the next generation. The giraffes with the longest necks could eat leaves that none of the other giraffes could reach, and they could also eat leaves at lower heights, competing with the giraffes that had shorter necks. The longer the neck, the more leaves a giraffe could reach, so the more environmentally fit the giraffe was, and the higher the probability that the giraffe survived to reproduce.

Figure 3 shows the histogram of the giraffes’ neck lengths at a certain time in history. Most of them had relatively short necks, but some had longer necks than others.

Now something else you need to know is that the giraffes’ neck length is hereditary. By that I mean that giraffes with short necks tend to produce offspring with short necks, while long-necked giraffes tend to have long-necked offspring. Since all of the giraffes that couldn’t reach any leaves died before reproducing, there were no giraffes with necks shorter than 11 inches in the next generation. And since the giraffes with the longest necks had the best chance to survive and reproduce, the next generation had fewer short-necked giraffes and more long-necked giraffes. Figure 4 shows the histogram for the next generation. Notice that there were no more giraffes with neck lengths less than 11 inches, and there were far fewer giraffes with neck lengths between 11 and 20 inches than there were in the previous generation.

I want to make sure you understand that there’s a degree of randomness here. Short-necked giraffes sometimes got lucky and found enough leaves to live long enough to reproduce. And the long-necked giraffes didn’t always reproduce successfully. But since the long-necked giraffes could reach more leaves than the short-necked giraffes, they had a better chance of surviving and reproducing.

Figure 4 shows that some of the giraffes in the second generation had necks longer than 40 inches, even though none of the giraffes in the previous generation did. This is possible, because giraffes sometimes grew up to have longer necks than both of their parents in the same way that human children may become taller than both of their parents. And the giraffes with relatively long necks could reach more leaves than those with relatively short necks, so they had a better change of getting enough food until they reproduced. The giraffes with the shortest necks still had a very low chance of reproducing, so the next generation had a few short-necked giraffes, as shown in Figure 5 with the histogram for the next generation.

In each generation, there were fewer short-necked giraffes and more long-necked giraffes, with a few of them having longer necks than any in the previous generation. Figure 6 shows the populations three generations later. Notice that some of the giraffes’ necks were more than 70 inches long. It turns out that 75 inches was an important threshold, because those giraffes with 75-inch necks could reach the highest leaves. So there was no benefit to having a neck longer than 75 inches. That doesn’t mean there couldn’t be any giraffes with 85-inch long necks. It just means that those giraffes didn’t have a better chance of reproducing than the giraffes with 75-inch long necks. Figure 7, which corresponds to the giraffes two generations after Figure 6, shows that the increase in neck length slowed down.

After three more generations, the histogram in Figure 8 shows that the giraffes with the shortest necks all died without reproducing, while the number of giraffes with necks that were about 75 inches in length kept increasing. And as a result, giraffes have the long necks that we see today.

What have we learned from this example? Over eleven generations, the giraffes’ neck lengths progressively increased. There was certainly a degree of randomness. If there wasn’t, then none of the short-necked giraffes could ever survive, while all of the giraffes with the longest necks would always successfully reproduce. But that’s not what happened. Some of the short-necked giraffes got lucky, and some of the long-necked giraffes were unlucky.

However, this is not pure randomness. If it was, then all giraffes would have had the same chance of reproducing, so every generation’s histogram would have looked similar to the first one. The reason that the average neck length increased across the generations is that the chance of surviving to reproduction was heavily biased in favor of those giraffes with longer necks — biased randomness.

I should note that, when I said, “Over eleven generations, the giraffes’ neck lengths progressively increased,” I didn’t mean that any particular giraffe’s neck length increased over time. What I was doing was abbreviating the following sentence: “From one generation to the next, the average neck length across the population of giraffes progressively increased.” This is what I mean by “increased” in the context of natural selection. I will also use words like “decreased,” “bigger,” “smaller,” “faster,” and “slower” in a similar way.

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