Natural Selection

You may have heard that giraffes stretched their necks to reach leaves high in the trees, and that's how their necks got so long. But just as a parent who works out hard doesn't pass those muscles straight to their baby, a neck that gets longer during an animal's life isn't passed on through its genes.

Here's how natural selection explains it. Long ago, giraffes had all kinds of neck lengths: short, medium, and long. Then a severe drought hit, and the short grass and low shrubs all dried up. The giraffes that happened to have slightly longer necks could reach the high branches and get through the hunger.

Those long-necked survivors mated and had young, and they passed along the genetic information for long necks too. This repeated over millions of years and countless generations. Short-necked giraffes faded away, and only long-necked ones were left, giving us the giraffes we see today. That said, scientists still debate exactly what pressure drove the long neck. Competition for food up high and males swinging their necks in fights are among the leading ideas.

For natural selection to actually happen, three gears have to mesh. The first is variation: even within one group, individuals differ a little in size, color, running speed, and so on. If everyone were an identical clone, a harsh change in the environment would wipe out everyone or spare everyone, and there would be nothing to select.

The second condition is heredity: those differences get passed to the next generation through genes. The third is that those differences change how well an individual survives and has offspring. This usually comes from competition for survival and reproduction, because food and living space are too scarce for every creature born to thrive.

Once all three are in place, nature's filter starts working. Individuals with even a slightly better fit for their environment push through the competition and spread more offspring. As time goes on, that helpful trait becomes the most common look in the population.

Natural selection is not a process that moves living things up a ladder toward "better, more perfect" beings. Evolution has no final destination and no form that is better in every situation.

Yesterday's winner can become today's loser when the environment changes. The woolly mammoth, with its thick coat, was a survival champion in the bitterly cold Ice Age. But when the Ice Age ended and the cold grasslands it lived on shrank, its numbers slowly fell. (Several explanations exist for its extinction, including climate change and human hunting.) In the end, natural selection doesn't mean the strongest survive. It means the one that best fits the environment right now survives.

Also, evolution isn't a magical change in which one animal reshapes its own body during its life. It means a statistical change, in which the mix of genes across a whole population slowly shifts over a long time.