How Natural Selection Works

Evolution is the result of the tendency for some organisms to have better reproductive success than others -- natural selection.

It's important to remember that differences between individuals, even individuals from different generations, don't constitute evolution. Those are just variations of traits. Traits are characteristics that are inheritable -- they can be passed down from one generation to the next. Not all traits are physical -- the ability to tolerate close contact with humans is a trait that evolved in dogs. Here's an example that helps explain these concepts:

Now imagine that some conditions arise that make it more likely for jockeys to reproduce successfully  than basketball players. Jockeys have children more frequently, and these children tend to be short. Basketball players have fewer children, so there are fewer tall people. After a few generations, the average height of humans decreases. Humans have evolved to be shorter.

Evolution is all about change, but what is the mechanism that causes these changes? Every living thing has everything about its construction encoded in a special chemical structure called DNA. Within the DNA are chemical sequences that define a certain trait or set of traits. These sequences are known as genes. The part of each gene that results in the varying expression of traits is called an allele. Because a trait is an expression of an allele, the tendency of a certain trait to show up in a population is referred to as allele frequency. In essence, evolution is a change in allele frequencies over the course of several generations.

Different alleles (and thus different traits) are created in three ways:

Sexual reproduction itself is a product of natural selection -- organisms that blend genes in this way gain access to a greater variety of traits, making them more likely to find the right traits for survival. For more detailed information on evolution, head over to How Evolution Works.

Fitness is the key to natural selection. We're not talking about how many reps a sea otter can burn through at the gym -- biological fitness is an organism's ability to successfully survive long enough to reproduce. Beyond that, it also reflects an organism's ability to reproduce well. It isn't enough for a tree to create a bunch of seeds. Those seeds need the ability to end up in fertile soil with enough resources to sprout and grow.

Fitness and natural selection were first explained in detail by Charles Darwin, who observed wildlife around the world, took copious notes, then sought to understand what he had seen. Natural selection is probably best explained in his words, taken from his landmark work "On the Origin of Species."

Organisms show variation of traits. "The many slight differences which appear in the offsprin­g of the same parents may be called individual differences. No one supposes that all the individuals of the same species are cast in the same actual mould."

More organisms are born than could ever possibly be supported by the planet's resources. "Every being … must suffer destruction at some period of its life, otherwise, on the principle of geometrical increase, its numbers would quickly become so … great that no country could support the product."

Therefore, all organisms must struggle to live. "As more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life."

Some traits offer advantages in the struggle. "Can we doubt … that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating?"

Organisms that have those traits are more likely to successfully reproduce and pass the traits on to the next generation. "The slightest differences may turn the nicely balanced scale in the struggle for life, and so be preserved."

Successful variations accumulate over the generations as the organisms are exposed to population pressure. "Natural Selection acts exclusively by the preservation and accumulation of variations which are beneficial under the conditions to which each creature is exposed. The ultimate result is that each creature tends to become more and more improved in relation to its conditions."

Let's delve deeper into the concept of population pressure. 

The process of natural selection can be sped up immensely by strong population pressures. Population pressure is a circumstance that makes it harder for organisms to survive. There's always some kind of population pressure, but events like floods, droughts or new predators can increase it. Under high pressure, more members of a population will die before reproducing. This means that only those individuals with traits that allow them to deal with the new pressure will survive and pass along their alleles to the next generation. This can result in drastic changes to allele frequencies within one or two generations.

Here's an example -- imagine a giraffe population with individuals that range in height from 10 feet to 20 feet tall. One day, a brush fire sweeps through and destroys all the vegetation below 15 feet. Only the giraffes taller than 15 feet can reach the higher leaves to eat. Giraffes below that height are unable to find any food at all. Most of them starve before they can reproduce. In the next generation, very few short giraffes are born. The population's average height has gone up by several feet.

There are other ways to quickly and drastically affect allele frequency. One way is a population bottleneck. In a large population, alleles are evenly distributed across the population. If some event, such as a disease or a drought, wipes out a large percentage of the population, the remaining individuals may have an allele frequency very different from the larger population. By pure chance, they may have a high concentration of alleles that were relatively rare before. As these individuals reproduce, the formerly rare traits become the average for the population.

The founder effect can also bring about rapid evolution. This occurs when a small number of individuals migrate to a new location, "founding" a new population that no longer mates with the old population. Just as with a population bottleneck, these individuals may have unusual allele frequencies, leading subsequent generations to have very different traits from the original population that the founders migrated from.

The difference between slow, gradual changes over many generations (gradualism) and rapid changes under high population pressure interspersed with long periods of evolutionary stability (punctuated equilibrium) is an ongoing debate in evolutionary science.

Evolutionary biologist Richard Dawkins wrote a book called "The Selfish Gene" in the 1970s. Dawkins' book reframed evolution by pointing out that natural selection favors the passing on of genes, not the organism itself. Once an organism has successfully reproduced, natural selection doesn't care what happens after. This explains why certain strange traits continue to exist -- traits that seem to cause harm to the organism but benefit the genes. In some spider species, the female eats the male after mating. As far as natural selection is concerned, a male spider that dies 30 seconds after mating is just as successful as one that lives a full, rich life.

Since the publication of "The Selfish Gene," most biologists agree that Dawkins' ideas explain a great deal about natural selection, but they don't answer everything. One of the main sticking points is altruism. Why do people (and many animal species) do good things for others, even when it offers no direct benefit to themselves? Research has shown that this behavior is instinctive and appears without cultural training in human infants [source: CBC]. It also appears in some primate species. Why would natural selection favor an instinct to help others?

One theory revolves around kinship. People who are related to you share many of your genes. Helping them could help ensure that some of your genes are passed down. Imagine two families of early humans, both competing for the same food sources. One family has alleles for altruism -- they help each other hunt and share food. The other family doesn't -- they hunt separately, and each human only eats whatever he can catch. The cooperative group is more likely to achieve reproductive success, passing along the alleles for altruism.

Biologists are also exploring a concept known as the superorganism. It's basically an organism made out of many smaller organisms. The model superorganism is the insect colony. In an ant colony, only the queen and a few males will ever pass their genes to the next generation. Thousands of other ants spend their entire lives as workers or drones with absolutely no chance of passing on their genes directly. Yet they work to contribute to the success of the colony. In terms of the "selfish gene," this doesn't make a whole lot of sense. But if you look at an insect colony as a single organism made up of many small parts (the ants), it does. Each ant works to ensure the reproductive success of the colony as a whole. Some scientists think the superorganism concept can be used to explain some aspects of human evolution [source: Wired Science].

We usually think of evolution as something we don't see happening right before our eyes, instead looking at fossils to find evidence of it happening in the past. In fact, evolution under intense population pressure happens so fast that we've seen it occur within the span of a human lifetime.

African elephants typically have large tusks. The ivory in the tusks is highly valued by some people, so hunters have hunted and killed elephants to tear out their tusks and sell them (usually illegally) for decades. Some African elephants have a rare trait -- they never develop tusks at all. In 1930, about 1 percent of all elephants had no tusks. The ivory hunters didn't bother killing them because there was no ivory to recover. Meanwhile, elephants with tusks were killed off by the hundreds, many of them before they ever had a chance to reproduce.

The alleles for "no tusks" were passed along over just a few generations. The result: As many as 38 percent of the elephants in some modern populations have no tusks [source: BBC News]. Unfortunately, this isn't really a happy ending for the elephants, since their tusks are used for digging and defense.

The bollworm, a pest that eats and damages cotton crops, has shown that natural selection can act even faster than scientists can genetically engineer something. Some cotton crops have been genetically modified to produce a toxin that's harmful to most bollworms. A small number of bollworms had a mutation that gave them immunity to the toxin. They ate the cotton and lived, while all non-immune bollworms died. The intense population pressure has produced broad immunity to the toxin in the entire species within the span of just a few years [source: EurekAlert].

Some species of clover developed a mutation that caused the poison cyanide to form in the plant's cells. This gave the clover a bitter taste, making it less likely to be eaten. However, when the temperature drops below freezing, some cells rupture, releasing the cyanide into the plant's tissues and killing the plant. In warm climates, natural selection acted in favor of the cyanide-producing clover, but where the winters are cold, non-cyanide clover was favored. Each kind exists almost exclusively in each climate area [source: Purves].

What about humans? Are we subject to natural selection as well? It's certain that we were -- humans only became humans because an assortment of traits (larger brains, walking upright) conferred advantages to those primates that developed them. But we're capable of influencing the distribution of our genes directly. We can use birth control, so that those of who are "fittest" in terms of natural selection might not pass on our genes at all. We use medicine and science to allow many people to live (and reproduce) who otherwise wouldn't likely survive past childhood. Much like domesticated animals, which we breed to specifically favor certain traits, humans are influenced by a sort of unnatural selection.

However, we're still evolving. Some humans have more reproductive success than others, and the factors that affect that equation have added a layer of human complexity on top of the already complicated interactions of the animal world. In other words, we don't really know what we're going to evolve into. Change is inevitable, but remember that natural selection doesn't care about making "better" humans, just more of us.