1. Evolution

Before life began, the oceans contained chemical compounds, referred to as the primordial soup. At some point during these hundreds of millions of years, something special happened. Completely by chance, a molecule with a unique property appeared—it was able to replicate itself. 

This molecule made good but not perfect copies. A few errors occurred during the process, so some of these reproductions were just a tiny bit different from the original. Again, by pure luck, the molecular arrangement of a few copies made them chemically more stable—less likely to break apart and longer lasting. Additionally, other copies happened to be good at acquiring and bonding to other molecules, making them especially proficient at replicating. These became more frequent than the copies that were less stable or less efficient replicators. 

Now consider that the primordial soup did not have an unlimited number of building blocks, or raw material, for infinite molecular replication. Therefore, the molecules that managed to remain intact longer or were good at replicating had an advantage. It became quite the competitive environment!  

These replicating molecules share similarities with modern-day genes. As complexity increased on earth, genes became the copying machines. Slight mutations, or differences in each generation, continued. Some of these differences were harmful while others were advantageous to the process of making more copies. For example, it was quite beneficial for genes to develop a kind of protective shell around them. These genes were more likely to survive long enough to copy, and so their offspring had shells as well. These shells are our bodies. 

Why stop there? If a body can move in a way that helps survival and reproduction, isn’t that an advantage? It would be even better if it provided directions on how to do so. A nervous system provides feelings to draw the body away from or toward what might be harmful or helpful. The ability to think and plan can advance this agenda. All this occurs with one purpose: to increase the genes’ chance of being able to copy themselves. 

At some point in this multi-billion-year journey, genes began teaming up and sharing a body, and instead of copying themselves using building blocks from the soup, they started to combine with other bodies of genes through male–female intercourse. This recombination gives the genes a better chance of creating a body that is adapted to the environment. The nature of the environment plays a big part in determining which genes are good for survival. Genes cannot think ahead, or predict (brains can, but they only last one generation). Genes only copy what was advantageous in the past. If the environment changes, then it’s possible different genes will have a new advantage. 

The moral so far is this: Life is about gene replication, not about the individual or the species. A body is just a means to an end, a temporary vessel among many in the gene’s longevity. Genes are made of segments of DNA, which contain instructions to build the proteins that comprise us. The genotype refers to an organism’s genetic instruction manual; the phenotype is the observable finished product. The phenotype might be a plant or an animal; the genotype is the set of genes that designed and built such an organism. 

The phenotypes we see today are the result of evolution. Evolution simply means change over time and occurs primarily through natural selection.

Natural selection is a process by which the frequencies of traits increase or decrease across a population depending on how they affect survival and reproduction. For example, fish with coloring that helps them blend into their surroundings are less likely to be food for predators. They are able to live long enough to reproduce and pass on their coloring to their offspring. Soon many fish have this color, and the ones that do not have it decrease in number. 

Natural selection operates through variation, inheritance, and differential reproductive success.

First, this means that there is natural variation among individual characteristics. Remember that each copy could have slight differences, which gives nature something to “select from.” Second, offspring inherit traits. Genes are the fundamental unit of heredity, meaning that only the genes—nothing else—replicate. Third, differential reproductive success means that some traits are more advantageous than others are when it comes to survival and reproduction. This is a cornerstone of evolutionary philosophy: Genes design and build organisms for the ultimately singular purpose of reproducing those genes. To do this optimally, genes design organisms for competitive problems. Selection and life in general are inherently competitive processes. 

You can see the echoes of natural selection in the primordial soup story. Whether we are talking about molecules or life forms, the message is the same: It’s an unintentional process without foresight, and what we see now reflects past environments. The fish did not choose to blend in to avoid becoming prey; they simply survived. If the color of their background changed in the future, they would stand out and be eaten. 

Survival is necessary but insufficient for genes to advance to the next generation. Reproduction is also necessary. Many life forms must reproduce sexually. Sexual selection is a type of natural selection that arises through preferences that confer a mating advantage. Because not all organisms will get a chance to mate, those that display traits indicative of genetic quality—either in appearance or character—are more likely to be chosen and pass on these traits. For example, male birds that produce songs more complex and melodic than those of their counterparts are more likely to be chosen as a mate. Song has nothing to do with survival—in fact, it can be harmful because it may attract predators—but it’s a fitness indicator, or a sign that the birds have valuable genes worthy of mating. Sometimes even survival and reproduction are at odds with each other! 

There are other types of natural selection (for example, stabilizing, disruptive, or directional selection) and processes other than natural selection that affect evolution (such as genetic drift through mutation, founder effect, or genetic bottleneck), but we won’t go into more detail now.

Adaptations are evolved traits in an organism and are solutions to challenges relating to survival and reproduction. They are not perfect because everything comes at a cost, and they must be as efficient as constraints allow. There is also a time lag. Evolution needs many generations to make genetic alterations, but the environment can change in an instant (much of evolution takes place in deep time, or billions of years). In addition to adaptations, there are by-products—characteristics that happen to be part of the design—and other random effects carried along but with no specific value or cost. 

Just as evolution creates physical adaptations in response to survival and reproductive challenges, it also creates psychological ones for the same reason. These psychological adaptations are the focus of evolutionary psychology, which we will look at in the next section. 

*The information in this section is primarily from The Selfish Gene by Richard Dawkins.