The Handy Psychology Answer Book. Lisa J. Cohen

      What is natural selection?

      Darwin’s primary contribution to the theory of evolution was his concept of natural selection. Natural selection refers to the effect of the natural environment on the likelihood that traits will be passed on from one generation to the next. The process goes like this: First there must be variation in a particular trait within a population. Secondly, the trait must be passed from parents to their offspring. Thirdly, not all traits are equally well adapted to the environment; one version of the trait is better adapted to the environment than another version. Consequently, the animals with the more adaptive trait will bear more young. As a result, the succeeding generations will have a larger proportion of animals with this adaptive trait than had previous generations. The trait will become more common.

      Let’s consider the example of light and dark moths first recorded by Charles Darwin. There were two varieties of moths in England, light-colored moths and dark-colored moths. Originally, there were more light-colored moths than dark ones, as the dark ones stood out against the light-colored tree bark and were easy prey for the local birds. At this point, light color was more adaptive than dark color.

      During the Industrial Revolution in England, however, the trees became covered with soot. This meant the dark-colored moths were better adapted to their environment than the light-colored moths, as they no longer stood out against the soot-covered tree bark. Now it was the light-colored moths that were easy prey for the birds. Hence, the population of dark-colored moths grew relative to the population of light-colored moths as more of the former survived to reproduce and pass their genes on to the next generation. Thus natural selection acted on the moth population as a result of their coloring. Of note, Darwin’s concept of natural selection does not explain how variation in the population comes to be in the first place, only how one trait comes to be more frequent in the population than another trait.

      What does evolutionary fitness mean?

      Evolutionary fitness is the ability to pass on one’s traits to the next generation. Although Darwin did not have a theory of genes, we now understand that inherited traits are coded in molecules referred to as genes. If there is a larger proportion of gene A in the present generation than in the previous one, then the organism with gene A has demonstrated evolutionary fitness. Conversely, if the proportion of gene B has decreased across generations, then the organism with gene B has poor fitness.

      How is reproductive success relevant to evolution?

      Evolution occurs through the process of reproductive success. Those organisms that pass their genes onto the next generation have succeeded; their genes and the traits associated with them have survived into the next generation. In evolution, success really means survival. If a trait is common in a population, this means that the genes of previous generations with that trait have survived to the present.

      What does survival of the fittest mean?

      Survival of the fittest means that those individuals of a species with genetic traits that are best adapted to the particular environment are most likely to mate and pass those traits on to the next generation. Importantly, survival of the fittest does not mean that the most aggressive and dominant will pass their genes on to the next generation. Dominance is one evolutionary strategy, but it is not the only one. For example, in some fish species, male fish can disguise themselves as females and then sneak into the dominant male’s territory to mate with his females. In this case, fish that are not the most dominant nevertheless reproduce successfully. Moreover, in many circumstances, cooperation and altruism can be useful evolutionary strategies—as effective, if not more effective, than competition and aggression.

      Does evolution have a goal?

      It’s important to recognize that natural selection is not goal oriented. Traits are favored that improve an organism’s adaptation to the environment. If the environment changes, the traits that were once an advantage may turn into a liability. In our own times of rapid climate change, many species are going extinct because they cannot adapt quickly enough to the new climate. Likewise, it is believed that mammals only began to thrive many millions of years ago after a giant meteorite wiped out the dinosaurs that had previously dominated the ecosystem. In this way evolution is greatly influenced by chance. That said, there does seem to be a trend for organisms to evolve from general simplicity to greater complexity, although this is not always the case.

      What did Gregor Mendel contribute to the theory of evolution?

      Darwin’s great contribution was to identify how nature acted on organisms to increase or decrease the likelihood of any given trait surviving into the next generation. But he also left many important questions unanswered. For example, how exactly are such traits passed from parent to offspring? The work of the monk and scientist Gregor Mendel (1822–1884) provided the foundation for the modern theory of genetics. Although his work was not well appreciated in his own day, by the twentieth century Mendel’s concepts were solidly integrated into modern biology. Mendel studied pea plants, which varied according to easily identifiable traits, such as height, pea shape, and flower color. By cross pollinating plants with various combinations of these traits and examining the proportion of offspring plants that displayed each trait, Mendel discovered two important principles which later became known as Mendelian Laws of Inheritance. Mendel posited that each parent carries two versions of some biological characteristic (what we now refer to as a gene) which determines a given trait. But each parent contributes only one version of this characteristic to the offspring. In this way, the offspring receives half of each parent’s genes.

      These genes can be either recessive or dominant; dominant genes would always be expressed over recessive genes. For example, a single gene determines whether a pea plant will be short or tall. If an offspring receives two recessive genes, the offspring will express the recessive trait (the pea plant will be short). If the offspring receives two dominant genes, the dominant gene will be expressed (the pea plant will be tall). If the offspring receives one dominant and one recessive gene, the dominant gene will be expressed (and the plant will be tall).

      What did Watson and Crick contribute to the theory of evolution?

      By the mid-twentieth century, when James Watson (1928–) and Frances Crick (1916–2004) discovered the structure of deoxyribonucleic acid (DNA), the concept of genetic transmission of inherited traits was well established. It was widely accepted that an organism’s genetic code was contained in a specific molecule in the nucleus of a cell. This molecule had already been identified as deoxyribonucleic acid (DNA). What was unknown was the shape of that molecule. A trans-Atlantic race took place, in which the English scientific pair of Watson and Crick competed with American rivals to identify the molecular structure of the gene. Building on the previous work of Rosalind Franklin, Watson and Crick identified the shape of DNA as a double helix, a kind of twisting ladder. Additionally, they determined that the vertical bars of that ladder were made up of sugar and phosphate groups, while the rungs were formed from four nitrogenous bases: adenine, thymine, guanine, and cytosine. Together, these components form nucleotides, which are the building blocks of the DNA molecule. With this information, an enormous body of information opened up, ultimately allowing us to directly manipulate the genetic code for the first time in history. Of note, as a woman scientist in the 1950s, Rosalind Franklin received minimal recognition for her scientific contributions, an oversight which was only rectified long after she died.

      The double helix shape of the DNA molecule was discovered by James Watson and Frances Crick with the help of Rosalind Franklin.

      How does the theory of evolution connect with natural selection?

      The modern theory of evolution goes something like this: The genetic code for all cellular organisms (organisms made up of cells) is found in each organism’s DNA. DNA gets transmitted from parent to offspring,