Artificial selection is the process of humans deliberately choosing certain varieties of an organism over others by implementing breeding programs that favor one variety over another. Natural selection is the evolutionary process leading to the extraordinary variation–including behavioral variation–that we see in nature. Once Darwin’s ideas were widely disseminated and integrated into the heart of biology during what is called “the modern synthesis“, animal behaviorists possessed a theory that helped explain not only what animals do, but why they do it.
When nature is the selective agent, traits, including behavioral traits, increase or decrease in frequency as a function of how well they suit organisms to their environments. If one variety of a trait helps individuals survive and reproduce better in their environment than another variety of the same trait, and if the trait can be passed down across generations, then natural selection will operate to increase its frequency over time. The phenotype is typically defined as the observable properties of an organism. An individual’s phenotype itself is the result of its genotype–its genetic makeup–and the way that a particular genotype manifests itself in the environment. Over evolutionary time, small differences in fitness can accumulate into large changes in gene frequencies.
Natural selection requires three prerequisites to operate:
- Variation in the trait–different varieties of the trait
- Fitness consequences of the trait–different varieties of the trait must affect reproductive success differently
- A mode of inheritance–a means by which the trait is passed on to the next generation
Technically speaking, a fourth requirement also exists, and that is that resources must be limited with respect to the trait being studied. Variation in a trait can be caused by environmental or genetic factors. Mutation–which is defined as any change in genetic structure–creates new variation in a population. Another factor that produces variation in a population is genetic recombination. New genetic variants of a trait can enter a population via nongenetic pathways; the most common way for this to occur is through migration. The fitness consequences of a trait refer to the effect of a trait on an individual’s reproductive success, which refers to the mean number of reproductively viable offspring an individual produces. Because genes are passed down from generation to generation, they are the most obvious candidate for a method of transmission (of inheritance). One way to study genes as a mode of transmission is by calculating narrow-sense heritability–a measure of the proportion of variance in a trait that is due to genetic variance. One means for measuring narrow-sense heritability is by designing a truncation selection experiment. It can also be measured through parent-offspring regression.
Sociobiology is the study of the evolution of social behavior. The sociobiological notion that genes–in our case, genes associated with behavior–are the units upon which natural selection acts, is often referred to as the “selfish gene” approach to ethology. Any allele that codes for a trait that increases the fitness of its bearer above and beyond that of others in the population will increase in frequency. So natural selection often, but not always produces genes that appear to be selfish. Apply this approach to animal behavior, particularly animal social behavior, and you have one of the main ways in which ethologists think about genes and and animal behavior. Adaptations are traits that natural selection molds and that often match organism to environment so exquisitely. Natural selection is the primary process generating adaptations.
To study common ancestry, evolutionary biologists construct phylogenetic trees, which depict the evolutionary history of a group of species, genera, families, and so forth. Species that share a recent common ancestor tend to have many traits in common for the very reason that they share a common ancestor. A homology is a trait shared by two or more species because those species share a common ancestor. A homoplasy is a trait that is not due to descent from a common ancestor shared by two or more species but instead is the result of natural selection acting independently on each species. Such homoplasies are referred to as analogies, and the process leading to the production of analogous traits is called convergent evolution. Homologous traits are used in phylogeny building because they reflect shared evolutionary histories. Homoplasies do not reflect the historical relationships between species and, in fact, distort and misrepresent those relationships when used in phylogenetic tree building. When we study the historic order in which different varieties of a trait appear, we are examining what is referred to as polarity, or the direction of historical change in a trait. For most of the history of evolutionary biology, the fossil record was the primary source for adding a temporal component to the phylogenetic trees. Today, molecular genetic data can be used to help date major changes that occur on phylogenetic trees. The most common technique to handle the problem of distinguishing between possible phylogenetic trees is parsimony analysis. Every phylogenetic tree is a hypothesis of the evolutionary history of the groups under study. When morphological and molecular genetic analyses produce similar phylogenetic trees, our confidence increases that our phylogenetic tree is correct. Once we know something about the phylogenetic history of the group we are studying, we can ask whether certain selective pressures consistently favor one combination of traits or another independent of phylogenetic history, or whether the co-occurence of the traits in certain species is the result of a common ancestry for those species.
—May 2022
Lance Jones 