Darwin's Observations and Inferences about Natural Selection
- There is variation in traits within a population, and some of that variation is heritable. For example, some organisms in a population may be one color and others are another color. The color of offspring is inherited from parents.
- More offspring are produced than can survive, so competition is inevitable.
He combined these observations with his previous knowledge to make two inferences:
- Individuals that are better suited to their environment will reproduce more. For example, those that can thrive, not just survive, in an environment will have more opportunities to mate and pass on their genetic traits.
- Over time, more individuals will carry traits that match their environment. If those that are better suited to an environment produce more offspring, the traits that make them better suited will more likely be passed on and increase within the population.
It is important to note that Darwin did not understand patterns of inheritance proposed by Austrian monk Gregor Mendel's work with pea plants. In his work, Mendel established the basic rules of inheritance and began the study of genetics. This work would not be published until 1866, seven years after Darwin published his work. Darwin built his idea of evolution by natural selection without any understanding of how offspring inherit a combination of traits from their parents.
Natural Selection Leads to Adaptation
As an example, imagine a population of butterflies in which some have a genotype (genetic makeup) for blue wings (BB), some have a genotype for purple wings (Bb), and some have a genotype for pink wings (bb).
The environment in which the butterflies live selects against blue butterflies because they are the preferred prey of a local bird predator. More blue butterflies are eaten by the predator than pink butterflies. During each generation, blue butterflies are selectively removed from the population. This means pink butterflies are surviving and reproducing at a higher rate. In each subsequent generation, there are fewer blue butterflies. Over many generations, blue butterflies may disappear completely. This environment selects for butterfly colors other than blue. The population adapts to the presence of this predator by shifting away from the preferred prey color.
Under conditions of sexual selection, mates are not randomly selected. There are two types of sexual selection: intersexual selection and intrasexual selection. Intersexual selection is a form of natural selection where mate choice by one sex results in different rates of success in reproduction between individuals. Mate choice is driven by the more selective sex, which is often the female because of higher investment of time and energy per offspring. Females often have to carry the offspring within their bodies or tend to eggs for long periods of time, as well as care for the offspring until they reach adulthood. Intersexual selection can lead to elaborate body structures and behavioral displays aimed to entice potential mates (e.g. male peacock tail feathers). This type of selection might include singing, dancing, or displaying bright colors. However, having elaborate ornaments is energetically costly and time-consuming. A brightly colored bird, for example, will stand out much more against green foliage than a duller colored bird and hence is more likely to be noticed by predators. Therefore, there is an increased survival risk associated with the increase in reproductive benefit.
With intrasexual selection, competition among individuals of the same sex results in different rates of success in reproduction. This type of selection can lead to the development of weapon-like structures, such as horns and antlers, as seen on moose, and large body size, as seen in elephant seals, to use when fighting competitors for access to mates. However, fighting is dangerous and the loser of the interaction often fails to mate at all.