Mechanisms of Evolution

Natural Selection and Adaptation

Natural selection is a process by which individuals with certain traits survive and reproduce better than other individuals in a given environment.
Charles Darwin proposed that the driving force behind evolution (genetic change between generations) is a process called natural selection. Natural selection is a mechanism of evolution in which individuals that are better adapted to their environment survive and reproduce more successfully than others. He asserted that this process shaped early life on Earth and that it is ongoing. The idea states that individuals with certain traits will survive and reproduce better than other individuals in a given environment. As an example of natural selection, an animal that blends in with its environment is less likely to become prey than one that is colored in a way that makes it stand out. The color of this animal is considered an adaptation, a process or trait that improves an organism’s ability to survive or reproduce in a given environment. The animal will then pass on these traits to its offspring. Each generation will inherit a greater proportion of these favorable traits because individuals without them do not have as many offspring. In this way, natural selection is an editing process, not a creative force. Natural selection requires genetic variation, which includes differences among segments of DNA (through which traits are inherited) within a population (interbreeding group). The organism's environment "selects" the best-suited traits from the available population by determining which individuals will reproduce the most. Those organisms that are very successful in a given environment are said to have higher fitness. Fitness is measured by the number of surviving offspring that an individual contributes to the next generation. It can be described as an average of all individuals with a certain genotype (the genetic makeup of an organism) or phenotype (the observable characteristics of an individual). This is the type of fitness referred to by the common phrase "survival of the fittest." The phrase does not mean that the strongest survive; rather, it means that those best suited to their environment survive. Giraffes, for example, have long necks that allow them to feed on vegetation that other animals in their environment cannot reach. This helps to ensure their food supply and, hence, the survival of the species. During Darwin's voyage, he observed finches with different beak shapes in the Galapagos Islands. Each island was home to finches with beaks that matched the type of food available.
Natural selection favors traits that help organisms survive and reproduce. Charles Darwin observed its effects in the differences among the beaks of finches that were geographically separated in the Galapagos Islands. The shapes of the different beaks helped each type of bird find food on the island where it lived but would be unfavorable on the other islands.

Darwin's Observations and Inferences about Natural Selection

Darwin observed that there is heritable variation (differences based on genes) in traits among individuals and there is competition for resources. From these observations he inferred that individuals who are better suited for their environment will be more successful than other individuals, causing their traits to spread in a population over time.
How did Charles Darwin arrive at his idea of evolution (genetic change between generations) by natural selection? During Darwin's travels and subsequent experiments, he made two important observations:
  1. 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.
  2. More offspring are produced than can survive, so competition is inevitable.

He combined these observations with his previous knowledge to make two inferences:

  1. 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.
  2. 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

Natural selection is the only mechanism of evolution that always leads toward adaptation (better survival and reproduction in an environment).
Natural selection is one of several mechanisms of evolution, which can be defined as a change in gene frequencies within a population. A gene frequency is how often a particular version of a gene appears in a population. Natural selection is the only mechanism that always leads toward adaptation; better-suited organisms reproduce more, spreading adaptive traits within a population. Adaptation is any process or trait that allows an organism to survive and reproduce better in its environment. For example, thick fur is an adaptation to a cold environment. Natural selection acts on individuals who either survive and reproduce or do not, but evolution happens at the population (interbreeding group) level, because it is measured by changes in the genetic makeup of a group. With each generation of natural selection, a population will become better adapted to its current environment.

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.

In a fast-changing environment, a population may not be able to evolve fast enough to maintain a high level of adaptation. Evolution happens between generations, so environments that change frequently within a single generation may not consistently select the same traits. Evolution is not always possible, because if genetic variation does not exist, natural selection cannot happen at all.
This population of butterflies lives in an environment that is more dangerous for butterflies with blue wings because they are preferred by predators. Consequently, the blue wing color genotype (BB) becomes rarer with each generation because there are fewer and fewer individuals with that genotype who survive long enough to reproduce.

Sexual Selection

Sexual selection increases the reproductive success of organisms with certain traits.
Sexual selection is a type of natural selection, which is a process wherein better-adapted individuals survive and reproduce more than others. Sexual selection is a form of natural selection that focuses on reproduction. Natural selection involves two components of fitness: reproduction and survival. Under sexual selection, traits that increase reproduction are selected for. In many cases, these traits can reduce survival of the individual, but the overall fitness of the organism is still high due to the enhanced fitness from reproduction. Fitness is based on number of offspring. Thus, it can be argued that reproduction is more important than long-term survival because a very long-lived organism that never reproduces has zero fitness.

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.