Natural Selection

• In reproduction isolation, there will be the emergence of new species.

The basis for Darwin’s evolution was that all organisms can trace their descent to a common ancestor and that, because of natural selection, there becomes a diversity of species. Natural selection proposes that the heritable traits developed by a generation will vary and that some of the traits confer a natural advantage that allows some variations to have a reproductive advantage, perpetuating those particular variations in later descendants, leading to changes in the population.

Many of Darwin’s theories came from the study of finches on the Galapagos Islands. He noted specific variations in the bird species, so that those that had certain types of beaks ate seeds and those that had other types of beaks ate insects. He argued that, through natural selection, changes in beak types were self-selected so that there was a reproductive advantage to certain birds in situations of limited resources. Divergences happened, he said, so that more resources (seeds and insects both) could be utilized by the different variations in the species.

The Galapagos Island is an isolated island off of Ecuador, containing species similar to those in the mainland but isolated enough that they needed to adapt to local conditions over long periods of time and over multiple generations. The process of the development of new species led to the formation of more than one distinct species of animals, including birds.

Darwin proposed that there can be changes of species over time, that new species can originate from pre-existing species, and that there must be a common ancestor to all species. He believed that the diversity of species is because of the inheritance over time of traits that diverged from the ancestor species. He referred to the change in heritable traits over many generations as “descent with modification”. This is now what we call evolution.

NATURAL SELECTION

One of the key aspects of Darwin’s theories was that of “natural selection”. It explains how populations evolve over time in certain ways that allow the population to be better suited to their environment over the generations. It is based on the idea that traits are

heritable and are passed from parent to child. It is also based on the theory that more offspring are created than can survive and that can be supported by the environment. This leads to competition for limited resources in the environment. In addition, the offspring will have differences in their heritable traits (and will appear different).

Some individuals in a population will have inherited traits that help them live and reproduce to a better degree than those with other traits. These better-adapted organisms will have more offspring than those that are less adapted to the environment. In other words, the trait must confer a reproductive advantage. This leads to an increase in the numbers of individuals who are better adapted to the environment.

The concept of natural selection isn’t random. The traits that get inherited are not in some way inherently superior to other traits. Instead, these traits are beneficial in relation to the environment. In other words, in rat species, for example, black rats stick out more than tan rats if the rocks they live on are tan. This means that more black rats get eaten by hawks so that tan rats survive better to reproduce. The reverse would be true if the rocks were black.

The traits need to be something that is heritable in the genes of the organism rather than something that develops over the lifetime of the organism. If a bird species gets smarter at collecting food over time and survive to reproduce, this may not be inherited by the next generation—unless bird intelligence itself is inherited and gets passed on as a heritable trait.

Finally, the origin of these gene variations is that of mutations that are basically random and that are passed onto offspring through sexual recombination. We all know that there are random variations in the act of meiosis that assure that the offspring are different from their parents. There will be mutations that lead to more different offspring than would typically be seen by meiosis and recombination alone.

Natural selection is the cornerstone of biology today. It is different from artificial selection (which involves the selection of desired characteristics by breeders) in that it is random and depends almost entirely on the environment that the selection occurs in. There is genetic drift, which is completely random; however, most genotype and

phenotype changes are not random and occur because the change confers an advantage in the environment the organism lives in.

Interestingly, there have been natural selection theories predating Darwin. In fact, classical philosophers had expressed these ideas in limited form. Islamic theorists also proposed natural selection as the struggle for existence. Darwin’s grandfather (Erasmus Darwin) reintroduced these classical arguments in the 18th century. Jean-Baptiste Lamarck suggested that there were inherited characteristics that acted as a mechanism for evolutionary change. Eventually the inherited changes caused the development of new species.

Darwin studied natural selection during his trip on the second voyage of the HMS Beagle between 1831 and 1836 but didn’t actually realize what he had discovered until he worked with an ornithologist who studied examples of the birds retrieved from the Galapagos Islands. In 1859, he came out with his book On the Origin of Species, which laid out the theories of natural selection. Darwin was inspired by the work of Thomas Malthus, who wrote an essay indicating that the food supply grows at a slower rate than the population that eats the food, leading to an inevitable shortage in the supply of food and the need to have adaptations that make some organisms more adaptable to the short food supply.

As mentioned, there must be a reproductive advantage to the organism in order for natural selection to take place. It doesn’t have to be a large advantage but it has to be heritable so that the advantage can increase over successive populations. There is no intentional choice as there is in artificial selection, just an evolutionary advantage of one trait over another.

The idea of fitness is important to natural selection. Organisms that are fitter have a better chance of survival, leading to the phrase “survival of the fittest”. Fitness is not just how long an organism lives but is more about how successful it is at reproducing. In other words, an organism can live a shorter period of time but, if it produces twice as many offspring that survive to reproduce themselves, it will be the fitter organism. It doesn’t necessarily mean there is an improvement in fitness but that it allows for the removal of those organisms that are less fit in the population.

With competition, the fitness of one organism is lowered by the presence of another organism. This is because both organisms must share territory, water, food, or other resources. Competition may occur between organisms of the same species or between other species. Competition plays a significant role in natural selection because, without competition, there would be no need for natural selection in a species as all species would survive equally.

There are three types of natural selection. The first is directional selection, in which a single “extreme” phenotype is favored over other species. The second is stabilizing selection, in which an intermediate species is favored over the extremes. The third is disruptive selection, there is more than one extreme favored over the intermediate organisms. Natural selection can act on any heritable phenotype with selective pressure produced by any aspect of the environment—including sexual selection and competition between those in the same or other species. Disruptive selection can be a precursor to the development of different species.

Selection can be classified by the life cycle it acts on. There is viability or survival selection, which increases the organism’s probability of survival, as well as fecundity or fertility selection, which increases the rate of reproduction of the organism. Within fecundity selection, there can be factors that act on the survival of the gametes themselves and factors that act on the ability or inability to create a viable zygote.

Other ways to classify natural selection include those at the level of the individual versus those at the level of the group. Selection, in other words, can mean that certain traits act on individual organisms that have or don’t have the advantage. The selection can also act on the group of organisms to change the entire group’s ability to survive. The later classification of group selection is probably less of a factor than individual selection.

Natural selection has played a role in the development of antibiotic resistance among microorganisms. Since the discovery of antibiotics, bacteria have developed ways to become resistant to the antibiotics. This is borne out in the development of methicillinresistant Staphylococcus aureus (MRSA), which developed the ability to be resistant to antibiotics that typically they used to be susceptible to. Those that were susceptible to penicillin/methicillin died out, leaving behind mutated organisms that survived and