Natural Selection



Natural Selection




Vocabulary: adaptations, common ancestors, competition, classification, evolution, extinction, fossils, fossil record, natural selection, overproduction, reproduction, selecting agent, speciation, species, survival of the fittest, variations

Natural Selection

Evolution, the change of species over time, is the central unifying theme of biology. Biological evolution occurs through natural selection. Natural selection is the result of overproduction of offspring having variations, these variations struggling for survival, and the subsequent survival and increased reproduction of those best adapted to a particular environment.  


Natural selection is the evolutionary process which selects the variation(s) of organisms best suited for a particular environment.  Natural selection and its evolutionary consequences provide a scientific explanation for the fossil record of ancient life, as well as for the molecular and structural similarities observed among the diverse species of living organisms.

The degree of kinship between organisms or species can be estimated from the similarity of their DNA sequences; this similarity often closely matches organisms' or species' classification based on anatomical similarities.

           DNA Sequence Similarities in Some Primates

The graphic above shows that:

  1. All of these primates had a common ancestor, the ancestral primate.
  2. The human and chimpanzee have the closest evolutionary relationship as their DNA is the most similar.

Charles Darwin, late in his life.


Theory of Natural Selection

  1. Overproduction: Within a population more offspring are born than can possibly survive.
  2. Competition: Since the number of individuals in a population tends to remain constant from generation to generation due to limited resources, a struggle for survival occurs.
  3. Survival of the Fittest: The individuals who survive are the ones best adapted to exist in their environment due to the possession of variations that best suit them to their environment. This genetic variability within a species is chiefly due to mutation and genetic recombination. The variation of organisms within a species increases the likelihood that at least some members of the species will survive under changed environmental conditions.
  4. Reproduction: Variations assist or hinder individuals in their struggle for survival. The best adapted individuals survive and reproduce, passing on the favorable variations to their offspring.
  5. Speciation: As time and generations continue, adaptations are passed on and new species may evolve from a common ancestor.

Small differences between parents and offspring can accumulate in successive generations so that descendants become very different from their ancestors.

An adaptation is a variation which assists an organism or species in its survival. Biological adaptations include changes in structures, behaviors, or physiology that enhance survival and reproductive success in a particular environment.

Some characteristics give individuals an advantage over others in surviving and reproducing, and the advantaged offspring, in turn, are more likely than others to survive and reproduce. The proportion of individuals that have advantageous characteristics will increase.

Behaviors have evolved through natural selection. The broad patterns of behavior exhibited by organisms have evolved to ensure reproductive success.

Modern Examples of Natural Selection

Peppered moth:

  • Two varieties of peppered moth existed, a light colored and a dark colored one.
  • As industrialization and coal burning increased, the environment in England where these moths lived became dirtier .
  • The dark colored variety of the moth blended into the trees and increased in numbers, while the light colored moth was less adapted and decreased in numbers.

Insect resistance to insecticides:

  • Insecticides kill insects not resistant to the insecticide, while insects resistant to the insecticide live to reproduce. The insecticide acts as a selecting agent.

Bacterial resistance to antibiotics:

  • Bacteria not resistant to an antibiotic are killed by it, while resistant bacteria live to reproduce. The antibiotic is a selecting agent for these bacteria.

Galapagos Island Finches:

  • Darwin developed his theory of natural selection by observing the many species of finches on the Galapagos Islands off the coast of South America. He notices similarities among all of the species, both on the mainland and the islands but noticed distinct differences among those on different islands which he attributed to the selecting agents of the varying foods the finches ate.

Classification of Galapagos finches and their principal functional differences.

  • After years of observations, he concluded that all of the island finches had evolved from the mainland finch. This has later been proven with DNA analysis of the different finches.

The pattern of adaptive evolutionary diversification among the Galapagos finches.


Evolution and Extinction
Evolution does not necessarily mean long term progress is going to go in a certain direction. Evolutionary changes often appear to be like the growth of a bush. Some branches survive from the beginning with little or no change, many die out altogether, and others branch out repeatedly, sometimes giving rise to more complex organisms.

              Direction of Evolution


Note the divergence of the various groups from a common ancestor and the fact that some branches became extinct.

Extinction of a species occurs when the environment changes and the adaptive characteristics of a species are insufficient to allow its survival. The fossil record indicates that many organisms that lived long ago are extinct.   Extinction of a species is common; most of the species that have lived on earth no longer exist.

The Fossil Record
are direct or indirect remains of organisms preserved in media such as sedimentary rock, amber, ice, or tar.  Fossils have been found that indicate organisms existed well over 3 billion years ago. These organisms were simple, single-celled organisms.
About a billion years ago, increasingly complex multi-cellular organisms began to evolve.

The higher up you go in an undisturbed rock stratum (rock layer), the younger the rock layers become and therefore it is believed the fossils within these layers, as compared to lower rock layers, are also younger.


Relative Dating of Undisturbed Sedimentary Rock and its Fossils

Upper strata generally contain fossils of younger, more complex organisms, whereas, the lower strata contain fossils of simpler life forms. This means there is a tendency toward increasing complexity in life forms over time.

When comparing fossils in undisturbed strata, fossils can be found in upper strata which, although different from fossils in lower strata, resemble those fossils. This suggests links between modern forms and older forms, as well as divergent pathways from common ancestors.

Biological classification is based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on structural similarities and evolutionary relationships.

The species is the most fundamental unit of classification. This is a group of organisms which are close enough in their evolutionary relationship to be capable of successful reproduction and having fertile offspring.                        

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