Introductory Biology

13.1 The Theory of Evolution by Natural Selection

Charles proposed the mechanism whereby evolution happens and proposed that all species come from a common ancestor. Lucretius of Rome wrote the first recorded writing about evolution, about 20 years ago. He noticed that there was a change, the book was called On the Nature of Things. Charles Darwin’s work improved the modern theory of evolution and it was supported through decades of observations and experimentations. At age 22, Darwin sailed around the world as a naturalist on the HMS Beagle but he spent more time on land studying fossils and brought many species back.

The traditional view within science was that the relationship between species and their environment was through divine creation. Traditional view was that species are unchanging over time since they have been perfectly created.  Some writers went against traditional view, proposing that species evolve/change over time and evolution occur naturally, in order to explain the fossil record. In 1809, Jean Baptiste Lamarck proposed a mechanism for evolution are changed during an individuals lifetime. The idea of Lamarckism: increasing through their use or decreasing with disuse, in which these acquired traits are then inherited by that organism’s offspring. Lamarck’s ideas are now discredited, but he was able to view evolution as a dynamic between organisms and their environments. Darwin knew about geology and was influenced by Charles Lyell’s book (Principles of Geology) and he thought the same change in geology is occurring in biology. He began to see the change of species through slow processes, similar to Lyell’s geologic change. During his voyage to South America, he discovered many fossils and took many specimens back to England (27 years old) and began to study these ideas.  Darwin was surprised by the similarities of plants and animals on the Galapagos Islands and on the coast of South America. Darwin proposed descent with modification as a description of gradual evolution after his studies.

Thomas Malthus’ Essay on Population provided information to understand natural selection. A population contains individuals of a species that live in a specific geographical area, which can interbreed. Overpopulation is one issue that leads to natural selection. Populations grows rapidly, whereas resources, such as food grow at only an arithmetic rate. And so, there will always be a shortage of resources, and organisms will struggle from starvation. Darwin observed that all populations apply to the same principles as the human populations Malthus wrote about. Natural Selection was a term used by Darwin to describe the differential rate of reproduction. Every organism is able to produce as many offsprings as possible, although only a few/limited number of offsprings can survive to reproduce. Individuals that have physical traits that suit their environment have a higher chance to survive and reproduce than others without these traits. Individuals that suit their environment survive and reproduce, passing their heritable traits to their offsprings. These favorable characteristics gradually increases in the population. Evolution is the resulting genetic change in the population over time. Organisms are different geographically because each species has evolved to its specific environment. Adaptations occur from natural selection of traits that best fit a particular environment. * Darwins Finches: Darwin realized how the different birds were closely related, even though each one eats different kinds of food and have different beaks. These organisms became different because they took different evolutionary paths and ate different food.

In 1844, Darwin noted his ideas about evolution and natural selection. Alfred Russell Wallace (biologist influenced by THomas Malthus) also had the idea of natural selection and wrote a paper containing ideas similar to Darwin’s ideas in 1858. Wallace and Darwin’s papers describing their ideas was presented at the same meeting (1858). Darwin was given more credit, as he continued to write On the Origin of Species by Means of Natural SelectionInherited variation occurs within the genes of every population/species, result of random mutation and translation errors. Some individuals of a specific population or species are better suited to survive (as a result of variation) and have more offspring (natural selection). Over time, the traits that support some individuals of a population to survive and reproduce tend to spread throughout that population. Evidence is provided from fossils and many other sources that living species evolved from organisms that are extinct.

Genes produce inherited traits and come in different forms (alleles). Variation is formed by mutation and genetic recombination, as a result of meiosis and random fertilization. Alleles that provide better adaptations to an environment increase over time, less fit alleles decrease. In short, evolution is the change in the frequency of alleles over time.  Populations of one species living in various locations tend to evolve differently. Reproductive isolation is the condition in which 2 populations of the same species do not breed with each other: because of geographic separation, difference in mating periods, and other obstacles to reproduction. Isolated populations become more different. The two populations end up being so different they can not interbreed anymore and become reproductively isolated. When two populations are reproductively isolated, they are considered different species. Example: the Kaibab squirrel living on the north of Grand Canyon and the Alber squirrel living in the south are separate species since they are reproductively isolated from one another.

Gradualism is the slow and continuous model of evolution. Punctuated  equilibrium is a model of evolution where periods of quick change, caused by major environmental changes, are separated by period of little/ no change. The concepts of Darwinism are: overproduction, variation, competition, survival of the fittest phenotype, and favorable combination increase. 


One thought on “13.1 The Theory of Evolution by Natural Selection

  1. Yurika,

    Great blog on 13.1 on Darwin’s conception of evolution. You’ve covered all the ideas in great detail and excellent organization. Keep up the great work!

    Mr. F.

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