Evolution, the change in gene frequency over time, has taken decades to fully understand and each day, bits of information come together to give it a stronger platform to stand upon. The theory of evolution, designed in part by Charles Darwin explores the adaptations of species, speciation, and the organization of the relationships between species. These components provide the heart of the theory of evolution. The evolution of species is witnessed through three methods- natural selection, genetic drift, and gene flow. All of the methods give rise to a varying gene pool with respect to the environment and frequency of certain alleles. Moreover, the gut of evolutionary theory revolves around speciation, which can arise in numerous ways. Two methods of speciation will discussed later on: allopatric and sympatric speciation. Finally, the organization of the relationships between species, phylogeny will be discussed in order to explain the importance of understanding who is related to who, and some species are not related to others, especially those that have similar characteristics(i.e. homologous vs. analogous). After completing the unit of evolution, I have gained a new found understanding on evolutionary theory through the use of lectures, text, and kinesthetic exercises, which have helped reinforce the information into my mind.
Evolution encompasses many notions, but there are three main components that comprise the theory: evolution of a species, speciation, and the representation of the relationships between species. The evolution of a species can occur through natural selection, genetic drift, and gene flow. According to Darwin, natural selection works to change a population over generations if individuals with heritable characteristics produce more offspring than other individuals (Campbell 443). For example, a population of red and white orchids inhabit a certain environment, but the white flowers are eaten more frequently than the red flowers. Over time, more red flowers will reproduce offspring(thus, more red flowers than white) as the white flowers are less populated in the environment. In addition to natural selection, genetic drift, the frequency of alleles in the gene pool each generation, can have a drastic impact of the evolution of species. Moreover, genetic drift can reduce the genetic variation in the gene pool of a population experience the founder or bottleneck effect. The bottleneck effect can be seen when a great disaster occurs and reduces the population size. For example, the recent oil leak in the Gulf of Mexico has significantly reduced the population of many species of fish- this decrease in numbers reduces the gene pool. A decrease in the variation of the gene pool allows for certain genes to be overrepresented(or vice versa). Beside the bottleneck effect, the founder effect reduces the genetic variation of a population, as well. The Campbell text cites that the founder effect is when a population is isolated from the original population and the new gene pool “is not reflective of the source population” (Campbell 462). An example is the early colonization of the Americas by Europeans; only a select group of men and women relocated, which significantly changed the gene pool. Both events can significantly evolve a species in a short amount of time because the gene pool was not changed due to natural selection, but by the change in allele frequency of the gene pool. Finally, evolution of species can occur by gene flow. A population experiences gene flow when it gains or loses alleles from its gene pool. Most commonly this happens when two populations are able to breed together, such as the seeds of a wildflower population blowing to another population of wildflowers, thus introducing a new allele. Over time, gene flow decreases the difference between populations as it spread the alleles throughout many populations.
The evolution of species is important to Darwin’s theory, but it is the development of new species(speciation) that is a pivotal concept because it produces biological diversity. There are many definitions for species, but for the biological concept, Campbell describes a species as a “group or group of populations that have the potential to interbreed…produce viable, fertile offspring, and…cannot reproduce with members of other populations” (Campbell 473). Speciation can occur through two methods, allopatric speciation and sympatric speciation. Allopatric speciation refers to an interruption in gene flow caused by a geographical isolation. Islands (like Hawaii) are great examples of allopatric speciation; the isolated populations evolve over generations producing endemic species unique to that region(again natural selection plays a role here). Isolation creates a perfect environment for speciation, but the development of new species does not require geographical barriers. Sympatric speciation accounts for new species arising within the same geographical region. Oftentimes, the two species reside in the same region, but prefer different regions of the habitat and have opposing sexual selection factors. For example, Hawthorn flies used to feed on Hawthorn trees(and mate their too), but the introduction of the European Apple enticed many of the flies to feed on these apples because of their quick growth rate(Causes of Speciation). Over time, the original population of flies split into two species- the Hawthorn flies and the flies that chose the European apples.
Lastly, the evolutionary relationships between species, or phylogeny, allows biologists to examine the common ancestry of many species with respect to their morphology. Phylogeny is a visual picture of the relationships that many species share with each other, as it also depicts the distance between each other, as well. A phylogenic tree shows the common ancestor and the species that have adapted over time to create new species. Trees are relatively simple to read, once the vocabulary and its purpose are clearly understood. First, two terms can cause confusion for the reader of a phylogenic tree: analogous and homologous. An analogous characteristic, is one that is similar in nature between two species that are not related to each other by a common ancestor. For example, the dorsal fin of a dolphin and a shark are similar in structure and function, yet the two species are not related. They are said to be analogous to one another(these creatures would not be seen on a tree with a recent common ancestor). In other words, two species that are on the same tree, but do not share a most recent common ancestor, are said to be polyphyletic(Campbell 498). On the other hand, a homologous characteristic is one that is shared between two species that share a common ancestor. Species that are homologous to one another share similar characteristics at the anatomical and molecular levels. For example, all vertebrates have a tail during the embryonic stage.
The vast information on evolution can be confusing, but there are many tools that help the biology student understand the material. The first section on evolution involved learning the evolutionary process, particularly natural selection. After several thorough readings of these chapters, I understood the differences in the concepts of natural selection and genetic drift. For example, figure 22.14 in the Campbell text explained the homologous structures between four different species and that the adaptations over time allowed these same structures to perform very different functions. Speciation was very easy to understand, yet I found several tools on the Internet that aided in the distinction between different forms of speciation. For example, I read “Evolutionary 101” developed by the evolutionary biology department at UC Berkeley. The web site provides clear images and explanations of allopatric and sympatric speciation. The last subject, phylogeny, required the most outside help in order to understand the difference between analogous and homologous, and monophyletic and polyphyletic characteristic with respect to the evolutionary tree. There were two items that I used to comprehend the structure and function of evolutionary trees. The first tool that explained the usefulness of phylogenic trees was the Mastering Biology quiz, “Trees/Evolution”, particularly the exercise that involved arranging the aliens on the tree. Once I was able to arrange them in order and place the correct adaptations of traits in the correct locations, the ability to create or reconstruct a tree seemed like a simple task. In addition, “Understanding Evolutionary Trees” developed by University of California Museum of Paleontology, provided the qualitative information that explained how to read the trees regardless of its positioning of the different species.
Campbell, Neil A. Biology. 7th ed. [S.l.]: Benjamin-Cummings, 2005. Print.
“Causes of Speciation.” Understanding Evolution. 2011. University of California Museum
of Paleontology. 22 August 2008 .
“Trees and Evolution.” MasteringBiology: Make Learning Part of the Grade. Web. 30 Jan. 2011.
“Understanding evolutionary trees: A Quick Review.” Understanding Evolution. University
of California Museum of Paleontology. 22 August 2008 edu/evolibrary/news/060101_batsars>.