Chapter Summary

     Section 23.2 is about the observations that demonstrate evolutionary change on earth. These observations are used to support the theory of evolution and describe the process in which species originate. Observations typically used to show the evolutionary process include observations of the fossil record, biogeography, convergent evolution and structural analogies, selective breeding, and homologies.

     The fossil record, while incomplete and unreliable at times, provides a history of life's evolution on earth. The fossil record provides a series of species successively similar to each other, providing clear evidence of evolutionary change, and demonstrating evolutionary relatedness between modern and historical species. Observations of the fossil record are further aided by discoveries of transitional forms. These transitional forms show intermediate states between a species and its known ancestor. For example, the newly discovered "fishapod" shows a transitional state between fish and the terrestrial tetrapod. The fishapod has the fins of a fish, but also the broad skull, and top-mounted eyes of a tetrapod. Its fins also show the beginning of the formation of finger-like bones.

     Biogeography is another observation made involving the geographic distribution of extinct and living species. Populations isolated on islands and other landmasses develop differently from populations on the mainland. These populations eventually become endemic species, retaining particular characteristics only found in that specific location. Two modern related species isolated from each other indicate the separation of a common ancestor due to changing geography. Such observations have led to the theory on the origin of marsupials, which may have risen from the isolation of early mammals in Australia.

     Certain organisms share similar characteristics, but are not very closely related, such as the anteater and the echidna, which both have long, pointed snouts, despite having very different lineages. These similarities are believed to arise from the process of convergent evolution. In convergent evolution, two unrelated species which share similar environments and adaptive pressures, develop similar traits. These traits are referred to as convergent traits. The echidna and the anteater both developed similar snouts in response to the high number of ants available as food within their environments.

     Selective breeding is a unique observation of evolution, as it is evolution caused by human involvement. Selective breeding, or artificial selection, refers to the human practice of phenotypic modification of domesticated animals. Humans control the reproduction of certain species, breeding for certain traits found desirable. Subsequent populations exhibit evolutionary changes corresponding to the traits selected by humans. In this sense, the traits that survive within the population are not necessarily those enabling the organism to survive and adapt better to its environment. Selective breeding can cause large variations in phenotype, giving rise to the many breeds of the one dog species, Canis lupus, and many of the different types of foods consumed by humans.

     Homologies are similar traits shared between species with a common ancestor. Such traits were found in the ancestor and were passed on and modified by its descendants. These homologies can be observed anatomically, developmentally, and molecularly. Anatomical homologies refers to structures similar to each other because the structure is derived from a common ancestor. For example, the forearm of vertebrates are homologous structures that have all been modified for different functions. This also gives rise to vestigial structures, structures that are derived from an ancestor but lose their functions in the new species. The vestigial structures are no longer useful, and are subject to degeneration through mutation, eventually disappearing. Developmental homologies refers to similarities present within the embryonic states of species that are very different when matured. These traits are normally present only in the premature state and disappear upon development. These traits represent ancestral traits and can be indicative of a distant ancestor For example, the presence of gill ridges in terrestrial animals (including humans) seems to suggest a common aquatic ancestor. Lastly, molecular homologies refer to homologies present in the analysis of molecular structures, such as DNA and biochemical pathways. Certain biochemical pathways, such as the metabolism of glucose, are found in every living organism, and are believed to be traits arising very early in the origin of life. Analysis of genetic sequences, such as the sequencing of enzymatic proteins such as p53 found in a vast number of organisms, reveals a gradation of evolutionary relatedness, as a large percentage of the genetic sequence remains identical. The more identical the sequence, the more closely related the organism. 

Useful Materials

Illustration demonstrating the convergent evolution of four species in four corners of the world. Despite being very geographically distant from one another, all four developed in similar environments, and therefore developed the traits for long snouts and tongues.

Evolutionary Biogeography

This webpage discusses evolutionary biogeography, attempting to explain the cause of evolution via geographical distribution of populations

http://sci.waikato.ac.nz/evolution/Homology.shtml

This webpage discusses anatomical homologies, providing examinations of several notable homologies present in modern species.

Observed Instances of Speciation

Article discussing instances of speciation through observations of fossil record, homologies, geographic distribution, and convergent evolution. Also discusses the methods of observation in general, as well as the process of speciation.

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