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=Evolution =

**Genetic Variation**
 * Definition:** difference in alleles and genomes among organisms of a population

Mutations are a source of genetic variation. Changes in the nucleotide base sequence of an organism can cause new alleles to arise. Sexual recombination, including crossing over and independent assortment, also cause genetic variation within a population. Genetic variation is preserved in several ways. One mechanism that perpetuates the presence of alleles that are unfavorable in the environment is diploidy. Often, the deleterious phenotype only affects individuals that are homozygous recessive. This means that the allele can be carried on by heterozygous individuals, as it only affects their fitness if they are recessive, which is unlikely if the frequency of the allele in the population is low. Natural selection may also enhance genetic variation. If a population exists in an environment where two or more phenotypes are equally able to survive and reproduce (balanced polymorphism), then all of the beneficial alleles will remain prevalent. In this situation, natural selection also maintains the heterozygote advantage. Organisms that are heterozyous are more fit than homozygotes, so the frequency of each allele remains stable. If by chance one of the balanced polymorphisms becomes more common, the fitness of those individuals decline. For example, the more common phenotype may become more easily recognizable to predators, decreasing those organisms' chance to survive to reproduce.



**Definition:** Organisms with traits that are most suited to their environment are more likely to survive and reproduce, passing on their favorable genes
 * Natural Selection **

All populations tend to reproduce faster than their food supply, which creates a competition for resources. Organisms which characteristics that better enable them to survive are more fit. There are several modes of selection that work to determine which phenotypes will be favored. Directional selection favors phenotypes on one end of the spectrum; disruptive selection favors phenotypes on either ends; stabilizing selection favors intermediate phenotypes. Natural selection cannot cause changes in an organisms genome; it cannot create a trait that will enable a population more viable; it only selects for organisms that are most suited to their surroundings.

**Microevolution vs. Macroevolution** **Definition (microevolution):** Change in the allele frequencies from generation to generation


 * Definition (macroevolution):** Change in the genetic composition of a species over a long period of time, including the development of major evolutionary structures or abilities

Microevolution measures change in the allele frequencies in a population over the course of a generation. Usually, no new major developments arise. Macroevolution occurs over a much longer period of time, and can include processes such as speciation. The species' genes may change drastically. The population may even evolve new characteristics, such as the ability to fly.



**Speciation** **Definition:** the evolution of a new species

Speciation is the source of biological diversity. Humans have never witnessed speciation because it takes much too long, but the fossil record provides evidence of it. There are two different methods of speciation: allopatric and sympatric. Allopatric speciation occurs when a population is separated by a geographic barrier and are therefore unable to mate with a portion of the original population. Each new population reproduces and, over time, evolves to their new environment. Sympatric speciation takes place without any geographic isolation. It is usually caused by a random mutation that does not prevent an organism from producing viable offspring, but keeps it from fertilizing/being fertilized by other organisms from its original species. Some scientists believe that evolution is a gradual, continuous process. The gradualism model state that species slowly accumulate more and more different adaptations. The other theory is known as "punctuated equilibrium." This model suggests that a speciation occurs comparatively quickly; the new species goes through a relatively quick succession of changes, and then the species undergoes almost no changes.

**Anagenesis vs. Cladogenesis** **Definition (anagenesis):** the accumulation of mutations and genetic changes over a long period of time give a species new characteristics


 * Definition (cladogenesis):** branching evolution, one species evolves into two or more sepate species

Anagenesis and cladogenesis are the two main patterns of evolution. Anagenesis results in only one new species; cladogenesis produces at least two. Cladogenesis increases biological diversity because it increases the number of different species.

**Biological Species**
 * Definition:** a species is a group of organisms that can reproduce with each other and produce fertile offspring

The members of a biological species are potentially reproductively compatible. Organisms of a species may be remarkably dissimilar, like dogs of different breeds that have widely varying characteristics but can still procreate.

**Reproductive Barriers** Prezygotic barriers prevent fertilization from ever occurring. Geography is one example of a prezygotic barrier. Organisms that have different habitats most likely will never procreate. Even species that live in the same geographical region may be separated by different mating times, courtship practices, or physical characteristics that prevent them from mating. For example, wood frogs and leopard frogs will not mate because the males of each species have mating rituals that attract only females of the same species. Postzygotic barriers prevent the fertilized egg cell from becoming a fertile adult organism. Many times when fertilization occurs, the offspring dies before reaching adulthood. Ones that do survive are very weak. If the hybrid offspring is healthy and strong enough to survive, it is probably infertile. Even if the first generation hybrid can reproduce, it will not be able to produce viable offspring. Different species of sunflowers can reproduce and form fertile offspring. However, these hybrid offspring produce progeny that are not viable.
 * **Prezygotic**
 * **Postzygotic**



**Patterns of Evolution** One species evolves into new, separate species, each suited to its environment. The new organisms likely have molecular and structural homologies. For example, the finches of the Galapagos islands are all descendants of one species that migrated from the mainland of South America. When the reached the separate islands, the new populations evolved independently until eventually they speciated. Two species with different ancestors independently develop similar features (analogous structures) because they live in similar environments. Birds and bats both have wings with somewhat similar structure, but they are not related. Parallel evolution occurs when separated populations evolve similar structures and appearances. One example is the similarity between the sugar glider and the flying squirrel. Although they live on different continents, they have a strong morphological resemblance. Two species interact and influence the evolution of the other population. A population of zebra is preyed upon by a pride of lions. Zebra that are able to run at higher speeds are more likely to escape predation and survive to reproduce, creating a population of zebra that gradually becomes quicker. The group of lions coevolves because only individuals that are fast enough to catch the zebra or able to prey on them by other means are able to survive. A myriad of differently adapted species evolve from a single common ancestor. Adaptive radiation is usually caused by the migration of a small portion of a population to a different geographical area or when one species suddenly goes extinct. The diverse plant-life of the Hawaiian Islands originated from a few plant species that spread all over the island and adapted to many different environments.
 * **Divergent Evolution**
 * **Convergent Evolution**
 * **Parallel Evolution**
 * **Coeveolution**
 * **Adaptive Radiation**

**Homeotic Genes**
 * Definition:** genes that control where organs are placed on the body


 * Hox genes are a specific type of homeotic genes. They control the specialization of cells by determining which cells should develop into a particular organ or structure. Mutations in the Hox genes could cause a fish to develop an extra fin or an invertebrate to create a backbone. **


 * Phylogeny and Systematics **
 * Definition (phylogeny):** the evolutionary diversification of a species


 * Definition (systematics):** a method to understand the diversity and unity of all life


 * Scientists use fossils and systematics (the study of physical and chemical similarities) to determine an organisms evolutionary history. **


 * Homologous vs. Analogous **
 * Definition (homologous):** similar in position, composition, and origin


 * Definition (analogous):** similar in function, but different in origin


 * Homologous structures, such as the forearm of a human, the front paw of a cat, and the front flipper of a whale, have very similar structures. Each is composed of one upper bone joined to two lower bones which are connected to a length of small pieces of bone. This is because humans, cats, and whales shared a common ancestor. However, each organism uses this structure is a different way. Analogous structures, such as the wing of a bat and the wing of a bird, are used for basically the same purpose, but the structural composition of the organ is different due to each organism having a different evolutionary history. **


 * Cladistics **
 * Definition:** method of classifying organisms based on evolutionary history


 * Using systematics to determine phylogeny, scientists can organize species based on how recently they shared a common ancestor. The organisms are placed into charts called cladograms that are divided into clades, groups that share an ancestor. **


 * Tree of Life **
 * When organisms are arranged into a cladogram, they form a “tree of life.” All life came from a common ancestor (the trunk) and has become increasingly diversified and expanded (the branches). **

**Sources** "3D 3ds fish fin swimmer." //3D Models, 3D Modeling Textures and Plugins at TurboSquid//. Web. 26 Feb. 2010. ."Bird Respiratory System." //People - Eastern Kentucky University//. Web. 26 Feb. 2010. .Campbell, Neil A., and Jane B. Reece. //Biology AP Edition//. AP 7 ed. Upper Saddle River: Prentice Hall College Div, 2004. Print."Chapter 5 - Ecological Genetics:Adaptation and Natural Selection." //MorrisCourse//. Web. 26 Feb. 2010. ."Clades and Ancestry-Adaptation-Form." //Mesa Community College, the resource for your continuing education.// Web. 26 Feb. 2010. ."Closing the University of Wyoming Geological Museum: A Travesty of Funding Misallocation." //Lovingthebigislands Weblog//. Web. 26 Feb. 2010. ."Jspivey - Charles Darwin JY." //Jspivey - home//. Web. 26 Feb. 2010. ."Macroevolution -, the encyclopedia of creation science." //CreationWiki//. Web. 26 Feb. 2010. ."Micro-Evolution." //Index - Truth Saves//. Web. 26 Feb. 2010. ."Myths About Bats." //Tharagors Bat Page//. Web. 26 Feb. 2010. ."Parallel Evolution." //Encyclopædia Britannica Online School Edition//. Encyclopædia Britannica, 2010. Web. 26 Feb. 2010. ."Photo # NH 45759 picture data." //Naval History and Heritage Command//. Web. 26 Feb. 2010. ."Poetry Friday The Flying Squirrel." //Neverending story//. Web. 26 Feb. 2010. ."SHAW PET HOSPITALS - Veterinarians In Victoria, BC, Canada :: Sugar Gliders." //Shaw Pet and Equine Hospitals Routine and 24 hr Emergency Veterinary Service in greater Victoria//. Web. 26 Feb. 2010. .Stanley, Desiree. "2009 March." //.:Dessays:.// Web. 26 Feb. 2010. ."Terragen" 2." //Planetside - Home//. Web. 26 Feb. 2010. .Thomas Learning, Inc. "Chapter 19 - Postzygotic barriers." //Biology Active Learning//. Thomas Learning, Inc., 2002. Web. 26 Feb. 2010. <http://www.dwm.ks.edu.tw/bio/activelearner/19/ch19c3.html>."Zorse - Zebra and Horse Cross Breed - weird Facts." //Funny Interesting Facts | Weird |Science Yoga bizarre|//. Web. 26 Feb. 2010. <http://cool-interesting-facts.blogspot.com/2008/05/zorse-zebra-and-horse-cross-breed-weird.html>.

Created by Erin Triplet