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= EVOLUTION =

The Preservation of Population Variation
Genetic variation is preserved during meiosis. Genetic recombination, a new chromosome combination created by the re-arrangement of maternal and paternal chromosomes, is the source of genetic variation. Genetic recombination occurs through independent assortment and crossing over. Independent assortment is the random arrangement of parental chromosomes along the metaphase plate. Crossing over is the exchange of genetic material between non-sister chromosomes during synapsis, the pairing of homologous chromosomes, in meiosis I (1).

The Imperfections of Natural Selection
Natural selection cannot produce perfect creatures, because:
 * Organisms are constrained by their ancestral history. Evolution has to use existing structures and adapt those structures to the environment; it cannot create a whole new structure.
 * Adaptations are often the result of compromises. Because organisms do a variety of activities for survival, some adaptations that are useful in one situation may be a hindrance in another.
 * Evolution is not always adaptive. Chance can have a greater affect on the genetic drift in a gene pool than adaptations. For instance, a natural disaster can cause the removal of or the addition of a group of organisms from/to a population. As a result, the new gene pool may not contain genes best fit for survival.
 * Natural selection can only edit existing organisms. The fittest variations of phenotypes are favored by natural selection, whether or not the traits are ideal (1).

Microevolution vs. Macroevolution
Microevolution is the small changes in the gene pool of a population over generations. Macroevolution is the evolutionary changes of gene pools on a large scale, such as the creation of a new species (1).

Speciation and Anagenesis vs. Cladogenesis
Speciation is the origin of a new species in evolution. There are two main ways that speciation can occur. The first way is allopatric speciation, which is when a population forms a new species after it becomes geographically isolated from it's parent population. The second way is sympatric speciation, which is when a small population becomes a new species without geographic separation. Anagenesis is the changes accumulated by organisms when one species transforms into another species. Cladogenesis is when one or more new species bud from an existing parent species, promoting diversity by increasing the number of species (1).

The Biological Species Concept
The biological species concept states that organisms within a population have the ability to interbreed with one another, because members of the same biological species are reproductively compatible; however, organisms are unable to reproduce with organisms outside of their species (1).

Prezygotic Isolation Barriers vs. Postzygotic Isolation Barriers
Prezygotic barriers stop the mating between species and the fertilization of ova if two different species attempt to mate. Types of prezygotic barriers include:
 * Habitat Isolation--two species live in different habitats (1). For example, two species of flies live in the same area, but if one of the species lives in water while the other lives in soil, it is very unlikely that the two species will mate (2).
 * Behavioral Isolation--each species has special signals to attract mates, as well as elaborate behavior, which are unique to the species (1). For example, some different species of crickets look identical, but the mating songs of the males are different, allowing females to respond to the mating song of their own species and ignoring the other males (2).
 * Temporal Isolation--two species mate during different times of the day, different seasons, or different years, making them unable to reproduce (1). For example, two species of plants would be unable to mate if each produced flowers during different seasons (2).
 * Mechanical Isolation--two species are anatomically incompatible, making it impossible for the organisms to breed (1). For example, Bush babies are divided into several species based on the shape of their genitals, which only fit into the genitals of members of their own species (2).
 * Gamete Isolation--gametes of different species are rarely able to form a zygote, because the proper conditions for fertilization may be different (1). For example, giant clams are hermaphrodites, who release both egg and sperm cells into the surrounding water. The giant clam sperm cells and the giant clam egg cells only recognize each other because of molecular markers (2).

Postzygotic barriers prevent the development of a zygote after a sperm cell from one species fertilizes the ovum of another species. Types of postzygotic barriers include:
 * Reduced Hybrid Viability--the genetic incompatibility between two species causes an abortion of the hybrid during the embryonic stage (1). For example, in crosses between different species of irises, embryos die before seeds are formed (3).
 * Reduced Hybrid Fertility--generally if different species reproduce, the gametes of the offspring are abnormal, because the parents' gametes may differ in number or structure. The abnormal gametes often leave the offspring sterile (1). For example, a Hinny is the sterile offspring of a male horse and a female donkey (4).
 * Hybrid Breakdown--first generation hybrids may be fertile, but when they mate with one another or with either parent species, the offspring are sterile (1). For example, sunflower hybrids that mate with each other have sterile offspring (3).

The Tempo of Speciation
There are two theories for the tempo of speciation: gradualism and punctuated equilibrium. The gradualism theory states that a species descended from a common ancestor will gradually diverge at increasing rates as the species acquires new and unique adaptations. The punctuated equilibrium theory states that a new species changes the most as it buds from a parent, but will then have little change for the rest of its existence (1).

Patterns of Evolution
The five patterns of evolution are:
 * Divergent Evolution--the differences accumulated between groups which can form a new species. For example, apple maggot flies, over the years, have separated into two different species, those who reproduce when apples are ripe and those who don't (5).
 * Convergent Evolution--the acquirement of the same trait by unrelated species. For example, birds and bats have similarly structured wings (6).
 * Parallel Evolution--the acquirement of the same trait by species that are not closely related, but do share a common ancestor. For example, many species of butterflies share a similarity in color (7).
 * Coevolution--two interacting species influence each others evolutionary adaptations. For example, a species of plant and an avirulent pathogen have a gene-for-gene recognition between each other (1).
 * Adaptive Radiation--the phenotypic diversity in a rapidly growing species lineage. For example, the beak shape of the different finch species found on the Galapagos Islands is thought to be a result of adaptive radiation (8).

Homeotic Genes
Homeotic genes are genes, which are responsible for determining where body parts form. The Hox genes assist the homeotic genes by determining the proper number and proper placement of embryonic segment structures, such as legs and eyes (9).

Phylogeny and Systematics
Phylogeny is the evolutionary history of a species. Systematics is the analysis of the diversity and relationships of organisms (1).

Homologous Structures vs. Analogous Structures
Homologous structures are structures in different species that are similar, because both species share a common ancestor. Analogous structures are structures which are similar but evolved separately (1).

Cladistics, Cladogram, and Clade
A clade is a group of species, including the original, ancestral species, followed by all of its descendants. Cladistics is the analysis of how species may be grouped into clades. A cladogram is a diagram, which shows the patterns of a species' shared characteristics (1).

Universal Tree of Life
The Tree of Life is the connection between all organisms through a common ancestor. In the beginning, prokaryotes (bacteria and archea) contributed DNA to the eukaryotes. Eukaryotes then went on to sexual reproduce, creating the branches of the Tree of Life (10).

Resources
1. //AP Edition Biology Seventh Edition// by Campbell and Reece 2. http://www.sparknotes.com/biology/evolution/reproductiveisolation/section1.html 3. http://www.dwm.ks.edu.tw/bio/activelearner/19/ch19c3.html 4. http://www.sciencedaily.com/articles/h/hinny.htm 5. http://en.wikipedia.org/wiki/Divergent_evolution 6. http://en.wikipedia.org/wiki/Convergent_evolution 7. http://en.wikipedia.org/wiki/Parallel_evolution 8. http://en.wikipedia.org/wiki/Adaptive_radiation 9. http://en.wikipedia.org/wiki/Hox_gene 10. http://en.wikipedia.org/wiki/Modern_evolutionary_synthesis#Trees_of_life

Photo Sources (In Chronological Order)
1. http://www.allstate-jobs.com/view/client/allstate/images/HEAD_citizenship_diversity.jpg 2. http://www.seerious.com/Palau%20Clams.jpg 3. http://artfiles.art.com/5/p/LRG/17/1723/GS53D00Z/philip-enticknap-sunflowers-field-umbria.jpg 4. http://www.animalcorner.co.uk/galapagos/graphics/finchbeakssml.jpg 5. http://2.bp.blogspot.com/_9XlVJY8__iE/RvhtAByjgNI/AAAAAAAAABs/ZAsl8ZP_2m4/s320/T010228A.gif 6. http://dn.duke.edu/images/primatetree_cptn.jpg