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Evolution Reading Class Set Models of Speciation Charles Darwin explained diversity of speciation through divergence. In divergent evolution, selection of successful adaptations eventually leads to separation of one species into two or more species. Divergent evolution is often depicted in a model known as a phylogenetic tree. Phylogeny proposes an ancestral evolutionary path of organisms over millions of years. Much of the information is based on the fossil record, which is continually updated. More modern trees are designed based on DNA nucleotide sequences and common genes for proteins like hemoglobin and cytochrome c. A phylogenetic tree is a graphic representation showing the evolutionary relationships between organisms and their common ancestor. A branching point of the tree shows a speciation event. At that point, changes occurred in the species that result in two separate lineages where previously there had been one. Speciation Common Ancestor A phylogenetic tree shows evolutionary relationships. A change in organisms that leads to a new species is called speciation. A species is defined as a group of organisms capable of interbreeding and producing fertile offspring. Organisms that interbreed share genes; thus for organisms to change over time, the distribution of genes wihin a population also changes. In order to form two different species, the organisms must become reproductively isolated, forming two separate gene pools. Reproductive isolation can occur in a variety of ways, which include geographic isolation, behavioral isolation, and temporal isolation. http://commons.wikimedia.org/wiki/File:Glen_Canyon.jpg A single species separated by geographical Geographic isolation occurs when physical barriers such as continents, mountains, canyons, rivers, highways, or poplins keep members of populations apart, preventing them from reproducing. The organisms of the Galapagos Islands are examples of species that developed due to the geographic barrier of water between islands. Each population of organisms developed adaptations over time that made them significantly different, so that if brought together, they would no longer reproduce. formations eventually diverges into a separate species Behavioral isolation can occur as a result of a geographic isolation. Organisms within a population that get separated may develop different courtship behaviors or mating calls. These different forms of behavior lead to the separation of gene pools because organisms within a population do not respond to the display, courtship dance, or song and thus do not interbreed. Temporal isolation occurs when there are differences in the timing of mating periods. For example, some species of tomato plants bloom early while other plants bloom later, reducing the chance for cross-pollination. Differences in timing, whether seasonal, monthly, or hourly, contribute to the maintenance of separate gene pools. Speciation can also occur through mutations. Speciation is limited in the sense that it does not address species that reproduce solely through asexual means. It is also difficult to apply the definition to organism that form hybrids in nature (mules). Speciation Occurs at Different Rates Scientists studying evolutionary patterns discuss the occurrence of gradualism and punctuated equilibrium as two hypotheses that explain speciation. Gradualism is the hypothesis that evolution occurs over a slow and steady rate. This is the idea that Darwin propose in his theory of evolution. Speciation occurs through the gradual build up of new adaptations. Gradualism is supported by the fossil record. Paleontologists have uncovered some types of fossils that show organism in the strata have not changed much in their variations for long periods of time. This is referred to as stasis. An example of an organism that has changed little in over 250 million years is the horseshoe crab. A more recent idea on the rate of speciation is known as punctuated equilibrium. This hypothesis was proposed in the 1970s. Punctuated equilibrium is the hypothesis that speciation will occur rapidly in bursts with long periods of stability in between. During stability, the populations have genetic equilibrium and remain unchanged for long periods of time. Punctuated equilibrium occurs after sudden environmental changes or if a mutation causes a shift in the gene pool. During this rapid change in the gene pool, speciation can occur. Examples of punctuated equilibrium occur in cases where the gene pool of a small population is suddenly limited to survivors of a catastrophic event. The fossil record shows that punctuated equilibrium is a form of gradualism. Paleontologists have uncovered fossils of organisms in strata that show significant changes that have occurred in short periods of geologic time followed by fossils that have remained unchanged for long periods of geologic time. Gradualism and punctuated equilibrium can also be graphically represented through phylogenetic trees. Phylogenetic trees also signify a change in speciation with regard to time. Gradualism models represent slow diversification of one line of descent. The punctuated equilibrium model displays rapid speciation and then stability of several lines of descent. A phylogenetic tree showing either type of branching is sometimes referred to as the tree of life. Equus Pliohippus Merychippus Equus Pliohippus Merychippus Mesohippus Mesohippus Hyracotherium Gradualism Model of Horse Evolution Hyracotherium Punctuated Equilibrium Model of Horse Evolution Patterns of Evolution Biologists refer to the large-scale history of life as macroevolution. Macroevolution is characterized by six different evolutionary patters: extinction, adaptive radiation, punctuated equilibrium, coevolution, convergent evolution, and developmental genes. Extinction Extinction is the complete loss of a species from Earth. Extinction of organisms is caused by habitat destruction, pollution, invasive species, or catastrophic events. Extinction occurs when organisms are unable to adapt to the changes in the environment. Scientists classify extinctions into two type: background extinctions and mass extinctions. Background extinctions are those that occur at a continuous slow, steady rate. This type of extinction usually only affects a small number of species in a small area. For, example, the effect of global climate change on the polar ice caps is diminishing the polar bear population. If their habitat continues to be destroyed, the number of polar bears that survive annually is reduced. Eventually, they may become extinct. Mass extinctions are those that affect large numbers of species over a large area for thousands or millions of years. These are thought to be caused by catastrophic events such as an asteroid impact or a severe climatic change. The fossil record shows five periods of time where mass destruction of multiple species drastically altered the gene pool. Adaptive Radiation Evolution of one species into several different species through natural selection is known as adaptive radiation. Adaptive radiation typically occurs after mass extinction when species move into new, unoccupied regions. After a mass extinction, there is a large amount of ecological space for a species to migrate with little to no competitors. As populations increase in size, competition within the population become will also increase. As a result, variations within a population become specialized and groups become smaller and more selective. This ultimately forms a new species. Darwin observed adaptive radiation with the birds of the Galapagos Islands. When Darwin observed the birds on the Galapagos Islands, he recognized what he thought was a relative of birds called grosbeaks that have large, seed-eating beaks. He collected and preserved specimens of each type and sent them to scientists in England for classification. His misidentified grosbeaks turned out to be finches. Darwin was amazed by the variety of finches living on the islands. Each type had a different type of beak specially adapted to eat a specific type of food. Darwin wondered how a family of finches could be so different yet live so close together. Eventually, he surmised the original founders of the Galapagos finch species gradually diversified through natural selection and evolved into the species he observed. Coevolution http://commons.wikimedia.org/ wiki/File:Purplethroated_carib_hummingbird_fee ding.jpg http://commons.wikimedia.org/wiki/File:Evolution_sm.png Darwin hypothesized that the founding finches on the Galapagos Islands had originally come from the mainland of South America and had diverged into different species due to natural selection. Coevolution is the evolution of two different species according to their interaction with one another. Each species places selective pressure on the other, so they evolve together. Coevolution is often represented by the adaptations of organism in feeding relationships. For example, birds like vultures do not have feathers on their heads because they feed on carrion (dead animals). The waste from the rotting carcass or bacteria from decomposing flesh would cling to feathers, eventually causing disease. Without head feather, vultures have less chance of contracting diseases. The relationships between pollinators and flowers are also examples of coevolution. Some flowers are red with long tubular shapes and provide high concentrations of nectar. These flowers are most likely pollinated by hummingbirds that can see the color red very well, have a high need for energy and have tubular-shaped beaks. Other symbiotic relationships such as predator-prey, parasitism, commensalism, and mutualism influence coevolution. Convergent Evolution Sometimes organisms face similar environmental demands, such as moving through air or water, and develop structures through natural selection that have similar functions yet are structurally different. Structures that have the same function but are physically different are called analogous structures. When species are not related yet have similar body morphology or shape, they represent convergent evolution. For example, bats have wings and can fly like birds, yet they are mammals and not closely related to birds. Animals that live in water, such as dolphins, seals, and sharks, share similar torpedo-shaped body styles that enable them to swim rapidly through the water. Each of these animals has body parts that allow it to move through the water rapidly, but the internal structure of each body part is very different among organisms. In fact, the internal structure is so different there is no evidence of a common evolutionary history. http://commons.wikimedia.org/ wiki/File:Parc_Asterix_20.jpg http://commons.wikimedia.org/wiki/File:Reef1372 _-_Flickr_-_NOAA_Photo_Library.jpg Organisms with analogous structures or comparable body styles enable them to function in similar ways. Developmental Genes Most animals have a specific sequence of nucleotides that are approximately 180 units long, called a homeobox, or Hox genes. Hox genes are also known as developmental or master control genes because they influence body plans of organism during embryological development by turning certain genes “on” and “off.” Basically, they control the head-totail pattern of development. For example, Hox genes in humans control the location of arm and leg buds. Hox genes in insects control wing placement and the number of wings. They are the genes that determine the head, thorax, and abdomen body style of insects. Embryos of dogs, fish, turtle, pigs, and humans show similarities in body plans due to similarities in the activation of developmental genes. Scientists believe Hox genes play an important role in evolutionary history because mutations that occur in a Hox gene lead o the development of a body structure in a different position, leading to diversity within animal species.