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Evolution Research Project

Mariama Sowe

on 26 September 2012

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Transcript of Speciation

Speciation Speciation can be categorized into two separate main ideas or topics ... Reproductive Isolation and Geographic Separation. Reproductive Isolation Reproductive isolation is the existence of biological factors that impede members of two species from producing viable, fertile, offspring. Such mechanisms of isolation block gene between species and inhibit the formation of hybrids (offspring that result from inter-specific mating). There are two significant barriers that facilitate reproductive isolation ... Prezygotic Barriers and Postzygotic barriers. Prezygotic Barriers These barriers block fertilization from ocurring, acting in one of three ways...(1) they inhibit members of different species from attempting to mate, (2) they prevent an attempted mating from being completed successfully, and (3) they hinder fertilization if mating is completed successfully. There are five prezygotic barriers ... habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gametic isolation. Habitat Isolation In habitat isolation, two species occupy different habitats within the same area, rarely encountering one another despite the absence of "obvious" physical barriers such as mountain ranges. Temporal Isolation Temporal isolation occurs when species that breed during different times of the day, different seasons, or different years are incapable of mixing their gametes. Behavioral Isolation Behavioral Isolation results from courtship rituals that attract mates and other behaviors that are unique to a certain species and enable mate recognition, a method of identifying potential mates of the same species. According to biologist Ernt Mays's description of the biological species concept in 1942, a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but don't produce viable, fertile offspring with members of other such groups. Members of a biological species are united by being reproductively compatible. Sub-topics of Speciation EXAMPLE :
One species of garter snake is primarily aquatic, whereas a closely related species of garter snake is primarily terrestrial. Both species of garter snakes belong in the genus Thamnophis. EXAMPLE :
One species of spotted skunk, the Spilogale putorius in eastern North America, mates in the late winter, while another species, the Spilogale gracilis in western North America, mates in the late summer. EXAMPLE :
Male fireflies of one species signal females of the same species by blinking their lights in a specific pattern, isolating other firefly species that have different patterns. Mechanical Isolation In mechanical isolation, morphological differences prevent the successful completion of mating. EXAMPLES :
1. The copulatory organs of two insect species don't fit together.
2. The shells of two snails in the genus Bradybaena spiral in different directions, making successful mating impossible due to the fact that the snails' genital openings aren't aligned. Gametic Isolation The sperm of one species may be incapable of fertilizing the eggs of another species. The sperm of one species is incapable of penetrating the eggs of a closely related species as a result of biochemical mechanisms. EXAMPLES :
Sea urchins release their sperm and eggs into the surrounding water, where they fuse and form zygotes. The gametes of different species are unable to fuse because protein on the surfaces of the eggs and sperm can't bind to each other. An example would be the gametic relationship between red and purple urchins. Postzygotic Barriers If the sperm cell from one species overcomes prezygotic barriers and fertilizes an ovum from another species, then a variety of post-zygotic barriers may contribute to reproductive isolation after the hybrid zygot has formed. There are three types of postzygotic barriers ... Reduced Hybrid Viability, Reduced Hybrid Fertility, and Hybrid Breakdown. Reduced Hybrid Viability The genes of different parent species may interact in ways that impair the hybrid's development or survival in its environment. EXAMPLES :
1. Nearly all embryos die when eggs of a bullfrog are fertilized artificially with sperm from a leopard frog.
2. Some salamander subspecies of the genus Ensatina share the same regions and habitats, allowing for occasional hybridization. However, most of the hybrids don't survive, and those that do are frail. Reduced Hybrid Fertility Hybrids can sometimes be sterile, and if the parent species possess different stuctures or a different amount of chromosomes, then meiosis in hybrids fail to produce normal gametes, resulting in infertile hybrids. EXAMPLES :
The hybrid offspring of a donkey and a horse is the mule, which is sterile. Hybrid Breakdown It's possible for some first generation hybrids to be both viable and fertile. However, offspring of the next generation are feeble or sterile when they mate with each other or with either parent species. EXAMPLE :
Hybrids between strains of cultivated rice consisting of different mutant recessive alleles at two loci are large and fertile, whereas plants in the following generation that carry excessive amounts of such alleles turn out small and sterile. Speciation can occur with or without Geographic Isolation The two types of speciation associated with geographic isolation are Allopatric (Other Country) Isolation and Sympatric (Same Country) Speciation. ALLOPATRIC SPECIATION This occurs when gene flow is interrupted when two populations of one species are divided into geographically isolated sub-populations. As a result, either population may experience evolutionary change while separated, which in turn establishes prezygotic and postzygotic barriers to the populations' reproduction. Natural selection under different environmental conditions, genetic drift, and sexual selection are examples of the various processes that produce the genetic changes that facilitate pre/post-zygotic barriers. EXAMPLES :
1. water in a lake subsides , creating multiple smaller lakes that become the homes of separated populations.
2. a river changes course, dividing a population of animals unable to cross it.
3. individuals colonize a remote area , making it so that their descendents become geographically separated from the parent populations. this is an example of allopatric speciation devoid of geographic remodeling EVIDENCE OF ALLOPATRIC SPECIATION 1. Harris's antelope squirrel inhabits the southern portion of the rims of the Grand Canyon while the white tailed (leucurus) squirrel, a species greatly remembling Harris's , inhabits the northern part of the Canyon just a few kilometers away.
2. Two present day groups of frogs belonging to the Mantellinae and Rhacophorinae subfamilies diverged nearly 88 million years ago when the island of Madagascar started separating from the Indian landmass, evidence supported by biographic and genetic data. The two frog species apparently shared a common ancestor that inhabited the Madagascar-India landmass before the landmass began to drift apart, which resulted in allopatric speciation that enabled the formation of many news species in each location. Evidence of Allopatric Speciation Cont. Mantelliane/Rhacophorinae frog species Species of antelope squirrels SYMPATRIC SPECIATION This type of speciation is less common than allopatric speciation, but sympatric speciation can occur if gene flow is diminished by factors such as polyploidy, habitat differentiation, and sexual selection. Sympatric speciation is present in populations that live in the same geographic area. In the process of sympatric speciation, a new species can develop while still inhabiting a geographically overlapping area with its parent species. POLYPLOIDY Polyploidy is a condition in which a species originates accidentally during cell division, resulting in an extra set of chromosomes. Plant species have evolved through polyploidy. There are two distinct forms of polyplody ... autopolyploid and allopolyploid An autopolyploid is an individual that has more than two chromosome sets that are all derived from a single species. As a species progresses from generation to generation, various mechanisms can change a sterile hybrid into a fertile allopolyploid. Such allopolyploids are fertile when mating with one another but are incapable of interbreeding with either parent species, meaning that they represent a new biological species. Sympatric speciation can also occur when a subset of a population becomes reproductively isolated due to natural selection brought about by a switch to a habitat or food source not utilized by the parent population. This is called habitat differentiation. Polyploidy Additional Definitions of Species While the biological species concept emphasizes the contrasts between species as a result of reproductive barriers, other definitions highlight the unity within a species. 1. Morphological Species Concept : Characterizes a species by body shape and other structural features.
2. Ecological Species Concept : Views a species in terms of it ecological niche.
3. Phylogenetic Species Concept : A species is defined as the smallest group of individuals that share a common ancestor.
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