BIOL 151 Final: Chapter 8 notes for final exam

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University of North Dakota
BIOL 151
Chris Felege

Speciation Introduction • Is gene flow ends, allele frequencies in two or more separate, isolated populations are free to diverge ◦ The populations can begin to evolve independently of each other, especially if they are under different selective pressures • Pressures can be anything in the environment ◦ Divergence may also occur as a result of mutation, natural selection, and genetic drift • This genetic divergence may eventually lead to speciation • Speciation is a splitting event that creates two or more distinct species from a single ancestral group • Speciation is the evolutionary process by which new biological species arise • Speciation results from ◦ Genetic isolation, from lack of gene flow ◦ Genetic divergence, due to selection, drift, and mutation What is a Species • In biology, a species is one of the basic units of biological classifications. It is also a taxonomic rank • Aspecies is often defined as the largest group of organisms where two individuals are capable of reproducing fertile offspring, typically through sexual reproduction • While this definition is adequate in many cases, there are also many times when it IS NOT ◦ The difficulty of defining species is known as the species problem ◦ For example, a species complex is a group of closely related species that are very similar in appearance to the point that the boundaries between them are often unclear. ◦ Differentiating measures include similarity of DNA, morphology, or ecological niche ◦ Specific, locally adapted traits may further subdivide species into "intraspecific taxa" such as subspecies or varieties How are Species Defined and Identified • A species is defined as an evolutionarily independent population or group of populations • Biologists commonly use three criteria for identifying species: 1. The biological species concept 2. The morphospecies concept 3. The phylogenetic species concept • The Biological Species Concept ◦ According to the biological species concept, different species are reproductively isolated from each other • They either do not interbreed or they fail to produce viable, fertile offspring ◦ Biologists categorize the mechanisms that stop gene flow between populations as either • Prezygotic isolation ▪ Individuals of different species are prevented from mating • Postzygotic isolation ▪ Individuals from different populations do mate ▪ The hybrid offspring do not survive or reproduce ◦ The biological species concept has disadvantages: • Reproductive isolation cannot be evaluated in ▪ Fossils ▪ Species that reproduce asexually • It can be applied only to populations that overlap geographically • The Morphospecies Concept ◦ According to the morphospecies concept, individual species differ in size, shape, or other morphological feature • Distinguishing features are most likely to arise if populations are independent and isolated from gene flow ◦ It can be widely applied • Knowledge of the extent of gene flow is not necessary • Equally applicable to sexual, asexual, and fossil species ◦ The morphospecies concept has disadvantages: • Asingle polymorphic species may be split into two species • It cannot identify cryptic species that differ in non-morphological traits • The features used to distinguish species under this concept are subjective • The Phylogenetic Species Concept ◦ The phylogenetic species concept identifies species based on evolutionary history ◦ On phylogenetic trees, a monophyletic group consists of an ancestral population plus all of its descendants • Also called a clade or lineage ◦ Monophyletic groups are identified by synapomorphies • Homologous traits inherited from a common ancestor • Unique the phylogenetic species concept, a species is defined as the smallest monophyletic group on the tree of life ◦ The phylogenetic species concept has two distinct advantages: i. It can be applied to any population ii. It is logical 1. Species have different synapomorphies due to independent evolution ◦ The phylogenetic species concept also has disadvantages: • Phylogenies are currently available for only a tiny (though growing) subset of populations on the tree of life • Critics point out that it would probably lead to recognition of many more species than either of the other species concepts ◦ In actual practice, researchers use all three species concepts to identify evolutionary independent populations in nature Species Definitions inAction • Subspecies are populations that ◦ Live in discrete geographic areas ◦ Have their own identifying traits ◦ Are not distinct enough to be considered a separate species Isolation and Divergence -Allopatry • Genetic isolation happens when populations become geographically separated Populations that live in different areas are said to be in allopatry ◦ Speciation that begins with geographic isolation is known as allopatric speciation • Geographic isolation occurs in one of two ways: ◦ Dispersal • Apopulation moves to a new habitat, colonizes it, and forms a new population ◦ Vicariance • Aphysical barrier splits a widespread population into subgroups that are physically isolated from each other • Biogeography is the study of how species and populations are distributed geographically ◦ It can tell us how colonization and range-splitting events occur • Allopatric Speciation by Dispersal ◦ Colonization events can lead to speciation • Peter and Rosemary Grant compared colonist offspring with the migrant population ▪ Average beak size in colonist population was much larger ◦ Two evolutionary processes could cause the change in beak size: 1. Genetic drift via the founder effect 1. Causes the old and new populations to diverge rapidly 2. Natural selection 1. May cause divergence if the newly colonized environment is
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