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Chapter 26

Unit 5 - Chapter 26 Bio 1M03 .docx

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James S Quinn

Bio 1M03 Unit Five: Evolutionary Processes and Patterns Chapter 26: Speciation Key Concepts  Speciation occurs when populations of the same species become genetically isolated by lack of gene flow and then diverge from each other  Divergence of populations and species can be a consequence of natural selection, genetic drift, mutation and/or biased mating  Populations can be recognized as distinct species if they are reproductively isolated from each other, if they have distinct morphological characteristics, and/or if they form independent branches on a phylogenic tree  Populations can become genetically isolated from each other if o They occupy different geographic areas o They use different habitats within the same area o If one population is polyploidy and cannot breed with the other  When populations that have diverged come back into contact, several outcomes are possible o Reinforcement through pre and post-zygotic isolation o Development of hybrid zones o Speciation by hybridization fusion o One population goes extinct 26.1 How Are Species Defined and Identified?  When gene flow is reduced between population, they may then diverge genetically  Divergence occurs as a consequence of natural selection, genetic drift, mutation and/or biased mating  The genetic divergence may eventually lead to speciation, the creation of new species  Usually, speciation creates two or more distinct species from a single ancestral group  Biologists commonly use the following three approaches to identify species: o The biological species concept o The morphospecies concept o The phylogenetic species concept The Biological Species Concept  The critical criterion for identifying species is reproductive isolation  The biological species concept (BSC) assigned individuals to the same species if they “actually” or “potentially interbreed”  The BSC considers populations to be evolutionarily independent if they are reproductively isolated from each other, i.e. they do not interbreed  Therefore, no gene flow occurs between these populations  Problem with BSC – it is difficult to apply to natural systems  If two different populations do not interbreed in nature or if they fail to produce viable and fertile offspring when mating’s take place – they are distinct species  Biologists categorize the mechanisms that stop gene flow between populations as being either prezygotic or postzygotic Bio 1M03  Prezygotic isolation – occurs when individuals of different species are prevented from mating  Postzygotic isolation – occurs when individuals from different populations do mate, but the hybrid offspring produced have low fitness and do not survive or produce offspring o Eg/ Ligers are the offspring from cross between male lion and female tiger; male ligers are sterile The Morphospecies Concept  Under the morphospecies concept, species are distinguished by differences in size, shape or other morphological features  Based on the idea that distinguishing features are most likely to arise if evolutionary lineages do not have gene flow  Features used to distinguish species are rather subjective The Phylogenetic Species Concept  Monophyletic Groups (Clades or Lineages) o For example: Sequence DNA of 14 individuals, recover 6 unique sequences: o A ,1A ,2A 3 B 1 B2, 1 , 2 ,1D ,2D 3 D 1 E1, 2 ,3F , F o A monophyletic group contains all descendants of their most recent common ancestor  The phylogenetic species concept is based on reconstructing the evolutionary history of populations  Under this concept, a species is defined as the smallest monophyletic group on a tree that compares populations o On such a phylogenetic tree, each clade is a phylogenetic species  One drawback is that this concept may end up over diagnosing species  Advantages o Can be applied to any population (fossil, asexual or sexual) o Is logical because populations are distinct enough to be monophyletic only if they are isolated from gene flow and have evolved independently Problems with Species Concepts  Biological Species Concept o Hard to demonstrate; difficult to assess if populations do not overlap geographically o Some “species” are capable of producing hybrids o Not applicable to asexual or fossil species  Morphospecies Concept o Subjective – how much morphological divergence represents species-level as opposed to population level differences  Phylogenetic Species Concept o Over-diagnosis – are all monophyletic groups really species? Bio 1M03 Species Definitions in Action: The Case of the Dusky Seaside Sparrow  Subspecies – populations that live in discrete geographic areas and have distinguishing features (such as coloration or calls) but are not considered distinct enough to be called separate species  Dusky Seaside Sparrows (ammodramus maritimus nigrescens), a subspecies, had only 6 males remaining in 1980 o Tend to breed near hatching site – believed that little to no gene flow occurred among populations  Attempt Rescue Program – goal to preserve as much genetic diversity as possible by reestablishing a healthy population of dusky-like birds o Dusky seaside sparrows were taken into captivity and bred with females from a nearby subspecies A. maritimus peninsulae  By comparing gene sequences, estimated phylogeny of seaside sparrow o Two distinct monophyletic groups – one native to Atlantic Coast, one native to Gulf Coast o Under the phylogenetic species concept, only two species of seaside sparrow exist o Biological and Morphospecies Concepts had misled a well-intentioned conservation program o Under phylogenetic species concept – should have let dusky sparrow go extinct and then preserved one or more populations from each coast; two monophyletic groups of sparrows (and more genetic diversity) would have been preserved 26.2 Isolation and Divergence in Allopatry  Genetic isolation happens when populations become physically separated  Allopatry (literally “different countries") – refers to populations or species that live in different places  Physical isolation occurs by dispersal or vicariance o Dispersal – occurs when a population moves to a new habitat, colonizes it and forms a new population o Vicariance – occurs when a physical barrier splits a widespread population into subgroups that
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