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BIOL 1500
Alexander Mills

TEXTBOOK NOTES 5 SPECIATION: Pg. 93-101, 103, 105-106  If gene flow ends, allele frequencies in isolated population are free to diverge (populations evolve independently of each other) (since gene flow makes allele frequencies more similar)  With no gene flow and with mutation, selection, and genetic drift cause isolated populations to diverge and distinct species form  the process of speciation takes place  Speciation: is a splitting event that creates two or more distinct species from a single ancestral group - When speciation is complete, a new branch is added to the “tree of life”  Speciation results from genetic isolation and genetic divergence - Genetic isolation results from lack of gene flow - Divergence occurs because of selection, genetic drift, and mutation How are Species Identified  Species: is defined as an evolutionarily independent population or group of populations  Four criteria for identifying species: - The biological species concept - The morphospecies concept - The ecological species concept - The phylogenetic species concept The Biological Species Concept  The critical criterion for identifying species is reproduction isolation  This is logical because no gene flow occurs between populations that are reproductively isolated  When species fail to interbreed or produce infertile offspring, they are considered distinct species  Species that can interbreed and are isolated from other populations, belong to the same species  Reproductively isolated populations are evolutionarily independent  Reproductive isolation can result from certain processes that stop gene flow: - Prezygotic isolation: prevents individuals of different species from mating - Postzygotic isolation: offspring of matings between members of different species do not survive or reproduce  Biological species concept has disadvantages - Cannot be evaluated in fossils or inspecies that reproduce sexually - Difficualt to apply when closely related populations do not overlap with each other geographically The Morphospecies Concept  Under this concept, researchers identify evolutionarily independent lineages by difference in size, shape, or other features  Logic behind this concept is that distinguishing features arise if populations are independent  Useful when biologists have no data on the gene flow and is applicable for sexual, asexual, AND fossil species  Disadvantages: - It cannot identify cryptic species (differ in non-physical traits) - This way of distinguishing species is subjective (depends on people’s view of the species) - ^ Some may agree, and some may not The Ecological Species Concept  It defines a species as a set of organisms using the same resources, having the same range of environmental tolerances and facing the same predators and parasites  This concept emphasizes the role of natural selection which favours characteristic traits that cope with a specific set of environmental challenges  Most useful for identifying the species of bacteria, archaea, or asexual eukaryotes - These species cannot be identified by reproductive isolation - Instead, distinctions between species are maintained by their adaptation to different ecological conditions The Phylogenetic Species Concept  Is based on reconstructing the evolutionary history of populations  It is widely applicable and precise  The reason behind this concept starts with Darwin’s claim that all species are related by common ancestors - Darwin said that all species form a monophyletic group (tree of life) - ^ This group is also called a clade or lineage - On any tree of life or smaller parts of the tree of life, there are many monophyletic groups - Monophyletic groups are identified by traied called synapomorphies - ^ Synapomorphies is a trait found in certain groups of organism and none others - It is a homologous trait (inherited from a common ancestor) - Example: fur and lactation are traits that identify mammals as a monophyletic group - Example: DNA sequences that are unique to each species - Example: Some animals have placenta (such as humans)  marsupials monophyletic group  Under the phylogenetic species concept, species are defined as the smallest monophyletic groups on the tree of life - Phylogenetic species are made up of populations that share one or more unique synapomorphies  Some of the tips of the tree represent populations within species - These populations may be separated geographically but their characteristics are so similar that they do not form independent branches on the tree - They are simply [art of the monophyletic group containing other populations  Advantages: - It can be applied to fossils, asexual, sexual - It is logical because different species have different synapomorphies only if they are isolated from gene flow and have evolved independently  Disadvantage: - Phylogenies are available for only a tiny subset of populations on the tree of life - It would probably lead to recognition of many more species but this recognition might better reflect the extent of life’s diversity  Researchers use ALL four species concepts to identify evolutionarily independent population in nature The Case of the Dusky Seaside Sparrow  They live in marshes along the Atlantic and Gulf courses of the U.S  Scientific name: Ammodramus maritimus ( Genus and Species)  Using morphospecies concept, researchers named seaside sparrow “subspecies”  Subspecies: are populations that live in discrete geographic areas and have distinguishing features  In 1960s, researchers were worried about seaside sparrows because salt marshes were being destroyed, especially the dusky seaside sparrow (A. m. nigrescents) - Only 6 males remained - U.S government brought up the Endangered Species Act in order to prevent the extinction of species - This is due to very little to no gene flow occurring since young seaside sparrows tend to breed near when they hatched - To launch the program, the remaining male dusky seaside sparrows were taken into captivity and bred with females from a nearby subspecies: A. m. peninsulae - The goal was to preserve as much genetic diversity by reestablishing a healthy population - This plan failed when group of biologists estimated the phylogeny of the seaside sparrows by coming gene sequences - Phylogenetic species concept shows that only two species of seaside sparrow population exist (Atlantic Coast and Gulf Coast species) - Dusky male sparrows belonged to the Atlantic coast and were accidently mated with the Gulf coast lineage - ^ This resulted in the loss of heritable traits like colouration and songs Isolation and Divergence in Allopatry  Speciation begins when gene flow between populations is reduced or eliminated  Genetic isolation happens when populations become physically separated  Physical isolation occurs in one or two ways: dispersal or vicariance  Vicariance: the physical splitting of habitat (when population disperse to new habitat)  After Pleistocene epoch, North America’s boreal forests were fragmented - British Columbia and University of Toronto compared the DNA of pairs of closely related species of birds from these forests - The comparisons showed that each pair of species had originated during the Pleistocene - Vicariance due to glaciation played a role in each of these speciation events  Allopatric: Speciation that begins with physical isolation via either dispersal or vicariance - Allopatry: populations that live in different area  Biogeography: the study of how species and populations are distributed geographically Dispersal and Colonization Isolate Populations  Peter Grant and Rosemary Grant witnessed a colonization event while working in the Galapogos Islands (off the coast of South America) - Grants also studied the medium ground finches - In 1983, five members of a new species called the large ground finch, arrived and began nesting - They came from the Daphne Major island - These colonists represented a new population, allopatric with their source population  Both of the Grants weighed and measured most of the parents and offspring produced on Daphne Major over 12 years - When they compared these data with measurements of large ground finches in other populations they discovered that the average beak size in the new population was much larger  The two evolutionary processes that could be responsible for the changes in beak size: 1. Genetic drift produced a colonizing population that happened to have particularly large beaks relative to the source population 2. Natural selection in the new environment could favour alleles associated with large beaks  General message: characteristics of a colonizing population are likely to be different from the characteristics of the source population due to chance (genetic drift)  Natural selection may extend the rapid divergence that begins with genetic drift Vicariance Isolates Populations  Vicariance takes place when for example, a mountain range or river splits the geographic range of a species  Geologists estimate that the isthmus (land with water on both sides) closed, for
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