Material from test 2 to now Lecture 11 Speciation Biologists describe a species as an evolutionary independent unit They consist of interbreeding populations that evolve independently of other populations mutation selection gene flow and drift all operate on populations separately Be able to explain the differences between the three major species concepts and the problems and advantages of each It is difficult to establish practical criteria for identifying when populations are evolving independently meaning they are a species but we use these 3 species concepts that portray this independence but each employ a different criterion for determining that independence 1 Morphospecies a Characterized by unique phenotype b Traditional method can easily be applied to museum specimen so it is widely applicable i Can identify extinct or living sexual or asexual organisms c Problem ignores evolution of species that are morphologically identical i Difficult to apply to groups like bacteria Archaea and many fungi that are small and have few measurable morphological characters ii Fossil species that differ only in color or anatomy of soft tissues cannot be distinguished iii Also populations that are similar in morphology but were strongly divergent in traits like songs temperature drought tolerance habitat use or courtship displays cannot be distinguished iv Populations like these are called cryptic species a Evolution of reproductive isolation b Species are reproductively isolated from other species i This means that they fail to produce fertile offspring when they hybridize therefore they are considered separate species c Strong concept because it confirms lack of gene flow which is the key test of evolutionary independence in organisms that reproduce sexually d Problem can t be applied to asexual species can be difficult to apply to geographically isolated species because these populations don t actually come in contact with one another many plants 2 Biological species 3 Phylogenetic species a Focuses on criteria for identifying species that is known as monophyly monophyletic groups are lineages containing all known descendants of a single common ancestor b Smallest monophyletic unit distinguishable characteristic c Can distinguish populations on a phylogeny only if the populations have been isolated in terms of gene flow and have diverged genetically and possibly morphologically Populations within species have shared derived traits that distinguish them from populations of other species d Can be applied to any type of organisms asexual and fossils included e Problem data limited produces huge numbers of species Be able to explain that all species concepts may not apply in all circumstances with examples All species concepts may not apply in all circumstances Examples The biological species concept is based on the ability for two individuals to mate but this would clearly not work for asexual individuals such as bacteria This is a case where the biological species concept would not apply to the study of species of bacteria Be able to give examples of cases where detailed studies of groups of organisms have revealed previously unknown diversity i e one species is actually multiple species Example of this is with the European common skate which was threatened at becoming extinct Its taxonomic history has been confusing because Morphospecies are difficult to identify Before the 1920 s it was regarded as two distinct species the blue skate and the flapper skate But in 1926 these were combined into a single species and have been viewed that way ever since Two recent studies used molecular data to challenge this single species classification Although the two studies were conducted independently and relied upon DNA sequence data from different loci the findings matched There are two distinct species of common skate Know that speciation is composed of separation e g isolation divergence and in some cases secondary contact Classically speciation has been hypothesized to be a three stage process 1 An initial step that isolates populations 2 A second step that results in divergence in traits such as mating system or habitat use 3 A final step that produces reproductive isolation According to this model the isolation and divergence steps were thought to take place over time and to occur while populations were located in different geographic areas The final phase was hypothesized to occur when these diverged populations came back into physical contact an event known as secondary contact Be able to explain the difference between allopatric and sympatric speciation and between allopatry due to dispersal and allopatry due to vicariance Allopatric speciation is when physical isolation creates an effective barrier to gene flow In many cases geographic isolation is an important trigger for the second stage in the speciation process genetic and ecological divergence Ex the water snakes from Lake Erie which migrated to the nearby island to increase genetic variation in the population But in this case there is a physical barrier ends gene flow such as if the current changed and the snakes had no way of getting to the island Allopatry due to dispersal is movement across a physical barrier followed by colonization of a new habitat such as when a group of organisms rides a raft of vegetation to an island Allopatry due to vicariance is when an existing range is split by the appearance of a new physical barrier which may be as small as a road or as large as an ocean Vicariance events split a species distribution into two or more isolated ranges and discourage or prevent gene flow between them o A specific example of a classic vicariance event the separation of marine organisms on either side of Central America The Isthmus of Panama closed 3m y a and as the isthmus rose and created a land bridge between N and S America populations of marine organisms became separated on the Atlantic and Pacific sides This created populations that speciated due to vicariance Be able to explain some ways in which species can become isolated geographical separation separation in time breeding season morphological isolation Isolation is the first step in speciation It is a reduction of gene flow between two populations It most commonly occurs as a result of geographic factors that cause populations to become physically separated Any factor however that reduces the probability or effectiveness of interbreeding between two populations can