Chapter 16.doc

Chapter 16- Mechanisms of Speciation 16.1- Species Concepts • Species is the smallest evolutionarily independent unit • Evolutionary independence occurs when mutation, selection, gene flow, and drift operate on populations separately • Evolution consists of changes in allele frequencies, and species form a boundary for the spread of alleles o As a result, different species follow different evolutionary trajectories o The essence of speciation is lack of gene flow • Species consist of interbreeding populations that evolve independently of other populations • The three most important “species concepts”: o Morphospecies Concept o Biological Species Concept o Phylogenetic Species Concept  Each of the three agrees that species are evolutionarily independent units that are isolated by lack of gene flow, but each employs a different criterion for the determining that independence is actually in effect The Morphospecies Concept • In traditional cultures, people name species based on morphological similarities and differences • In biology, careful analyses of phenotypic differences are the basis of identifying morphospecies • Morphospecies can be identified in species that are extinct or living, and in species that reproduce sexually or asexually • Fossil species that differed in color or the anatomy of soft tissues cannot be distinguished • Neither can populations that are similar in morphology but were strongly divergent in traits like songs, temperature, or drought tolerance, habitat use, or courtship displays o Species like these are called cryptic species  Species that are indistinguishable morphologically, but divergent in songs, calls, odor, or other traits The Biological Species Concept • Under this concept, criterion for identifying evolutionary independence is reproductive isolation • If populations of organisms do not hybridize regularly in nature, or if they fail to produce fertile offspring when they do, then they are reproductively isolated and considered good species • Great strength of this concept is that reproductive isolation is a meaningful criterion for identifying species because it confirms lack of gene flow • Lack of gene flow is the litmus test of evolutionary independence in organisms that reproduce sexually The Phylogenetic Species Concept • Systematists are biologists who are responsible for classifying the diversity of life • This concept is also known as the genealogical species concept o This approach focuses on a criterion for identifying species called monophyly • Under this concept, species are identified by estimating the phylogeny of closely related populations and finding the smallest monophyletic groups o On a tree like this, species form the tips • Rationale behind the phylogenetic species concept is that traits can only distinguish populations on a phylogeny, if the populations have been isolated in terms of gene flow and have diverged genetically, and possibly morphologically as well • To be called separate phylogenetic species, populations must have been evolutionarily independent long enough for the diagnostic traits to have evolved • Populations within species have shared, derived traits that distinguish them from populations of other species • Species are named on the basis of statistically significant differences in the traits used to estimate the phylogeny • Recent analyses have found that the phylogenetic species concept often distinguishes a series of cryptic species in populations that were formerly considered a single species. Applying Species Concepts: Two Case Histories • Species can be identified by distinctive morphological traits, reproductive isolation, and/or phylogenetic independence. Each species concept has advantages and disadvantages. • Employing more than one species concept can help biologists recognize diversity and organize research on its consequences • Read example on Marine Copepods and Elephants on Pages 609-610 16.2- Mechanisms of Genetic Isolation • Speciation has been hypothesized to be a three-stage process: o Initial step that isolates populations o Second step that results in divergence in traits such as mating system or habitat use o Final step that produces reproductive isolation • Isolation and divergence steps were thought to take place over time and to occur while populations were located in different geographic areas • Final phase was hypothesized to occur when these diverged populations came back into physical contact- an event known as secondary contact o Secondary contact- when two populations that have diverged in isolation from a common ancestor are reunited geographically • It is now clear that, isolation and divergence steps that initiate speciation frequently take place at the same time and in the same place o Also it appears likely that in a significant number of speciation events or even a majority, the third phase never occurs • Isolation/divergence/secondary contact hypothesis provides a useful framework for analyzing how speciation takes place • First step in speciation is genetic isolation • Physical separation or changes in chromosome complements can reduce gene flow between populations • Once gene flow is dramatically reduced or ceases, evolutionary independence begins and speciation is underway • Therefore, speciation process begins when gene flow is disrupted and populations become genetically isolated Physical Isolation as a Barrier to Gene Flow • Geographic isolation produces reproductive isolation, and thus genetic isolation • Allopatric Model- the hypothesis that speciation occurs when populations become geographically isolated and diverge because selection and drift act on them independently • Essence of allopatric speciation is that physical isolation creates an effective barrier to gene flow • Geographic isolation has been an important trigger for the second stage in the speciation process: genetic & ecological divergence • Geographic isolation can come about through dispersal and colonization of new habitats or through vicariance events, where an existing range is split by a new physical barrier • Dispersal is when a population in one area splits into two and one moves to another isolated area like from mainland to an island • Vicariance is when a large area is populated and some natural cause occurs and divides the area into two separate lands, dividing one from the other by nature • Look at page 612 for better understanding. • The dispersal-and-colonization hypothesis makes two predictions based on these facts: o Closely related species should almost always be found on adjacent islands o At least some sequences of branching events should correspond to the sequence in which islands were formed • Dispersal to novel environments has proven to be a general mechanism for initiating speciation • Populations can become geographically isolated when individuals colonize a new habitat • Vicariance events split a species’ distribution into two or more i