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Chapter 16

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University of Waterloo
BIOL 359
Jonathan Witt

BIOL359 - Evolution Winter 2013 Chapter 16: Mechanisms of Speciation - 1.5 million species described today, total may be about 3-5 million - Mutation, natural selection, migration & genetic drift causes populations to diverge & form new, independent species 16.1. Species Concepts - Species: The smallest evolutionarily independent unit o Species forms a boundary for the spread of alleles & different species follows different evolutionary trajectories o Species consists of interbreeding populations that evolve independently of other populations o The 3 species concept  The Morphospecies Concept  Criterion for identifying evolutionary independence is morphological similarities & differences  Pro: Widely applicable, can be identified in extinct or living species, sexual or asexual reproducing species  Con: If not applied carefully, species definitions can become arbitrary & idiosyncratic, there could be incompatibilities between species designations made by different investigators & concepts will be hard to apply in groups with few measurable morphological characters  Cryptic species: Groups that were or are actually independent of one another appear to be members of the same species based on morphological similarity  The Biological Species Concept (BSC)  Criterion for identifying evolutionary independence is reproductive isolation  Reproductive isolation: Population of organisms do not hybridize regularly or if they fail to produce fertile offspring  Pro: Reproductive isolation = lack of gene flow = Independent evolution in organisms that reproduce sexually due to interbreeding  Con: Difficult to apply in certain situations, such as when nearby populations that do not actually overlap  Subjective judgment; Can never be tested in fossil forms; Not on asexual populations; Difficult to apply in plant group (hybridization between strongly divergent populations is routine)  The Phylogenetic Species Concept (PSC)  Statists: Biologists responsible for classifying the diversity of life  Focuses on criterion for identifying species  monophyly  Species are identified by estimating the phylogeny of closely related populations & finding the smallest monophyletic groups  Statistically significant o For species that cannot be clearly distinguished in a phylogeny by unique, derived characters, then they will form clusters  populations that make up these clusters are considered part of the same species  Pro: Can be applied to any type of organisms (Sexual vs. Asexual, living vs. fossil)  Cons: Populations must have been evolutionarily independent long enough for the diagnostic traits to have evolved  separate phylogenetic species (must share synapmorphic/homologous characters) o Difficult to put this criterion into practice, carefully constructed phylogenies are available for only a relative handful of groups so far o Might result in great deal of confusion of traditional names  Now, PSC often distinguishes a series of cryptic species in populations that was originally considered to be a single species - Applying Species Concepts: Two Case Histories BIOL359 - Evolution Winter 2013 o Major species concepts that have been proposed are productive when applied in appropriate situations  Species can be identified by distinctive morphological traits, reproductive isolation, and/or phylogenetic independence. Each species concept has advantages and disadvantages o Diversification in Marine Copepods  Copepods: Small crustaceans extremely abundant in the world’s oceans  E.g. Eurytemora affinis, the most important grazing animals in many of the world’s largest estuaries (ecosystems that form where rivers flow into the sea – nutrient rich, most productive habitats in marine environments)  Morphospecies concept grouped all population into the same species  Experiment: E. affinis was collected - 38 population throughout the Northern Hemisphere, tested with the Biological Species Concept & Phylogenetic Species Concept  Tested individual’s ability to mate & produce fertile offspring  Sequenced 2 genes to estimate their evolutionary relationships o BSC: Populations from different phylogenetic species are unable to produce fertile offspring & thus reproductively isolated o PSC: At least 8 independent species exist, each occupying a distinctive geographic area  Species diversity in copepods is far greater than previously thought o How Many Species of Elephant Live in Africa?  Traditionally classified into 2 species: African (Loxodonta africana) & Asian (Elephas maximus) elephants  Morphospecies concept: African elephants live in forest habitats vs. African elephants live in savanna/grassland have distinctive morphological features  Rare interaction Rarely interbreed (No BSC)  Experiment: Collected tissue samples from 165 elephants in 21 populations, isolated DNA, sequenced 4 genes from each individual (PSC)  Forest and savanna elephants are distinct phylogenetic species  Population of elephants are declining due to habitat loss & illegal hunting  Conservation programs focused on preserving both forest & savanna populations as distinct entities Box 16.1. What about bacteria and archaea? - Asexual reproduction “No exchange of genetic material” - Gene flow is unidirectional, may result in genetic recombination o Alleles are routinely transferred between members of widely diverged bacterial & archaeal lineages o Gene flow plays a relatively minor role in homogenizing allele frequencies among bacteria populations (e.g. Antibiotic resistance) - Acquiring novel alleles through lateral gene transfer is the primary mechanism for speciation in bacteria (gene flow triggers divergence in bacteria population (16%), not eukaryotes (2%)) - Bacterial and archaeal species are best defined in an ecological context, based on their ability to thrive in a particular environment o Dependent on which genes are present in the genomes of these organisms 16.2. Mechanisms of Genetic Isolation - Speciation is a 3 stage process: Isolation  Divergence in traits  Reproductive isolation o Process begins when gene flow is disrupted and populations become genetically isolated o Isolation & Divergence: Take place over time & occur when populations are isolated in different geographical areas o Reproductive isolation: When diverged populations came back into contact “Secondary contact” - The first step in speciation: Genetic isolation (Dramatic reduce or cease of gene flow) BIOL359 - Evolution Winter 2013 - Physical Isolation as a Barrier to Gene Flow o Gene flow/Migration tends to homogenize gene frequencies & reduce differentiation of population (e.g. Water snakes, genetic isolation of mainland & island snakes is prevented due to migration) o Experiment: What if lake currents stopped the migration of banded snake from mainland to island?  No more gene flow (migration), & natural selection works against migration  Allopatric model: Geographic isolation produces reproductive isolation & thus genetic isolation  Dispersal of habitats: Individuals colonize an isolated area  Vicariance of habitats: Formation of a physical barrier in-between habitat o Geographic Isolation through Dispersal & Colonization  E.g. Hawaii drosophilids (relative of D. Melanogaster), huge diversity (body size, wing coloration & other traits)  Dispersal-and-Colonization Hypothesis: Hawaiian islands  Magma  Island formation  Continental drift  H1: Closely related species should almost always be found on adjacent islands  H2: At least some sequences of branching events should correspond to the sequence in which islands were formed  PBS: Dispersal to new habitats triggered speciation o Geographic Isolation through Vicariance (Slow or fast process)  Vicariance: Splits a species’ distribution into 2+ isolated range & discourage/prevent gene flow between them  E.g. Rise of the Isthmus of Panama (~3 million years ago), separated North & South America, marine organisms are separated on the Atlantic & Pacific sides  Experiment: Snapping shrimp (Altheus) populations from either side of the Isthmus  Morphospecies Concept: 7 pairs of closely related species pairs “sister species” (1 from each pair found on different sides of the Isthmus)  PSC: DNA sequencing confirmed morphospecies concept  BSC: Strong correlation between the degree of genetic distance & how interested the shrimps were & the production of fertile clutches (Longer isolation = less interested)  Data X Vicariance hypothesis: If Isthmus formed rapidly, 7 species should have identical degree of reproductive isolation, but DNA sequence divergence varied from 6.5% to 19%+  Different species of shrimps are isolated in a staggered fashion as the land rose gradually (depending on the depth of water & the motility of larvae) o Deeper water & less motile larvae  First (e.g. 6 & 7) o Shallow water & more motile larvae  Last (e.g. 1-4)  Final break - A Role for Mutation: Polyploidy and Other Chromosome Changes as a Barrier to Gene Flow o Changes in chromosome number isolate populations genetically within an instant (Possible to separate with no physical isolation)  Tetraploid population  Diploid gametes + Diploid population  Haploid gametes = Triploid o Flower shape & timing of flowering is different in different species (different pollinators) o 300,000 species of land plants  2 to 4% from polyploidization events  Less common in animals (E.g. ~25 species in Barbus - fish)  E.g. Small African antelopes (dik-diks): Distinguished cryptic species based on chromosome # o Most data is only correlative, it’s likely that chromosome difference arose after speciation occurred due to other causes (physical isolation & divergence) 16.3. Mechanisms of Divergence - Genetic drift & Natural selection after dispersal, vica
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