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Speciation 2 Speciation 2 Carol Lee University of Wisconsin Copyright©2020; do not upload without permission Today’s OUTLINE: (1) Geographic Mechanisms of Speciation (What circumstances lead to the formation of new species?) (2) Species Concepts (How are Species Defined?) Mechanisms of Speciation Last Time: Genetic Models: The roles of: Mutations Natural Selection Genetic Drift This Time: Geographic Models: Allopatric Model (difference place) Sympatric Model (same place) Parapatric Model (adjoining) (1) Mechanisms of Speciation Last Time: Genetic Models: How do Genetic Drift, Natural Selection, Mutations, etc. create new species? Are there “speciation” genes? This Time: Geographic Models: How does speciation occur in Nature? Is geographic isolation required? Mechanisms of Speciation Geographic (Ecological) Models: Allopatric Model (different place) Disperse to Another Location Vicariance: a barrier is formed This geographic split could lead to Dobzhansky- Müller incompatibilities Sympatric Model (same place) Polyploid speciation Mate Choice (sexual selection) Niche Partitioning (e.g. different food source, Host Plant) Parapatric Model (adjoining) Geographic Models of speciation ■ Allopatric speciation: geographic isolation ■ Sympatric speciation: no geographic isolation ■ Parapatric speciation: geographic separation (or gradient), but not isolation Allopatric Models Involves Geographic Isolation Dispersal Vicariance Allopatric Models • Following geographic separation between populations, Dispersal Vicariance • This geographic separation provides the setting that allows speciation at the molecular level to occur (last lecture) Allopatric Models Dispersal Vicariance • Random Mutations would arise in the separated populations, and then selection or genetic drift would lead to fixation of those mutations • If different mutations are fixed in the different populations, reproductive isolation could arise through Dobzhansky-Müller incompatibilities (last lecture) Allopatric Speciation Examples (see book) ■ Dispersal: ◆ Colonization of islands ◆ Colonization of lakes ■ Vicariance: ◆ Highway going through a forest ◆ Fragmentation of habitats ◆ Formation of Panama splitting the Caribbean &Pacific Oceans Sympatric models Speciation with no geographic separation Speciation despite gene flow (1) Formation of polyploids (discussed in previous lecture) (2) Natural Selection due to Niche Partitioning Sexual Selection Sympatric Model (1) Formation of Polyploids (covered in previous lecture) Important mechanism for plants Occurs rarely in animals Autopolyploidy: happening in one spot (in the plant) Allopolyploidy: the different plant taxa have to be in the same location to hybridize Sympatric Model (2) Selection in the face of gene flow: • Niche Partitioning • Strong assortative mating and sexual selection (disruptive selection) Example of Niche Partitioning: • Soapberry bugs that have adapted to two different host plants Selection drives beak length apart Evolutionary change in beak length on the new small fruit trend toward smaller beaks on smaller fruit Niche Partitioning Soapberry bugs mate on different host plants ■ The populations that live and mate on different fruit are unlikely to encounter each other ◆ Reduces gene flow ◆ Isolation ■ Disruptive Natural Selection ■ Adaptation to alternative hosts leads to reproductive isolation (through the genetic mechanisms discussed earlier, such as Dobzhansky-Müller model) Sexual selection ■ Color preference during mating ■ But, sometimes hybrid zones do form between populations that are in the process of speciating ■ Sometimes hybridization between different species results in vigorous new species or populations, especially in plants (hybrid vigor, or heterozygote advantage) ■ The effects vary depending on how distant the two species or populations are… and whether the different alleles at different loci are able to work together (coadapted gene complexes) ■ Hybrids between different populations within a species do tend to have an advantage (Heterozygote advantage). However, mating between very distant populations (different species) can lead to hybrid breakdown. Increasing genetic distance Mating between Fitness different species (Lions x tiger, Horse x donkey) Will not mate or Mating between Populations Produce inviable or relatives within a species sterile hybrids Outbreeding Inbreeding Hybrid Vigor Depression = Depression (due to Heterozygote advantage) Hybrid Breakdown (2) How are Species Defined? How are species defined? So, what criterion should be used? Historically, the most common criteria had been using morphological characters (external phenotype) Speciation is a messy process ■ Rates of molecular, phenotypic (morphological) evolution and reproductive isolation are not necessarily concordant, but often discordant ■ Speciation is a jagged messy idiosyncratic process, where species boundaries are often difficult to define ■ Problem: Populations are in the process of speciating from one another, and species boundaries are often difficult to define until the populations are sufficiently divergent by all measures ■ So then, how do you define species??? Darwin’s view: Species are arbitrary constructs of the human mind imposed on a continuum of variation Species are dynamic rather than static entities, with boundaries changing constantly Many groups are in the process of speciation Three Main Species Concepts 1. Biological Species Concept 2. Phylogenetic Species Concept 3. Phenetic Species Concept (includes Morphological Species Concept) 1. Biological Species Concept (Ernst Mayr, 1942) A group of interbreeding populations that are evolutionary independent of other populations 1. Biological Species Concept (Ernst Mayr, 1942) Example: all human populations belong to the same biological species Biological Species Concept Strengths An unambiguous empirical criteria which is clearly linked to speciation (if populations can’t intermate they can’t belong to the same species) Using reproductive isolation as the criterion is meaningful as it confirms the lack of gene flow between groups Biological Species Concept PROBLEMS: ■ Many ‘species’ are asexual and do not intermate (viruses, bacteria, protists) ■ Many highly divergent species can hybridize (plants) ■ Only applicable to present (not fossil taxa) ■ Ability to intermate sometimes drops off gradually (“ring species”) Ring Species 2. Phylogenetic Species Concept The smallest group that is monophyletic is called a species 2. Phylogenetic Species Concept There are several monophyletic groups here Monophyletic group: A group with a shared derived (descendant) character A group that contains a common ancestor and all its descendents Phylogenetic Species Concept Typically, a phylogeny is constructed using DNA or heritable traits (proteins, morphological traits) The phylogeny reveals hierarchical relationships among groups The smallest group that has a shared derived character and is monophyletic is called a species Phylogenetic Species Concept There is a derived character that is shared by the 4 populations Monophyly The smallest monophyletic group is called a species A monophyletic clade consists of an ancestral taxa and all its descendants A A A B Group I B B C C C D D D E E Group II E Group III F F F G G G (a) Monophyletic group (clade) (b) Paraphyletic group (c) Polyphyletic group 37 Phylogenetic Species Concept Strengths Easy to see evolutionary relationships on large and small taxonomic scales It can be used on any species (sexual, asexual) for which there is phylogenetic information (molecular, morphological, biochemical data) on extant or fossil species It can be applied at any hierarchical level to define groups: species, genus, family, order, phylum, domain.... Phylogenetic Species Concept Problems: ■ Need a good phylogeny – time consuming and can be expensive ■ Not recognize paraphyletic groups (a monophyletic group that does not include all the descendents; reptiles are paraphyletic, as they do not include birds, because birds emerged from within reptiles) ■ A trivial trait (single mutation or trait) can make a group monophyletic, and may not warrant calling a group a new species Examples of Paraphyletic Groups Paraphyly: a group which either does not include all its descendants or the ancestor. Phylogenetic Species Concept Problems: ■ A trivial trait (single mutation or trait) can make a group monophyletic, and may not warrant calling a group a new species ■ The cut off for a “species” is often arbitrary. For example, 3% sequence divergence is often used for bacteria Phylogenetic Species Concept Monophyly Sometimes a trivial trait, like a single point mutation could make a group monophyletic, and a “species” according to the phylogenetic species concept The smallest monophyletic group is a species 3. Phenetic Species Concept ■ Traditional Definition: Populations that are phenotypically similar to one another but different from other sets of populations. ■ Identifies species using overall similarity (often “a key”), but not in a phylogenetic context… no hierarchy – no branching pattern, no ancestral-derived relationships ■ Encompasses the “Morphological Species Concept” ■ Most often morphological traits are used, but any phenotype could be used Phenetic (often Morphological) Species Concept Strengths ■ Most intuitive; the way we recognize species ■ Easiest. Easier than constructing phylogeny or intermating Phenetic Species Concept Problems: ■ Different species can look similar due to convergent evolution ■ Populations that look distinct sometimes
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