Speciation 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 Niche Partitioning (e.g. different food source, Host Plant) Geographic Models of speciation Allopatric speciation: geographic isolation Sympatric speciation: no geographic isolation Parapatric speciation: geographic separation (or gradient), but not isolation Parapatric Model (adjoining) Title goes here 1
Allopatric Models Involves Geographic Isolation Dispersal Allopatric Models Following geographic separation between populations, Dispersal Vicariance 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 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 If different mutations are fixed in the different populations, reproductive isolation could arise through Dobzhansky-Müller incompatibilities (last lecture) 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 Title goes here 2
Sympatric Model (2) Selection in the face of gene flow: Niche Partitioning Example of Niche Partitioning: Soapberry bugs that have adapted to two different host plants Strong assortative mating and sexual selection (disruptive selection) 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) 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. Title goes here 3
Fitness Mating between relatives Increasing genetic distance Populations within a species Mating between different species (Lions x tiger, Horse x donkey) Will not mate or Produce inviable or sterile hybrids (2) How are Species Defined? Inbreeding Depression Hybrid Vigor (due to Heterozygote advantage) Outbreeding Depression = Hybrid Breakdown 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??? Title goes here 4
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. 3. Phenetic Species Concept (includes Morphological SC) 1. Biological Species Concept (Ernst Mayr, 1942) 1. Biological Species Concept (Ernst Mayr, 1942) A group of interbreeding populations that are evolutionary independent of other populations 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 ) Title goes here 5
Ring Species 2. The smallest group that is monophyletic is called a species 2. 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 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 There is a derived character that is shared by the 4 populations 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 F F F Group III Monophyly G G G (a) Monophyletic group (clade) (b) Paraphyletic group (c) Polyphyletic group The smallest monophyletic group is called a species 36 Title goes here 6
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 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. 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 Monophyly The smallest monophyletic group is a species Sometimes a trivial trait, like a single point mutation could make a group monophyletic, and a species according to the phylogenetic species concept Title goes here 7
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 belong to the same species Speciation can occur without changes in morphology or other traits (cryptic species) Which species concept to use? When we discuss animals we often use the biological species concept as the gold standard... complemented with the phylogenetic and phenetic species concepts Plants: it depends, since very distant plants can hybridize phylogenetic species concept is often used. Bacteria: poses difficult problems for classification. Bacteria do not interbreed ( Biological Species concept). In some cases massive exchange of genetic material (horizontal gene transfer) leads to phylogenetic confusion. Often a combination of the Phylogenetic and Phenetic Species Concepts (biochemical and morphological [like cell wall] traits) are used. 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 However, concept of species is still useful: Species are considered the largest group with a common evolutionary fate Title goes here 8
Concepts Geographic Models Allopatric Sympatric Reinforcement Problems with the concept of Species Species Biological Phylogenetic Phenetic (Morphological) Monophyly 1. Which of the following is a species according to the biological species concept? (A) All hominin species (most are fossil species). (B) A population of bacteria for which 80% of their DNA sequences are identical. (C) All allopolyploid plants. (D) A set of populations of beetles that can intermate and produce offspring for multiple generations, but cannot intermate with other populations. 2. Which of the following is NOT a reason that Species are difficult to define? (A) Many plants that are genetically divergent are able to mate (B) Many organisms that are morphologically similar are genetically distinct (C) Many organisms are asexual (D) Sometimes groups split off from within a monophyletic group (such as birds splitting off from the reptiles) (E) Sometimes sexual populations that are unable to interbreed could still be the same biological species 3. Which of the following is most likely to be a "species" according to the? (a) A population of bacteria that has a gene that allows glucose metabolism (b) Bird populations, which share a unique heritable feather structure (c) Spider populations that can interbreed and produce fertile offspring (d) Crustacean populations that form a clade (geneticallyrelated group), except for one population within the clade that colonized land and became insects (e) Populations of deer that share similar antler shape answers Optional Slides 1D 2E 3B Title goes here 9
Reinforcement So, when hybrids are formed between different species, they are often costly and maladaptive because of hybrid breakdown (the hybrids are maladaptive) Reinforcement So, when hybrids are formed between different species, they are often costly and maladaptive because of hybrid breakdown In such cases, you would predict that mechanisms to avoid mating would evolve to avoid the production of maladaptive hybrids (= Reinforcement) Predict the formation of prezygotic isolation to prevent the creation of maladaptive hybrids Reproductive isolation could occur at different stages of reproduction Prezygotic Reproductive Isolation (before egg is fertilized) Failure to Mate Genetic drift and divergence in bird songàwon t mate Selection on coat coloràdon t recognize each other Sperm-egg incompatibility Postzygotic Reproductive Isolation (after egg is fertilized) DM incompatibilities cause embryo to not develop (Example: enzymes don t work together) Prezygotic barriers Gametic Isolation Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Fertilization Postzygotic barriers Viable, fertile offspring Reinforcement So, the prediction is that in sympatry (when two different species are in the same place), mechanisms to avoid mating (prezyotic isolation) would be strong Whereas in allopatry, prezygotic isolation would not be needed because the different species would not come into contact Title goes here 10
4. Under which of the following scenarios is reinforcement most likely to evolve? (a) Different fish species, with each living in a separate pond (b) Two snail species, where each lives on opposite sides of a freeway (c) Different species of crickets living together in a park, where hybrids between them have low survival rates (d) Different insect species, each living on a different species of fruit in a forest (e) Different species of allopolyploid plants living in a field answers 4C Title goes here 11