Speciation

(1) Geographic Mechanisms of Speciation (What circumstances lead to the formation of new species?) (2) Species Concepts ( How are Species Defined? ) Today’s OUTLINE:

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) Parapatric Model (adjoining)

Geographic Models of speciation n Allopatric speciation: geographic isolation n Sympatric speciation: no geographic isolation n Parapatric speciation: geographic separation (or gradient), but not isolation

Allopatric Models Involves Geographic Isolation DispersalVicariance

Allopatric Models Following geographic separation between populations,DispersalVicariance This geographic separation provides the setting that allows speciation at the molecular level to occur (last lecture)

Allopatric Models DispersalVicariance 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) n Dispersal: u Colonization of islands u Colonization of lakes n Vicariance: u Highway going through a forest u Fragmentation of habitats u 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 Polyploidy (extra chromosomes) Important mechanism for plants Also occurred possibly in vertebrates, some crustaceans (covered in previous lecture)

Sympatric Model (2) Selection in the face of gene flow: Niche Partitioning Niche Partitioning Strong assortative mating and sexual selection (disruptive selection) Strong assortative mating and sexual selection (disruptive selection)

Example of Niche Partitioning: Soapberry bugs 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 n The populations are unlikely to encounter each other u Reduces gene flow u Isolation n Disruptive Natural Selection n Adaptation to alternative hosts leads to reproductive isolation (through the genetic mechanisms discussed earlier, such as Dobzhansky-Müller model) Soapberry bugs mate on different host plants

n But, sometimes hybrid zones do form between populations that are in the process of speciating n Sometimes hybridization between different species results in vigorous new species or populations, especially in plants (hybrid vigor, or heterozygote advantage) n 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) n 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.

Inbreeding Depression Hybrid Vigor (due to Heterozygote advantage) Outbreeding Depression = Hybrid Breakdown Increasing genetic distance Fitness Mating between different species (Lions x tiger, Horse x donkey) Will not mate or Produce inviable or sterile hybrids Populations within a species Mating between relatives

Reinforcement n So, when hybrids are formed between different species, they are often costly and maladaptive because of hybrid breakdown

Reinforcement n So, when hybrids are formed between different species, they are often costly and maladaptive because of hybrid breakdown n In such cases, you would predict that mechanisms to avoid mating would evolve to avoid the production of maladaptive hybrids (= Reinforcement)

Prezygotic barriers Gametic Isolation Fertilization Reduced Hybrid Viability Reduced Hybrid Fertility Postzygotic barriers Hybrid Breakdown Viable, fertile offspring n Reproductive isolation could occur at many different levels n Prezygotic (before the egg is fertilized) u Genetic drift and divergence in bird song-won’t mate u Selection on coat color-don’t recognize each other n Postzygotic (after the egg is fertilized) u DM incompatibilities cause embryo to not develop (enzymes don’t work together)

Reinforcement n 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 n Whereas in allopatry, prezygotic isolation would not be needed because the different species would not come into contact

(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 (how an organism looks)

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

n 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

n So then, how do you define species???

Species are dynamic rather than static entities, with boundaries changing constantly Many groups are in the process of speciation Darwin’s view: Species are arbitrary constructs of the human mind imposed on a continuum of variation

Three Main Species Concepts 1.Biological Species Concept 2.Phylogenetic Species Concept 3.Morphological (Phenetic) 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 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 Strengths

Biological Species Concept PROBLEMS : n Many ‘species’ are asexual and do not intermate (viruses, bacteria, protists) n Many highly divergent species can hybridize (plants) n Only applicable to present (not fossil taxa) n 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 Monophyletic group: A group with a shared derived (descendant) character A group that contains a common ancestor and all its descendents There are several monophyletic groups here

Phylogenetic Species Concept Typically, a phylogeny is constructed using DNA or other types of data (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 Monophyly The smallest monophyletic group is called a species There is a derived character that is shared by the 4 populations

Phylogenetic Species Concept 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 Strengths

Phylogenetic Species Concept Problems : n Need a good phylogeny – time consuming and can be expensive n 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) n A trivial trait (single mutation or trait) can make a group monophyletic, and may not warrant calling a group a new species

Phylogenetic Species Concept Problems : n A trivial trait (single mutation or trait) can make a group monophyletic, and may not warrant calling a group a new species n The cut off for a “species” is often arbitrary. For example, 3% sequence divergence is often used for bacteria

Phylogenetic Species Concept The smallest monophyletic group is a species Monophyly Sometimes a trivial trait, like a single point mutation could make a group monophyletic, and a “species” according to the phylogenetic species concept

Identifying species using overall similarity (but not in a phylogenetic context… no hierarchy – no branching pattern, no ancestral-derived relationships) Most often morphological traits are used, but any phenotype could be used 3. Morphological (Phenetic) Species Concept

n Most intuitive; the way we recognize species n Easiest. Easier than constructing phylogeny or intermating Morphological (Phenetic) Species Concept Strengths

Problems : n Different species can look similar due to convergent evolution n Populations that look distinct sometimes belong to the same species n Speciation can occur without changes in morphology or other traits (cryptic species) Morphological (Phenetic) Species Concept

Which species concept to use? n When we discuss animals we often use the biological species concept n Plants: it depends, since very distant plants can hybridize… phylogenetic species concept is often used. n Bacteria: difficult problem. u Bacteria do not interbreed (≠ Biological Species concept). In some cases massive exchange of genetic material (horizontal gene transfer) leads to phylogenetic confusion. u Often a combination of the Phylogenetic and Phenetic Species Concepts (biochemical and morphological [like cell wall] traits) are used.

Species are dynamic rather than static entities, with boundaries changing constantly Many groups are in the process of speciation Darwin’s view: Species are arbitrary constructs of the human mind imposed on a continuum of variation

However, concept of species is still useful: Species are considered the largest group with a common evolutionary fate

Concepts Geographic Models Allopatric Sympatric Reinforcement Problems with the concept of “Species” Species Biological Phylogenetic Phenetic (Morphological) Monophyly

3. 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 group of beetles that can intermate and produce offspring for multiple generations.

4. 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 Phylogenetic Species Concept? (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 (genetically- related group), except for one population within the clade that colonized land and became insects (e) Populations of deer that share similar antler shape

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 (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

5. Which of the following scenarios is likely to lead to the most rapid formation of new species? (a) Two populations become geographically separated, and there is continued migration between the populations (b) Two populations become geographically separated, and then new mutations arise in each population that become fixed due to genetic drift (c) Two populations that are in the same location diverge due to sexual selection for different traits in the two populations (d) Two populations become geographically separated, and then new mutations arise in each population that become fixed due to selection favoring different egg coat proteins in the different habitats (e) All of the above would on average lead to equivalent rates of speciation

answers n 3D n 4E n 3B n 4C n 5D