Speciation - University of Wisconsin–Madison · Species Concepts (biochemical and morphological [like cell wall] traits) are used. Species are dynamic rather than static entities,

Carol Eunmi Lee 11/8/18

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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 SpeciationLast Time: Genetic Models:

The roles of: MutationsNatural SelectionGenetic 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 SpeciationGeographic (Ecological) Models:

Allopatric Model (different place)Disperse to Another LocationVicariance: a barrier is formedThis geographic split could lead to Dobzhansky-Müller incompatibilities

Sympatric Model (same place)Polyploid speciationMate ChoiceNiche 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


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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 ModelsDispersal

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 SpeciationExamples (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 modelsSpeciation with no geographic separation Speciation despite gene flow

(1) Formation of polyploids (discussed in previous lecture)

(2) Natural Selection due toNiche PartitioningSexual Selection

Sympatric Model(1) Formation of Polyploids

(covered in previous lecture)

Important mechanism for plantsOccurs 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


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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

■ 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)

Soapberry bugs mate on different host plants

■ 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.


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InbreedingDepression

Hybrid Vigor(due to Heterozygote advantage)

OutbreedingDepression =

Hybrid Breakdown

Increasing genetic distance

FitnessMating between different species

(Lions x tiger,Horse x donkey)

Will not mate orProduce inviable or

sterile hybridsPopulations

within a speciesMating between

relatives

(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???


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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. Phenetic Species Concept (includes Morphological SC)

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 ConceptPROBLEMS:

■ 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”)


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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) characterA 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 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


MonophylyThe smallest monophyletic group is called a species

There is a derived character that is shared by the 4 populations

A A A

BBB

C C C

DDD

E E E

FFF

G G G

Group IIIGroup II

Group I

(a) Monophyletic group (clade) (b) Paraphyletic group (c) Polyphyletic group

A monophyletic clade consists of an ancestral taxa and all its descendants

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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 ConceptProblems:

■ 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


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


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■ 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

3. Phenetic Species Concept

■ Most intuitive; the way we recognize species

■ Easiest. Easier than constructing phylogeny or intermating

Phenetic (often Morphological)Species Concept

Strengths

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)

Phenetic Species Concept 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.

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


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Concepts Geographic ModelsAllopatric Sympatric

ReinforcementProblems with the

concept of “Species”

SpeciesBiological PhylogeneticPhenetic (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 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

answers

■ 1D■ 2E■ 3B

Optional Slides


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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

Prezygotic barriersGametic Isolation

Fertilization

Reduced Hybrid Viability Reduced Hybrid FertilityPostzygotic barriers

Hybrid Breakdown

Viable,fertileoffspring

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)

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


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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

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Speciation - University of Wisconsin–Madison · Species Concepts (biochemical and morphological [like cell wall] traits) are used. Species are dynamic rather than static entities,