Species, Speciation, and Advanced Phylogenetics

Why can it sometimes be difficult to clearly delineate when a new species arises?

Because evolution is a continuous process

What are three fundamental questions that interest biologists about species?

1) What criteria can we use to define a species?, 2) How do new species arise, and why?, 3) How do separate species remain separate species?

What are the two main approaches scientists use to define species?

1) Morphological Species Concept, 2) Biological Species Concept

What is the basic idea of the Morphological Species Concept?

Members of a species look alike because they share many alleles in common

Is the Morphological Species Concept a perfect criterion?

No, this is not a perfect criterion

Why isn't morphological similarity always reliable for defining species?

Members of the same species can sometimes look quite different

Who proposed the Biological Species Concept?

Ernst Mayr

What is Ernst Mayr's definition of species (Biological Species Concept)?

"Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups"

What type of organisms does the Biological Species Concept only apply to?

Sexually reproducing organisms

Is reproductive isolation the same as geographic isolation?

No, reproductive isolation is not the same as geographic isolation

What is speciation?

The divergence of biological lineages and emergence of reproductive isolation between lineages over time

How long does speciation typically take?

It often takes a very long time

What does each node represent in a species tree?

A "speciation event"

What doesn't a species tree tell us about speciation?

It does not tell us how rapidly or slowly speciation occurred

What are the two main types of speciation?

1) Allopatric speciation, 2) Sympatric speciation

What is allopatric speciation?

Occurs when interbreeding populations are separated by a physical barrier

Which type of speciation is thought to be the dominant mode?

Allopatric speciation is thought to be the dominant mode of speciation

What is sympatric speciation?

Does not require separation by physical barriers

How does allopatric speciation occur through barrier formation?

Interbreeding ancestral populations may become physically separated into two or more descendant populations due to formation of natural barriers, which leads to reproductive isolation over time

How can allopatric speciation result from founder populations?

When a population of organisms move across an existing barrier and form a new, isolated "founder" population elsewhere

What are examples of allopatric speciation through founder populations?

Darwin's finches on the Galapagos Islands, or fruit flies in Hawaii

What happens to new founder populations over time?

The new population(s) will adapt and may become reproductively isolated from the ancestral population over successive generations

What recent scientific finding challenges traditional species concepts?

Recent scientific findings suggest that neanderthals and modern-day humans interbred at some time in the past

What is required for a species to be maintained?

There must be gene flow (exchange of alleles) among members of that species

What does speciation lead to regarding gene flow?

Speciation leads to the disruption of gene exchange

When are populations reproductively isolated?

If they have genetic differences that prevent them from successfully interbreeding with other species in the wild

What are the four different types of traits discussed in phylogenetics?

1) Homologies, 2) Synapomorphies (specific type of homology), 3) Evolutionary Reversals, 4) Convergent Evolution

What are synapomorphies in relation to homologous traits?

Synapomorphies are a type of homologous trait that were derived from the MRCA of a given clade and are characteristic and unique to the entire clade

What must a true synapomorphy include?

All descendants of a given MRCA

What is convergent evolution?

Superficially similar traits that arose independently in different lineages and were NOT derived from a common ancestor

Why can't convergent traits be considered homologous?

Because they arose independently in different lineages and were NOT derived from a common ancestor

How do convergent traits often differ despite similar appearances?

They often differ in how the outcome is achieved

What is an example of convergent evolution in flight?

Bat wings vs. bird wings - both allow for flight, but the design is different (birds have feathers, bats do not)

In the bat vs. bird wing example, what was inherited from a common ancestor?

The bone structures that support the skin or feathers were inherited from an ancestor common to both taxa

What does the bone structure inheritance in bat/bird wings imply?

The overall arrangement of the support structures (bones) are homologous traits

How should bat and bird wing bone structures be classified?

As homologous traits

How should the flight adaptations of bat and bird wings be classified?

As convergent evolution

What is an evolutionary reversal?

A character that was derived from an ancestor is subsequently lost, or else it reverts from a derived state back to an ancestral state

What is the first step in constructing phylogenetic trees?

Select a group of organisms to classify (the ingroup) along with an appropriate outgroup

What is an outgroup?

A closely related taxon that serves as a point of reference but is not part of the ingroup

What is the second step in phylogenetic tree construction?

From a list of characters, determine the most likely evolutionary relatedness among the taxa of interest, using synapomorphies to group taxa into clades

What is the third step in phylogenetic tree construction?

When using the parsimony principle, construct the simplest tree possible by minimizing the number of homoplasies

What are homoplasies in phylogenetic analysis?

Convergent evolution and/or reversals

What should be minimized when using the parsimony principle?

The number of character changes

What should be used first when constructing parsimonious trees?

Try to use homology first

What is the goal of the parsimony method?

Draw the most parsimonious tree with the least number of character changes

Besides academic research, where else do phylogenies become important?

In other aspects of human life, such as forensics and legal cases

What determines whether populations are actually different species under the Biological Species Concept?

Whether they can successfully interbreed and produce fertile offspring in nature

What is the key difference between allopatric and sympatric speciation?

Allopatric requires physical separation by barriers; sympatric does not require physical separation

Can both evolutionary appearances and reversals be shown on phylogenetic trees?

Yes, both appearances and reversals can be labeled on a tree

What makes a trait a synapomorphy versus just a homologous trait?

Synapomorphies are homologous traits that are unique to and characteristic of an entire clade

Why is gene flow important for species maintenance?

It allows exchange of alleles among members, maintaining genetic connectivity within the species

What happens when gene flow is disrupted?

It can lead to speciation as populations become reproductively isolated