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ENVS 1200 Lecture Notes - Species, Species Problem, Species Complex

Environmental Studies
Course Code
ENVS 1200
Rebecca Jubis

of 7
Chapter 16- Mechanisms of Speciation
16.1- Species Concepts
Species is the smallest evolutionarily independent unit
Evolutionary independence occurs when mutation, selection, gene flow, and drift operate on
populations separately
Evolution consists of changes in allele frequencies, and species form a boundary for the spread of
oAs a result, different species follow different evolutionary trajectories
oThe essence of speciation is lack of gene flow
Species consist of interbreeding populations that evolve independently of other populations
The three most important “species concepts”:
oMorphospecies Concept
oBiological Species Concept
oPhylogenetic Species Concept
Each of the three agrees that species are evolutionarily independent units that are
isolated by lack of gene flow, but each employs a different criterion for the
determining that independence is actually in effect
The Morphospecies Concept
In traditional cultures, people name species based on morphological similarities and differences
In biology, careful analyses of phenotypic differences are the basis of identifying morphospecies
Morphospecies can be identified in species that are extinct or living, and in species that reproduce
sexually or asexually
Fossil species that differed in color or the anatomy of soft tissues cannot be distinguished
Neither can populations that are similar in morphology but were strongly divergent in traits like
songs, temperature, or drought tolerance, habitat use, or courtship displays
oSpecies like these are called cryptic species
Species that are indistinguishable morphologically, but divergent in songs, calls,
odor, or other traits
The Biological Species Concept
Under this concept, criterion for identifying evolutionary independence is reproductive isolation
If populations of organisms do not hybridize regularly in nature, or if they fail to produce fertile
offspring when they do, then they are reproductively isolated and considered good species
Great strength of this concept is that reproductive isolation is a meaningful criterion for
identifying species because it confirms lack of gene flow
Lack of gene flow is the litmus test of evolutionary independence in organisms that reproduce
The Phylogenetic Species Concept
Systematists are biologists who are responsible for classifying the diversity of life
This concept is also known as the genealogical species concept
oThis approach focuses on a criterion for identifying species called monophyly
Under this concept, species are identified by estimating the phylogeny of closely related
populations and finding the smallest monophyletic groups
oOn a tree like this, species form the tips
Rationale behind the phylogenetic species concept is that traits can only distinguish populations
on a phylogeny if the populations have been isolated in terms of gene flow and have diverged
genetically, and possibly morphologically as well
To be called separate phylogenetic species, populations must have been evolutionarily
independent long enough for the diagnostic traits to have evolved
Populations within species have shared, derived traits that distinguish them from populations of
other species
Species are named on the basis of statistically significant differences in the traits used to estimate
the phylogeny
Recent analyses have found that the phylogenetic species concept often distinguishes a series of
cryptic species in populations that were formerly considered a single species.
Applying Species Concepts: Two Case Histories
Species can be identified by distinctive morphological traits, reproductive isolation, and/or
phylogenetic independence. Each species concept has advantages and disadvantages.
Employing more than one species concept can help biologists recognize diversity and organize
research on its consequences
Read example on Marine Copepods and Elephants on Pages 609-610
16.2- Mechanisms of Genetic Isolation
Speciation has been hypothesized to be a three-stage process:
oInitial step that isolates populations
oSecond step that results in divergence in traits such as mating system or habitat use
oFinal step that produces reproductive isolation
Isolation and divergence steps were thought to take place over time and to occur while
populations were located in different geographic areas
Final phase was hypothesized to occur when these diverged populations came back into physical
contact- an event known as secondary contact
oSecondary contact- when two populations that have diverged in isolation from a common
ancestor are reunited geographically
It is now clear that, isolation and divergence steps that initiate speciation frequently take place at
the same time and in the same place
oAlso it appears likely that in a significant number of speciation events or even a majority,
the third phase never occurs
Isolation/divergence/secondary contact hypothesis provides a useful framework for analyzing how
speciation takes place
First step in speciation is genetic isolation
Physical separation or changes in chromosome complements can reduce gene flow between
Once gene flow is dramatically reduced or ceases, evolutionary independence begins and
speciation is underway
Therefore, speciation process begins when gene flow is disrupted and populations become
genetically isolated
Physical Isolation as a Barrier to Gene Flow
Geographic isolation produces reproductive isolation, and thus genetic isolation
Allopatric Model- the hypothesis that speciation occurs when populations become geographically
isolated and diverge because selection and drift act on them independently
Essence of allopatric speciation is that physical isolation creates an effective barrier to gene flow
Geographic isolation has been an important trigger for the second stage in the speciation process:
genetic & ecological divergence
Geographic isolation can come about through dispersal and colonization of new habitats or
through vicariance events, where an existing range is split by a new physical barrier
Dispersal is when a population in one area splits into two and one moves to another isolated area
like from mainland to an island