Speciation occurs when populations of the same species become genetically isolated by lack of gene flow and then diverge
from each other due to any among the other 4 facts described in the previous chapter (most remarkably natural selection,
genetic drift or mutation.
Populations can be recognized as distinct species if they are reproductively isolated from each other, if they have distinct
morphological characteristics, or if they form independent branches on a phylogenetic three,
Populations can become genetically isolated from each other if they occupy different geographic areas, if they use different
habitats within the same area or if one population is polypoid and cannot breed with the other.
When populations that have diverged come back into contact, several outcomes are possible (fusion, reinforcement, through
pre and post zygotic isolation, development or hybrid zones, extinction and speciation by hybridization).
When gene flow is reduced between populations, they may then diverge genetically as a result of any among the other 4
factors (most remarkably natural selection, genetic drift, or mutation).
This genetic divergence may eventually lead to speciation.
o Speciation: the evolution of two or more distinct species from a single ancestral species.
How Are Species Defined and Identified?
Species: an evolutionary independent population or group of populations
Gene flow eliminates genetic differences among populations, so evolutionary independence starts with lack of gene flow,
If gene flow between populations stops, then the other 4 factors (most remarkably natural selection, genetic drift, or mutation)
begin to act on population independently.
Allele frequencies and other characteristics of these independent populations diverge, and, over time, the populations can
become distinct species.
Biologists commonly use the following three approaches to identify species:
o The biological species concept
o The morph species concept
o The phylogenetic species concept
The Biological Species Concept
Biological Species Concept: considers population to be evolutionarily independent if they are reproductively isolated from
each other (they cannot breed).
Therefore, no gene flow occurs between these populations
Biologists categorize the mechanisms that stop gene flow between populations as being either prezygotic or postzygotic.
Prezygotic Isolation: occurs when individuals of different species are prevented from mating.
o Timing, location, behaviour, genetics, mechanics
Postzygotic isolation: occurs when individuals from different populations do mate, but the hybrid offspring produced have
low fitness and do not survive or produce offspring.
The criterion of reproductive isolation cannot be evaluated in fossils or in species that reproduce asexually. In addition, it can
be applied only to populations that overlap geographically.
The Morphospecies Concept
Under the morphospecies concept, biologists identify evolutionarily independent lineages by differences in size, shapes, or
other morphological features.
This concept is based on the idea that distinguishing features are most likely to arise if populations are independent and
isolated from gene flow. The morphospecies concept can be applies widely, but the features used to distinguish species under
this concept are rather subjective.
The Phylogenetic Species Concept
Phylogenetic Species Concept: is based on reconstructing the evolutionary history of populations.
Monophyletic group (clade): an ancestral population plus all of its descendants on a phylogenetic tree
Under this concept, a species is defined as the smallest monophyletic group on a tree that contains populations. On such a
cladogram (phylogenetic tree), each tip is a phylogenetic species.
Populations comprise of individuals in the same species that live in the same areas. Populations may be identical
(genotypically, phenotypically) by separated geographically each G could represent and individual.
The Phylogenetic Species Concept
The phylogenetic species concept can be applied to any population.
However, phylogenies are currently available for only a tiny *though growing) subset of populations on the tree of life.
In practise, biologists use all three species concepts, the biological morphospecies, and phylogenetic.
Species Definitions In Action: Dusky Seaside Sparrow Speciation
Subspecies are populations that live in discrete geographic areas and have their own identifying traits but are not distinct
enough to be considered a separate species.
Several subspecies of seaside sparrow live along the Atlantic and Gulf Coasts and are isolated physically from one another;
scientists believed that there was little or no gene flow between populations.
Based on the biological species and the morphospecies concepts these subspecies were considered to be separate species.
Scientists launched a conservation program for one subspecies thought to be nearing extinction, the dusky seaside sparrow.
o In 1980, only 6 males remained conservations decided to mate with the A. maritimus.
However, phylogenetic analysis of gene sequences from different seaside sparrow populations showed that only two distinct
monophyletic groups of seaside sparrows exist.
The dusky sparrow was shown to be genetically indistinguishable from the other Atlantic Coast sparrows and thus did not
need to be individually preserved to preserve the genetic diversity of the species.
Isolation and Divergence in Allopatry
Genetic isolation happens when populations become physically separated.
Physical isolation occurs by dispersal or vicariance.
Dispersal occurs when a population moves to a new habitat, colonizes it, and forms a new population.
Vicariance occurs when a physical barrier splits a widespread population into subgroups that are physically isolated from
Speciation that begins with physical isolation via either dispersal or vicariance is known as allopatric speciation.
o Occurs to species that are living in different geographical area.
Populations that live in different areas are said to be in allopatry.
Biogeography: the study of how species and populations are distributed geographically. Can tell us how colonization and
range-splitting events occur.
Dispersal and Colonization Isolate Populations
Colonization events often cause speciation because the physical separation reduces gene flow, and genetic drift via the founder