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Part 1: Types of Speciation
Speciation

Recall from Darwin’s 6 main points of his
evolutionary theory that speciation is :
nOrigin of new species.
nOver numerous generations, new
species arise by the accumulation of
inherited variations.
nWhen a type is produced that is
significantly different from the original,
it becomes a species.
nA species can reproduce successfully
with its own kind.

Speciation:

Not Speciation:

Three-Spined
Stickleback Fish

Liger

Cannot reproduce,
therefore not a
new species of
animal

Evolved from salt
water into fresh
water varieties.
Examples of Speciation

Occurs when a new species evolves in geographic isolation
from its ancestor.

Example: One species could split into two if a physical
barrier, such as a new river, divided its geographic range. If
the barrier is large enough, gene flow between them would
cease and the two separate populations would evolve
independently. Over time, different alleles would be fixed in
them, either because of the hazards of mutation and drift,
or because selection favored different characters in the two.

If the two populations are separated long enough for
significant divergence to have taken place, then if the
barrier is removed and the populations are reunited, they
might remain distinct from each other. There would now be
two species where there was formerly one.
Allopatric Speciation

When a population of a species gets separated by
geographic means (distance, mountains, rivers, oceans,
etc.) that leads to reproductive isolation of that population.

Example: New World Drosophila flies that have been
extensively studied by evolutionists for several decades.
Two subspecies live in continental South America—D.
willistoni quechua lives west of the Andes and D. willistoni
willistoni east of the Andes. They are effectively separated
by the Andes because the flies cannot live at high altitudes.
It is not known whether their geographic separation is as
old as the Andes, but it has existed long enough for
reproductive isolation to occur. When the two subspecies
are crossed in the laboratory the males that result are
completely sterile.
Geographic Isolation – How
Allopatric Speciation Can
Happen

Speciation that occurs without geographic isolation of a
population. The speciating populations generally share the
same territory.

This type of speciation occurs when the gene flow between
members of a population is restricted due to ecological
isolation (niche differentiation). Some members of a
population may be better adapted to a slightly different
habitat in an ecosystem, and begin to specialize in that
habitat. Different selective pressures in the two habitats
lead to genetic changes in the organisms. The two
populations become reproductively isolated, and two
distinct species result, even though there are no physical
barriers separating the population.
Sympatric Speciation

Populations may occupy the same territory but
live in different habitats and so not meet.

Example: The Anopheles maculipennis group
consists of six mosquito species, some of which
are involved in the transmission of malaria.
Although the species are virtually
indistinguishable morphologically, they are
isolated reproductively, in part because they
breed in different habitats. Some breed in
brackish water, others in running fresh water, and
still others in stagnant fresh water.
Ecological Isolation (niche
differentiation) - How
Sympatric Speciation Can
Happen

Among sexual organisms, individuals that are
able to interbreed belong to the same species.
The biological properties of organisms that
prevent interbreeding are called reproductive
isolating mechanisms (RIMs).

Oaks on different islands, minnows in different
rivers, or squirrels in different mountain ranges
cannot interbreed because they are physically
separated, not necessarily because they are
biologically incompatible. Geographic separation,
therefore, is not a RIM.
Reproductive Isolation - How
Sympatric Speciation Can
Happen

There are two general categories of
reproductive isolating mechanisms:
prezygotic, or those that take effect before
fertilization, and postzygotic, those that
take effect afterward. Prezygotic RIMs
prevent the formation of hybrids between
members of different populations.
Postzygotic RIMs reduce the viability or
fertility of hybrids or their progeny.
Reproductive Isolation
 Examples
of prezygotic would be:
alterations in behaviour such as a
group of nocturnal mammals that
become active during the day so that
they no longer interbreed with
counterparts who are active at night.
 Example
of postzygotic would be a
horse and a donkey mate to produce a
mule. The mule is sterile.
Reproductive Isolation
Part 2: Patterns of Evolution

Occurs when different organism that live
in similar environments become more alike
in appearance and behaviour.

The environment selects similar
adaptations in unrelated species.

Organisms develop analogous structures
(same function, but different origins)
Convergent Evolution

Examples:
◦ Bird wings and insect wings
◦ Shark fins and dolphin fins
Placental vs Marsupial Animals

When 2 species evolve together.

There is mutual evolutionary influence
between 2 species

Their species have a symbiotic relationship
(interaction between members of 2
populations)
Co-evolution

Examples
◦ Birds and Flowers
◦ Leafcutter ant and the fungus it farms
◦ Newts and garter snakes
Examples of Coevolution

The process by which an ancestral species
gives rise to a number of new species that
are adapted to different environmental
conditions.

Often occur when a species colonizes a
new environment.

Organisms develop homologous structures
(similar or related structures in organisms
– ex. Human arm, bird wing, dog foreleg).
Divergent Evolution

This often happens when a species
colonizes a new environment in which
there are unoccupied ecological niches.

For example, the adaptive radiation of
Hawaiian honeycreepers and Darwin’s
finches occurred on islands.

In other cases, it occurred after the
extinction of many other species. The
rapid increase in the number of species of
mammals took place after the mass
extinction of the dinosaurs.
Divergent Evolution