Download File

Speciation
Speciation: splitting of one species into 2 different species.
What is a species? Based on ability to reproduce.
“Biological species concept”: a species is a group of organisms that
interbreed under natural conditions and that are reproductively isolated from
each other.
Reproductively isolated: don’t produce fertile hybrids.
Natural conditions: artificial breeding doesn’t count. For example,
artificial insemination, keeping 2 species locked up together.
.
Speciation
In contrast, the older “morphological species
concept”: members of the same species look similar
to each other. Many examples of organisms that
look similar but can’t produce fertile offspring.
Problems with biological species concept:
Doesn’t work with fossils or extinct species.
Doesn’t work with asexual species , such as most
bacteria.
How to deal with what is “natural”.
Reproductive Isolation
How do populations become reproductively isolated?
Result of gene mutations and altered allele frequencies due to selection and
genetic drift.
To maintain as a single species, there must be gene flow between populations:
matings between members of separated populations that allow mixing of alleles.
In the absence of gene flow, mutations in different populations will be independent,
and allele frequencies will change independently of each other.
In most cases, migration is the key to gene flow.
Once gene flow stops: genetic divergence occurs. The two populations gradually
become genetically different.
Speciation sometimes occurs very quickly, other times more slowly. In most cases
it is not an instantaneous event.
Genetic Divergence
Reproductive Isolation
Mechanisms
Pre-zygotic (before mating) vs. postzygotic (after mating).
Pre-zygotic: different breeding
seasons (either yearly or time of day),
different signals: sight, sound, smell,
behavior, mechanical problems—
genitalia don’t fit together. Also
gametic isolation: sperm don’t detect
the egg or can’t fertilize it.
Post-zygotic: hybrid inviability:
hybrids don’t survive to birth or
adulthood; hybrid sterility: as with the
mule, a sterile hybrid.
Even having hybrids that are
significantly less fit (able to survive
and reproduce) than purebreds is an
isolating mechanism, a way to prevent
the two populations from fusing into
one population.
Allopatric Speciation
The simplest and
most common
mechanism of
speciation is
allopatric
speciation: 2 groups
of one species are
isolated
geographically, and
diverge into
separate species.
More Allopatric Speciation
Geographical barriers:
mountains, oceans, rivers. A
few members of a species
manage to cross by a rare
chance event.
This is the mechanism by which
Darwin’s finches evolved into
separate species in the
Galapagos islands. Only very
rarely can birds cross the
ocean to get to other islands.
Or, the barrier develops slowly
as conditions change: the
gradual formation of the Grand
Canyon split a population into 2
isolated groups, that have
diverged into separate species,
the Kaibab and Albert
squirrels.
Sympatric Speciation
Geographical isolation is the
easiest way for species to form, but
there are other possible
mechanisms. “Sympatric
speciation” means speciation that
occurs within the same
geographical location.
An example: cichlid fish in Lake
Barmobi Mbo in Cameroon,
Africa—an isolated volcanic lake.
Nine species, all more closely
related to each other (by DNA
evidence) than to similar fish in
other lakes. Lake has no distinct
geographical zones, and the fish
can easily swim anywhere in it.
They feed in different locations, but
all breed in the same location, close
to the bottom. An example of
sympatric speciation, but the
mechanism is not clear.
Sympatric Speciation by
Polyploidy
About half of all flowering plants are
polyploid: more than 2 copies of each gene.
Polyploids are the result of failure of cell
division (mitosis or meiosis) to separate the
chromosomes into 2 cells.
New polyploids are usually sterile, or their
offspring are sterile: extra chromosomes
with no homologue to pair with in meiosis
leads to unbalanced (aneuploid) gametes,
which will produce dead offspring.
Triploids (3 copies of each gene) are
sterile—the source of seedless fruit.
In animals, sexual reproduction is essential
every generation, so most polyploids fail to
reproduce: need a similarly polyploid mate
to produce fertile offspring.
In plants, vegetative propagation is possible
for many generations without sex, and in
many species one plant contains both
sexes. So, reproduction with a polyploid
partner is easier in plants than in animals.
Hybrid Zones
When two populations of a species are
separated by a geographical barrier, they
diverge genetically. Sometimes the
barrier is removed and the two groups
come into contact with one another. The
region of contact is a “hybrid zone”.
Several possibilities exist:
If the two groups have only diverged a bit,
fertile offspring will result, and the two
groups will merge back into a single species.
Geographical differences may exist within
the species: different subspecies or
varieties, but all can interbreed freely.
If the two groups have diverged to the point
that no fertile offspring result from their
matings, sexual selection will occur to deter
further matings. New pre-zygotic
reproductive isolation mechanisms,
especially behavioral differences, arise to
reinforce the division between the two
species.
This is called “parapatric speciation”: 2
species forming while in contact with each
other in a restricted region.
•Shrimp in Panama. Up until 3
million years ago, North and South
America were separated by the
ocean. As dry land rose between
the continents, shrimp who once
freely interbred were separated by
a barrier. When put together in the
lab (or by going through the
Panama Canal) now, they attack
each other rather than mating.
Parapatric Example
Patterns of Speciation
What happens after 2 species separate
from each other?
In some cases, the species exists for
millions of years, gradually changing in
response to external conditions but always
maintaining as a single distinct species.
In other cases, many new species will form
from a single species in a very short time:
this is “adaptive radiation”. This often
happens on isolated islands, where a new
species is blown in by a storm, and finds
many different ecological niches to fill.
Darwin’s finches are an example of this.
They are thought to have originated with a
small group of finches that blew over about
1 million years ago, to islands with no
dangerous predators and very few other
land birds.
Also, sometimes a “key innovation” will
arise, and new niches will suddenly be open
to the possessors of the innovation. Or, a
disaster will allow a sudden expansion of a
few surviving species. Mammals grew
rapidly in number of species following the
extinction of the dinosaurs.
Extinction
Extinction can happen: none
left of the species.
Various events can cause
extinction: being outcompeted
for a critical resource, having
the climate change too rapidly
to adapt.
“Mass extinctions” are caused
by catastrophic events. The
Earth has had several mass
extinction events, where the
vast majority of species die out
over a short period of time.
This is what is seen when one
moves between various
geological ages. Asteroids
hitting the Earth are
responsible for at least some of
these, but probably not all.