Speciation
The species is the basic biological unit around which classifications are based. However, what
constitutes a species can be difficult to define and there are multiple definitions of species in use today.
What is a species?
The species is a basic biological unit and humans seem to intuitively recognize species.
However, why do we see discrete species? Why don’t we see a smooth continuous blending of one
species into another?
Because intermediate forms between closely related organisms are usually selected against. If they
were not selected against, then the two forms would merge into one as their gene pools mixed.
Organisms are, as we know, very well adapted to their environments having evolved over millions of
years (remember this is why stabilizing selection is by far the most common form of selection). Each
organism has highly specialized characteristics such as camouflage, feeding structures, behavior, and
genitalia all of which equip the organism to survive well in its environment.
Any offspring that result from a a cross between members of two different species or between members
of different populations that have been evolving in isolation from each other and adapting to different
environments, will probably have traits intermediate between its parents. As a result, the hybrid is
likely to be less well adapted to its environment than either parental form and be selected against.
Thus, we see distinctively different species.
Definitions of species
John Ray (1627-1705) gave first general definition of a species.
“A species consists of all individuals that can breed together and produce fertile offspring.”
A female donkey mated to a male horse produces what? A mule (which is sterile). Hence, donkeys and
horses are separate species.
Biological Species Concept
Ray’s idea was updated into the Biological Species Concept. Two definitions of the BSC are given below:
“Species are groups of actually or potentially interbreeding natural populations, which are
reproductively isolated from other such groups.” Ernst Mayr.
“A species is a reproductive community of populations (reproductively isolated from others) that
occupies a specific niche in nature.” Ernst Mayr.
The biological species concept emphasizes that a species is an interbreeding population of individuals
sharing common descent and that members of that community because they share a niche constitute an
ecological entity in nature.
We expect members of a species to be similar to each other but different from other organisms.
Criticisms of the Biological Species Concept
The BSC has been criticized for several reasons:
1. Applies only to sexually reproducing species.
2. It can be difficult to decide how much reproductive separation is needed to distinguish between
species.
3. The definition refers only to current populations and ignores the species status of ancestral
populations.
How many species of African elephants are there?
Traditionally one species of elephant Loxodonta africana has been recognized in Africa (a second species
Elephas maximus occurs in Asia).
However, recent morphological studies have pointed out that forest dwelling elephants in West Africa
appear to differ from elephants found in Savannah habitats elsewhere on the continent.
A comparison of DNA from 21 populations suggests that two species exist. Roca et al. (2001) that forest
elephants be named Loxodonta cyclotis.
Whether the two populations are capable of interbreeding is unclear, but the clear genetic differences
between populations suggest that conservation biologists should be attempting to conserve members of
both populations.
How species form
Classically, speciation has been viewed as a three stage process:
1. Isolation of populations – populations become separated from each other.
2. Divergence in traits of separated populations (e.g. mating system or habitat use). Isolated populations
adapt to their local environments and become different from the ancestral population
3. Reproductive isolation of populations occurs. The populations become so different that they can no
longer mate successfully.
These differences result in mating isolation between populations being maintained if and when
populations come into contact again (secondary contact).
Speciation by geographic isolation: Allopatric model of speciation
The three-step process described is the essence of Ernst Mayr’s allopatric model of speciation.
A physical barrier isolates a population or populations from the rest of the species and selection favors
genetic divergence of that population.
Separation of populations can occur by two major means:
Dispersal of some individuals across a barrier creating new isolated populations.
Development of a new barrier that separates populations [Vicariance] (the vicariance event could be
e.g. change in flow of a river, lava flow, development of a mountain range, habitat destruction)
Geographic isolation through dispersal
We have already encountered example sof speciation after individuals crossed a barrier. The ancestors
of Darwin’s finches colonized the Galapagos Islands after dispersing from South America and speciated
into the current range of species. Similarly, the Hawaiian Islands were colonized by ancestral
Drosophila fruit flies that appear to have speciated to produce more than 500 endemic species of
Drosophila on the islands.
Evidence for the founder hypothesis of speciation in Hawaiian Islands
The main hypothesis for how the Hawaiian Islands became populated with so many endemic species
(most of which occur on only a single island) is the founder hypothesis. According to the founder
hypothesis new species form when a small population of individuals disperses to a new island and after
being separated diverges from the ancestral form.
The Hawaiian Islands were formed by a stationary geological “hot spot” over which the continental plate
drifts northwest. Periodically, the hot spot produces magma flows, which form islands that are then
carried away on the plate and ultimately erode away. The newest islands are close to the hot spot and
the oldest further northwest.
Based on the geological information the founder hypothesis makes two predictions about the pattern of
speciation that should be observed.
1. Closely related species should be found on adjacent islands and
2. Speciation sequences should match the sequence in which islands formed.
A study of mitochondrial DNA of four species of closely related Drosophila by DeSalle and Giddings
(1986) found the predicted patterns. The most recent species occur on the youngest islands and
several of the branching events match the order of island formation.
Geographic isolation through vicariance events
There are many ways in which a species distribution may be split into two by a physical event. Some
processes, such as mountain formation, are slow, others such as a lava flow are rapid.
The Isthmus of Panama closed about 3 million years ago separating marine populations of shrimp on
either side. Did these populations speciate?
A DNA sequence study of snapping shrimp populations from both sides of the isthmus suggests they did.
(Knowlton et al. 1993)
Seven pairs of morphologically closely related species pairs occur. One of each pair occurs on each side
of the isthmus and the DNA sequence results show they are each others closest relatives, which is
consistent with the vicariance hypothesis.
Phylogenetic tree of numbered species of snapping shrimp. P and C refer to Pacific and Caribbean
species respectively.
Mating experiments with the snapping shrimp found that males and females with the greatest genetic
divergence were least interested in each other and almost none of the pairs produced fertile young.
Speciation without physical isolation (Sympatric speciation)
Not all speciation requires physical isolation of populations. Such speciation without physical isolation is
called sympatric speciation.
Polyploidization as a mechanism of sympatric speciation.
Polyploidy (production of multiple sets of chromosomes) appears to have played a major role in the
speciation patterns of plants. In such an event a new species can be formed in a single or just a few
generations and no physical separation is required. An estimated 70% of flowering plants appear to have
had polyploid events in their evolutionary history as have 95% of fern species.
Natural selection and sympatric speciation in apple maggot flies
The apple maggot fly (Rhagolestis pomonella) is a major pest of apples that occurs throughout the
northeastern U.S. It also parasitizes hawthorn trees a close relative of apples. Maggot flies recognize
trees on the basis of visual, tactile and olfactory cues and mate on or near the fruit. Eggs are laid on
fruits and larvae develop in them. When the fruit falls the larvae burrow into the ground and pupate
emerging as adults the next year.
Apple trees are a novel food source for these native flies, which exploited apples after they were
introduced into the United States about 300 years ago.
The question is does the new food source represent an island and are the populations that breed on
apples genetically distinct from those that breed on hawthorn trees? Do apple and hawthorn
populations interbreed or not and are they diverging? Hawthorn and apple trees are often in very close
proximity so it would seem hard for the populations to diverge. However, a protein electrophoresis
study by Feder et al. (1988,1990) showed that the populations are genetically distinct. Each population
shows a strong preference for its own fruit type, which because mating takes place on fruit, results in
strong nonrandom mating.
There is gene flow between populations because about 6% of matings are cross-population matings, but
despite this gene flow natural selection appears to driving the populations apart.
Natural selection favors divergence because hawthorn fruits ripen 3-4 weeks after apples. As a result
hawthorn fly larvae experience cool temperatures before pupating whereas apple fly larvae experience
warmer temperatures. Hawthorn flies and apple flies thus depend on different temperature signals to
time their pupation and emergence the next spring and have different developmental timetables.
Experimental tests show that these developmental schedules have a genetic basis and individuals need
the correct alleles to develop under each temperature regime.
Individuals born of crosses between apple and hawthorn flies are thus selected against and the
populations continue to diverge.
Secondary contact
Theodosius Dobzhansky (1937) argued that when populations diverged and later came back into contact
they would produce hybrid offspring with lower fitness. There should then be strong selection favoring
assortative mating (individuals mating within their own population). As a result, isolating mechanisms
should evolve to reduce the likelihood of interbreeding.
Isolating mechanisms
Isolating mechanisms fall into two categories:
prezygotic (those that reduce chances of mating and fertilization taking place) and
postzygotic (those that reduce the viability or hybrid offspring).
Examples of prezygotic isolating mechanisms:
Different habitat choice.
Activity at different times of day
Differences in sexual advertisements: calls, displays, pheromones.
Examples of postzygotic isolating mechanisms:
Failure of zygote to develop
Reduced viability of zygote
Sterility