 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.

 The species is a basic biological unit and humans seem to intuitively recognize species.
 However, why do species exist?
 Why don’t we see a smooth continuous blending of one species into another?

Why do we see discrete species?
 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.

Why do we see discrete species?
 Organisms are very well adapted to their environments having evolved over millions of years.
 Each organism has specialized characteristics such as camouflage, feeding structures, behavior, and genitalia that equip it to survive well in its environment.

Why do we see discrete species?
 An offspring that results from a cross between members of two different species or between members of different populations that have been evolving in isolation from each other, will probably have traits intermediate between its parents.
 As a result, it likely will be less well adapted to its environment than either parental form and be selected against.
 Thus, we see distinctively different 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.

 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.
 Members of a species we expect to be similar to each other but different from other organisms,

Criticisms of the Biological Species
Concept
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
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The BSC has been criticized for several reasons:
1. It applies only to sexually reproducing species.
2. Distinguishing between species on the basis of reproductive separation is problematic because it can be difficult to determine 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.

How many species of African elephants are there?
 A comparison of DNA from 21 populations suggests that two species exist and it has been suggested by 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.

 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 environemnts 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).

 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 isoloated 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)

 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 founder hypothesis of speciation in Hawaiian Islands
 The main hypothesis for how the Hawaiian
Islands became populated with a diverse variety of endemic species most of which occur on only a single island is the founder hypothesis.
 According to the founder hypothesis new species are formed when a small population of individuals disperses to a new island and after being separated diverges from the ancestral form.

Evidence for founder hypothesis of speciation in Hawaiian Islands
 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.
Thus the newest islands are close to the hot spot and the oldest further northwest.

Evidence for founder hypothesis of speciation in Hawaiian Islands
 Based on the geological information the founder hypothesis makes two predictions about the pattern of speciation that should be observed.

Closely related species should be found on adjacent islands and

Some speciation sequences should match the sequence in which islands formed.

Evidence for founder hypothesis of speciation in Hawaiian Islands
 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.

15.7

Geographic isolation through vicariance events
 There are many ways in which a species distribution may be split into two by a physical event. Some 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 on either side. Did these populations speciate?

Geographic isolation through vicariance events
 A DNA sequence study by Knowlton et al. 1993 of snapping shrimp populations from both sides of the isthmus suggests they did.
 Seven pairs of morphologically closely related species pairs occur, one of each pair on each side of the isthmus and the DNA sequence results confirm that these 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.

Geographic isolation through vicariance events
 Mating experiments with the snapping shrimp fund that males and females with the greatest genetic divergence were least interested in each other and almost none the pairs produced clutches that yielded fertile young.

 Not all speciation requires physical isolation of populations.
 Such speciation without physical isolation is called sympatric speciation .

Polyploidization as a mechanism of 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.

Natural selection and sympatric speciation in apple maggot flies
 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 about 300 years ago.

Natural selection and sympatric speciation in apple maggot flies
 The question is does the new food source represent an island and are the populations that breed on apples genetically distinct form those that breed on hawthorn trees?
 Do apple and hawthorn populations interbreed or not and are they diverging?

Natural selection and sympatric speciation in apple maggot flies
 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.

Natural selection and sympatric speciation in apple maggot flies
 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 crosspopulation matings, but despite this gene flow natural selection appears to driving the populations apart.

Natural selection and sympatric speciation in apple maggot flies
 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.

Natural selection and sympatric speciation in apple maggot flies
 Experimental tests show that these developmental schedules have a genetic basis and individuals need the correct alleles to develop under each temperature regime.
 Individuals that are the result of crosses between apple and hawthorn flies are thus selected against and the populations have diverged and continue to do so.

 Theodosius Dobzhansky (1937) the famous geneticist reasoned that for populations that had diverged and come back into contact hybrid offspring between them would have reduced fitness.
 As a result there should be strong selection favoring assortative mating (individuals mating within their own population) and as a result a variety of isolating mechanisms should evolve to reduce the likelihood of interbreeding.

 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