1. How do new species originate?
How do we account for
the great diversity of life
in the biosphere?

2. What is a species?
We say that all dogs are in the same species, but that Norway maples and sugar maples are in different species. How do we know that? What do we mean?
What we mean by species is a group of individuals with the potential to interbreed under natural conditions and produce fertile offspring. By this definition each species has its own gene pool that does not mix with the gene pools of other species.

3. Shortcomings of this definition
Sturnella
neglecta
As discussed earlier, this species definition does not work for species that reproduce only asexually.
Moreover, it is not always feasible to determine whether mating is occurring between two groups in the same habitat.
And sometimes members of two species look so much alike that we have a hard time telling them apart.
We sometimes make mistakes in concluding that two groups are in different species, when in fact they do share genes through mating.
Sturnella
magna
The eastern and western meadowlarks (above, S. magna and S. neglecta, respectively) look similar, but their songs and other behaviors are sufficiently different that they would not interbreed, if they met in nature.

7. And mimicry in nature can confuse biologists even more
As an adult, the butterfly on the left, Monarch danaus, is similar to the viceroy butterfly on the right, Limenitis archippus.
But compare their appearance as larvae!

8. How do gene pools of similar species remain separate from each other?
A catastrophe (earthquake, fire, new highway, and so on) may separate one population in two smaller populations.
A famous example is the two populations of Kaibab squirrels, isolated geographically from each other by the Grand Canyon (perhaps 6 to 7 million years ago).
We do not know if these two populations have been isolated for enough generations that mutations would have accumulated that have made the two populations reproductively isolated from each other.
North rim
Kaibab
squirrel* 
South rim
Abert
squirrel **
* Sciurus aberti kaibabensis and
**Sciurus aberti
Geographical Isolation

9. Habitat Isolation
Two other ways to ask this question are as follows: What keeps different species from breeding with each other? How to they maintain their reproductive isolation?
Such barriers may occur before fertilization has a chance to occur, or after fertilization.
A first step toward reproductive isolation is often habitat isolation within the same area.
An example is two garter snake species in the genus Thamnophis. One species spends most of its time in water, while the other is primarily terrestrial.

10. An example of ecological isolation by different habitats
There exist 750 wasp species (!) each of whose members lay their eggs in the flower of a different fig species. These fig species share the same habitat but bloom at different times. This keeps the different wasp species reproductively separate from each other.
Note the long
ovipositor on the
fig wasp. Why is
this adaptive?
Fig wasp laying her eggs
amongst the ovules of a
developing fig fruit
(www3.imperial.ac.uk/)
Emerging new generation of
fig wasps (image from

11. Temporal isolation is another pre-zygotic reproductive barrier
Two species may breed at different times of the day, or in different seasons, or in different years.
For example, even though the territories of the western and eastern spotted skunks, Spilogale gracilis, and S. putorius, overlap, S. gracilis mates in the late summer while S. putorius reproduces in the late winter.
Spilogale gracilis
Image from
Spirogale putorius
Image from

12. Different courtship behaviors may isolate two species
Below, four similar looking beetles with very different courtship behaviors–
Posterior courtship
behavior in Linsleya
beetles 
 Mounting behavior in
Meloe beetles
Linear courtship
behavior in subfamily
Meloinae beetles 
 Anterior courtship
behavior in
Tegrodera beetles
Images from

13. Sexual selection by coloration can act as a reproductive barrier
Biologists at the University of Leiden in the Netherlands raised two cichlid species (Pundamilia pundamilia and P. nyererei) together in two different aquaria. One included natural light and the other monochromatic orange light.
Under natural light the two species looked very different from each other, but under orange light they were indistinguishable from each other.
Campbell’s Biology, 7th ed.

14. Cichlid experiment, concluded
RESULTS:
With natural light, in which the two species looked different from each other, the females mated only with males of their species.
However, under orange lighting the females mated with males of either species indiscriminately and produced fertile offspring.
INTERPRETATION AND CONCLUSION:
The species are very similar to each other, differing only in their coloration.
This color difference acts as a prezygotic reproductive barrier.
The two species very likely separated from each other only recently in their evolutionary past.
P. pundamilia
Cichlid images from
P. nyererei

15. Mechanical isolation may keep two species reproductively separate
In such a situation, even if mating is attempted, structural differences in their reproductive organs prevent copulation.
Both shells must
coil in the same
direction
A boa’s
hemipene
Four-headed
penis of the
spiny ant-
eater
And be sure to check out the amazing video of
the four-headed penis of the spiny anteater  at
2007/10/experimenting_with_a_fourheaded_penis.php
(scroll halfway down)

16. Coiling direction matters to a snail!
Shells of three closely related snail species (image a)
Two snails whose coiling directions are the same are able to mate (image b)
The genitalia of two snails with opposite coiling direction do not match up (see image c).

17. Gametic isolation
For example, sea urchins of more than one species might reproduce in the same place at the same time, but the sperm of species A might be incompatible with the ova of species B and vice versa.
Sperm of one species might not be able to survive in the reproductive tract of another species.
Or biochemical differences might prevent the sperm of one species from getting through the protective barrier of another species’ ova (in both plants and animals).
Thus, even though members of two such species copulated, fertilization would not occur.

18. Postzygotic barriers may exist between two “subspecies”
This is true of some of the salamander subspecies of the genus Ensatina in California.
For example, if cross fertilization does occur, the resulting hybrid offspring may not complete embryonic development or may be frail.
We describe this situation as hybrid inviability or reduced hybrid vigor. 

19. Reduced hybrid fertility
The well known example of reduced hybrid fertility is the mostly sterile mule (see image b, below), produced by a mating between a male donkey (a) and a female horse (c)*:
Another example comes from the work of a friend of mine, Sandra Anagnostakis. She crosses different chestnut species in an attempt to produce a tall, straight, blight-resistant tree with chestnut fruits. Her world-famous chestnut plantation is at the east end of the Sleeping Giant.
* But see the following: !
Even if hybrids between two species or subspecies (two emerging species) are survive and are healthy, they might not be fertile.
Chromosome incompatibilities might prevent pairing at Prophase I of meiosis and thus limit gamete production.
This keeps the two gene pools effectively separate from each other.
(a) (b) (c)

20. Hybrids might have low fertility due to chromosomal incompatibilities
For example, in a hybrid between a parental stock and a stock with translocations, the chromosomes will not match up well at Prophase I of meiosis.
Only some of the resulting gametes will have full sets of genetic information.

21. Hybrid breakdown: A final reproductive isolating mechanism
Some hybrids are robust and fertile and mate either with each other or with a parent (!), but the offspring of such matings may be either weak or infertile.
That minimizes the gene flow between the two species.
For example, some rice species follow this pattern. Healthy hybrids have been produced experimentally, in attempts by farmers to increase crop yield by combining genetic traits of both parents.
However, attempts to cross two different such hybrids often fail; the two rice strains that produced these hybrids are well on their way to becoming different species.

22. Explanation for hybrid breakdown in the case of rice crosses
Various strains of cultivated rice have been produced by artificial selection.
Over time they have diverged genetically from their common ancestor and have each accumulated different recessive mutations.
Crosses between such strains often bring together harmful mutant alleles, often resulting in weak or sterile hybrid offspring. * *
* “N” means normal, and “S” means sterile.
** The Rf allele restores fertility, but the rf allele does not.

23. Summary of the different mechanisms for reproductive isolation of species*
(or have different niches) *
* The terms “prezygotic” and “postzygotic” are sometimes used instead of
“premating” and “postmating”, respectively.

24. How do new species form?
In addition to distilling Darwin’s theory into a few succinct observations and inferences, Ernst Mayr has given us the two major conditions under which speciation can occur:
(1) Interbreeding between the populations of two emerging species must be blocked, by their becoming isolated from each other.
(2) Enough time must pass during this time of isolation for mutations to accumulate and for the two populations to become genetically distinct from each other. They must become sufficiently different from each other that if the two populations were brought together again, they would not be able to share their gene pools.
These differences might be caused by genetic drift or accumulation of different mutations (as we have discussed), or through different environmental pressures that result in natural selection within each of these now separated populations.

26. Biologists believe that most new species arise from allopatric
Biologists believe that most new species arise from allopatric* speciation
We will consider the following examples of allopatric speciation:
Adaptive radiation, resulting from small groups leaving a larger population and colonizing new environments
Geological and climactic changes separating members of a population
Human activities that split a population into separate subgroups
* literally, “other country”

27. “Adaptive radiation” is well known to occur in island populations
By this means, small subsets of a common ancestral population* leave at different times, each traveling in its own direction.
Upon arrival in a suitable habitat**, each such “founder” group will become established, responding to the unique environmental conditions of its new location.
* Often on the mainland
** often an island

28. The Hawaiian islands provide many such examples of rapidly evolving populations
The five silversword species below are all very closely related, based on molecular studies, yet look quite different from each other.
Most Hawaiian species are endemic; they can be found nowhere else.
freight_analysis/state_info/hawaii
The Hawaiian islands are located ~3,500 km (about 2200 miles) from the nearest continent.
Very young, these islands arose volcanically; the youngest (“Big Island”) appeared only about a million years ago and still demonstrates active eruptions.
Campbell’s Biology, 7th ed.

29. Dams built by beavers or humans create ponds and lakes and separate populations
Studies of striped bass (Morone saxatilis) were carried out by Irwin Beitch 10 years after the completion of a large dam in southeastern Virginia.
Kidney morphologies of populations above (freshwater) and below (near the Chesapeake Bay) the dam had become significantly different from each other.

30. Check out the very cool animation at
Land masses that were connected during the Triassic Period later separated, taking life forms with them.
“Gondwana”
Check out the very cool animation at

31. Ratite birds– an example of early evolution after Gondwana broke apart
Cassowary
( )
Rhea
geographic.com
Emu
The above map shows the distribution of living and recently extinct* ratite birds. Key: C = Cassowary; E = Emu; e = Elephant Bird*; K = Kiwi; m = Moa*; O = Ostrich; R = Rhea (see
Kiwi

32. Alternative models of speciation
* In such a situation,
however, there might
exist subtle micro-
habitat differences
within the same
space. *

33. Ecological isolation within the same space ( sympatric speciation)
One of many examples of how microhabitat differences might provide environmental pressures for genetically different members of a species to specialize in different ways: palm trees in a habitat that contains both volcanic soil (high in ash) and calcareous soil (of coral reef origin)

34. (www.sciencecases.org/maggot_fly) 
Another example of microhabitat differences leading to sympatric speciation
The maggot fly Rhagoletis pomonella has a stable parasitic relationship with the American hawthorn tree, in which the fly lays its eggs.
In the seventeenth century apple trees were introduced to North America by the early colonists.
Over the years scientists began to observe the fly infecting apple trees, as well as hawthorns.
The two fruit fly populations tend not to intermingle, even if in the same habitat, and thus are unlikely to interbreed. When offspring become adults, they tend to lay their eggs in the same kind of fruit from which they had hatched (perhaps based on a type of olfactory imprinting).
Over time these two populations have become genetically different from each other and may be on their way to becoming separate species.
♂ ♀
Apple maggot flies
( 

35. Sudden speciation by polyploidy (happens mostly in higher plants)
Sometimes DNA replication occurs normally (during the S) phase in (especially in plants), but problems arise during meiosis.
If chromosomes failed to separate from their homologs, then diploid (2n) reproductive cells could result.
Self fertilization of such diploid gametes would then produce tetraploid (4n) offspring.
This occurs surprisingly often in plants; it is estimated that 25 to 50% of all plant species evolved through polyploidy.

36. Speciation by polyploidy

37. Polyploidy can occur during mitotic cell division

38. Other common examples of polyploid plant species
* Triploid crops: banana, apple, ginger
* Tetraploid crops: durum wheat, macaroni wheat, maize, cotton, potato, cabbage, leek, tobacco, peanut
* Hexaploid crops: chrysanthemum, bread wheat, oat
* Octaploid crops: strawberry, dahlia, pansies, sugar cane
CHALLENGE: Why are polyploids with odd numbers of sets of chromosomes (e.g., triploid species) sterile? And bananas are sterile, so how do we get new banana plants?

39. Why triploid species are sterile