SOC. SCIENCE : SPECIATION

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Speciation is the process whereby new species are formed. The following events are

thought to occur in most cases. In the first step, extrinsic isolation, an existing species becomes

subdivided by some extrinsic event, such as a climatic change, the formation of a physical barrier

(such as a mountain range), or its invasion of a new habitat or island. Subdivision may also occur

merely because many hundreds of generations may be required for individuals to disperse from

one end of the species' geographic range to another. In the second step, differentiation, the

isolated populations diverge genetically, which they can do more rapidly than populations

exchanging individuals with other populations. Populations may diverge either at random or as a

result of natural selection. In the third step, intrinsic isolation, some form of isolation evolves

among the populations; this is dependent on the organisms rather than the environment. Such

isolation may result from preferences during courtship or from genetic incompatibility, in which

offspring of matings between the differentiated populations are no longer viable or fertile; the

mule is an example. In the final step, independence, the newly separated populations continue to

evolve independently and may subsequently invade each other's geographic ranges without

hybridization. Each of these steps has been demonstrated in the field and laboratory with various

organisms.

Two major modes of speciation are theoretically possible: geographic and nongeographic. In

geographic speciation, initial isolation results from geographic separation of the populations. In

nongeographic speciation, initial isolation results from changes in behavior or genetics of part of

a local population. For example, many insects will eat only one species of plant and will use this

plant's shape, color, or odor as cues for location of mates and egg laying. If a group of these

insects accidentally invades a new plant species and mates there, then it is as isolated as if it were

far away. A great deal of controversy exists about the relative frequency of various modes of

speciation, but the geographic mode is generally considered more common.

The biological-species definition is not infallible. A few borderline cases will always exist for

which species identification is arbitrary. This arises because species are not static entities; they

can change in time and sometimes give rise to new species. The intermediate stages in speciation

cause the most difficulty in classification and identification, just as, during cell division, whether

one cell or two exist is a matter of opinion. The nonexistence of borderline cases would mean

only that evolution had run its course and was no longer taking place.

Ways new species originate

Speciation, in sexual organisms anyway, occurs when two genetic groups no longer

intersperse by interbreeding. This can happen in the following ways.

1. Instantaneous speciation. This tends to occur mainly in plants through hybridization or a

duplication of the number of chromosomes at conception. Hybrids will have asymmetric

chromosomes to begin with but a process of doubling and lack of secondary reduction through

meiosis (or the formation of haploid, or half complement of chromosome-containing, cells). So,

when you get a hybrid, the chromosomes of the two parent forms don't match up, but if you then

double the chromosomes and don't halve them again, you can pair them up. In the case of

duplication of one species' chromosomes, you end up with a 2n (or 3n or 4n...) set of already

paired chromosomes.

Sometimes these polyploids, as they are called (many-numbered), are able to reproduce by

selfing, or the polyploid is a common enough event that there are two or more individuals that

can cross, and away you go. Secondary selection eliminates genes that are less fit.

2. Allopatric speciation. A bit of terminology to begin with - "patris" means "country" or

"homeland". It comes in flavours ranging from "sym" (together) through "peri" (next to) to "allo"

(separate). So allopatric speciation is "speciation that happens when the populations are isolated

geographically". We'll encounter some other patrises later.

In allopatric speciation, a species' range is divided, say, by a river or mountain range or desert or

currents in the ocean, etc. Once this happens, they are adapting to novel conditions and also a

process of more or less random (stochastic) processes of sampling the genetic variants leads to a

population that is rather different from the parental one. The stochastic process is called "random

genetic drift". Although reproductive infertility or isolation is not something that selection "aims

for", it is often a byproduct of changes made to the developmental cycle of the isolate

population. So, when the two get back in sympatry, they either can't interbreed (are isolated) or

they can but the hybrids are not as fit as either of the parental variants (lowered hybrid fitness),

and so they are then subjected to "reinforcing selection" to maintain isolation. [Of course they

may just end up going extinct as well, due to lowered fitness.]

3. Peripatric speciation. In this case the isolated population is not entirely genetically isolated,

but because it is on the periphery of the main population, a local population (caled a "deme")

may have unusual genetic variants (called alleles") that get established for a mix of selective and

drift reasons to the point where hybrids between them and the main population are less fit,

causing reinforcing selection. This occurs because the rate of interdeme crossing is lower than

the rate of intrademe crossing, and so it is able under certain conditions to form novel genotypes

and developmental sequences. [Technical note: when the rate of migration between populations

is less than 50%, it is parapatry. When the geographical isolation is less than 100% it is peripatry.

So para- and peri-patry can be the state of the same population. This is a nusiance piece of

confusing terminology.]

4. Sympatric speciation. This is controversial (and was Darwin's preferred view). In this view, a

variant form reaches a new "adaptive peak", and reinforcing selection selects against hybrids

with the prior form. This is thought to happen in a couple of ways. One is through the evolution

of novel mating systems, such as calls (the case of the electric signalling fishes, for example; the

classical case is the mating calls of Rana pipiens). Another way is through the adaptation to a

new host, such as when Rhagoletis fruitflies started breeding on apple trees in California, which

flower at a different time of year, causing selection to isolate the older hawthorn-breeding

developmental cycle from the newer apple-breeding developmental cycle. This is sometimes

called "host-race" speciation.

5. Statsipatric speciation (in-place). This is like the instantaneous case above, although the

chromosomal variants are able to interbreed with the original chromosome count individuals.

Selection takes the form of inviable developmental cycles.

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Speciation is the process whereby new species are formed. The following events are thought to occur in most cases. In the first step, extrinsic isolation, an existing species becomes subdivided by some extrinsic event, such as a climatic change, the formation of a physical barrier (such as a mountain range), or its invasion of a new habitat or island. Subdivision may also occur merely because many hundreds of generations may be required for individuals to disperse from one end of the species' geographic range to another. In the second step, differentiation, the isolated populations diverge genetically, which they can do more rapidly than populations exchanging individuals with other populations. Populations may diverge either at random or as a result of natural selection. In the third step, intrinsic isolation, some form of isolation evolves among the populations; this is dependent on the organisms rather than the environment. Such isolation may result from preferences during courtship or from genetic incompatibility, in which offspring of matings between the differentiated populations are no longer viable or fertile; the mule is an example. In the final step, independence, the newly separated populations continue to evolve independently and may subsequently invade each other's geographic ranges without hybridization. Each of these steps has been demonstrated in the field and laboratory with various organisms. Two major modes of speciation are theoretically possible ...
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