In
another diary I began examining challenges to evolution. While it is true that some of the challenges have been posed by creationists, it would be the very definition of ad hominem to suggest that just because creationists pose them, they should not be discussed. As I endeavored to make clear, discussing challenges to evolution should be part of open debate, and not construed as some sort of evil agenda to undermine evolution. If evolution should fail to meet the challenges, then whatever explanation replaces it, if it is scientific, certainly will NOT be creationism.
www.sullivan-county.com :
So even if evolutionary theory turns out to be completely wrong and the whole thing was a big mistake, that does not mean that, ergo, creationism is right.
To be scientific, a hypothesis needs to be a testable, repeatable,
falsifiable, measurable, and observable. The only data accepted by science is that which can be collected by the five senses. A hypothesis may become a theory as it produces predictions that "come true." Even after a hypothesis, or a group of hypotheses become a theory, the theory may be destroyed by just ONE creatively designed experiment. One way to design these experiments is to test the "null" hypothesis.
Debunking of the Theory of Spontaneous Generation (Skip if you are familiar with this)
The Theory of Spontaneous Generation was destroyed by a "null" hypothesis experiment. The theory held that nonliving things could produce living things. Rags could produce mice; dead meat could produce maggots. Innumerable observations over countless years had confirmed the theory again and again. In every trial, dead meat produced worms. Then Fransciso Redi designed an experiment to test the null hypothesis, in this case, nonliving things could NOT produce living things.
His results can be reproduced by setting out a sealed jar with meat, a cheesecloth-covered jar with meat, and an open jar with meat. The sealed jar will have no maggots, the cheesecloth-covered jar will have maggots on top of the cheesecloth, and the open jar will have maggots crawling around on the meat. Clearly the meat did not produce the maggots. When a theory fails a challenge, it must be discarded or refined. Sometimes this means reevaluating and reinterpreting previous data and experiments.
Speciation a Foundational Concept
Since evolution is an explanation for the origin of species, a challenge to speciation challenges evolution at its very core. In my last diary, I first summarized definitions of evolution and speciation. Two widely accepted definitions of evolution are www.dailykos.com :
[1.] When biologists say that they have observed evolution, they mean that they have detected a change in the frequency of genes in a population...[2.] When biologists say that humans and chimps have evolved from a common ancestor they mean that there have been successive heritable changes in the two separated populations since they became isolated.
If the first definition (change in frequency of genes in a population) is used, then of course, evolution has been well observed and documented. But if evolution is the explanation for the diversity of species, then the second definition, which posits separation of a genetic line, is more appropriate. So any definition of speciation must include reproductive isolation. Now the question comes down to the degree of isolation. There are basically two types: prezygotic and post zygotic. Prezygotic prevents formation of the zygotes either because the organisms occupy different habitats, breed at different times, have different mating behaviors, or biologically cannot produce a viable zygote. Post zygotic prevents the line from continuing through natural abortion, sterile offspring or other reproductive weakness. Strict interpretation requires that members of different species CANNOT produce fertile offspring, not that they simply do not.
The problem with trying to observe speciation is that it usually does not occur within the lifetime of a scientist, but since bacteria have such short generations, sometimes as short as 20 minutes, there is potential for actually observing speciation. The problem is that strict bacterial speciation has not occurred. The supposedly divergent species are not reproductively isolated and have been observed reproducing by a sexual mechanism known as "conjugation." So some scientists have proposed another definition of species which basically means nothing more than "genetically different."
There are now two definitions of speciation, a biological one and a phylogentic one. www.dailykos.com :
...the two concepts only differ where there are groups of organisms that can reproduce with each other and produce healthy, fertile offspring but have distinctly different derived traits. When this occurs, the biological concept lumps these organisms into the same species, and the phylogenetic concept splits them into separate species
To recap, evolution that produces widely divergent species means "that there have been successive heritable changes in the two separated populations since they became [reproductively] isolated." Therefore it is fair to employ a rigorous definition of speciation which requires the actual inability of members of different species to reproduce. In other words, phylogenetic differences are a necessary, but insufficient, feature of speciation. To illustrate, a pesticide-resistant cockroach, while certainly genetically different from a non-resistant one, CAN and will successfully breed with the non-resistant one. Thus it is not a separate species. Same with the super-bug bacteria. Many cited examples of speciation fail the rigorous definition and fail to receive taxonomic recognition.
What about Those Fascinating Ring Species (about time)
Sometimes an organism encounters a geographic barrier, like a mountain range. Some individuals extend their range around one side, while other individuals extend around the other side, and eventually meet at the other end of the barrier. Over time, the separated population sometimes develop genetic differences and do not breed when the populations meet. Of the 20-something known cases, only three are considered to have enough merit to evaluate for genuine speciation.
1.Artic gulls
www.geology.ucdavis.edu :
The classic example of ring species is provided by Arctic gulls (Larus species). These birds form a set of populations which live around the Arctic Ocean. In Western Europe the lesser black-backed gull (L. fuscus) is a familiar species whose range extends east into the Soviet Arctic, through populations that are interbreeding but which can be arranged into four subspecies, each slightly different. The easternmost subspecies interbreeds with populations of the so-called Siberian skua. In turn, the Siberian skua interbreeds with populations farther east, through five more subspecies. The easternmost subspecies is so far east that it ranges into Western Europe as the herring gull, where it exists alongside the lesser black-backed gull. But the herring gull does not interbreed with the lesser black-backed gull in Western Europe, and no amateur ornithologist there would ever place these two in the same species. In fact the herring gull is called Larus argentatus. But there is gene flow round the 20,000 km of the Arctic ring that forms the range of these gulls. The interbreeding is so free, and the color patterns of the plumage and the behavior is so smoothly transitional between these successive sets of populations that it is clear they all belong to the same species, even though the end members are distinct.
In other words, these Artic gulls are the same species for much the same reasons that wolves and chihuahuas are considered the same species.
www.homestead.com :
The official change from Canis familiaris to Canis lupis familiaris was formalized in 1993 by the publication of Mammal Species of the World: a Taxonomic and Geographical reference, edited by D.E .Wilson and D.E.A Reeder. Published by the Smithsonian Institute in association with the American Society of Mammalogists, this reference book is the final authority of the scientific community on mammal classification.
2.Ensatina Salamanders
The Ensatina escholtzii salamanders form a north-to-south ring around California's Central Valley. Their appearance has changed probably due to founder effect so that the southernmost salamanders no longer resemble each other. The two southernmost groups, klauberi and eschscholtzi do not interbreed. All around the ring are various intermediate hybrids that breed more often. The more geographically separated, the less likely intermediate hybrids will interbreed. David Wake, the most prominent researcher, hesitates to recommend a separate species classification for the two salamanders, and concludes
While the complex appears to be in a state of incipient species formation, which makes taxonomy problematic, it provides an instructive evolutionary example.
One problem with classification is that the genetic differences within a single population are greater than the genetic difference between populations. www.santarosa.edu :
genetic distances among populations of either class exceed those measured between the classes where they are sympatric in southern California.
The second problem is that,
www.santarosa.edu :
There is evidence of renewed genetic interactions upon recontact, with greater genetic differentiation within xanthoptica than between it and some of the interacting populations.
The question remains, will the salamanders overcome their reluctance and resume interbreeding? For the two southernmost groups to be considered to be separate species, it must be shown that they CANNOT successfully interbreed. Therefore,
www.santarosa.edu :
for the present we recommend continued recognition of the Ensatina complex as a single taxonomic species.
3.The Greenish Warbler
Another species separated by a mountain range, the green warbler started in central Siberia and extended their range around the Himalayas. Where they meet, they do not interbreed. The most prominent researcher, Darren Irwin, thinks that as the birds extended their range their birdsong changed, and now the birds no longer respond. www.zoology.ubc.ca :
It is relatively easy to hear the song differences in the field, and playback experiments have shown that the birds distinguish between types of songs. A male greenish warbler will aggressively respond to songs that it recognizes as belonging to its own species, intending to chase an intruding male out of its territory, but it usually will not respond to song from a distantly related population. Females probably also use the song differences to distinguish among potential mates.
The greenish warbler is the best candidate for demonstration of both speciation and an alternative mechanism to natural selection called
genetic drift
Questions still remain. Will the birds overcome their song aversion now that they share habitat? Would artificial insemination produce fertile offspring? The classification scheme becomes ever more detailed: Kingdom, Phylum, Class, Order, Family, Genus, Species. Most common names are at the Genus level (dog, cat) or above (lizard). Is speciation at the Species level sufficient to demonstrate the wider divergence necessary to account for widely separated life forms? Speciation is observably extremely fluid at the species level. Is there a genetic barrier between the Species and Genus levels?