Since this was aimed at me by name, I may as well make a reply. Please note, though, that a fair proportion of this will be copy-pasted from elsewhere; other threads where I have given Chuz the same information and have been ignored for it. I can't see this thread changing all that much, but...
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First, I would say that the question "Is a zygote an organism" is not even a scientific one. The question deals almost exclusively with semantics - it shows how people use the word, not whether the word is applied appropriately or not. However, as arguments from authority go there is legitimacy to be had from a scientific consensus, so we shall push ahead with the term itself.
Based purely on dictionary use - arguably the most legitimate of authoritative sources, a zygote is not an organism. This can be shown from the following chain (bold always added by me):
Organism
An
individual form of life, such as a plant, animal, bacterium, protist, or fungus; a body made up of organs, organelles, or other parts that work together to carry on the various processes of life.
Individual.
5. Biology.
a. a single organism
capable of independent existence.
Viability
2 Physiology.
a. physically fitted to live.
b. (of a fetus) having reached such a stage of development as to be
capable of living, under normal conditions, outside the uterus.
In short, pre-viability a foetus cannot live outside the womb, which is a requirement for it being biologically 'individual', which is needed for it to be an organism. Further evidence to back up this line of reasoning comes from the placenta later on in pregnancy...
Placenta
The vascular organ in mammals except monotremes and marsupials that
unites the fetus to the maternal uterus.
Unite
1. to join, combine, or incorporate so as to form a
single whole or unit.
...and the frequent phenomena of microchimerism, which shows that DNA is largely irrelevant to this issue (even ignoring twinning issues, which I might come back to later) and that the mother/foetus often exchange cells:
Microchimerism
the most common form is ... whereby immune cells ... from a fetus pass through the placenta and establish cell lineages within the mother. Fetal cells have been documented to persist and multiply in the mother for several decades [1] After giving birth,
about 50-75 % of women carry fetal immune cell lines. Maternal immune cells are also found in the offspring ... though this phenomenon is about half as frequent as the former .[2]
Judging by these references, a zygote is at best equivalent to an organ, at worst equivalent to a blood cell. The fact that it has different DNA is irrelevant - a transplanted heart would have this just as much, and many of the blood cells in your body have
no DNA in them (hence my use of them as an example). The only thing which makes a zygote significantly different is the fact that it will one day develop
into an organism if the right conditions are met - but then, so will a sperm cell (again, if the right conditions are met) and this also does not make the zygote an 'individual', just different from the other parts of the mother. In fact, that a zygote is defined this way...
Zygote
1. The cell formed by the union of two gametes, especially a fertilized ovum before cleavage.
2. The organism that develops from a zygote.
... broadly : the developing individual produced from such a cell
...implies that the single cell itself is not an organism, nor an individual.
Even more backup for this line of reasoning (zygote =/= individual) comes from a consideration of reproduction:
Reproduction
Biology. the natural process among organisms
by which new individuals are generated and the species perpetuated.
Reproductive cycle
The cycle of physiological changes that begins with conception and
extends through gestation and parturition.
If reproduction generates new individuals, and only ends at parturition (birth) then a zygote is definitely ruled out.
Moving on from this line... many of Chuz's sources disagree with the dictionary use shown above. However, the vast majority of them only show scientific
opinions, not the consensus which is needed to get a definition into a widely used dictionary as I have been using. The problem with relying on individual people/sites is that to argue based on such sources naturally biases the argument towards false positives - it's far more likely that someone will consider a zygote to
be an organism and use that on their website than for someone to consider a zygote
not to be an organism and post it - after all, a zygote is not a great many things, why should 'not an organism' be specifically listed unless an agenda is present, making for an easy dismissal of the source? Most of the places to be found where a scientist states that a zygote is 'not an organism' are when they have been directly asked the question - as such, I present two such cases, just to show that not all scientists agree with Chuz's sources.
"a zygote is not an organism.
...
I don't know any biologist who would classify a single cell from a Homo sapiens as a Homo sapiens. Even a zygote, which may have the *potential* to become a Homo sapiens, but is not an organism by any stretch of the imagination, is not considered an individual Homo sapiens by any members of the scientific community that I know.": Dr Krempels PhD,
allexperts.com (
x2)
"
a zygote is not an organism": Dr Wolpe PhD,
email exchange. [NB: This exchange was with Chuz Life himself, who emailed Dr Wolpe in a manner similar to his university emails in this thread following a debate with me on another forum. Dr Wolpe's disagreement with Chuz appeared to have little effect on Chuz's stance, though he did post the reply]
...and just to be complete, an expert agreeing with my opening statement.
"[Whether a zygote is an organism or not] isn't a topic of concern in the scientific community. In fact, it isn't even really science! Science is testable and falsifiable; my view is neither. This does not make it a religious view, however; consider the fact that neither is your position in this argument! It is philosophy and semantics; taxonomy with some deep thoughts, if you will!": Dr Kalstrom,
allexperts.com.
...I think that's probably enough for now.