My education was in biology rather than anthropology, so I can't speak as an expert. I have, however, done a great deal of reading in anthro/archeo/history over the last 20 years or so. The term "ethnic" just refers to race. Since a race has a language, it would therefore fall under ethnic concerns. Many different scientific specializations are used to study different groups of people through their time on this planet. Aside from linguistics, there are anthropological and archeological artifacts, as well as the recently popular DNA studies. They usually agree with each other, but linguistic evidence is often read differently by many experts, which results in a lot of arguments with not only anthropologists, archeologists and geneticists, but even with other linguists. As soon as a group of people splits, such as the Slavs and the Celts, their languages start to change, as well as their tools, beliefs, foods, homes, and chromosomes. As far as language is concerned, they add new words for new things they come across, for new items in the landscape, for new weather conditions, etc. They also add new words by adopting some from other cultures they encounter. Much less often, they even change their sentence structure. In fact, all the aspects mentioned three sentences ago can be changed by encountering new environmental conditions and objects or a new people.

Linguistic studies of ancient groups of people have many advantages (and supporters) and disadvantages (and antagonists). One of the main disadvantages is that, of course, you can't carbon-date the spoken word. Linguistics have been used successfully in many places. Nobody seemed to have any idea where the earliest Sumerians came from till it was mentioned that their language showed the most similarity to Caucasian (from the Caucasus Mtns). The early Etruscans claim to have migrated there from Lydia while the early Romans claim to have migrated from Troy. In neither case does their language resemble the language in the land they claim to have come from, so linguists propose that they were simply claiming to come from lands which had more stature at the time of their birth for the sake of pride. There is some supportive evidence for the Etruscan's claim, but it is much weaker than the major differences in the languages. One author, using linguistic evidence, made observations based on the people of ancient New Guinea. These were later almost exactly born out in the carbon-14 dating of the finds in that region. When the same author (I can't remember her name) used linguistic studies to place the peopling of the New World at older than 30,000 years, nobody accepted her hypothesis. It went against most people's pre-set ideas about when the New World was peopled, as well as all the evidence of the people at a later date. The unfortunate thing about that evidence is that it only proved that people were there at that later time, not that they weren't there at an earlier time. The evidence until recently showed that people were here as far back as the closing of the Aleutian passage by ice (about 13,000 years ago). It has now been proven that there were people here much earlier (archeological evidence going back as far as 50,000 years). At Topper in South Carolina, when they reached the level where they expected evidence to stop, they stopped digging. When Monte Verde and other sites proved there was earlier habitation, they went back to the same site, dug deeper and found that this site continued back to (I think) 18 or 19,000 years ago. As I said, I am not an anthropologist, so I can only make observations on what the true experts say. When one of those aspects (language, archeology, anthropology, or genetics) argues with the others, I reserve the right to form my own opinion which is closer to the truth. In other words, I'm not specifically a linguist, and I would say Slavs are both an ethnic group AND a linguistic group - the latter falls within the former. I believe their early chromosomes were as a subgroup within the Indo-Europeans, or the closely related Linear Pottery Culture.