Tuesday, July 21, 2015

The Hairy Problem of Defining Species, with Hoary Vervain -- Verbena stricta -- Verveine veloutée

Yesterday, I posted about Verbena hastata, which is native to this region (Montreal area). Today, I post about Verbena stricta, which is native in Ontario but actually introduced here in Quebec. US range map here, Canada range map here. Now, it is possible that Verbena stricta is native to parts of Quebec and introduced to others. Unfortunately, I can't find any resources about it so I only know that it is listed as introduced in Quebec [1]. It is native to much of North America [2], and is considered as weedy/invasive in parts of the US. As with Verbena hastata, this designation within the native range of a species indicates that it is a strong competitor (eg an aggressive spreader) and under some circumstances can end up squeezing out other native plants. And, as demonstrated by its presence in Quebec where it was not originally native, the weedy classification does indicate a capacity for invading new habitats as well.

Verbena stricta inflorescences
Though this and yesterday's plant are both vervains, they are not the same. Verbena stricta has considerably larger flowers and inflorescences, and has a lot of trichomes [3,4,5] (fine hairs on the surface of the plant).

Verbena stricta full plant
Trichomes are an interesting anatomical feature of many plants. They can serve many different functions: they can deter herbivory (trichomes of this variety will often be sharp and stiff, or will deliver a painful irritant to the unwary brusher-by); they can protect against frost; they can reflect excessive sunlight; they can reduce evaboration; and they can even enhance fog drip in order to improve water collection.

Verbena stricta inflorescence
One issue which complicates matters a bit when trying to distinguish species of Verbena is that many members of this genus can readily hybridize [3,6].

This statement may immediately twig some concern in the minds in those who have taken an intro to biology course. After all, aren't species largely defined by reproductive isolation (the inability to produce offspring by crossing two populations)? Well, sort of. Defining species is a complex issue.

To an extent, the definition of species (by which I mean the point of genetic relatedness beyond which we identify groups of organisms as "same"), is a matter of pragmatism and judgement. This doesn't mean that we lack any standards for determining whether populations are members of a single species, but it does mean that exactly what judgement we make will be influenced by context. Depending on the purpose, we might make it a bit more stringent in some way, eg produce viable offspring and be morphologically/genetically similar to a given degree (a common standard with plants).

So where some might see a single species [eg], others might see a species complex [eg] (a group of closely interrelated species which are so similar as to sometimes make it difficult to distinguish between them). Depending on the purposes and interests of the individuals involved, both of these interpretations can be valid.

Verbena stricta inflorescence
So am I actually saying that depending on the context, Verbena stricta could be either a single species or a species complex (or even part of the broader group "Verbena")? Yes, I am. This kind of fuzzy imperfection of definition stems from our practical need to categorize the inter-relatedness of life, which is functionally much more continuous than categorical. People don't like it because it's messy, but that messiness is a product of the attempt to categorize a continuum. The lines we draw will always be in some sense arbitrary. That doesn't mean there's no value in categorizing, as long as we're clear about what we're doing and how we're making the call.

In many aspects of biology, we use the standard of intercompatibility (the ability to produce viable offspring) to define the species because reproduction is central to relatedness (genetic/functional relatedness is a pillar of many avenues of scientific inquiry), so it is a place to draw the line which has a lot of practical applications. But there are also other places to draw the line that are useful or informative in a variety of applications.

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