"Country" is to "compatriot" as "species" is to what?

I was about to write the following sentence:

I actually have some young seedlings of this species. It will be interesting to compare them to their older ...?

Is there a word for something that belongs to the same species? I am thinking something like "fellow X" but I don't want to name X, and I'm not sure just "fellow" will work.

Solution 1:

The word is conspecific. It is used both as an adjective and a noun*; and you can use as a noun in your example.

Two or more individual organisms, populations, or taxa are conspecific if they belong to the same species.

_{[Wikipedia / Biological Specificity]}

Etymology of the word:

1859, from conspecies (1837), from con- "with" + specific, here representing species (n.). From 1962 as a noun.

_[Etymonline]

Solution 2:

I would use the word specimen.

Google defines specimen as:

an individual animal, plant, piece of a mineral, etc., used as an example of its species or type for scientific study or display.

Your example would be:

I actually have some young seedlings of this species. It will be interesting to compare them to the older specimens.

Solution 3:

Your sentence looks like it needs a less technical word than some of the other answers. I would go with this:

I actually have some young seedlings of this species. It will be interesting to compare them to their older cousins.

of course cousins refers to close family members (but not as close as brothers and sisters) so here it is used as a metaphor. But it is an extremely common metaphor.

The disadvantage is that it is vague and only tells us that the other plants are closely related, not that they of the same species. But you already told us that they are of the same species in the previous sentence. Nature documentaries frequently use the word cousin in this (rather literary) way to refer to organisms of the same species, the same genus, or even the same phylum when it is clear from the context what is meant.