The story of a butterfly that explains how species evolve.
People often wonder or rather doubt how new species are formed. Some people doubt the whole phenomenon of evolution because it's "a theory of evolution". However, in reality, evolution happens and we can find evidence of it in nature. Here, for instance, a species of butterfly in Amazon region was found to have evolved from the hybridization of two other species of butterflies. Interestingly, the descendants of all these three lineages of butterflies coexist in same geographical region and two of these occasionally mate with each other.
Images are taken from Wikipedia and www.ucl.ac.uk/taxome/neil_rosser/
If a species is still mating with another species, can they still be called as two distinct species. Well yes, these two species look very different from each other and you and me could distinguish between them.
The species which is the subject of this post, the Heliconius elevatus, has red, black and yellow pattern and the closest parent species, the Heliconius pardalinus, has a pattern similar to butterflies with tiger type pattern. H. elevatus has inherited the colouring pattern from its another parent species, the Heliconius melpomene.
The researchers compared genomes from several individuals of the three species and constructed a phylogenetic tree. A phylogenetic tree is a tree shaped diagram. Each species would be the tip of the branches and closely related species will be joined to a close-by common node.
The H. pardalinus and H. elevatus were found to be very closely related. Specifically, the individuals of these two species that occupied same geographical niches , the Amazon region, were very close in evolutionary terms. About 99% of the genome of H. elevatus was found to be similar to H. pardalinus.
It was inferred that about 180,000 years ago, the lineage of H. elevatus separated from its parent H. pardalinus, after hybridization with H. melpomene. Now, after thousands of years, the qualities acquired from this hybridization are still expressed in this new species and makes it stand distinct from its more closely related parent.
H. pardalinus and H. elevatus individuals that share the same geographical region do sometimes mate and produce viable offsprings. Although there is no report of them mating in captive conditions. The DNA of H. elevatus offers evidence of recent hybridization with it parent. Also, forced mating in lab confirms that they can produce fertile progeny.
In contrast to the above, the DNA composition tells us that H. elevatus individuals almost never mate with individuals of their other parent, the H. melpomene species, even though their colour pattern is very similar. It is plausible, that the pheromones secreted by H. elevatus are very different from than that of H. melpomene. Also, there is something called as "female-limited hybrid sterility" which means that the hybrids cannot produce progeny.
To summarize, the evolution of H. elevatus goes like this:
Long time ago, about 180 thousand years ago, the common ancestor of both H. elevatus and H. pardalinus mated with the ancestor of H. melpomene. The hybridization may have occurred several times and the hybrids gave rise to a new lineage that 720 thousand generations after would form today's H. elevatus. During this long time, individuals of the new lineage occasionaly mated with their cousins from the original common ancestor who slowly spread their DNA into the new lineage. Yet after this long time the individuals of H. elevatus maintain their distinct lineage and properties, still sharing the same space and environment with its two distant cousins, the H. pardalinus and the H. melpomene who went their separate ways thousand of years ago.
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