InsectNet forum

The question was asked of me why the Monarch only uses four legs. I searched, without finding an answer.

98% of butterflies must find six legs to be useful (I picked 98% I don't know the actual ratio.)

Considering natural selection, one would think that if most butterflies were more successful with six legs, how could some succeed and even over time differentiate into many species that have only four legs? If four legs is good enough, why would most bother with six?

Much has been made about natural selection and often it's cited for why some animal is the way it is. Sometimes I'm suspicious of this; I wonder how many features (including four legs) aren't from selection but from an anomaly, maybe even a defect, that is largely immaterial yet gets passed down. If this is the case, what else is immaterial? The blue of morphos?

Some traits are neutral and are not selected for or against. They persist. Nearly all Nymphalidae have reduced front legs. Humans have vestigial organs too. Think about the appendix.

Recent research has indicated the appendix does have a function
https://www.frontiersin.org/articles/10 ... 24980/full

Some theorizing on four legs

https://blogs.scientificamerican.com/la ... -six-legs/

More technical
https://www.researchgate.net/publicatio ... utterflies

Sure, gut microbiome can change due to appendectomy, but there has never been conclusive demonstration that this leads to anything clinically significant. It’s not like people without an appendix are less likely to procreate. Similarly, people with an appendix are not more likely to procreate. Thus, no selection pressure either way.

With regard to the legs, it’s not surprising that sensory function is involved. In nymphalid species with reduced first pair of legs, the males invariably have hairier legs than females. A common ancestor lost the walking function of the first pair of legs but evidently this trait was not critical enough to impact selection on a large scale. Loss of leg function didn’t lead to extinction and gain of sensory function didn’t lead to this trait taking over in all butterflies. This is what I mean by “neutral.” The trait survived and was passed down because if you balance the pros and cons, it comes out to more or less neutral so the trait persists without dying out or exploding.

Mike thanks for finding that paper on Researchgate. I suppose it shouldn't be a surprise that somebody thought to investigate.

I do wonder what other features are "neutral." Color of Ornithoptera priamus/urvilleanus/croesus I suspect would be one, just demonstrated by the different colors. To go further, do the various geographically isolated, morphologically different ssp of Ornithoptera victoriae indicate evolution to be beneficial in a specific environment? I think not; just morphological changes that happened over time and are "neutral" meaning immaterial.