Question for you biologists...

[crawfish]

I'm having a conversation on evolution on another board where this came up:

Is it against any scientific law you know for one fossil to show attributes of two different species without the need for common ancestry? I mentioned above the scientific concept of “homoplasty” where different organisms share common environmental conditions and for this reason have similar anatomies.

Essentially, the poster claims that this effect negates the need for common descent & gene replication as an explanation for the similarity of gene sequences and the adding of genetic information during microevolutionary changes.

Not knowing a lot about the subject, and not finding a great deal of information about it online, what do the intelligent posters here think of the idea?

 

[sbvera13]

Look up convergent evolution. This is where similar selective pressure (environment) produce similar physical structures. It happens from time to time. Think of bat wings vs. bird wings. They are similar, and work in a similar way, but they are still fundamentally different, reflecting their different history and different source.

Even convergent structures will have physical and/or genetic differences to show their lineage. Common descent still applies.

 

[Vene]

Oh sure, use flattery

It sounds like the poster has misunderstood what homology means. Homologous structures can serve very different purposes or similar purposes. For this post, I'm going to talk about bones (because they are easy). The bones in the human arm are the humerus, radius and ulna (I'm ignoring the hand and wrist). Here is what the bones actually look like (link):

The top bone is the humerus, the right forearm bone is the radius and the left forearm is the ulna. Each bone has specific sites were tendons attach to it and where ligaments attach. This makes it possible to identify homologous bones on different organisms. Like the seal (link):

Again, the humerus is furthest from the flipper (which also looks amazingly like a human hand). The radius is the topmost forelimb bone and the ulna is the bottommost forelimb bone.

Humans and seals live in very different environment, and yet we share the same skeletal structure and arrangement. Curious, isn't it? I think that poster has homology confused with convergent evolution. Homologous structures can be found in separate organisms, even if they have very different lifestyles. Convergent evolution is when two structures are superficially similar (because they serve the same purpose). But, depending on how far away their common ancestor was, the more or less similar they actually are. For example, wings have evolved independently in insects and birds. And humans are closer in the evolutionary tree to birds than insects.
The skeleton of the bird wing has the same bones that are found in humans (link):

Again, the humerus, radius, and ulna can be identified.
The wing of an insect, a solution to the same environmental problem (flight) is much different. The most obvious, they have no bones. This is the wing of a fly (link):

So, no, similar environments and selection pressure do not negate homology as evidence of common descent. And this doesn't even cover the much more objective genetic evidence (phylogenetic trees) that indicate common ancestry. Amazingly, genetic evidence and homological evidence match.

 

[crawfish]

Look up convergent evolution. This is where similar selective pressure (environment) produce similar physical structures. It happens from time to time. Think of bat wings vs. bird wings. They are similar, and work in a similar way, but they are still fundamentally different, reflecting their different history and different source.

Even convergent structures will have physical and/or genetic differences to show their lineage. Common descent still applies.

Thanks! That's what I figured, but I wanted to hear from somebody who knew what they were talking about. And thanks for the top on covergent evolution, at least I can study it a bit now.

 

[crawfish]

Oh sure, use flattery

Despite the fact that MythBusters proved that vinegar attracts more flies than honey, I'm still going to follow the advice of that adage.

It sounds like the poster has misunderstood what homology means.

Yes, this is likely. Some are so desperate to disprove evolution that they will use any data that seems promising without caring about understanding it. I didn't want to make the same mistake from my side.

Thanks so much for the info!

 

[Naraoia]

I'm having a conversation on evolution on another board where this came up:

Is it against any scientific law you know for one fossil to show attributes of two different species without the need for common ancestry? I mentioned above the scientific concept of “homoplasty” where different organisms share common environmental conditions and for this reason have similar anatomies.

Essentially, the poster claims that this effect negates the need for common descent & gene replication as an explanation for the similarity of gene sequences and the adding of genetic information during microevolutionary changes.

Not knowing a lot about the subject, and not finding a great deal of information about it online, what do the intelligent posters here think of the idea?

Vene has covered homology versus convergence, but I think something specifically about gene sequences is worth adding.

Similarity of function doesn't always imply similarity of sequence even in genes/proteins. Apparently it doesn't even imply similarity of structure (which, I've heard, is a better indicator of homology in proteins than sequence similarity). Here's one abstract I've found, but search "analogous enzymes" and you'll probably find lots of useful reading. So the sort of superficial convergence you see in anatomical structures is also present at much deeper levels.

Moreover, much evidence for common descent is entirely independent of function because the traits involved have no function.

Pseudogenes such as the oft-quoted human vitamin C pseudogene are a good example [look section 2.2.1 here] - why on earth do we have something that resembles a very functional gene found in other mammals when we can't carry out its function?

Endogenous retroviruses, another pro-common descent favourite, are similar to the pseudogene example but even better. There are thousands of them in us and other primates and they fit perfectly on a single primate family tree - what is the chance of having the same randomly-placed inactive viruses in exactly that pattern?

And the third favourite, human chromosome 2. Putting two chromosomes together doesn't do anything to the functional bits - genes, regulatory regions, whatnot - on the chromosomes. AFAIK it has no functional consequence. So having a chromosome that look exactly like two chimp chromosomes stuck together by the ends is hardly explainable by any functional argument.

 

[Molal]

Thank you to everyone for helping crawfish