Genetic similarity and nomenclature question

[mycosis]

Hello,

I was wondering how close genetically can two organisms be related and still be considered different species?

How different can tow organisms be and be considered the same species?

How close genetically can two organisms be and be considered different Genera?

How different can two organisms be and be considered same Genus?

 

I am wondering in regards to basidiomycetes fungi specifically.

Thanks!

[mistermack]

I can't be specific about a particular type of fungus, but "species" is the one classification that isn't arbitrary. At least, in organisms that reproduce sexually, species are separated by whether they can and do regularly interbreed and produce viable offspring. If they don't, they are considered different species. 

At least, that's the general principle, although there can be fuzziness and debate over whether two groups are different species or not. 

Classification of genus is a question of evolutionary history and that subject is constantly evolving with new discoveries in the field of genetics, so there is often debate about what genus an organism belongs to. In general, genera differ after an evolutionary split but that can get murky, because sometimes there is occasional interbreeding and mixing of genes after a split, so it's not black and white.

We modern humans, for example, split from Neanderthals and were considered separate species, but later there was some interbreeding, so humans North of the Sahara carry some Neanderthal genes, and Sub-Saharan Africans don't.

[Arete]

7 hours ago, mistermack said:

At least, in organisms that reproduce sexually, species are separated by whether they can and do regularly interbreed and produce viable offspring. If they don't, they are considered different species. 

Unfortunately, not even this holds. Many sexual species can and do hybridize, especially in plants. 

To answer the original post - as evolutionary divergence is continuum, any demarcations on it are to a degree arbitrary. The justifications for such delimitations have been a topic of considerable discussion, with multiple conceptualizations of what species represent being presented. e.g. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5910646/

The one I would consider the predominant concept among taxonomists is the evolutionary species concept - whereby the ultimate goal of taxonomy is to put name to independently evolving lineages of organisms using multiple lines of objective measurement (morphology, genetic distance, reproductive isolation, ecology, geographic distribution, etc) to validate the lineage. Of course, that leaves little that is generalizable about what makes a species - apart from that it has a evolutionary trajectory distinct from its closest relatives. Clear as mud.  

Edited March 4, 2023 by Arete

[mistermack]

38 minutes ago, Arete said:

Unfortunately, not even this holds. Many sexual species can and do hybridize, especially in plants.

I mentioned that, with modern human/neanderthals as an example. But what I said does hold, in that if they don't regularly interbreed, and produce viable offspring, they are considered a separate species. 

The term viable offspring is the fuzzy, debateable part. It really means, produce a viable population of offspring. If two species can do that, then it's very debateabe whether they are separate species or not.

If you take the example of the carrion crow, and the hooded crow, then as a general rule, there will be an area where it's all hoodies, and an area where it's all carrion crows, with a narrow border area between them. Around the border area, you will find hybrids, but away from the border, you get one, or the other. So you don't get a successful self-sustaining population of hybrids, so they are still considered separate species. 

The only substantial difference between them is the colour pattern, but that's enough so that a hoodie that migrates into the territory of carrion crows will not attract a mate, and vice versa, because they look out of place. But in the narrow border area, the birds get confused, because they are regularly encountering both colours, and they don't know which they are, so they don't know which to mate with. So you do get some hybridization, but not enough to form a self-sustaining population, so they still get classed as separate species.

In the plant world you get a similar situation, but for different reasons. The Linden tree for example, in my area, has two common species, the broad leaf and the small leaf lime. They do readily hybridize, but even so, they tend to separate on a population level into areas where nearly all trees are one, or the other, and the hybrids don't form a self-sustaining population. I don't think it's known why that should be, probably something to do with the soil or small climate differences, enough to favour one over the other. So even though they readily hybridize, they still maintain separate populations. But like the crows, you do get areas where you will find both, and hybrids. 

[mycosis]

Thank you, this has been somewhat helpful. Hybrid species in some of these examples may be sterile like a donkey that would explain why complete integration does not occur or hybrids are only found in small specific areas.

I am looking into some fungi were "separate" species are showing 98-100% similarity. With a single species there is deviation`2%. Also, mushrooms of the same Genus are showing genetic similarity of 90-100%.

It seams like a 100% match for two separate species would indicate they are synonymous, is there any exceptions to this.

2% difference for two samples of the same species seams large to me, is that the case?

Also, two species in the same Genus being 10% different seams like a big difference, is this normal?

Thanks again!

[TheVat]

22 minutes ago, mycosis said:

Thank you, this has been somewhat helpful. Hybrid species in some of these examples may be sterile like a donkey that would explain why complete integration does not occur or hybrids are only found in small specific areas.

Donkey is neither sterile nor hybrid.  You are probably thinking of the mule, a hybrid of donkey and horse, which has an odd number of chromosomes and thus cannot produce haploid cells for reproduction.  

[mistermack]

2 hours ago, mycosis said:

I am looking into some fungi were "separate" species are showing 98-100%

You need to define what the figures actually mean. I see various claims of percentage difference, for the same species. 

For example, I've seen claims that humans and chimps are genetically 98 % identical, and other claims of 99+ for the same thing. 

[Genady]

There are about 2% genetic difference between human males and females. They considered the same species, though.

[CharonY]

1 hour ago, Genady said:

There are about 2% genetic difference between human males and females. They considered the same species, though.

Both values indicate different things. The 98% similarity is, IIRC based on comparing coding regions (genes) only, while the 2%, if accurate, likely refers to differences in the overall DNA sequences.

Historically for prokaryotes we used 70% DNA-DNA hybridization similarity as a cut-off to define species. But again, the species concept is overall a very muddled one as is often applied differently, depending on the research question.

3 hours ago, mycosis said:

am looking into some fungi were "separate" species are showing 98-100% similarity

What is this similarity based on. Total genome sequence, marker genes the percentage of  shared genes, or...?

[Genady]

3 minutes ago, CharonY said:

while the 2%, if accurate, likely refers to differences in the overall DNA sequences.

Yes, it refers to the bp number in Y chromosome compared to the total bp number in the genome.