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Ken Fabian

Clarify terminology; Homologous

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I've managed to get confused about terminology around evolution and am not sure I have it correct. Is a trait Homologous if all descendants from a common ancestor have it? Or do all members of the species have to have this trait?

For the example I have in mind - human "hairlessness" (this misleading term referring to smaller hairs than related apes, not actual absence).

All juvenile humans are "hairless" before puberty (except for head hair, which they also share in common)  but adults vary from male to female (dimorphism) as well as vary across different populations. The hairless juvenile trait is universal within our species and is therefore homologous? But patterns of hairiness in adults are not all the same so is not homologous within the species, though it may be homologous within a sub-population? Whilst dimorphism may have predated the hairlessness, the extent of variability should be evidence of genetic changes that came after the change that made juveniles "hairless" and adult overall less hairy?

Edited by Ken Fabian

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Does this help?:

Quote

Convergent evolution is the independent evolution of similar features in species of different periods or ephos in time line. Convergent evolution creates analogous structures that have similar form or function but were not present in the last common ancestor of those groups. The cladistic term for the same phenomenon is homoplasy. The recurrent evolution of flight is a classic example, as flying insects, birds, pterosaurs, and bats have independently evolved the useful capacity of flight. Functionally similar features that have arisen through convergent evolution are analogous, whereas homologous structures or traits have a common origin but can have dissimilar functions. Bird, bat, and pterosaur wings are analogous structures, but their forelimbs are homologous, sharing an ancestral state despite serving different functions. https://en.m.wikipedia.org/wiki/Convergent_evolution

 

Edited by StringJunky

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On the subject of hairlessness  in humans, it's very relevant to view it in the context of the neoteny that our species exhibits. 

Basically, it means that humans, particularly males, never fully develop in the way that other apes do. We found  some kind of advantage in retaining juvenile characteristics into adulthood, and our hairlessness in adulthood may be part of that. 

https://en.wikipedia.org/wiki/Neoteny_in_humans    

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16 hours ago, Ken Fabian said:

I've managed to get confused about terminology around evolution and am not sure I have it correct. Is a trait Homologous if all descendants from a common ancestor have it? Or do all members of the species have to have this trait?

For the example I have in mind - human "hairlessness" (this misleading term referring to smaller hairs than related apes, not actual absence).

All juvenile humans are "hairless" before puberty (except for head hair, which they also share in common)  but adults vary from male to female (dimorphism) as well as vary across different populations. The hairless juvenile trait is universal within our species and is therefore homologous? But patterns of hairiness in adults are not all the same so is not homologous within the species, though it may be homologous within a sub-population? Whilst dimorphism may have predated the hairlessness, the extent of variability should be evidence of genetic changes that came after the change that made juveniles "hairless" and adult overall less hairy?

Homology us above the species level. I.e. if you see a trait in two extant species that is derived from a common ancestor it would be considered a homologous trait. The important bit is the inheritable (I.e. genetic) aspect of the trait. Many traits are require many genetic elements, there are variations of the said gene and a the actual phenotype can also vary based on environmental input. However if the relevant genetic elements are shared between several related species (and therefore likely derived from the same ancestor) they are considered homologous.

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2 hours ago, CharonY said:

Homology us above the species level. I.e. if you see a trait in two extant species that is derived from a common ancestor it would be considered a homologous trait. The important bit is the inheritable (I.e. genetic) aspect of the trait. Many traits are require many genetic elements, there are variations of the said gene and a the actual phenotype can also vary based on environmental input. However if the relevant genetic elements are shared between several related species (and therefore likely derived from the same ancestor) they are considered homologous.

Thanks CharonY. I had thought the term was more widely applicable than that. That answer leads me to ask what the correct term is for a trait that is unique to - and shared by all members of - a species? I would expect such traits to come down from a common ancestor.

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Well all members of a given species share essentially the same core genome. In cases of mammals we basically all have the same genes and related traits. The phenotypes can be different, due to variations of the genes (i.e. individuals have different variants of the same gene) or external factors. In other words, a species is basically defined by the genetic traits that they share. 

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14 hours ago, Ken Fabian said:

That answer leads me to ask what the correct term is for a trait that is unique to - and shared by all members of - a species? I would expect such traits to come down from a common ancestor.

Firstly, it's virtually certain that such traits would come from a common ancestor. Since all members of a species have common ancestors. The only way I can think of for a trait NOT to come from a common ancestor, is if it comes from a very recent mutation. 

I think that a trait that is unique to a species would also be pretty rare. You would generally find it in others, maybe less developed, but still there in some form. Like language, it's tempting to regard humans as the only animal to use language, but when you look again, there is signs of it in lots of species, but less advanced. 

So if there is a single word for what you described, it's not going to get a lot of use. I think the way you expressed it is fine. 

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5 hours ago, mistermack said:

he only way I can think of for a trait NOT to come from a common ancestor, is if it comes from a very recent mutation. 

A simple mutation would not change the fact that the locus is still shared by the whole population (and was derived from the same ancestor). The typical scenario for relatively rapid acquisition of new genetic elements are typically horizontal gene transfer events (e.g. via viruses). 

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