Evolution and attempts to describe full taxa

Just pondering out loud, but we often see those statistics proposed for the diversity of undescribed species (particularly invertebrates) which can be in the tens to hundreds of thousands.

But high frequency reproducing species evolve quickly, so I wonder how much things change by the time we measure them? For instance, if there is assumed to be hundreds of thousands of undescribed species of hymenoptera out there and we have ~150K described species, how many of these existing species will be extinct by the time another 50K are described? Or, how many newly described future species will just be evolutions of existing species that didn’t exist at the time of the original descriptions?

In other words, do forward estimations of unknown species account for the evolution of the known taxa into such species (or extinction) as part of it, or do they generally assume distinct, currently existing unknown diversity?

Is there any good material out there that dives into this?

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My understanding is that most species are hundreds of thousands or even millions of years old, so emergence of new species on short timescales (e.g. 100 years) is virtually zero. Current rates of extinction because of habitat loss, introduced species and other human-caused problems are much higher than rates of appearance of new species.

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Since industrialisation and globalisation in particular took hold, is that really still true? What’s the evidence that newly discovered species are still mostly hundreds of thousands/millions of years old, particularly as mentioned within invertebrates and other high fecundity/rapid lifecycle taxa?

Is it because most of the unknowns are likely within the wilderness and poorly understood locations?

I haven’t seen any published evidence that urbanization or climate change have caused an uptick in speciation (and there are researchers looking for adaptation to these processes). It’s certainly possible, but there’s no current evidence for this that I’m aware of. So I would say that, until such evidence is presented, it’s reasonable to work with our knowledge that most species are older.

I wouldn’t guess that estimations of unknown species try to account for this - most that I have seen discuss how many species there are now, and aren’t trying to estimate how many will be discovered.

I agree with @deboas that it’s probably much more important to think about how many species will be lost/go extinct than how many will be created. Extinction rates appear to be much higher than the historical baseline, and this seems to be the case in deep time for periods of rapid climatic change in the past - ie, dramatic changes are associated with increased extinction rates. This is true even though species diversity increases over long time scales.

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Globalization can actually lead to quite the opposite since reproductive isolation is one factor that often contributes to speciation. In some cases, subspecies that were isolated geographically historically are no longer.

While many invertebrates do have higher generational frequency and do speciate faster than many vertebrates, I agree that this process still takes a very long time. Microorganisms with generational frequencies of days or hours are the most likely to show speciation within a relatively short timeframe.

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It’s true that most undescribed species are likely in relatively remote and less degraded ecosystems, rather than in heavily modified environments, where biotic homogenization is more the norm (e.g. cities around the world have House Sparrows and Feral Pigeons, whereas adjacent natural habitats will have different bird species depending on location).

I found a paper that takes a look at this question. It includes an estimate of speciation rate: “Plants have median diversification rates of 0.06 new species per species per million years, rates for birds are estimated at 0.15, and mammals at 0.07.” Assuming there are ten million species, and taking a speciation rate of 0.1, that would imply one new species appearing per year, on average.

The authors cite some evidence that human activities can speed up rates of phenotypic change in populations, so in theory, our activities might accelerate formation of some new species in some circumstances. However, they also point out that we might suppress evolution by reducing the size of populations and eliminating distinct populations. And, emergence of some new species that are closely-related to other existing species would not offset the loss of evolutionary history and genetic variation lost with the extinction of distantly-related species.

Another paper looks at the potential for introduced mammals to evolve into new species in the future, and whether this would offset mammal extinctions. The conclusion is that yes, perhaps, on a timescale of several millions of years, although phylogenetic diversity would remain permanently reduced with the loss of distinct ancient lineages.

A third paper argues that because we are reducing the amount of space available to wild species, we are likely reducing the speciation rate, because taxa with large distributional ranges (i.e., before they have been decimated by human activities) speciate faster.

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In general, speciation is a slow process, and we can only realize that a new species has formed in retrospect. The usual process of speciation is so slow that it hasn’t had time to produce new species since the present taxonomic system began (and it began in 1753 for plants, some time very close to that for animals).

However, speciation can be sudden and that has been detected a few times. It can occur through polyploidy. For example, an error in cell division can produce a tetraploid offspring from diploid parents. (Tetraploid = 4 copies of each chromosome; diploid = 2 copies. Humans are diploids.) In general, offspring with one diploid parent and one tetraploid parent have 3 copies of each chromosome and therefore can’t do meiosis properly and can can’t reproduce. Therefore, if both the tetraploid and the diploid populations can reproduce among themselves, they can form separate lineages, separate species. (Oddly, many plants can work around this problem so that a difference in chromosome number may not mean that the individuals are different species. Some plant species exhibit many different chromosome numbers.)

More often, sudden speciation involves hybridization followed by chromosome doubling. Here in the Pacific Northwest of North America, three species of Tragopogon have been introduced, T. dubius, T. porrifolius, and T. pratensis. They have produced hybrids in all possible combinations and two of these have developed into sexually reproducing species.

Other cases of hybridization leading to speciation have been observed. Often human introduction of species to new places is involved, but some are entirely natural.

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