How many species on Earth: differentiation problem (Introduction)

by David Turell , Wednesday, May 19, 2021, 18:31 (1081 days ago) @ David Turell

Sometimes organisms are so similar it is difficult to tell:

https://www.sciencedaily.com/releases/2021/05/210513142353.htm

"Evolutionary biologists have developed a new approach to genomic species delineation that improves upon current methods and could impact similar policy in the future.

"This approach is based on the fact that in many groups of organisms it can be problematic to decide where one species begins and another ends.

"'In the past, when it was challenging to distinguish species based on external characters, scientists relied on approaches that diagnosed signatures in the genome to identify 'breaks' or 'structure' in gene flow indicative of population separation. The problem is this method doesn't distinguish between two populations separated geographically versus two populations being two different species," said Jeet Sukumaran, computational evolutionary biologist at San Diego State University...

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"In cases where it is difficult to sort the variation between individuals into differences due to variation within a species as opposed to between two species, they often turn to genomic data based approaches for the answer. This is when scientists often use a model that generates a population phylogeny, or an evolutionary tree relating different populations.

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"Whether some of the population lineages in the sample are assigned to existing species or classified as entirely new species depends on two factors. One is the age of the population isolation events such as the splitting of an ancestral population into multiple daughter populations, which is how species are "born" in an extended process of speciation. The other is the rate of speciation completion, which is the rate at which the nascent or incipient species "born" from population splitting events develop into true full species.

"'We're coming to realize now that many organisms are cryptic species," Sukumaran said. "Many of them are similar looking even though they are actually distinct species separated by many tens or hundreds of thousands or even millions of years of evolution."

"This is either due to strong selection pressures to maintain the same morphology, or, more typically, due to very recent speciation resulting in insufficient time for external differences to develop.

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"Currently, scientists apply a model based on multispecies coalescent theory to genomic data to identify the disruption of gene flow between different groups of organisms. This disruption is fundamental to species formation, but it can also occur between two different populations as well as two different species.

"While scientists agree that it is critical to distinguish between populations and species boundaries in genomic data, there is not always a lot of agreement on how to go about doing it. "If you ask ten biologists, you will get twelve different answers," Sukumaran said." (my bold)

Comment: We don't understand how speciation happens, and this problem of minor differences cannot be decided by splitter or lumper dispositions of researchers. Darwin's finches come to mind, all based on beak size.