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Archive for October, 2019

guatemala black howler

Hybridization between species is aspect of evolution that is only just now becoming recognized as a force in evolution. It is sort of taking a biological app from one species and adapting it another, and most studies on this phenomenon look at the app adaptation aspect of hybridization.

However, hybridization is more often than not less advantageous from a natural selection standpoint. Although these “new apps” and heterosis might be good for hybrids, many hybrids are sterile. Or if they aren’t sterile, one sex will either be absent or sterile.

Species generally have mechanisms that prevent hybridization. Many of these are behavioral.   For example, related bird species often won’t exchange genes because the female are simply not attracted to the males’ songs.  But there are molecular responses against hybridization as well.

One of the most contentious hypotheses about hybridization between species is that of reinforcement. What this hypothesis contend is that when two species begin to hybridize readily, there will be a strong selection for greater genetic distance between the two hybridizing species. With greater genetic variation, it will be less likely that the two species will be able to produce viable offspring, and over time, there will be fewer hybrids in the population.

This hypothesis has not been tested much. However, a study of two species of howler monkey in the Mexican state of Tabasco revealed that, yes, reinforcement is a thing.

Mantled and Guatemalan black howler monkeys diverged from a common ancestor about 3 million years ago. The two species have only a narrow contact zone, which is thought to have formed only 10,000 years ago in this tiny part of Mexico.

The researchers examined loci of the genomes of specimens of both species, including those in the hybrid zone. They found that the genetic difference between the two species was greater at the hybrid zone than from monkeys that lived in other regions. This discovery supports the hypothesis of reinforcement.  The greater genetic difference between the two species at the hybrid zone means that this greater genetic difference likely has evolved as a way of keeping the two species from producing lots of hybrids, which might not be as fit or  as good at reproducing in the wild as pure ones.

This discovery of reinforcement means that we have another tool in sorting out whether two species make sense. If we discover that there is greater genetic difference at a hybrid zone between the two species, then we know that they really are quite taxonomically distinct.  If we find the opposite, it means that hybrids aren’t deleterious in the population, and hybridization is either advantageous or neutral for the populations.

Yes, I would like to see this hypothesis tested on the various hybridizing canid populations in the gray wolf species complex. My guess is that it doesn’t exist in these animals, because hybridization isn’t that deleterious. And the genetic divergence isn’t that great to start out with.

But this study gives us a good idea of how hybridization operates in populations, and how some populations evolve to restrict gene flow.

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jolly ball the best ball

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forest dog

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cheetah

India’s supreme court is now seeing an interesting case in which taxonomy and endangered species politics converge to have real world consequences. The question is whether African cheetahs can replace Asiatic cheetahs on India’s plains.

Yes, for there were once cheetahs in India. Their traditional quarry was the blackbuck antelope, and many nobles in India kept cheetahs or “hunting leopards,” as the British colonizers called them, for coursing blackbuck.

Cheetahs were not just found in India.  They ranged throughout the Middle East up into the Caucasus and Central Asia. In the wild, this lineage of cheetah is found only in Iran, where they exist in only relict numbers.  In Iran, the situation is made even more complicated with an international human rights scandal in which several cheetah researchers were imprisoned.  Cheetahs have since been extirpated from all of Asia, except for that tiny Iranian population.

So India, a nation with growing wealth and a growing conservation ethic, cannot turn to Iran to reintroduce its former cheetahs.  With Iran out of the question, some experts have suggested that African cheetahs be used as stand-ins.

And this is where things get interesting. African cheetahs are not exactly like the ones in India. There is a bit of a debate about when the two lineages of cheetah split, with one set of papers and researchers suggesting a very recent split (5,000 years ago) and another suggesting a more ancient one (44,000-47,000 years ago).

40,000 years suggests way too much evolutionary distance between the two cheetah populations for African cheetahs to be equivalent of the Asiatic ones.

But even if we accept this later date, it is still not that much of a divergence. Currently, most experts recognize only a single species of red fox, but Old World and North American red foxes diverged 400,000 years ago.

African cheetahs have evolved to hunt on open plains. Various small antelopes comprise the majority of their diet. They are not ecologically that different from cheetahs that lived on the plains of India.

So they aren’t that genetically distinct from each other, and they aren’t ecologically that different either.

It would make sense to bring African cheetahs to India. Of course, the legal system and the interpretation of statutes often goes against sound conservation policy.

But if cheetahs are ever to return to India, the question is now in the hands of India’s supreme court.

I hope they decide that those from Africa can stand in. They are far from exact, but they are far from ersatz.

 

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looking good

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