Carl zimmer how many species

Scientists have named and cataloged 1. How many more species there are left to discover is a question that has hovered like a cloud over the heads of taxonomists for two centuries. On Tuesday, Dr. Worm, Dr. Mora and their colleagues presented the latest estimate of how many species there are, based on a new method they have developed. They estimate there are 8. The new paper , published in the journal PLoS Biology, is drawing strong reactions from other experts.

In , a British entomologist named John Obadiah Westwood made the earliest known estimate of global biodiversity by guessing how many insect species there are. He estimated how many species of insects lived on each plant species in England, and then extrapolated that figure across the whole planet.

Today, scientists know the Westwood figure is far too low. In recent decades, scientists have looked for better ways to determine how many species are left to find. In , Robert May, an evolutionary biologist at the University of Oxford, observed that the diversity of land animals increases as they get smaller.

He reasoned that we probably have found most of the species of big animals, like mammals and birds, so he used their diversity to calculate the diversity of smaller animals. He ended up with an estimate 10 to 50 million species of land animals. Other estimates have ranged from as few as 3 million to as many as million. Parmesan said. Parmesan and her colleagues have continued to expand their database since then.

But other researchers have been moving in the opposite direction, seeking to attribute changes in individual species to climate change. Last year, for example, Michael Kearney of the University of Melbourne and his colleagues published a study on the common brown butterfly of Australia. From to , adult butterflies had been emerging from their pupae 1.

To see if the brown butterfly is actually responding to climate change, Dr. Kearney and his colleagues first analyzed historical temperature records in Melbourne. Temperatures have gradually risen over the past 60 years. Computer models indicate that natural climate cycles can explain only a small part of the change. The scientists then observed how temperature affects how brown butterflies develop. The warmer the temperature, the faster the butterflies emerged from their pupae.

Kearney and his colleagues used those results to build a mathematical model to predict how long the butterflies would develop at any given temperature. In the journal Nature Climate Change, Dr. Parmesan and her colleagues argue that trying to attribute specific biological changes to global warming is the wrong way to go. While the global fingerprint of climate change may be clear, the picture can get blurry in individual species.

In Europe, for example, the map butterfly has expanded its range at both its northern and its southern edge. Global warming probably has something to do with its northern expansion. But the butterflies are also benefiting from the mowing of roadsides, which allows more nettle plants to grow. A number of experts applaud the commentary from Dr. Parmesan and her colleagues. Tracking the effects of climate change on species today can help show how nature may respond to it in decades to come.

And many scientists think that the future looks grim. As temperatures rise, many species may not be able to shift their ranges to stay in a comfortable environment.

Most rotifers reproduce sexually, but bdelloid rotifers abandoned sex about million years ago. All bdelloid rotifers are female, and they make embryos without any need for sperm. By the standards of the biological species concept, the rotifers went from being a species to being not a species, whatever that means. This kind of dissatisfaction led some scientists to devise new species concepts.

Each concept was crafted to capture the essence of what it means to be a species. One of the strongest rivals to the biological species concept, called the phylogenetic species concept, takes sex out of the equation and puts descent from a common ancestor in its place.

Related organisms share traits because they share the same ancestry. Humans, giraffes and bats all descend from ancient mammals, and as a result they all have hair and milk. Within mammals, humans share a closer common ancestry with other primates. From the common primate ancestor, primates inherited other traits, such as forward-facing eyes. You can zoom in on smaller and smaller sets of organisms this way.

Eventually, though, the zooming in comes to a stop. There are organisms that form groups that can no longer be split. These, according to the phylogenetic species concept, are species. The phylogenetic species concept has been embraced by researchers who need to identify species rather than just contemplate them.

Recognizing a species is a matter of finding a group of organisms that shares certain clear-cut traits. Scientists do not have to depend on slippery qualities like reproductive isolation. Recently, for example, the clouded leopards on the Indonesian island of Borneo were declared a species in their own right, distinct from the clouded leopards of southern Asia.

All the Bornean clouded leopards shared certain traits not found in the cats on the mainland, including a distinctively dark coat. Some critics think that there is far too much species splitting going on these days.

A single mutation might, at least theoretically, be enough to earn a small group of animals a species name. Mace also argues that that a population should be considered ecologically distinct—as defined by geography, climate and predator-prey relations—before someone decides to split it off as a new species.

But other researchers think that they should go where the data lead them rather than worrying about oversplitting. A few years ago the endless arguments of this kind convinced Kevin de Queiroz, a biologist at the Smithsonian Institution, that the species debate had gone too far.

Most of the competing species concepts actually agree on some basic things. They are all grounded in the notion that a species is a distinct, evolving lineage, for instance. For de Queiroz, that is the fundamental definition of a species. Most of the disagreements about species are not actually about its concept but are about how to recognize a species.

De Queiroz thinks that different methods work best in different cases. Strong reproductive isolation is good evidence that a population of birds is a species, for example. But it is not the only yardstick that can be used. For bdelloid rotifers that do not have sex, scientists just have to use other kinds of criteria. Instead of trying to use just one gold standard, they are testing new species against several different lines of evidence. Jason Bond, a biologist at East Carolina University, and his student Amy Stockman took this approach in a survey of an enigmatic genus of spiders, Promyrmekiaphila , found in California.

Taxonomists have long struggled to determine how many Promyrmekiaphila species there are. The spiders resist easy classification because they look almost identical. And yet scientists also have known that they probably form very isolated populations, thanks in large part to the fact that each spider is unlikely to move very far from home.

He has dug up Promyrmekiaphila burrows containing three generations of female spiders that have lived there for years. Males will leave their birthplace burrows, but they will not move far before mating with a female from a neighboring burrow.

To identify the species of the spiders, Bond and Stockman adopted methods developed by Templeton. For the evolutionary history, Bond and Stockman sequenced parts of two genes from spiders at 78 sites in California.

They surveyed the DNA for genetic markers that showed how the spiders were re-lated to one another. The evolutionary tree of the spiders turned out to be made up of a number of distinct lineages. Bond and Stockman then looked for versions of genes in different populations to find evidence of gene flow. And finally, they recorded the climate conditions in which each group of spiders lived. In the end, they identified six species that met all three criteria. If accepted, these findings would double the number of Promyrmekiaphila species.

This kind of approach is allowing scientists to study organisms that once seemed not to fit into species concepts. Because bdelloid rotifers do not have sex, they do not fit well under the biological species concept.

Tim Barraclough of Imperial College London and his colleagues used other methods to determine whether the rotifers belong to species-like groups. They sequenced the DNA and built an evolutionary tree.

The tree had just a few long branches, each one topped by a tuft of short twigs. Then they examined the bodies of the rotifers on each tuft and found that they had similar shapes. The diversity of rotifers, in other words, is not just a blur. The animals form clusters, which are probably the result of separate lineages adapting to different ecological niches. If those clusters are not species, they are awfully close. Most of the work that has been done on the species concept in recent years has been directed at animals and plants.

That bias is the result of history: animals and plants were the only things that Linnaeus and other early taxonomists could study. But today scientists know that the vast majority of genetic diversity lies in the invisible world of microbes.

And microbes have long posed the biggest puzzle of all when it comes to the nature of species. When microbiologists began naming species in the s, they could not inspect feathers or flowers like zoologists and botanists can. Microorganisms—especially bacteria and archaea—generally look a lot like one another.