The scissortail sergeant fishes (Abudefduf sexfasciatus) above, found in the Indo-Pacific, and sergeantmajor fishes (Abudefduf sexfatilisus), at right, found in the Caribbean,  are sibling species.

1. THE GENERAL DEFINITION OF SPECIES IS THAT THEY ARE GROUPS OF ORGANISMS that are like each other and can produce offspring with the same traits

They’re both members of the Damselfish family (Pomacentridae) but differing species that evolved separately due to geographic isolation from each other.
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2. While they sometimes can produce hybrid offspring with other species, those offspring should not be able to reproduce. 

3. Except…within each species of gene-directed traits, there are inevitably genetic variations that favor some as environments change and work against others. As succeeding generations adapt to these changing circumstances, they become increasingly distinct.

4. Eventually, accumulated genetic differences preclude successful interbreeding with members of the original group. This is speciation, the evolution of new species. Because these genetic variations can occur only among members of each species, it’s at the species level that evolution takes place.

Three very different commensal species – a pink anemonelfish (Amphiprion penderaion), a  magnificent sea anemone (Stichodactylia gigantea) and an anemone shrimp (species uncertain).

5. Recognized species of plants, animals and microorganisms on our planet today total around 1.5 million, including 750,000 insects, 60,000 fungi, 500,000 plants and 45,000 vertebrates.

 6. It’s clear that many more species have yet to be identified, but estimates of the numbers vary widely – from five million to as high as 100 million. Many experts put the number at around 30 million.


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7. To keep track of them, we rely on the concept of taxonomy. There were originally many systems in use to identify living things but the 18th Century Swedish scientist Carl Linnaeus standardized the binominal naming system that, with improvements, we use today. Linnaeus kept the Latin  used for many of those early systems, so we’re stuck with it.

8. Linnaeus’ real name was Carl von Linné. Linnaeus is the Latinized version of von Linné, so he was stuck with it, too.

9. The protocol for the binominal naming system is to combine an organism’s genus name and its species name, officially referred to as its species epithet, as in Homarus americanus, the New England lobster. On second reference it would be H. americanus. And, in both cases, italicized.

10. Modern taxonomy is a complex cascade that starts with the broad categorization of kingdoms (animals, plants, fungi, protists and bacteria) and proceeds through increasingly detailed ranks (phyla, classes, orders, families, genera and then species) that each take into account some level of characteristics like cellular type, body symmetry and reproductive methods.

11. There can also be subgroups tucked in between. Sometimes a subgroup, like the subphylum Vertebrata, which oversees more than 45,000 species of fishes, birds, mammals, reptiles and amphibians, can be more numerically significant than the phylum it’s under.

12. We humans may feel a closer kinship for other vertebrates, especially cute furry animals, but 90 percent of the diversity in the world is represented by invertebrates.

Lionfishes are swell in the Indo/Pacific, where they are part of a balanced habitat, terrible as an invasive species in the Atlantic/ Caribbean, where they have no predators – so far.

13. The largest number of species by far are insects. There are about 250,000 species of beetles alone identified, many of them from solitary specimens.

14. With some 25,000 species, more than half of all species of vertebrates are bony fishes.

15. Tropical zones have at least twice as many species as temperate latitudes and 70 percent of the world’s species are found in a dozen countries, all with tropical regions – Australia, Brazil, China, Colombia, Ecuador, India, Indonesia, Madagascar, Mexico, Peru and Zaire.

16. It’s not uncommon to have subspecies below the species level. These are often variations that have developed in geographic or ecological isolation. Although they can mate with the originating species, their male offspring are often sterile. A step away are incipient species, which rarely interbreed and whose interbred male offspring are all sterile.

17. Sibling species don’t mate at all – even when virtually identical in appearance, they have come to represent different species (see sergeant fishes, above).

An estimated 500-600 species of stony corals call the Indo/Pacfic basin home (perhaps 10 times as many as in the Caribbean), too many for your average field guide to identify. Like this one.

18. Sometimes, distinct organisms are connected by intervening variations to create “ring species,” in which traits gradually change more and more with each additional level of geographic or ecological separation. At each extreme are two species that can’t interbreed. And sometimes new species develop through quantum speciation, when chromosomal mutations occur in a particular population within a group and spur a rapid process of trait changes.

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19. The International Commission on Zoological Nomenclature estimates that 2,000 new genera and 15,000 new species are added to the zoological literature each year. 

20. There are several international organizations working to identify and record all the species on Earth, including Species 2000 and the All Species Foundation, which admits that there will never be certainty that every species on the planet has been found.  

Principal Sources: Marine Biology, Fourth Edition, Peter Castro, Michale Huber; All Species Foundation,; Explore Biodiversity, Topics in Diversity and Evolution,; Britannica Online Encyclopedia,

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