A lot of ink has been spilled about what species are, but for animals in a given area this is not a difficult question: species are individuals that recognize each other as conspecifics and thus exchange genes through reproduction. Different species are delineated by lack of gene flow – no, very little, or unsuccessful exchange of genes between species (hybridization). Within species, DNA is mixed up each generation through sexual reproduction, but remains separate from other pools of DNA in other species. As a result, when we look at the relationship of individuals to each other in a phylogenetic tree, individuals within species differ little, whereas individuals from other species differ substantially:
A lack of gene flow and divergent evolution of genomes means that species end up differing in phenotype as well, sooner or later. However there is a catch – it may be sooner or later. When there is selection favoring different phenotypes – for ecological or reproductive reasons, for example – then morphologic divergence happens sooner. When there is no selection, or there is stabilizing selection favoring the current phenotype, then morphological divergence happens later.
Mate recognition is important for each species; it is how individuals within one species decide to exchange genes with each other rather than with individuals from another species. For animals like many vertebrates, visual cues can be very important in mate recognition, thus there is a strong tendency for species to look different. As vertebrates, we are used to this, and expect species to look different. However, this does not have to be the case; mate recognition can be accomplished through other sensory pathways. Thus fish crows and American crows, species that are very difficult to distinguish visually, have very different calls – mate recognition is through sound. In the ocean, light can be limiting, and many marine animals have poorly developed vision. For this reason, chemical communication for mate recognition is often the dominant mode in marine invertebrates.
It is, therefore, not surprising that we are finding that many “good species” – i.e. species whose DNA shows that they are distinct from each other – are not easily distinguished based on their appearance. These are called cryptic species. Cryptic species are turning out to be very common in the sea, and many common marine organisms are now being recognized as complexes of similar species. A major challenge for taxonomists of the coming decades is to tease out and characterize this cryptic diversity. Nevertheless, most species are identifiable visually, and even cryptic species discovered through DNA sequencing often turn out to be visually distinguishable when studied closely and guided by DNA sequence data.
While species (when properly delineated) are natural units, other taxonomic categories (subspecies, genera, families, etc.) are arbitrary human constructs. Hierarchical classifications in part reflect the philosophical views and taxonomic opinions of the time. Much of our knowledge about biodiversity is based on species occurrence data: where and when a particular species was documented to occur. Knowing the natural world that surrounds you begins with identifying the relevant players. A good exercise for any naturalist – or group of students – is to sort out and identify the species that live in the area. If we all did this for the coastal zone in Florida, we would rapidly get a very useful field guide for the marine biota, including a distribution map of component species. This would be an extremely useful outcome of a statewide student and citizen science effort.
Occurrence records are records of a species in a particular place and time. Minimally they consist of a location (latitude / longitude), time, and identification. Occurrence records can be based on expert observations, photographs, specimens, DNA sequences, and other types of records. Several organizations are drawing together occurrence records from many sources to provide global overviews of the distribution of species. Check out the following resources: GBIF, OBIS, Map of Life, Discover Life (see links in Appendix IV).