3 “individuals”, “populations”, “species”, “families”,
organized in a nested hierarchy: one fits within the next, not unlike those Russian dolls where inside each
one is another, smaller one.
The system that biologists use to classify species reflects this nested hierarchy. As one moves up the
hierarchy, the categories become more general and include more and more lower-level categories.
Domains, Kingdoms, Phyla (singular: Phylum), Classes, Orders, Families, Genera (singular: Genus), and
Species (singular is also species, not specie).
ariability at these levels is one of the things we mean when we talk about “biodiversity”
. Of course, it is also true that there are differences within species. If there weren’t, there would be no
evolution — and thus, no biodiversity — as we know it.
Diversity Among Species
biologists use the word “diversity” to refer to the number of species in a taxonomic group or a
Some taxonomic groups are very diverse (i.e., contain a lot of species) and others are not.
factors such as invasive species, agricultural practices, and habitat destruction are posing significant
challenges to the survival of many of the native mussel species in this area.
Differences Within Species: Sexual Dimorphism and Age
possible to identify many of the native mussels just by their outward appearance.
This is possible because each species has morphological (physical) traits that can be used to distinguish
it from other mussels. Most of the time, anyway.
challenges in doing field work on mussels is that these morphological characters do not always hold up
when you try to use them for identification. The reason is that species themselves are variable, and many of the morphological traits of interest exist along a range of variation among conspecifics (members of the
con·spe·cif·ica member of the same species.
Our development — what biologists call “ontogeny” — involves significant changes in morphology from
youth to old age.This is also true of mussels!
Like humans, mussels may exhibit clear differences among individuals within species due to sexual
dimorphism (in some species) and age.
it is both challenging to identify some species on the basis of immature specimens and also because
finding gravid (pregnant) females and juveniles is a good sign that recruitment (production of the next
generation) is succeeding.
Differences Within Species II: logy
species are named after people or places. In other cases, the scientific name conveys important
information about the species itself.
“polymorpha” means “many forms” Zebra mussel infestations may look like little more than a mass of shells, but closer inspection reveals
that there are differences in size, colour, stripe pattern, and other characteristics.
unlikely that anyone would misidentify the zebra mussel as one of the native mussels because they are
efforts to identify species of native mussels can be confounded by the variation that exists in
morphological traits within species.
trained researchers can encounter difficulties because of intraspecific variation (though the students were
very knowledgeable when it came to mussel species).
Differences Within Species: Cryptic Variation
To be evolutionarily relevant, variation among individuals must be heritable. In other words, there must be
a genetic basis for the observed trait such that it will be passed on in similar form to offspring.
In some cases, we can observe morphological variability that is not due primarily to genetic differences,
but to the environment.
Cases in which the same set of genes (genotype) can result in differing features (phenotype) according to
environmental conditions are known as “phenotypic plasticity”. Ie. Same genotype, different phenotype.
Eg. Water flow affects same species and different species.
there can be only minor phenotypic differences among individuals, but plenty of genetic variation.
use microsatellites — regions of the genome that do not code for proteins and vary widely in quantity
among individuals — to study variation that is invisible to the eye. This is the same technology used in
DNA fingerprinting for forensic analysis. It really is “CSI for mussels”.
Same genotype =~>>> different phenotypes
Abundance vs. Variation vs. Diversity vs. Disparity
seen examples of differences in abundance (number of individuals) among mussels — for example, the
massive abundances of zebra mussels versus the very low abundance of endangered native mussels.
seen examples of both morphological and genetic variation (differences among individuals within a
species) in the mussels.
noted differences in diversity (number of species) in different areas, and of course this also applies to
differences in diversity among taxonomic groups.
The phylum Mollusca is the second largest group of animals after arthropods (insects, crustaceans,
spiders, etc.), and is the largest primarily aquatic phylum. Around 85,000 known species of clams,
oysters, mussels, limpets, slugs, snails, squids, and octopi have been described so far. About 30,000 of
these species are classified in the class Bivalvia, of which mussels are members. This is a very large
group, especially compared to our own class Mammalia with a mere 5,000 species. However, the class
Gastropoda, which includes snails and slugs, contains more than 60,000 species. (For the record, the
class Insecta contains over a million described species).
There is one other key type of difference among groups that we must consider: disparity.
disparity. Whereas diversity is a measure of the number of different species, disparity refers to how
physically different those species are from each other. mammals are not very diverse but they are very disparate — consider shrews vs. bats vs. giraffes vs.
blue whales. Bivalve molluscs, though much more diverse than mammals, are not as disparate — on the
whole, they all maintain a relatively similar morphology.
Biological scale Examples
Differences in diversity, abundance, and Bacteria vs. eukaryotes; Plants vs. animals;
distribution among major groups of life Arthropods vs. chordates
(Domains, Kingdoms, Phyla, etc.)
Differences in diversity, abundance, and
distribution within particular groups (Classes, Amphibians vs. mammals; Rodents vs.
primates; Old World monkeys vs. hominids
Orders, Families, etc.)
Genetic and/or phenotypic differences among Human populations in Africa vs. in southeast
populations within a species. Asia.
Genetic and/or phenotypic differences among Your classmates vs. you.
individual organisms within populations.
We Are All Related
In fact, you are a member of a lineage of ancestors and descendants that has never once been broken in
more than 3.8 billion years of life on Earth. The same is true of every living thing today. more and more of
these lineages converge into common ancestors. Fi