Species and the three domains
Classifying living things
- Biologists sort all life into groups — classification.
- First we need to define a species.
- Then we place every organism into one of three domains.
What is a species?
- biological species — members can breed together to make fertile offspring.
- morphological species — members look alike.
- ecological species — members fill the same role in their surroundings.
Practice
The biological species concept defines a species as a group whose members:
The biological concept uses interbreeding to produce fertile offspring; other concepts use appearance or ecological role.
The three domains
- Archaea and Bacteria — both prokaryotes (no nucleus). They differ in their membrane lipids, ribosomal RNA, and cell-wall make-up.
- Eukarya — all organisms whose cells have a nucleus.
Practice
The three domains of life are:
The largest groups are the three domains: Archaea, Bacteria (both prokaryotes) and Eukarya.
Practice
Which is true of the domains?
Archaea and Bacteria lack a nucleus (prokaryotes); Eukarya have a nucleus.
Where viruses fit
- Viruses are not placed in these groups.
- They are classified by the type of nucleic acid they contain (DNA or RNA), and whether it is single- or double-stranded.
Practice
Viruses are classified by:
Viruses are non-cellular, so they are grouped by their nucleic acid (DNA/RNA, single/double-stranded).
You've got it
Key idea
- species can be defined biologically (fertile offspring), morphologically (look alike), or ecologically (same role)
- three domains: Archaea + Bacteria (prokaryotes) and Eukarya (have a nucleus)
- viruses are classified by their nucleic acid type (DNA/RNA, single/double-stranded)