Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 27.4 Prokaryotes Have Radiated into a Diverse Set of Lineages
Prokaryotes have evolved extraordinary structural, genetic, and ecological diversity. Thanks to advances in molecular systematics and metagenomics, scientists now recognize the immense scale of prokaryotic diversity and how deeply these organisms are woven into the evolutionary tree of life.
The Expanding Tree of Prokaryotic Life
- Molecular techniques (e.g. ribosomal RNA sequencing, PCR, metagenomics) have revealed many previously unknown lineages.
- Metagenomics allows whole prokaryotic genomes to be sequenced from environmental samples—no need to culture in a lab.
- Though ~16,000 species are officially named, one handful of soil may contain 10,000 prokaryotic species.
- Phylogenetic analysis shows that many bacterial groups are not monophyletic—some traditional categories (like Gram-negative bacteria) are genetically scattered.
Horizontal Gene Transfer (HGT)
- HGT has played a massive role in prokaryotic evolution.
- On average, 75% of genes in a bacterial genome were transferred from other species at some point.
- HGT blurs phylogenetic lines, making classification difficult, but it underscores the mosaic nature of prokaryotic genomes.
Overview of Bacterial Diversity
- Proteobacteria: Extremely diverse gram-negative clade; includes:
- Autotrophs (e.g. Thiomargarita), pathogens (Neisseria, Vibrio, Helicobacter), and ancestors of mitochondria.
- Chlamydias: Animal cell parasites lacking peptidoglycan. Cause blindness and STDs.
- Spirochetes: Helical, internal flagella. Include free-living species and pathogens (Treponema pallidum, Borrelia burgdorferi).
- Cyanobacteria: Photosynthetic bacteria that produce O₂. Ancestors of chloroplasts. Abundant in phytoplankton.
- Gram-positive bacteria: Includes:
- Soil decomposers (e.g. Streptomyces, source of antibiotics).
- Pathogens (e.g. Staphylococcus, Clostridium, Bacillus).
Archaea: Unique and Versatile
- Archaea share traits with both bacteria and eukaryotes.
- Extremophiles thrive in extreme conditions:
- Halophiles: Live in highly salty environments (e.g. Halobacterium).
- Thermophiles: Survive at high temperatures (e.g. Sulfolobus, “strain 121”).
- Methanogens: Anaerobic archaea that produce methane, found in swamps and animal guts.
- Many archaea are not extremophiles and live in moderate habitats like oceans and soils.
TACK Supergroup and Lokiarchaeotes
- Newly discovered archaeal lineages (Thaumarchaeota, Aigarchaeota, Crenarchaeota, Korarchaeota) form the TACK supergroup.
- Lokiarchaeotes, close relatives of TACK archaea, may be the sister group to eukaryotes, shedding light on the origin of complex cells.
- These discoveries suggest our understanding of prokaryotic phylogeny will continue to evolve.
In a Nutshell
Prokaryotes represent some of the most diverse life forms on Earth. Advances in molecular biology have revealed vast genetic diversity, frequent horizontal gene transfer, and many lineages still unknown to science. Both bacteria and archaea exhibit a wide range of lifestyles, from extreme environments to everyday ecosystems, and continue to shape life’s evolutionary story.