Chapter 22
In a phylogenetic tree, what does a branch point (node) represent?
The most recent common ancestor
The three types of symbiosis are...
mutualism, commensalism, and parasitism
What process allows Eukaryotic cells to engulf food particles into vesicles?
phagocytosis
In which of the following groups did multicellularity evolve independently? Animals only, Plants and Fungi, Algae only, or All of the above
All of the above
Which fungal structure provides the large surface area needed for nutrient absorption?
Hyphae
Which type of group contains a common ancestor and all of its descendants?
monophyletic
What group of prokaryotes includes species that thrive in very salty environments?
Halophiles
What organelle is believed to have originated from proteobacteria through endosymbiosis?
mitochondria
What three processes are required for complex multicellularity?
Cell Adhesion, Communication, Genetic Programming
True of False: The parasexual cycle in fungi allows genetic diversity without meiosis
True
The molecular clock hypothesis is powerful in estimating divergence times. Why is it particularly useful when fossil evidence is scarce, and what is its key underlying assumption?
Molecular mutations accumulate at relatively constant rates
Assumes mutation rates can be used as “clocks” for divergence time estimates.
A researcher adds purified DNA containing an antibiotic resistance gene to a culture of non-resistant bacteria. After incubation, some bacteria survive on antibiotic plates. Which process most likely explains how these cells acquired the resistance gene, and what does this suggest about horizontal gene transfer in natural environments?
Transformation — uptake of naked DNA; shows how bacteria can adapt quickly by acquiring genes from dead cells or free DNA in the environment.
Comparative genomic studies show that both fungi and animals share a single posterior flagellum in their motile cells, unlike plants and algae, which typically have anterior flagella. Molecular phylogenies using actin and tubulin genes also group fungi and animals together. Based on this evidence, to which eukaryotic supergroup do they belong?
Opisthokonts
During early embryonic development in animals, rapid mitotic divisions (cleavage) occur without significant growth, producing a hollow sphere of cells. This stage precedes gastrulation, when germ layers begin to form. Developmental biologists often use this stage to study cell fate determination and polarity. What is this structure called?
Blastula- the hollow ball of cells that marks the end of cleavage and the beginning of gastrulation.
A soil ecologist observes that fungi in a forest floor spread extensively through the soil, increasing contact with organic matter and plant roots. This network of filaments enables efficient nutrient absorption and forms mutualistic associations such as mycorrhizae. What is this interconnected structure called?
Mycelium — the collective network of hyphae that maximizes surface area for absorption and ecological interactions.
What does a paraphyletic group include and provide an example.
a common ancestor and some, but not all of its descendants
A medical team isolates a bacterial strain from a patient infection. The cells appear pink after a Gram stain. The strain is more resistant to penicillin than typical Gram-positive bacteria. What reasoning would best explains both the staining result and the increased antibiotic resistance?
Gram-negative bacteria have a thin peptidoglycan layer and an additional outer membrane containing lipopolysaccharides, which blocks many antibiotics and causes them to stain pink.
Scientists sequenced the DNA of an organelle and found it to be circular, with genes most closely related to those of α-proteobacteria. The organelle is also surrounded by a double membrane and contains its own ribosomes. Based on these data, which organelle most likely originated from endosymbiosis with a proteobacterium?
Mitochondrion — supported by genomic, structural, and biochemical evidence of its proteobacterial ancestry.
In multicellular organisms, cells must communicate to coordinate growth and development. In plants, cytoplasmic connections allow direct exchange of signaling molecules, ions, and nutrients across rigid cell walls. In animals, specialized protein channels connect adjacent cells for rapid communication. Which structures perform these roles in plants and animals, respectively?
Plasmodesmata in plants; gap junctions in animals
During fungal sexual reproduction, two haploid cells first fuse their cytoplasm but keep their nuclei separate, resulting in a prolonged dikaryotic (n+n) stage. Only later do the nuclei merge to form a diploid zygote nucleus. Which terms describe these two sequential fusion events?
plasmogamy; karyogamy
The principle of maximum parsimony assumes evolutionary events are minimized. How might this principle conflict with cases of convergent evolution?
Parsimony can be misleading when convergent evolution produces similar traits independently (e.g., wings in birds vs. bats).
A wetland restoration project is struggling because plants show severe nitrogen deficiency despite adequate sunlight and water. Soil analysis reveals very low populations of symbiotic bacteria such as Rhizobium. The restoration team considers adding industrial fertilizer but also tests for microbial activity capable of reducing atmospheric N₂. Which biological process is missing in this ecosystem and which organisms normally carry it out?
The missing process is nitrogen fixation, the conversion of N₂ gas into ammonia (NH₃) and it is carried out by diazotrophs such as Rhizobium, some cyanobacteria, and free-living soil bacteria, using the enzyme nitrogenase.
Comparative genomics reveals that plastid DNA in Archaeplastids (plants, red algae, green algae) shares the greatest similarity to cyanobacterial genomes. Additionally, plastids are surrounded by two membranes, consistent with engulfment of a free-living prokaryote rather than secondary endosymbiosis of an already symbiotic eukaryote. Which prokaryote was engulfed during this primary endosymbiotic event? How does the number of plastid membranes provide evidence for the nature of this event? Why is this primary endosymbiosis considered one of the most important evolutionary innovations in Earth’s history?
The symbiont was a cyanobacterium.
Two membranes indicate direct engulfment of the prokaryote: one from the cyanobacterium and one from the host phagosome.
This innovation allowed oxygenic photosynthesis in eukaryotes, driving the diversification of algae and plants, altering global carbon and oxygen cycles, and enabling the rise of complex ecosystems.
Fossil evidence shows that large, multicellular eukaryotes appear in the record roughly 600 million years ago, shortly after the “Great Oxidation Events.” Molecular and physiological studies reveal that complex multicellularity requires high-energy metabolism to support bulk flow, specialized tissues, and signaling networks. Which atmospheric change enabled the evolution of complex multicellularity? How did this shift alter the efficiency of ATP production compared to anaerobic metabolism? Why was this change a prerequisite for the diversification of animals and plants during the Precambrian and Cambrian periods?
A rise in atmospheric oxygen levels.
Oxygen enabled aerobic respiration, which produces ~18× more ATP than anaerobic glycolysis.
This energy surplus allowed the evolution of larger, more complex multicellular organisms, facilitating differentiation, tissue specialization, and ecological expansion.
Place the following fungal innovations in the correct evolutionary order: 1. Septa 2. Hyphae 3. Fruiting Bodies 4. Chitin in cell walls
4, 2, 1, 3