Major Evo Transitions (Ch15)

Define Major Evolutionary Transitions

Refers to the events that have changed the way life is organized

How does life change from a prokaryotic cell to a blue whale?

The answer is probably one evolutionary step at a time

What 9 events changed the way life is organized?

1. Origin of self-replicating molecules capable of heredity

2. Transition from RNA to DNA

3. Origin of the first cells

4. Emergence of eukaryotic cells (ft. membrane-bound organelles and nuclei)

5. Evolution of sexual reproduction

6. Evolution of multicellularity (Evolved independently many times)

7. Evolution of developmental complexity

8. Evolution of individuality

9. Evolution of group living (Evolved independently many times)

→These transitions often share a common structure where individuals give up independent reproduction to form larger groupings that reproduce collectively, allowing them to benefit from economies of scale and efficiencies of specialization.

Explain the evolution of the eukaryotic cell

The eukaryotic cell evolved 1–2 billion years after prokaryotes (>3 billion years) through a singular event that introduced more complexity, membrane-bound organelles and extensive within-cell communication networks that coordinate interactions.

Explain the Endosymbiotic Theory (Symbiogenesis)

It proposes that eukaryotic organelles like mitochondria and chloroplasts originated as independent bacteria that were engulfed by and came to live inside larger ancestral cells.

What confirmed the Endosymbiotic Theory?

It was confirmed by genetic evidence showing that these organelles maintain their own DNA and replicate independently, with mitochondria likely evolving from alpha proteobacteria and chloroplasts from cyanobacteria, which both evolved once in eukaryotic evolution.

Explain the conundrum of the eukaryotic nucleus

The conundrum of the eukaryotic nucleus arises bc it was viewed as a singular evolutionary event, but new discoveries suggests a more complex evolutionary process than a simple, one-time occurrence.

New discoveries include:

1. Emerging knowledge of bacterial internal structure

2. Metagenomic identification of new Archaea that carry homologs of
   genes associated with internal cell membrane structure in eukaryotes

3. Recent evidence suggests frequent organelle to nucleus migration

Explain the 3 domain hypothesis and the eocyte hypothesis

1. The 3 domain hypothesis: Suggests that life is organized into 3 distinct, primary lineages: Bacteria, Archaea, and Eukaryota

→Eukaryotes positioned as a sister group to the entire archaeal domain.

2. The eocyte hypothesis: Suggests that eukaryotes originated from within the archaeal domain, specifically branching off as a sister group to the Eocytes (Crenarchaeota).

→Implies there are only 2, primary domains, with eukaryotes representing a specialized lineage that evolved from an archaeal ancestor.

Explain common structure and common consequences

Major evolutionary transitions share a common structure where individuals give up independent reproduction to form larger groupings that reproduce collectively.

This results in common consequences, allowing these groups to benefit from:

1. Economies of scale: Groups perform a task more efficiently than a single individual or groups can do things individuals cant do at all.

2. Efficiencies of specialization: Once groups are engaged in a task, they benefit from larger numbers and from division of labor.

These transitions facilitate the development of increasingly complex ways for organisms to acquire, process, and store information.

Explain the evolution of multicellularity

Multicellularity evolved convergently more than once approx ~2 billion years ago. The two routes include: Coming together & Staying together

Explain the coming together route in multicellularity

Independent, formerly free-living cells aggregate into a group. This route allows group to immediately benefit from economies of scale, like improved environmental navigation, but it faces greater challenges due to the potential for genetic dissimilarity among members.

→This route doesn’t explain many multicellularity cases

Explain the staying together route

The most common/favored route, where daughter cells fail to separate even though parent cells divide and subsequent growth follows the same pattern. Because the resulting cluster consists of genetically identical clones, it has the effect of reducing genetic conflict among cells and cell lineages. The more genetically similar cells are leads to their reproductive interests being more closely aligned.

→Natural selection would favor a well-integrated cluster of cells. This route explains most cases of multicellularity.

Explain the staying together example with yeast and multicellularity

In experimental yeast populations, they manipulate selective conditions that favors “settling selection”, which yeast reproduction and forced proximity led to the formation of "snowflake clusters" where cells remained attached following division.

→Bc these cells are clones, their reproductive interests are aligned, which reduces genetic conflict and favors the evolution of a well-integrated multicellular individual.

Explain the coming together example with slime molds and multicellularity

The transition is found in slime molds, where individual cells coordinate their behavior to function as a single unit. The cells that make up the slug coordinate their behavior by releasing cAMP, which signals cells to orient to the source, and as the cells aggregate, they produce sticky proteins to form the slug.

The transition into a multicellular state (slugs) provides the group with several economies of scale, including:

1. Can orient to light, temp, ammonia, and chemicals, allowing the slugs to move toward the soil surface (individual cells cant do this)

2. Can travel through environment more efficiently than individual cells.

3. Forms a slime sheath around itself to protect it from nematode predators, which is not present in individual cells.

4. Evolved to manage the genetic dissimilarity conflict, they exhibit cell discrimination, ensuring that the cells making up the final fruiting body (the stalk and spores) are close genetic relatives.

Explain the evolution of individuality.

The evolution of individuality occurs when groups of cells become integrated and indivisible wholes that reproduce and pass on heritable variations to their offspring.

This transition is defined by two key factors:

1. Cells becoming unable to persist independently

2. Differentiation of function (i.e the separation of somatic and germ cell lines).

Eplain the study done with Volvox carteri to investigate the Evolution of Individuality

It examines how colonies transitioned into integrated organisms through the differentiation of somatic and germ cell lines.

1. Division of Labor: A colony consists of ~2,000 small somatic cells that use flagella for movement and survival but never reproduce, alongside about 16 large germ cells specialized exclusively for reproduction.

2. Genetic Regulation: The fate of these cells are determined by the regA gene, which is expressed in somatic cells to inhibit growth and division, while its absence in germ cells allows them to undergo the multiple rounds of division necessary for reproduction.

3. Survival Necessity: Somatic cells provide a critical economy of scale by using flagellar motion to keep the colony from sinking, which is essential for photosynthesis. Colonies with non-functional somatic cells fare poorly.

Explain the transition from solitary to group living

The transition from solitary to group living involves individuals joining together to form higher-level collectives that reproduce and function together, allowing them to benefit from economies of scale and specialization. Unlike some other transitions, group living has evolved independently many times across diverse species. Has a degree of sociality requirement.

→Benefits: Group foraging, protection from predators, decreasing an individual’s danger, and confusing a predator.

→Costs: Resource competition, cheaters, increased parasite transmission

There are benefits and costs to to group living

• Benefits: Groups achieve greater foraging efficiency—either passively, like bluegill fish flushing prey, or through coordinated roles, like chimpanzee hunts—and increased safety from predators through tactics like "flash explosions".

• Costs: Living in groups introduces trade-offs, such as competition for resources, the rise of "cheaters" who take advantage of the group, and increased parasite transmission, which is seen in the high number of swallow bugs in larger cliff swallow colonies.

• Evolutionary Pattern: Unlike some other transitions, group living has evolved independently many times across diverse species.

Explain the following benefits in group living

→Group foraging

→Protection from predators

→Decreasing an individual’s danger

→Confusing a predator

1. Group foraging: Groups increase efficiency either passively (bluegill fish flushing prey from vegetation), or through coordinated & communicated efforts (chimpanzees using specialized roles to capture prey)

2. Protection from Predators: Groups can develop defenses impossible for individuals

→Like the protective slime sheath formed by slime mold slugs against nematodes

3. Decreasing an Individual’s Danger: Groups reduce the risk to any single member and allows for collective vigilance (

→Like prairie dogs’ vigilance

4. Confusing a Predator: Groups employ tactical maneuvers

→like flash explosions, where members of a school of fish scatter simultaneously to disorient and evade an attacker

Explain the following costs in group living

→Resource competition

→Cheaters

→Increased parasite transmission

1. Resource Competition: As more individuals join a group, they must compete for the same limited food, space, or other environmental resources.

2. Cheaters: Gruops are vulnerable to individuals who exploit group benefits for their own fitness without contributing, creating social conflict.

3. Increased Parasite Transmission: High-density living facilitates the spread of pathogens and pests.

→Ex: Cliff swallows demonstrates a direct correlation between larger colony sizes and an increased number of parasitic swallow bugs per nest.