Genetic Drift & Macroevolution - Study Notes

Genetic Drift happens when allelic frequencies in a population change due to chance; the smaller the population, the greater the impact.
In a small population, it might only take a few generations before individuals are all the same color or share other traits, reducing variation quickly.
Consequence: Genetic drift tends to eliminate variation more quickly in small populations; large populations tend to maintain greater genetic diversity.

A type of genetic drift that occurs when most of the population is killed off by chance events (fire, flood, volcano, earthquake).
Result: Many alleles carried by the removed individuals are lost; the survivors represent only a subset of the original genetic variation.
After a bottleneck, there is decreased genetic variability; once alleles are lost, it is extremely difficult to regain diversity through mating.

A type of genetic drift that occurs when a very small group colonizes a new area.
The small group becomes the founders of a new population; their limited genetic variation shapes the genetic makeup of future generations.
The concept is often illustrated with a chart showing population evolution from the founding generation to later generations.

If a population loses all genetic variation (only one form of every gene), crossing over during meiosis cannot restore new alleles.
With all individuals genetically alike, environmental changes may lead to universal disadvantage, increasing extinction risk.
Variation is crucial for adaptability to changing environments and for long-term survival.

Historical context: Cheetahs once existed across Asia and Africa but have been driven out by human expansion and overhunting.
Current estimates: about $7{,}000$ cheetahs remain in Africa and $50$ in Asia.
Genetic consequence: The population is highly homozygous due to past bottlenecks and genetic drift.

Both wild and zoo cheetah populations are nearly identically homozygous.
The cheetahs have only one form of almost all of their genes, reducing genetic diversity.

New alleles arise through mutations; however, the mutation rate for animals and plants is extremely low.
Given the small population size and low mutation rate, reliance on mutation alone is unlikely to restore genetic diversity quickly.
The slide suggests: "The Only Hope: Random Mutations" as a conceptual note on how new variation might arise.

Extinction can be caused by a variety of factors including:
- meteor impact
- climate change
- predation
- disease
- competition
- degradation of habitat
Mass extinctions have occurred frequently in Earth's history, eliminating multiple species at once.
Scientists recognize five mass extinctions in the past; some propose a sixth, and perhaps starting a seventh, may be underway.

Ordovician–Silurian (about $440$ mya): affected small marine organisms
Devonian (about $365$ mya): affected larger marine organisms
Permian–Triassic (about $250$ mya): the largest extinction event in history; many species disappeared; openings for dinosaur ancestors
Triassic–Jurassic (about $210$ mya): affected marine life and some land insects and vertebrates
Cretaceous–Tertiary (about $65$ mya): extinction of the dinosaurs

Many scientists propose we are in the midst of the sixth mass extinction, called the Holocene Epoch, which began about $11{,}700$ years ago and is linked to human activity and population growth.
The extinction rate is about $1000$ times what it would be without human impact.
The extinction of many animal and plant species is attributed to human hunting and climate change.
The extinction rate has risen during the 20th century, leading some scientists to suggest a new epoch, the Anthropocene Epoch.
epoch: a geologic time segment with a distinctive characteristic.

Four types of macroevolution:
- Adaptive Radiation
- Divergent Evolution
- Convergent Evolution
- Coevolution

Definition: The emergence of numerous species from a common ancestor.
Occurs when organisms move to a distinctly new environment or when major environmental change occurs (e.g., Darwin's Finches).

Adaptive radiation can lead to divergent evolution; new species arise from a common ancestor.
Divergent evolution can also arise from disruptive selection.

Convergent evolution occurs when different organisms, from different ancestors, develop similar characteristics due to adapting to similar environments.
Example: Sugar glider (a marsupial) and flying squirrel (a placental mammal) show similar traits despite different lineages.

Definition: When an evolutionary adaptation in one species affects the evolution of another species.
It requires linkage of genetic changes between the two species.
Example: Butterflies and flowers have evolved mutually compatible characteristics; a mutualistic relationship where each party benefits.