21.4 Convergence, Extinction, Polymorphism, and Drift
Key Takeaways
Sharks and dolphins have streamlined bodies because both are active swimmers. Dolphins are mammals, so the similarity is convergence.
Background extinction is ongoing, while a mass extinction removes many lineages in a geologically short interval.
The end-Cretaceous extinction is associated with the loss of non-avian dinosaurs. Humans did not cause the ancient mass extinctions.
Balanced polymorphism keeps both alleles common when heterozygotes have an advantage, as with the sickle-cell allele where severe malaria is present.
Genetic drift is random allele-frequency change, strongest in small populations. A bottleneck is a crash, a founder effect is colonization by a few individuals, and drift is not selection.
21.4 Convergence, Extinction, Polymorphism, and Drift
Convergent evolution makes distant relatives look alike. Extinction removes lineages. Balanced polymorphism keeps two alleles common because heterozygotes have an advantage. Genetic drift changes allele frequencies by chance. Do not treat every similarity as close ancestry, every ancient loss as a human act, or every frequency change as survival of the fittest.
Convergence in the water
Convergence is the evolution of similar traits in lineages that are not close relatives and that live in similar ways. Sharks and dolphins have streamlined bodies, a dorsal fin, and paired front fins or flippers because both are active swimmers. They did not inherit that shape from a recent common ancestor.
A shark is a cartilaginous fish with gills and a vertical tail fin. A dolphin is a mammal with lungs and milk, driven by horizontal tail flukes, and its ancestors lived on land. The dolphin flipper contains tetrapod forelimb bones, so that skeleton is homologous to a human arm. As a swimming surface it is analogous to a shark pectoral fin. Which word applies depends on the comparison. Radiation splits one lineage into different niches. Convergence makes already separate lineages more alike.
Background extinction and mass extinction
Extinction ends a lineage. Background extinction is ongoing species loss at a comparatively low rate as climates and local conditions change. One species can vanish this way without a global crisis.
A mass extinction removes many lineages, across many habitats, in a geologically short interval. The end-Permian event is the largest in the fossil record. The end-Cretaceous event is the one to link to dinosaurs. It is associated with a large asteroid impact and with severe disruption, and with the loss of the non-avian dinosaurs. Birds are living dinosaurs, so it is wrong to say every dinosaur died out. Mammals were already present and later radiated into roles that loss had opened.
Humans did not cause the ancient mass extinctions. Those events came before our species. Today's conservation losses are a different subject and do not explain the end of the Cretaceous.
Balanced polymorphism and the sickle-cell allele
A polymorphism means two or more alleles are both common. It is balanced when selection maintains both. The mechanism here is heterozygote advantage: the heterozygote leaves more surviving offspring in that setting than either homozygote.
The sickle-cell allele changes hemoglobin. People with two copies have sickle-cell disease. Their red cells can sickle when oxygen is low, which causes pain, anemia, and organ damage. People with two typical alleles do not have that disease. Heterozygotes carry one sickle-cell allele and one typical allele and usually do not have sickle-cell disease. Where severe malaria caused by Plasmodium falciparum is present, heterozygotes are less likely to develop severe malaria. People with two typical alleles are more vulnerable to severe malaria there.
The heterozygote therefore does better than either homozygote where severe malaria is a major danger, so both alleles stay common. That is selection, not drift. Where malaria is absent, the advantage goes away and two copies of the allele are selected against because they cause serious disease. The person with sickle-cell disease has two sickle-cell alleles. The heterozygote is the carrier.
| Genotype | Sickle-cell disease | Severe malaria, where that disease is present |
|---|---|---|
| Two typical hemoglobin alleles | Does not have the disease | More vulnerable to severe malaria |
| Heterozygote, one sickle-cell allele | Usually does not have the disease | Less likely to develop severe malaria |
| Two sickle-cell alleles | Has sickle-cell disease | Illness from sickled cells is the severe cost of this genotype |
Genetic drift, bottlenecks, and founders
Genetic drift is random change in allele frequency caused by sampling. It is strongest in small populations, where chance draws a poor copy of the parental frequencies. In a large population the random swing is small.
A bottleneck is a population crash. The few survivors are a sample of the old gene pool, not automatically the fittest animals. After recovery, the population keeps the frequencies that sample left behind. Northern elephant seals were hunted to a very small population, later became numerous, and still have little genetic variation.
A founder effect starts a population from a few colonists. Their alleles, not the whole source population, set the new frequencies. A rare allele in the source can be common in the colony if the founders carried it. In some Amish communities an allele for Ellis-van Creveld syndrome, a condition with short stature and extra fingers, became common that way. Helpfulness is not required.
Drift can fix an allele or lose one. Fixation means the allele reaches every individual. A harmful allele can be fixed by chance while the population is small. Natural selection sorts by heritable differences in reproductive success. Drift does not. A crash can change frequencies even when survival had nothing to do with the allele in question.
Warning
Genetic drift is not survival of the fittest. A bottleneck or a founder event can change allele frequencies by chance, and drift can fix a harmful allele. The sickle-cell heterozygote is not the person with the full disease. Heterozygotes usually do not have sickle-cell disease, and where severe malaria is present they are less likely to develop it.
Keep the four patterns separate
- Sharks and dolphins look alike because of life in water, and dolphins are mammals.
- Background extinction continues. A mass extinction removes many lineages in a geologically short interval. The end-Cretaceous loss of non-avian dinosaurs was not caused by humans.
- Sickle-cell heterozygotes keep both alleles common where severe malaria is present. That balance is selection.
- Drift is strongest in small populations. A bottleneck shrinks a population. A founder effect begins one from a few individuals.
Similar bodies in distant relatives are convergence. A random shift after a crash is drift. An allele maintained by heterozygote advantage is balanced polymorphism.
A population of island birds crashes to a few survivors after a storm, then recovers. The survivors happened to carry mostly one allele that had been uncommon, and that allele is harmful. What happened?
Natural selection fixed the fittest allele, because every bottleneck keeps only the best traits.
Genetic drift changed the allele frequency. A bottleneck is a population crash, and the survivor sample can shift frequencies by chance.
The birds converged on the body shape of sharks and dolphins.
Humans caused an ancient mass extinction, and this storm is that same event.
Why do sharks and dolphins both have streamlined bodies?
The similarity is convergent. Both are active swimmers, and dolphins are mammals rather than close relatives of sharks.
The pattern is balanced polymorphism of one gene inside a single species.
Dolphins are cartilaginous fishes, so the match is expected from a close relationship.
They inherited the body shape from a recent common ancestor that was already a dolphin-like shark.
Where severe falciparum malaria is present, what is true of the sickle-cell allele?
The allele stays common by genetic drift alone, and heterozygote advantage plays no role.
People with one sickle-cell allele and one typical allele have full sickle-cell disease, so the allele cannot remain common.
Heterozygotes usually do not have sickle-cell disease and are less likely to develop severe malaria, so both alleles can remain common.
People with two typical alleles have sickle-cell disease, and people with two sickle-cell alleles only carry the trait.
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