Lecture 5: Molecular Genetics II

What this lecture covers

This lecture continues directly from the previous one, moving from small point mutations that tweak a single protein to mutations that reshape entire networks of genes. It asks how macroevolutionary change, the kind Gould's punctuated equilibrium describes, could actually happen at the molecular level, since the previous lecture left that question open as the main objection to punctuated equilibrium.

You come away understanding how mutations in splicing enzymes, promoters, transcription factors and transposable genetic elements can create entirely new proteins or gene networks in a single step, why such large changes are usually harmful, and why they can nonetheless spread rapidly during evolutionary bottlenecks. The lecture ends by proposing that gradualism and punctuated equilibrium are not rival theories but descriptions of different mechanisms operating together, closing out the course's first disciplinary detour into molecular biology before moving to behavioral genetics.

Key ideas

• Splicing enzyme mutations: a mutation that changes how an enzyme cuts and joins exons can produce entirely new proteins that never existed before, not just a modified version of an existing one.
• Promoter mutations: changing a regulatory sequence upstream of a gene changes when and where that gene is switched on, altering behavior (as in vole monogamy) without changing the gene itself.
• Transposons: Barbara McClintock's discovery that segments of DNA can move to new locations in the genome, first found in plants and later shown to occur in animals, including in the immune system and in neurons.
• Gene duplication: an extra copy of a gene lets one copy keep its original function while the other is free to mutate and potentially acquire a new one, without losing existing capabilities.
• Evolutionary bottleneck: an event so selective that a large-effect mutation's overall risk stops mattering, letting a rare trait sweep through a population very quickly.
• Micro vs. macro evolutionary change: small point mutations tend to fine-tune existing systems (such as immune resistance to specific diseases), while regulatory mutations tend to reshape development and produce large, qualitative differences between species.
• Reconciling gradualism and punctuated equilibrium: many traits changing gradually at once, each in its own punctuated bursts, can look like smooth gradual change at the population level.

Walkthrough

New proteins from splicing and promoter mutations (6:04)

Building on the exon/intron model from the prior lecture, the lecture shows that a mutation in a splicing enzyme can cause it to cut a gene's transcript at different points, producing entirely novel proteins rather than a variant of an existing one. Similarly, a mutation in a promoter changes which genes a transcription factor activates, or when. The vasopressin receptor promoter is used as the running example: a different promoter variant distinguishes monogamous from polygamous vole species, and swapping the promoter experimentally changes mating behavior. Related human research links a variant of this promoter to relationship stability, and a variant near the dynorphin gene has been linked to drug addiction risk.

Transposons and Barbara McClintock (19:13)

The lecture tells the story of Barbara McClintock, who in the mid-20th century proposed that genes can physically move within the genome, based on inheritance patterns in corn. Ridiculed for decades, she was eventually vindicated when molecular techniques confirmed transposable genetic elements in the 1980s, and she won a Nobel Prize. The lecture explains why plants, which cannot flee stress, were the right organism to reveal this: transposons let them shuffle DNA under stress in search of useful new traits. The same mechanism was later found in animals, including in the immune system's generation of novel antibodies, in a parasite (trypanosome) that uses it to evade immune detection, and notably in neural progenitor cells, where a specific transposable element becomes unusually active while new neurons are being generated.

From transposons to new if-then rules (33:19)

The lecture works through hypothetical and real examples of how a transposon moving a promoter or an exon can invent a new regulatory rule instantly. Moving a "dehydration" promoter upstream of a fertility gene could create seasonal mating tied to rainfall; moving a "smells like me" promoter upstream of a cooperation gene could be an early mechanism for kin selection. A real example follows: steroid hormone receptors have separate exons for hormone-binding and DNA-binding domains, so swapping a hormone-binding domain between receptor genes could explain how immune suppression, originally tied to glucocorticoids, became linked instead to progesterone, a plausible mechanism for suppressing the immune system during pregnancy (with autoimmune flare-ups after birth as a side effect).

Gene duplication and irreducible complexity (46:29)

Gene duplication, including the larger-scale "copy number variants" now linked to conditions like schizophrenia, lets one copy of a gene keep performing its original job while the second is free to mutate without immediate cost. The lecture cites researcher Joe Thornton's work showing that pairs of related steroid receptor genes evolved this way, and argues this undermines the "irreducible complexity" objection to evolution: intermediate forms of a trait, such as a partial eye, do not need to be dramatic evolutionary jumps, because duplicated genes can carry a nonfunctional variant along for many generations until it stumbles into a new use.

Bottlenecks and reconciling the two models (54:41)

Because large regulatory or network-level mutations usually harm many traits at once, most of them are eliminated by stabilizing selection, producing long periods of evolutionary stasis. But during an evolutionary bottleneck, when survival depends on one overriding trait, a rare large-effect mutation can sweep through a population rapidly regardless of its other costs. Cheetahs and points in hominid history show genetic signatures consistent with past bottlenecks. The lecture then reviews directly observed rapid evolution: a century of genetic change in Chicago rats, shifting finch beak traits in the Galapagos, the rapid rise of adult-onset diabetes in populations (such as the Pima Indians and Yemenite Jews) switching to Western diets, and the 35-generation domestication of Siberian silver foxes bred purely for tameness, which also produced floppy ears, patchy coats and other juvenile traits as side effects. The lecture concludes that gradualism and punctuated equilibrium likely both occur, since many traits changing in their own punctuated bursts can add up to something that looks gradual at the level of a whole population.

Before you watch

• Watch the previous lecture in this course (Molecular Genetics I), since this lecture continues directly from its discussion of exons, introns, promoters and transcription factors.
• Recall the earlier critique of punctuated equilibrium, particularly the demand for a molecular mechanism that could produce rapid evolutionary change, since this lecture is built around answering that challenge.

Check your understanding

1. Why can a mutation in a splicing enzyme create an entirely new protein, rather than just a modified one?
2. How did the vasopressin receptor promoter experiment demonstrate that behavior can change without a change in the gene itself?
3. Why did Barbara McClintock's discovery of transposons take decades to gain acceptance, and what convinced the field she was right?
4. What role does gene duplication play in explaining how new functions can evolve without losing old ones?
5. Why do most large-effect regulatory mutations get eliminated by selection, and what circumstance allows them to spread rapidly instead?