Gizem

There is also a genuine trade-off, which is worth knowing because it explains why pigmentation is not simply maximised everywhere.

Skin needs some ultraviolet exposure for vitamin D synthesis. Heavy pigmentation reduces that, which at high latitudes with weak winter sun becomes a real cost. The global pattern of skin pigmentation tracks latitude reasonably well, and the usual explanation is exactly this balance — enough protection where the sun is strong, enough transmission where it is weak.

That is a nice illustration of the general point: evolution rarely produces maximal anything. It produces compromises tuned to a particular environment, and moving the population moves them off their compromise.

Two more mechanisms worth knowing, because they explain why small populations sometimes do better than the simple model predicts.

Rapid growth reduces the damage. Inbreeding effects are worst when a population stays small for a long time. A founding group that expands quickly into empty habitat spends few generations at low numbers, so less diversity is lost to drift than the founding count alone suggests.

Purging. In a small population, harmful recessive variants are exposed in the homozygous state more often, and individuals carrying them are removed. Over generations that can strip out some of the worst variants, leaving a population that is genetically impoverished but not obviously sick. It is a brutal mechanism and it is real.

Neither means the population is healthy in any general sense. Reduced diversity leaves it fragile — poorly equipped for a new disease or an environmental shift — and that fragility does not show up while conditions are good.

The main thing you are missing is time, measured in generations rather than in years.

For a large, long-lived mammal, sexual maturity may be several years away and generation times are long. Four decades might be only four to six generations. Inbreeding depression accumulates across generations, and at that count you are still early — the effects are present and typically not yet severe enough to stop population growth.

Compare with a fruit fly, where forty years would be hundreds of generations and the outcome would be entirely different. People intuitively convert years to "a long time" when the relevant clock is generations.

So the case you are describing is not evidence that founder effects are weak. It is a population early in a process whose consequences are still ahead of it.

Your records are probably fine. Sharing no detectable DNA with a genuine fifth cousin is not merely possible, it is the more likely outcome.

The mechanism is that inheritance is a lottery at every step. You get half your DNA from each parent, but which half is chosen essentially at random, in large chunks. Each generation halves the expected share from any given ancestor, and — the crucial part — the variance around that expectation is enormous.

Rough expectations for autosomal DNA:

• First cousins: about 12.5%, and always detectable.
• Second cousins: about 3%, essentially always detectable.
• Third cousins: about 0.8%, detected roughly 90% of the time.
• Fourth cousins: about 0.2%, detected around half the time.
• Fifth cousins: around 0.05%, detected perhaps 10 to 15% of the time.

So for fifth cousins, not matching is the normal result and matching is the exception.

One evolutionary note that completes the picture: a virus that destroys its host tissue rapidly is generally not doing well by its own standards.

Transmission requires a host who is up and about, breathing on people, for as long as possible. Killing the local tissue quickly shortens that window. There is real selective pressure on many pathogens towards being contained enough to keep the host functional — which is not benevolence, it is arithmetic about how many further hosts get infected.