You may have heard of the idea of a genetic bottleneck. It is when genetic variation is lost by sampling a very small subset of a population. Just by chance, the frequencies of alleles on the subset of the population will be different to those in the whole population, and overall genetic variation is likely to be lost by population sampling.
This is why small population sizes are not good, and species on conservation lists are required to not drop to too small population sizes. The limited genetic variation remaining in a small population poses a risk because it makes the species susceptible to whatever relies on genetic variation for adaptation, such as environmental changes or diseases.
Here is a visualisation of a bottleneck.
A more recent way to think about bottlenecks is that you don't need a catastrophic reduction in population size to achieve them. Population expansion can have the same result, because expansion is usually from a subset of the population, for example the individuals in the edge of the species range.
Furthermore, if the expanded individuals establish a population locally, it is difficult for genetic variation to be reintroduced at the expanded range, by more individuals migrating from the original population centre, because there is no space for them by the time they arrive. That is why, in terms of genetic variation, there is “Southern richness and Northern purity” in European species (see Hewitt 2000).
It is quite obvious once you think about it.
But you don't need to think anymore! You can now run a simulation illustrating this on your browser. Head here and press the play button and see for yourself that the multicoloured centre of a disk, representing ancestral genetic variation, quickly turns into random sized unicolour pizza slices, i.e. genetic variation is lost by population growth as the disk grows.
The authors have a more formal explanation...
Here](https://youtu.be/nkvVR-sKJT8) is an essential video that distils a lot about geometry, that anyone finishing school should know.
A bit of history, a bit of philoophy, a bit of math, from ancient Greece all the way to our modern understanding of spacetime and the universe.
Well worth the time of watching the whole series.
You may have heard that the moon should appear the same size regardless of its position on the horizon, since its distance to the observer varies minimally with its position on the horizon. Therefore the fact that it appears larger when close to the horizon is an illusion.
What I did not know is that we don't fully understand the illusion. Three explanations are offered in this video.
One has to do with our brains performing a relative comparison with nearby objects of known size, such as trees, which is only possible when the moon appears on the horizon. But the explanation fails to account for the illusion also working for sailors and pilots, even though they don't have such objects to confuse them.
Another explanation involves perspective correction, where things that are far away are assumed to be bigger. The video has a nice illustration of this.
Yet a third explanation is that the brain judges distance by taking into account how much our eyes need to focus to observe something. This focus is different for the horizon and for looking at the top of the sky.
No single explanation works in all cases, so amazingly we still don't know why the moon appears larger. Check out the video for a more visual explanation of the same points.
And here is wikipedia’s take.
I was reminded in the Natural History Museum in New York that the Mediterranean sea was dry at some point.
If the connection to the Atlantic Occean was cut off, it would dry again. That is why it is so salty, especially in the East, it evaporates more than it can be replenished from rivers.
It did not take too long to go from there, to this video with graphics and some speculation about the consequences of no water in the Mediterranean.
How much of Europe would be a desert, how much deeper than the Grand Kanyon would it be, and how much higher the rest of sea level would be?