I start with 5,000 cards, but I end up with approximately 5,000 cards. The population varies because you are flipping a coin. It is unlikely that you will have exactly 5,000 cards. Maybe 4,948 or 5,217 for example.I got a little lost in this one.(the new becoming the old pile). But your population didn't increase. You started with 5000 cards and ended with 5000 cards. You may have successful breeders but the numbers are stagnant. How does this reflect evolution that starts out with one naked self replicating cell and eventually slowly blossoms into what we have now?
Evolution is a little more complex. Depending on how well the organism is adapted to its environment there may be as much as a 60% chance of heads, or as little as a 40% chance of heads. In the first case, the population will increase while in the second case the population will decrease. An increasing population will eventually be limited by the food supply or perhaps by predators. A decreasing population will go extinct unless something changes.
For most populations for most of the time, the population size stays about the same. That does not mean that there cannot be temporary changes. Human populations dropped during the Black Death, people died before having children, thus rolling tails. Human populations increased after the development of agriculture, the chance of rolling heads increased because there was a larger and more reliable food supply.