In addition, because the hosts are also evolving, they occasionally develop immune mechanisms that free them from parasites. However, the parasites generally return after a time, and the hosts and parasites show alternating patterns of advantage, as the parasites overcome the defenses of the hosts, and the hosts evolve new defenses.
Figure 9: Evolutionary race between hosts and parasites in a soup of the Tierra program.
Hosts, red, are very common. Parasites, yellow, have appeared
but are still rare.
Hosts, are now rare because parasites have become
very common. Immune hosts, blue, have appeared but are rare.
Immune hosts are increasing in frequency, separating the
parasites into the top of memory.
Immune hosts now dominate memory, while parasites and susceptible
hosts decline in frequency. The parasites will soon be driven to
extinction. Each image represents a soup of 60,000 bytes, displayed
as 60 bars of 1000 bytes each. Each individual creature is
represented by a colored bar, colors correspond
to genome size (e.g., red = 80, yellow = 45, blue = 79).