Evolutionary arms race
An evolutionary arms race is when two species (or populations) are so linked that a change in one indirectly causes a change in the other. It is a metaphor drawn from the so-called arms race between the West and the Soviet Union, after the second world war. Evolutonary arms races are a kind of co-evolution.
- Predator–prey evolution is the classic example. A new twist by the predator must be answered with a new defence by the prey.
- Conifers grew tall, to some extent out of the reach of sauropod dinosaurs. Dinosaurs grew taller and continued eating the conifers.
- Gazelles run fast and so do cheetahs; both have been under a continual series of similar changes which increase their running speed.
- The Rough-skinned newt, the Garter snake and toxin resistance is a well-researched case study. Where the two species live together, the newt produces more toxin, and the snake has evolved to resist it; where they live separately the newt toxin is milder, and the snake has little resistance. The snakes pay a price for their resistance: their digestion and body metabolism is slower than related species. "Really resistant snakes have slower crawl speeds than snakes with little or no resistance".
- Parasite–host evolution is another classic. As a host species splits into two, so do its parasites. And when the host immune system defends against the parasites, so they adapt and survive.
- Sexual selection can be a kind of evolutionary arms race.
- Female mating preferences are known to be responsible for the evolution of male secondary sex characteristics. Selection is between males in species where female choice is the pairing method.
- Sexual conflict between breeding partners is quite common. The basic interests of the male and female can be quite different, and each develops adaptations to get the best 'deal' in terms of offspring. Males: their interest is to mate with a large number of completely faithful females, thus spreading their genes widely in the population. Females: Their interest is to mate with a large number of fit males, thus producing a large number of fit and varied offspring.
- In the sense that, the selection pressure on the second partner changes, and counters the change in the first partner.
- Bakker, Robert T. 1986. The dinosaur heresies: new theories unlocking the mystery of the dinosaurs and their extinction. Morrow, New York. Chapter 9 When dinosaurs invented flowers. p181 "The warfare between plants and herbivores...".
- Science Daily 
- Garter snake info: 
- Price, Peter W. 1980. Parasites. Princeton University Press, Princeton N.J. p34: "The stepwise coevolutionary process results in extreme specialization and complex defense mechanisms".
- Andersson M 1994. Sexual selection. Princeton Univ Press, Princeton, NJ.
- Arnqvist G. and Rowe L. 2005. Sexual conflict. Princeton University Press, Princeton New Jersey
- Schilthuizen, Menno 2001. Frogs, flies and dandelions: the making of species. Oxford University Press, p92. ISBN 019850392X
- Crudgington H. & Siva-Jothy M.T. 2000. Genital damage, kicking and early death. Nature. 407: 855-856.