9: Beyond the Mk Model
The simple Mk model provides a useful foundation for a number of innovative methods. These methods capture evolutionary processes that are more complicated than the original model, including models that vary through time or across clades. Modeling more than one discrete character at a time allows us to test for the correlated evolution of discrete characters.
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- 9.1: The Evolution of Frog Life History Strategies
- Frog reproduction is one of the most bizarrely interesting topics in all of biology. Across the nearly 6,000 species of living frogs, one can observe a bewildering variety of reproductive strategies and modes. As children, we learn of the “classic” frog life history strategy: the female lays jellied eggs in water, which hatch into tadpoles, then later metamorphose into their adult form. But this is really just the tip of the frog reproduction iceberg. Many species have direct development.
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- 9.5: Threshold Models
- Threshold models work by modeling a discrete character as underlain by some other, unobserved, continuous trait (called the liability). If the liability crosses a certain threshold value, then the discrete state changes. More specifically, we can consider a single trait, y, with two states, 0 and 1, which is in turn determined by some underlying continuous variable, x, called the liability. If x is greater than the threshold, t, then y is 1; otherwise, y is 0.