![]() However, many characters of interest to biologists are best defined as characters with a set number of fixed states. So far, we have only dealt with continuously varying characters. Section 7.2: Modeling the evolution of discrete states Fitting this model to comparative data will help us understand the evolution of traits like limblessness where species can be placed into one of a number of discrete character states. In this chapter, I will discuss the Mk model (Lewis 2001), which is a general approach to modeling the evolution of discrete traits on trees. Analyzing the evolution of discrete traits requires a different modeling approach than what we used for continuous traits. ![]() Limblessness is an example of a discrete trait – a trait that can occupy one of a set of distinct character states. User:Marek_bydg / Wikimedia Commons / CC-BY-SA-3.0. User:Smacdonald / Wikimedia Commons / CC-BY-SA-2.5, F. User:Bladerunner8u / Wikimedia Commons / CC-BY-SA-3.0, E. User:Gionorossi / Wikimedia Commons / CC-BY-SA-4.0, D. Photo credits: A: User:Raul654 / Wikimedia Commons / CC-BY-SA-3.0, B. Figure 7.1E and F), with some estimates that squamates have lost their limbs at least 26 times in the past 240 million years (Brandley et al. In fact, lineages within squamates have lost their limbs over and over again through their history (e.g. And snakes are not the only limbless lineage of squamates. ![]() In other words, snakes are lizards – highly modified lizards, but lizards nonetheless. 2006) – and also underwent a suite of changes to their head shape, digestive tract, and other traits associated with their limbless lifestyle. The squamate lineage that is ancestral to snakes became limbless about 170 million years ago (see Hedges et al. Even the snakes (Figure 7.1C and D), extraordinarily diverse in their own right (~3,500 species), are actually a clade that is nested within squamates (Streicher and Wiens 2017). From the gigantic Komodo dragon of Indonesia (Figure 7.1A, Varanus komodoensis) to tiny leaf chameleons of Madagascar (Figure 7.1B, Brookesia), squamates span an impressive range of form and ecological niche use (Vitt et al. Squamates, the clade that includes all living species of lizards, are well known for their diversity. R markdown to recreate analyses Chapter 7: Models of discrete character evolution Section 7.1: Limblessness as a discrete trait
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