It is hard to have patience with people who say 'There is no death' or 'Death doesn't matter.' There is death. And whatever is matters. And whatever happens has consequences, and it and they are irrevocable and irreversible.
In the last chapter we saw how new evolutionary lineages may arise, increasing the number of branches on the tree of life. We will now consider how the number of branches might be depleted, thus 'pruning' the tree of life; the process known as extinction. At its limit, extinction is a population level phenomenon: it is the specific case of population dynamics when the number of individuals becomes zero. This sounds like a purely ecological process with little evolutionary perspective. Yet, as we shall see in both this and the next chapter, extinction has evolutionary causes and consequences. First, as populations decline, changes in genetic composition often occur, some with impacts on the phenotype of the organism. These changes may affect the probability of extinction, and are the subject of this chapter. Second, certain characteristics may incidentally make some species more likely to decline than others, or more vulnerable to extinction once in decline. These differences have come about through evolution. Finally, once a lineage has gone extinct, that may alter the likelihood of extinction or speciation in other species. The latter topics are dealt with in the next chapter.
There are several ways in which the processes of extinction might be classified. From a phylogenetic perspective, extinction can occur by a lineage simply ceasing to exist altogether, or alternatively by merger with another lineage (e.g. hybridization). Another framework is to consider what extrinsic processes in the environment might be causal agents (e.g. Purvis et al. 2000). We will explore all these aspects of extinction. Another useful and frequently-made distinction is between factors that make a species rare and factors that cause extinction once a species is rare (Soule 1987; Caughley 1994; Lawton 1995). This will form the framework for the chapter.
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