In evolutionary biology, one of the goals is to understand the origins of biodiversity. To that end, one of he most powerful approaches we can deploy in this effort is to understand the evolutionary relationships of iving things. This helps us to not only trace the origins of biodiversity, both extinct and extant, but to gain a more fundamental perspective on the rest of the biological sciences. For example, as a developmental biologist we might be interested in whether a lineage, such as the sponges, exhibit one or two tissue types. We can approach this in the classical way of conducting detailed developmental studies to determine the fates of cell differentiation during development, and indeed for years this has suggested that sponges have 1 tissue type (although recent studies suggest that there could actually be 2!). But, we can also approach this developmental biology question in a tree-thinking approach, or by applying our knowledge of evolutionary biology to the questions as well. For the sponge example, and as we will discuss in more detail in lecture, the evolutionary relationships of the sponges to the rest of the metazoans might have a profound impact on the question regarding the number of tissue types that exist in this group. If sponges are the earliest-diverging lineage of the metazoans, then we might expect that multiple tissue types evolved among the metazoans after the sponge lineage began diverging. Alternatively, if we find that another group, such as the Ctenophores, which have two tissue layers, are the earliest diverging lineage, followed by the sponges, then we expect that the sponges have potentially lost the ancestral condition of having two tissue layers. Why does this matter? Well, an understanding of the evolutionary relationships gives us an a priori expectation regarding what to expect in our studies of metazoan development. Prieri -.

1. In evolutionary biology, one of the goals is to understand the origins of biodiversity. To that end,
one of he most powerful approaches we can deploy in this effort is to understand the evolutionary
relationships of iving things. This helps us to not only trace the origins of biodiversity, both extinct
and extant, but to gain a more fundamental perspective on the rest of the biological sciences. For
example, as a developmental biologist we might be interested in whether a lineage, such as the
sponges, exhibit one or two tissue types. We can approach this in the classical way of conducting
detailed developmental studies to determine the fates of cell differentiation during development,
and indeed for years this has suggested that sponges have 1 tissue type (although recent studies
suggest that there could actually be 2!). But, we can also approach this developmental biology
question in a tree-thinking approach, or by applying our knowledge of evolutionary biology to the
questions as well. For the sponge example, and as we will discuss in more detail in lecture, the
evolutionary relationships of the sponges to the rest of the metazoans might have a profound
impact on the question regarding the number of tissue types that exist in this group. If sponges are
the earliest-diverging lineage of the metazoans, then we might expect that multiple tissue types
evolved among the metazoans after the sponge lineage began diverging. Alternatively, if we find
that another group, such as the Ctenophores, which have two tissue layers, are the earliest
diverging lineage, followed by the sponges, then we expect that the sponges have potentially lost
the ancestral condition of having two tissue layers. Why does this matter? Well, an understanding
of the evolutionary relationships gives us an a priori expectation regarding what to expect in our
studies of metazoan development. Prieri -