Developing Theory

Not until the mid-1900s did scientists begin to integrate the theory of natural selection with new understandings of genetics. This blending has been called the modern synthesis of evolutionary theory. As in all areas of science, hypotheses and theories about evolution continue to be formed, challenged, and revised. Many aspects of evolution remain poorly understood, and some observations remain unexplained. Altough modern evolutionary theory, like all theories in science, can never be "proven," it is widely accepted and applied by scientists because it explains the broadest range of observations and makes useful predictions.

figure 15-10

This phylogenetic diagram, or "tree," shows one hypothesis of the ancestral relationships among whales and the group of hoofed mammals known as artiodactyls. Scientists construct models such as these by comparing the anatomical or biochemical traits of many organisms. For updates on phylogenetic information, visit go.hrw.com and enter the keyword HM6 Phylo.

For example, scientists try to model phylogeny, the relationships by ancestry among groups of organisms. To do so, they must analyze many kinds of evidence. Sometimes, different evidence supports different models. Figure 15-10 shows a phylogenetic diagram, or "tree," which models a hypothesized phylogeny. This particular phylogeny was based on an analysis of biological molecules in animals that seem closely related to whales. The "trunk" represents a past species that could have been the ancestor of all these animals. Each "branch" over time represents a separate population or lineage. More closely related groups appear closer to each other on a branch. For example, whales are more closely related to hippopotamuses than to camels. Hypotheses such as these are supported when independent anatomical and biochemical data match the same model.

figure 15-10

This phylogenetic diagram, or "tree," shows one hypothesis of the ancestral relationships among whales and the group of hoofed mammals known as artiodactyls. Scientists construct models such as these by comparing the anatomical or biochemical traits of many organisms. For updates on phylogenetic information, visit go.hrw.com and enter the keyword HM6 Phylo.

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