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How does anagenesis change a lineage without splitting it?

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A diagram showing the tentative evolutionary relationships among hominid species over the past four million years, with skull images and labeled species names connected by solid and dotted lines to indicate proposed evolutionary pathways. — phylogeny Tentative phylogenetic scheme for the evolution of the human lineage. Solid bars indicate the time ranges during which species are thought to have existed, based on fossil evidence. Dotted lines signify evolutionary relationships between hominin species that have been proposed on the basis of the fossil evidence.

Anagenesis is an evolutionary process in which changes accumulate within a single, continuous lineage over time, eventually resulting in one taxon replacing another without the lineage splitting into multiple branches.12

Unlike cladogenesis, which involves the branching of a lineage into two or more separate, reproductively isolated species, anagenesis represents a transformation of the entire population. This process occurs as genetic changes—such as those driven by natural selection—gradually accumulate across generations. Because these modifications occur throughout the lineage, the descendant population eventually becomes distinct enough from its ancestors to be classified as a new taxon, even though no speciation event (splitting) has occurred.1234

Historical examples of this process include the doubling of the human cranium size over two million years and the reduction of the horse foot from four toes to one.2