- Historical Views
- Equidae Through the
Ages - Extant
Equids - Glossary - References
Modified from Campbell and Reece 2002.
Equidae has long been held as a classic example of orthogenesis (Gould 1987)
for several reasons:
1) Fossils of Equidae are very abundant (MacFadden 1992). This
means that there were enough of them to be studied at the time when evolution
was first being studied, and when orthogenesis was the prevailing evolutionary
2) There is only one surviving genus, Equus, which may suggest
the concept of an evolutionary "goal" and a single line of evolution (Hunt
3) Straight-line evolution of horses has been taught since the late
17th century (Gould 1987), which makes it a long-standing theory which
is hard to shake from the public consciousness despite scientific evidence
that refutes it.
We know now that the evolution
of horses did not proceed in a straight line, but in a complexly branching
pattern in which many genera may have coexisted (Hunt 1995). Evolutionary
change within a single lineage occurred slowly, if at all: most change occurred
where one genus branched off into two (Gould 1987). Although a trend
of larger size, longer limbs, fewer toes, and longer teeth may have occurred
in many different lineages, they did not all occur at the same time or
at the same rate, and in some cases did not occur at all (Hunt 1995).