CH 21 INTRO TO EVOLUTION
21.2 Observations of Evolutionary Change
Table 21.1 Observations of Biological Evolution
Type of observation Description
Fossil record When fossils are compared according to their age, from oldest to youngest, successive
evolutionary change becomes apparent
Biogeography Unique species found on islands and other remote areas arise because the species have evolved in
isolation from the rest of the world
Convergent evolution Two different species from different lineages sometimes become anatomically similar because
they occupy similar environments. This indicates that natural selection promotes adaptation to a
Selective Breeding Traits in domesticated species have been modified by artificial selection
Homologous trait Homology – fundamental similarities due to descent from common ancestor
Anatomical Evolutionarily related species possess homologous structures that have been modified in ways that
allowed them to be used differently by each species. In some cases, homologous structures are no
longer needed and degenerate to non-functional vestigial structures.
Developmental Analysis of embryonic development often reveals similar anatomical features that point to past
Molecular At the molecular level, certain characteristics are found in all living cells = derived from common
ancestor. Closely related species tend to have similar DNA sequences.
Fossils show Successive Evolutionary Change
- Fossils of Tiktaalik roseae (fishapod) show the steps that led to the evolution of tetrapods (4 legged animals).
- T. roseae is a transitional form because it provides a link between earlier species and later species.
o Fish T.roseae Tetrapod (terrestrial animals)
o Unlike fish, T. roseae had a broad skull, flexible neck and eyes on top of its head as well as lungs.
o Pectorial fins on side of the body showed it had a primitive wrist and five finger-like bones.
o Was able to peek its head above water and look for prey.
- Certain oysters undergone a change in shell structure 200 million years ago from small, curved shells large flat
shells in response to stronger currents.
- Fossil record showed adaptive changes of horses’ size, foot anatomy, and tooth morphology.
o First horses – Hyracotherium – were size of dogs, had small teeth , had four toes on front feet and 3 on hind
that were enclosed in fleshy pads.
o Modern horses have a single toe enclosed in a tough bony hoof and larger teeth with ridges on molars.
o Horse evolution occurred in North America where large areas changed from dense forests to grasslands,
increase in size and foot structure allowed escape from predators and travel long distances in search of food.
o Dietary shift to eating more tender leaves to grasses that need more chewing.
Biogeography indicates that Species in a Given Area have evolved from pre-existing species
- Biogeography: the study of geographical distribution of extinct and modern species.
- Isolated continents and island groups have evolved their own distinct plant and animal communities.
o Species of finches on Galapagos Islands had unique characteristics, such as beak shapes when compared with
similar finches found on mainland.
o These species probably migrated to the islands and became adapted to a variety of feeding habits.
- Islands often have many species of plants and animals that are endemic – naturally found in a particular location with
closely related species on nearby islands or mainland.
o Ex. Island fox lives on Channel Islands off the coast of southern California and is found nowhere else in the
o Mainland grey fox crossed the Santa Barbara channel when it was frozen Ice age ended and sea levels rose
foxes were cut off from mainland and evolved into smaller size. Mar 5/11
- Island dwarfing: when large animals isolated on island shrinks dramatically – natural selection; smaller size = survival
advantage due to limited food.
- Placental mammals probably first arose somewhere other than Australia and the barrier of a large ocean prevented most
terrestrial placental mammals from migrating there.
- Australia is home to Marsupials are a group of mammal species in which young are born in a immature condition and
then develop further in mother’s abdominal pouch.
Convergent Evolution Suggests Adaptation to