Frietson Galis is an evolutionary developmental biologist at Naturalis Biodiversity Center. She studies innovations and mechanisms that facilitate or constrain evolutionary changes—in particular the role of developmental constraints and selection in shaping the evolution of body plans.
The main focus of our research is the evolution of body plans in vertebrates. The key to the understanding is the early organogenesis (or phylotypic) stage. Most organs appear during this highly conserved stage. As a consequence, the number and early development of many organs is also highly conserved, e.g. the number of eyes, ears, limbs, digits, and lungs in vertebrates, and the number of neck and trunk vertebrae in mammals. We have found strong support for the important role of developmental constraints in shaping the evolution of body plans. Our data on developmental constraints have implications for human morbidity and lethality.
One of the riddles of mammal evolution is the strong conservation of the number of trunk vertebrae. We argue on biomechanical and developmental grounds that evolutionary change is virtually impossible in fast running and agile mammals. The rationale is that several mutations are necessary to change trunk vertebral counts, with single mutations usually leading to irregular lumbosacral joints that severely hamper running and jumping capability. Our findings show that selection against these initial changes is strong in fast and agile mammals and weak in slower and sturdier ones.
Fingers & Toes
Evolutionary changes in the number of digits and other limb elements appear to be severely constrained, probably as a result of a low level of modularity during early limb development (phylotypic stage). Amniotes have evolved many digit-like structures rather than actual extra digits, such as the panda’s thumb. Evolutionary loss of limbs and digits typically occurs via earlier and earlier developmental arrest, followed by degeneration. In amniotes, limb development occurs during the crucial phylotypic stage, when many inductive interactions are occurring throughout the body. As a result, changes in limb development usually engender changes in other body parts. Thus, mutations that change the number of limb bones are expected to have many pleiotropic effects, which severely reduces the chance of such mutations being successful.
Why five fingers? Evolutionary constraints on digit numbers (Trends Ecol Evol)
Giant Dog Breeds
The size of giant dog breeds (Great Dane, Newfoundland, St. Bernard dog, Irish Wolfhound) has remarkably increased during the last century, as shown here for St. Bernhard dogs. The breed standard for St. Bernard dogs now specifies a shoulder height of between 70-90 cm and these dogs weigh 65-85 kg, whereas a typical 19th century dog was approx. 60 cm high and weighed less than 50 kg. Artificial selection for extremely high growth rates in large breeds appears to have led to developmental diseases that seriously diminish longevity. The extremely high growth rates in large breeds are associated with serious health problems, such as bone cancer and hip dysplasia.
Do large dogs die young? (JEZ-B)
Mammals normally have seven cervical vertebrae, regardless of the length of the neck. Deviant numbers of neck vertebrae are generally associated with congenital abnormalities in humans and other mammals. These abnormalities probably cause strong selection against changes of the number of neck vertebrae in mammals. Exceptionally sloths and manatees have a different number of neck vertebrae. Their extremely slow lifestyle allows them to tolerate some of the normally deleterious side-effects.
Some people are born with ribs on the seventh neck vertebra, so-called cervical ribs. This implies a change in the normally highly conserved number of neck vertebrae in mammals, from seven to six. Cervical ribs are rare in the general human population, but common in deceased fetuses and infants (~1% vs. ~50%). There is strong, often prenatal, selection against individuals with cervical ribs. The early selection is mainly indirect and probably caused by a strong association of cervical ribs with deleterious side-effects, which include major congenital abnormalities of all organ systems. The strong association of cervical ribs with many types of abnormalities appears to be a result of high global interactivity during the phylotypic stage — the early embryonic stage when the number of neck vertebrae is determined. This strong interactivity also explains the large heterogeneity of genetic and environmental causes of cervical ribs. Our findings indicate that prenatal assessment of the vertebral pattern using 3D ultrasound could be of added value in predicting fetal and neonatal outcome.