Turtle Biology, Evolutionary History, and Natural Selection
The Biology and Classification of Turtles (Testudines)
Turtles (Testudines) are an order of reptiles (Sauropsida) characterized by having a wide, short trunk, and a shell that protects the internal organs of the body. The head, front legs, and tail protrude from the shell.
Key Anatomical Features
The most important feature of the turtle skeleton is that a large part of the spine is fused to the back of the shell. This skeletal structure makes breathing via rib cage movement impossible. Respiration is achieved primarily by the contraction of modified abdominal muscles that function similarly to the mammalian diaphragm, along with pumping movements of the pharynx. The skull exhibits characteristics of great primitivism. Although turtles lack teeth, they possess a **horny beak** covering their jaws.
Ectothermy and Skin Shedding
Like all reptiles, turtles are **ectothermic**, meaning their metabolic activity depends on external or environmental temperature.
Turtles shed their skin, but unlike lizards and snakes, they do so gradually. They also shed the shields (scutes) of the carapace, individually and seemingly in no particular order.
Evolutionary History of Testudines
In the early **Triassic period**, approximately 260 million years ago, *Captorhinus* appeared, a small reptile about 60 cm long. This animal was the first reptile to possess a shell covering its chest, organs, and ribs. This development paved the way for animals like turtles to evolve a bony shell.
The oldest known Testudines species is ***Odontochelys***, which lived in South Asia during the Triassic, 220 million years ago. This places turtles among the oldest reptile groups, significantly older than lizards and snakes. *Odontochelys* was aquatic and had a well-defined plastron, but its dorsal shell was primitive.
The next significant turtle was ***Proganochelys*** (formerly *Triassochelys*), which lived in the Upper Triassic of Eurasia about 210 million years ago. It was a primitive turtle with a carapace similar to existing species, but it still had teeth in the palate. Furthermore, its head, tail, and legs could not retract into the shell, but were protected by thorns.
The Theory of Natural Selection
In its initial form, the Theory of Evolution by Natural Selection is the great contribution [1] of Charles Darwin (and, independently, by Alfred Russel Wallace). It was subsequently reformulated into the current theory of evolution, the Modern Evolutionary Synthesis. In evolutionary biology, natural selection is often considered the main cause of the origin of species and their adaptation to the environment.
Defining Natural Selection and Its Premises
Natural selection is an essential phenomenon of evolution, defined as the differential reproduction of genotypes within a biological population. The classic formulation states that environmental conditions either help or hinder—that is, they *select*—the reproduction of living organisms based on their peculiarities. Darwin proposed natural selection as the mechanism to explain biological evolution. This explanation is based on two core premises:
- **Premise 1:** Among the descendants of an organism, there is variation (non-random, non-deterministic) that is partly heritable.
- **Premise 2:** This variability leads to differences in survival and reproductive success, allowing certain advantageous features to spread throughout the population.
The accumulation of these changes over generations produces all evolutionary phenomena.
The General Law of Natural Selection
Natural selection can be expressed as the following general law, taken from the conclusion of The Origin of Species:
- Organisms reproduce, and offspring inherit characteristics from their parents.
- Variations in features exist.
- If the environment cannot support all members of a growing population, those members with less adapted features (as determined by their environment) are more likely to perish.
- Consequently, those members with better-adapted features are more likely to survive and reproduce.
(Darwin, The Origin of Species)