Convergent Evolution Examples
Throughout the 3.8 billion year history of life on Earth, plant and animal species from vastly different evolutionary backgrounds have come to possess very similar traits. This convergent evolution often results from these species living in ecosystems such as deserts, rain forests, and tundra that force these varied species to adapt to their climatic conditions in very similar ways. Modern genetic analysis is beginning to show that these adaptations not only have similar appearance and function, but have significant genetic similarities as well.
There are dozens of examples of convergent evolution in current species and in the fossil record. This article will examine three well-known ones in detail: evolution of the eye in vertebrates and cephalopods, evolution of caffeine production in coffee and tea plants, and evolution of echolocation in dolphins and bats.
NCSE: Evolution of the Eye
Convergent Evolution Example One: The Eye
The independent evolution of the camera eye, which uses a lens to focus light on a retina, in at least two distinct groups of animals is perhaps one of the best examples of convergent evolution. Since the Precambrian approximately 500 million years ago, the eye has evolved separately multiple times in cephalopods (such as octopus and mollusks) and vertebrates (such as fish and humans) from the simple photoreceptor spot of our common ancestor to the complex camera structure of the modern eye.
The evidence for common ancestry of cephalopod and vertebrate eyes lies in the aspects they have in common. Though the exact structure of photoreceptor cells varies widely across the animal kingdom, they have the same basic operating procedure. At the heart of all photoreceptors contain a light-sensitive pigment bound to a protein called opsin. When hit by a photon, this bond is broken, and the opsin binds with another protein known as G-protein. This then sets off a complicated cascade of chemical reactions that ends with the production of a nerve signal. The presence of this basic procedure in all multicellular animals with vision suggests that it results from a common ancestry. This is further confirmed by genetic evidence
Though these eyes all evolved from a common ancestor, they developed quite differently. The camera eye of cephalopods, for example, fixes a major "design flaw" in vertebrate eyes by putting the photoreceptor layer of the retina above the nerve layer. Vertebrate eyes evolved with the nerve layer above the photoreceptor layer, producing a blind spot where the nerve fibers pass through to form the optic nerve.
The second key difference is in the types of photoreceptor cells lining the retina. There is a wide range in the structure of photoreceptors across different types of animals, but they can be categorized into two main types: ciliary and rhabdomeric. In addition to some major differences in the protein cascade sequence, the main structural difference is in the shape of the receptor area. Ciliary receptors are horizontal structures resembling a stack of discs. Rhabdomeric receptors are vertical structures resembling bristles on a paintbrush. While the eyes of cephalopods mostly use rhabdomeric receptors, the eyes of vertebrates contain only ciliary ones.
Convergent Evolution Example Two: Caffeine
A number of plant genera produce the chemical caffeine in their leaves and fruit seeds as a deterrent against pests - the coffee, tea, yerba maté, kola, and guarana plants being the most well-known examples. These plants tend to grow in similar tropical regions of Southeast Asia and South America, but are only distantly related. Their independent evolution of the complex, multi-step process needed to produce caffeine is a classic example of convergent evolution.
The tea plant (Camellia sinensis) belongs to the order Ericales, a large and diverse group of flowering plants that includes blueberries, brazil nuts, persimmons, and azaleas. The oldest fossil belonging to this order has been dated to approximately 90 million years ago, and researchers have estimated that this order diverged from its common ancestor with other plant orders between 92 and 109 million years ago.
There are three main species of coffee plant used to produce most of the world's coffee supply: Coffea arabica, Coffea canephora, and Coffea liberica. These belong to the order of flowering plants known as Gentianales, which also includes gardenias and milkweeds. Researchers have estimated that this order first emerged between 83 and 89 million years ago.
Given the ages of the orders containing both coffee and tea plants, their last common ancestor must date back to at least 100 million years ago, probably much older. As caffeine production is not common among other plants in either order - there are no caffeinated gardenias or blueberries, after all - this appears to be a relatively recent trait.
Caffeine production is a rather complex five-step process converting the organic compound xanthosine into the 1,3,7 trimethylxanthine we know as caffeine. The last step in the process is accomplished by the production of an enzyme known as caffeine synthase that enables the chemical reactions to produce caffeine from the predecessor chemical theobromine.
Recently, scientists have isolated the genes CCS1 in coffee plants and TCS1 in tea plants that produce the caffeine synthase enzyme, and found both genes to have a 40% agreement in the amino acid sequences they code for. Thought these genes aren't exactly the same, they have enough in common that they can produce the same result - caffeine - even though they evolved via vastly different genetic pathways.
Is echolocation too complex to have evolved by natural processes?
Convergent Evolution Example Three: Echolocation
Bats and dolphins use echolocation for similar purposes - navigating and catching prey. These distant mammal families both evolved these powers at different times and in different ways, but with some interesting commonalities, making echolocation a fascinating example of convergent evolution.
The earliest bats evolved around 52-54 million years ago, in the late Eocene. Although for a time there was some disagreement among paleobiologists whether flight or echolocation evolved first, the emerging consensus is that flight indeed evolved first in the common ancestor of all bats, and that echolocation evolved twice - once in the common ancestor of all yangochiropteran microbats using a vocal ultrasound chirp, and then re-evolved in the Egyptian Rousettus bat using tongue clicks.
Microbats use a variety of vocal strategies for echolocation, depending on their environment. In open environments, they will use constant frequencies as a form of sonar ping for distant objects. In confined cave environments, they will use frequency modulation, changing pitch to keep the echos from overlapping each other.
Dolphins are part of the odontoceti , or "toothed whale" suborder of whales. This suborder includes sperm whales, orcas, porpoises, and beaked whales. Nearly all of the toothed whales are capable of echolocation, indicating that this is a trait that evolved at or just prior to the time of their split from the baleen whale branch some 34-36 million years ago.
Toothed whales produce a beam of high-pitched clicks using a structure in their skulls known as the bony nars and the phonic lips. These sounds are then amplified and pitch-shifted by a structure known as the melon. The melon is a round fatty protrusion from the cetacean's head, giving dolphins, orcas, and sperm whales their distinctive forehead shape. Echoed sounds are received through fatty structures in the lower jaw that transfer the sound vibrations to the inner ear. Like many species of bats, dolphins and other toothed whales can modulate the frequencies of their clicks depending on the situation.
Although dolphins and bats evolved at different times and under vastly different conditions, there are some very similar strategies of producing sound and mechanisms for receiving it. There are also similarities at the molecular level. The protein prestin is a critical component of the hair cells in the cochlea of the mammalian inner ear. In both dolphins and echolocating bats, the gene that codes for prestin has undergone a very similar series of mutational changes, altering the amino acids used in the protein. This seems to indicate that prestin is critical to the sensitive hearing required for echolocation. Though this relationship is still being studied, if demonstrated to be valid it would represent one of the best examples of molecular convergent evolution.
But Wait, There's More...
The examples above are just a few of the many examples of convergent evolution that abound in modern plants and animals and in the fossil record. Though at first glance the phenomenon of convergent evolution can seem like evidence of a designer, upon inspection of the details it confirms the haphazard and very organic nature of natural evolution.
Sources and Further Information
- How many genes does it take to make a squid eye? - Pharyngula
This is an article about cephalopods and eye evolution, but I have to confess at the beginning that the paper it describes isn’t all that interesting. I don’t want you to have excessive expectations!
- ScienceDirect.com - Developmental Cell - Role of Pax Genes in Eye Evolution: A Cnidarian PaxB Gene U
Our data suggest that the ancestor of jellyfish PaxB, a PaxB-like protein, was the primordial Pax protein in eye evolution and that Pax6-like genes evolved in triploblasts after separation from Cnidaria...
- Fresh Fossil Evidence Of Eye Forerunner Uncovered
Ancient armored fish fossils from Australia present some of the first definite fossil evidence of a forerunner to the human eye. Researchers analyzed the fossilized remains of 400-million-year-old Devonian placoderms -- jawed ancestors of modern fish
- Scientific Commons: Caffeine synthase gene from tea leaves, [Kato, Misako, Mizuno, Kouichi, Crozier
Caffeine synthase is an enzyme that catalyses the final two steps in the caffeine biosynthesis pathway. We have cloned the gene encoding caffeine synthase from young leaves of tea (Camellia sinensis)...
- Angiosperm Phylogeny Website
The focus of this site is on angiosperm families, although treatments of gymnosperm groups were added in 2005.
- Distribution and biosynthesis of caffeine in plants. [Ashihara H, Suzuki T.]
The diversity of biosynthesis and accumulation between species and between tissues of different age is considered. We also discuss the physiological function of these purine alkaloids in plants and the biotechnology for creating caffeine-free plants.
- Animal echolocation - Wikipedia, the free encyclopedia
- Prestin and high frequency hearing in mammals