Encyclopedia of Evolution.pdf - Online Reading Center

fishes, evolution of
his obvious visual defect) into practice: he championed government
subsidies for rich families to have more children; got
married and had eight children; and left academia to start a
dairy farm. He justified this last move by saying that farming
was the only occupation in which having many children was
an advantage. The farm was disastrous and Leonard Darwin
had to rescue him financially.
Modern evolutionary science continues to benefit from
Fisher’s insights, both through population genetics and by
using statistical methods of which Fisher was the pioneer.
Fisher died July 29, 1962.
Further Reading
Fisher, R. A. The Genetical Theory of Natural Selection. Oxford,
U.K.: Oxford University Press, 1930.
fishes, evolution of Fishes were the first vertebrates.
Fishes are the only group of vertebrates that is almost exclusively
aquatic, which was the primitive condition of all life.
When referring to more than one individual of the same species,
“fish” is the correct plural; for more than one species,
“fishes” is correct.
All vertebrates are chordates and have evolved from
invertebrate ancestors that resembled modern lancelets (see
invertebrates, evolution of). Lancelets have a cartilaginous
rod along the back (a notochord) in association with
the main nerve cord and gill slits. All vertebrates have these
features, although in most adult vertebrates the notochord is
replaced by a backbone of vertebrae and may possess the gill
slits only during the embryonic stage. Lancelets do not have
jaws. An animal of the Cambrian period known as Pikaia
closely resembled a lancelet (see Burgess shale).
The earliest known fishes lived during the Cambrian
period. One example is Myllokunmingia, found from fossil
deposits in China. Conodont animals also appeared in
the Cambrian period and were probably also classifiable as
fishes. Neither they nor the fishes of the early Ordovician
period had jaws. Two classes of modern agnathan (jawless)
fishes survive: lampreys and hagfishes, which today live by
sucking blood from larger fishes.
One lineage of fishes evolved jaws during the Ordovician
period. This allowed a major evolutionary advancement in
the efficiency of predation. Many of the earliest jawed fishes,
including the placoderms, were covered with bony armor, as
the predatory arms race became severe. Placoderms apparently
became extinct without descendants. Other lineages of
jawed fishes survived to become the Chondrichthyes (cartilaginous
fishes) and the Osteichthyes (bony fishes).
Cartilaginous fishes include the sharks and rays. Their
skeletons consist only of cartilage. Teeth are not bones; shark
teeth are structurally similar to their scales and are often the
only part of the shark to be preserved as fossils. Many kinds
of sharks diversified throughout the Paleozoic era. Today,
some of them have very specialized features, such as the ability
to navigate by electrolocation.
Bony fishes evolved in freshwater conditions and were
restricted to freshwater for the first 160 million years of their
existence. Evolutionary scientists speculate that their bones
were more important as a way of storing calcium, a mineral
that could be scarce in freshwater, than as a skeletal reinforcement.
Bony fishes diverged into two major lineages:
• the ray-finned fishes, which have fins reinforced with bony
rays that do not correspond to the fingers or toes of other
vertebrates. The major lineage of ray-finned fishes is the
teleosts, which includes most modern fish species.
• the flesh-finned fishes. These fishes diverged into two
lineages: the lungfishes and the crossopterygians. Some
lungfishes survive today in shallow tropical ponds. When
the ponds have water, the oxygen levels are low and the
lungfishes gulp air. When the ponds dry up, the lungfishes
continue to breathe while estivating in the mud. The crossopterygian
fishes have bones at the bases of their fins that
correspond to the one upper and two lower limb bones of
tetrapods (four-legged animals). Two branches of crossopterygian
fishes became the coelacanths and the tetrapods.
Coelacanths were thought to have been extinct since
the Cretaceous extinction 65 million years ago. However,
in 1938, Captain Hendrick Goosen of the trawler Nerine
brought a fish that had been caught deep in the Indian
Ocean to Marjorie Courtenay-Latimer, curator of a museum
in East London, South Africa. She identified the fish, now
named Latimeria chalumnae, as a crossopterygian. Subsequent
searches revealed that native fishermen of the Comoros
Islands (in the Indian Ocean off the coast of Africa) reported
that they had caught these fish for years and had thrown
them back as inedible. In 1998 another species, Latimeria
menadoensis, was found 10,000 kilometers away, in Indonesia,
by a scientist on his honeymoon.
The lineage that became tetrapods included fishes such
as Eusthenopteron, which had skeletal characteristics intermediate
between those of earlier fishes and later amphibians.
Later species in this lineage had legs but still retained many
fish skeletal characteristics (see amphibians, evolution of).
As is the case with any so-called missing links, there is no
clear line of division between ancestor and descendant.
Lungs are a primitive condition for bony fishes. Lungs
began as expansions of the upper digestive tract that were
open to the pharynx, allowing the fish to gulp air. Gars and
bowfins can still do this. In most bony fishes today, the lung
has evolved into the air bladder, a pocket not connected to
the pharynx, which the fish uses for buoyancy.
Teleost fishes represent tremendous evolutionary diversity.
Some groups of fishes, such as the cichlids of African
lakes, are some of the best examples of rapid evolution (see
speciation).
Further Reading
Barlow, George. The Cichlid Fishes: Nature’s Grand Experiment in
Evolution. New York: Perseus, 2002.
Forey, P. L. “Golden jubilee for coelacanth Latimeria chalumnae.”
Nature 336 (1988): 727–732.
Long, John A. The Rise of Fishes: 500 Million Years of Evolution.
Johns Hopkins University Press, 1996.