Red tailed black shark, Epalzeorhynchos bicolor
The first vertebrates to appear 500,000,000 years ago were primitive jawless fishes which resembled an armoured hagfish. From these not only did all other fishes evolve but all other vertebrates evolved.
The first known vertebrates to evolve were Agnathans, Class agnatha these fish lack jaws but they have the ability to rasp at food. These fish are now mostly extinct but there are several modern species of Agnathans still alive in the form of hagfish and lampreys.
During the Silurian period 443 million years ago there were many species of armoured jawless fishes These jawless fishes were the ‘ostracoderms’ (‘bony skinned ones’), so called because they were covered in a protective armour made out of plates of bone. Typically between 15 and 60cm long, they had gills and balancing organs, and are thought to have sucked food into their mouths using a muscular pharynx. The ostracoderms shared their environment with the shelled relatives of squid and octopus - of which the chambered nautilus is the only surviving example.
Some of the jawless fishes slowly changed over time, and it is thought that the first jaws evolved as a means of helping regulate water flow through the mouth and throat. This happened at a time when the environment was very dynamic and many changes were happening. Jaws gave the gnathostomes (vertebrates with jaws) many advantages and this probably helped them to displace many of the Agnathans.
Today the gnathostomes make up about 99% of all living vertebrates.
Evolutionary changes don't always take thousands of years. Studying steelhead trout (Onchorhynchus mykiss) in Oregon's Hood River for 19 years, Mark Christie and his colleagues showed in their genetic analysis that first-generation captive-reared trout raised in hatcheries had significantly lower survival and reproductive rates when reintroduced into the wild. In fact, the more successful the fish were at surviving hatchery conditions, the worse they fared when returned to the wild.
This study shows that hatcheries can exert a profound influence on the genetic makeup of salmon populations, allowing for the selection within a single generation of traits that would confer advantages in hatcheries at the cost of the ability to thrive and reproduce in the wild.
Some mbuna (Rift Valley Cichlids) can show changes in dentation after just one generation in captivity. Many aquarists have long held the belief that commercially bred aquarium fish adapt to captive conditions far better than their wild caught relatives and that some of these fishes may not fair to well if released in to the wild where they would find a very alien environment than they are used to.
This has a big impact on the way we should keep fish, wild fish should be kept in conditions which replicate their natural environment whereas fish with a long history of being commercially bred may do far better if kept in fairly neutral conditions where extremes of hardness and pH are avoided.
Agnathans - is a superclass of jawless fish in the phylum Chordata
Ostracoderms - extinct fish-like jawless vertebrate having a heavily armored body.
Gnathostoma - is the group of vertebrates with jaws.
Serendip.brynmawr.edu - Evolutionary steps of fish
ox.ac.uk - Jaw Vs jawless the: battle for the sea
Wikipedia - Agnatha
Christie, MR, ML Marine, RA French and MS Blouin (2012) Genetic adaptation to captivity can occur in a single generation. Proceedings of the National Academy of Sciences of the United States of America 109, pp. 238–242