The Puzzles of Fish Evolution

Every group of animals and plants has puzzles that scientists try to solve. A group as large and old as fish has many, many puzzles. It sometimes seems that every time scientists solve one puzzle, they find ten more!

One of the biggest puzzles about fish is “What even are fish?” You, and most people, may think you have a good idea of what a fish is, but scientists who study them think it is pretty confusing and not clear at all. One famous scientist, Stephen Jay Gould, who studied animal evolution his whole life, once got so frustrated he actually said “There is no such thing as fish!”.

He wasn't really talking mainly about the fish we keep in aquariums – or even most of the fish that are in oceans, rivers and lakes today – but about the amazing fossil fish that have been found and how some of the things we call “fish” have quite different an evolutionary histories.

Here's what (we think) we know about the story of fish:

More than half of the vertebrate animal species alive today are things we call fish (in one form or another). Most of the nearly 30,000 species of modern fish have hard bones, a skull and jaws and they have fins supported by at least a few bones called rays -these are the “Ray-finned fishes”. A small number of modern fish (the sharks and rays) don't have hard bones – they have a softer skeleton called cartilage and they are called the “Cartilaginous fishes”. A very small number (the lampreys and hagfish) don't have jaws (they have a really awful looking sucker mouth instead and they look like eels). An even smaller number of fish have fleshy fins and tails – these are called “lobe-finned fishes

500 million years ago, the story was quite different. According to the fossils, fish were just getting started – they were the first animals to evolve a backbone and a skull. By around 400 million years ago, they were going strong and there were six quite different groups of fish – the four we have still today and two that did pretty well for a while and then became extinct. That period, which scientists call “The Devonian” or “The Age of Fishes” lasted for around 60 million years.

Around the end of the Devonian, a small group of the lobe-finned fishes started to come out onto the land for a look around – at the weird ferns and insects, I guess. These were the ancestors of all the other vertebrate groups we have today – the amphibians, reptiles, birds and mammals. Fish didn't stop evolving just because there were a bunch of land-based newcomers. Have a look at the diagram below and you'll see that one group in particular – our “ray-finned” friends have actually done quite well – in fact there are more species of ray-finned fish around today than ever before.

The earth has changed a lot and many, many times since the Age of Fishes. It has had Ice Ages and Tropical Ages and the continents have moved around and broken up and the chemicals dissolved in water have changed a lot. Fish have rolled with the punches and evolved into all sorts of weird and wonderful forms to adapt to those changes. Lots and lots of species that once existed have become extinct but new ones have arisen to take their place in the changed ecology of our watery planet.

How and why new species develop, and what happens to the “old” ones that don't change, is the stuff that scientists study. While their understanding of evolution, and the tools they use to gather and interpret evidence, has changed a lot since Charles Darwin worked out the basics in the 1850s, his fundamental insights still hold true.

But what did Stephen Jay Gould mean when he said “There is no such thing as fish!”? Maybe he was just pointing out that, unlike all the other recognised “classes” of animal and plant life (amphibians, mammals, etc.), “fish” doesn't refer to one class – but is a broad description of things that we once believed were somehow closely related. As you can see from the diagram, the relationship between, say, a guppy (a Ray-finned fish) and a hammerhead shark (a Cartilagenous fish) is really no closer than between the guppy and us humans.

Other Puzzles of Fish Evolution

The puzzles of evolution are not all about the far distant past, at least not in terms of hundreds of millions of years. As Charles Darwin pointed out, evolution is not something that happened long ago and then stopped – it is happening all around us and all species are potentially “mutable” - they can change.

Before Darwin, people liked to think of species as “fixed” and easily categorised. We now know that is not always true.

Evolution happens on different timescales for different groups and different species and can only “work” with what any species has to start with – new features just don't appear, they evolve from something the species already has.

Those big changes in the planet – the climate, the sealevel and the landforms – are big “drivers” of evolution. The environement keeps changonging and species need to evolve to survive. There are also pressures because of the thousands of changes that other species are making all the time - because a change in one species affects others through the foodchain and through competition for resources.

There are many examples of places and species that we believe are changing rapidly (at least on timescales of hundred or thousands of years). For fish, we see this on coral reefs, and in big lakes and some forest river systems. These habitats provide resources for long-term survival but have lots of change and variety of habitat too.

One freshwater example is the massive “rift” lakes of Africa, especially Lake Tanganyika which is over one kilometre deep in the middle. Lake Tanganyika has had water in it for more than 5 million years so provides one of the longest known continuous evolutionary histories for freshwater fish anywhere in the world. It has also changed dramatically – especially in terms of water level, over that history and it has had very little connection with the other major water systems in Africa. It is a unique environment and has become a “living laboratory” for evolutionary biologists because the diversity of species and forms of fish in Lake Tanganyika is truly unique and amazing. Scientists think that from possibly only six original species of cichlid fish in the lake 5 million years ago, there are now over 300 species adapted to life in different ecological niches throughout the lake. Many of these species are still in the process of “speciating” further and some of the changes can be dated at only a few thousand years.

We can also look at a fish where evolution has apparently stood still since the Age of Fishes. The Australian Lungfish (Neocetrodus forsteri) is one of the very few surviving “lobe-finned fishes” that gave rise to the amphibians. Scientists have found quite ancient fossils of fish just like the lungfish so we know it has changed very little in 350 million years. Animals and plants that still survive in their ancient form are called “Living Fossils”.

Also close to home (for us Australians), we have some wonderful examples of evolution in action in our small fish. Australia is relatively poor in freshwater fish species (compared to Asia, Africa and South America) because the regular drying out of the continent over the past several million years means that freshwater species have a hard time surviving. We do however have some quite remarkable fish that have adapted to life in very unreliable inland waters. A group of species called Desert Gobies can be found in the springs that dot inland Australia. There are five different, but closely related, species in different drainage basins.

We also share some quite spectacular freshwater fish with Papua New-Guinea. Some of the fish, like the Threadfin Rainbowfish (Iriatheria werneri) and the Spotted Blue-eye (Psuedomugil gertrudae) live in quiet swamps and small streams and don't go into salt water at all – yet populations of both species are found either side of the Arafura Sea and Torres Strait to the north of Australia. This type of distribution pattern provides evidence of a past connection between the two land masses and one that had substantial and interconnected river systems. The last time that happened was around 14,000 years ago, towards the end of the most recent ice age. These fish have not been studied very much at all and there are still lots of questions about how and why the fish in the different isolated populations are still so similar when they have been separated for so long.

Project Ideas and Further Research

  1. Pick a fish. Any fish. That fish has an evolutionary history stretching back to the Devonian era. Can you find out (approximately) how old that species is and what it is related to?
  2. Stephen Jay Gould wrote a lot of books about evolution. They are mostly very easy to read and have a lot of excellent ideas and information about all sorts of evolutionary puzzles. Most of his books are made up of short articles and stories that explain a single puzzle.
  3. What is a “species” anyway? Charles Darwin threw everything up in the air when he published his book “The Origin of Species”. Scientists still debate what is meant by “species”. Can you find a few different definitions and understand why each might “work” for different purposes?
  4. Darwin's theories on the way evolution works were very controversial when he first published them. Why? Should scientists not publish their work if it upsets some people? What other areas of science do some people object to?
  5. If you know anything about conservation, you'll see the word “extinction” and think “that's bad”. Why is the current rate and process of species extinction considered undesirable? What's so different to the “normal” process of evolution where species go extinct “all the time”? What is actually “lost” when a species goes extinct?
  6. Darwin spoke about “natural selection” as the main way that evolution works to produce new species. What do you think that means? What other ways have scientists proposed that the form of animals and plants can change?
  7. A lot of people other than Charles Darwin (and Stephen Jay Gould) have made big contributions to our understanding of evolution. Can you research and write a brief biography of one of them? What did they contribute?
  8. Many aquarium fish have been bred for particular features or colours and are not much like their wild ancestors at all. How does “selective breeding” relate to evolution?
  9. What scientific “tools” are available now that weren't available to people like Charles Darwin? How have they changed the way scientists study evolution?
  10. Stephen Jay Gould is responsible for an evolutionary theory called “Punctuated Equilibrium” which challenged Darwin's original idea that evolution was always a slow and gradual process. Find out more about Gould's theory and discuss how it either adds to or disproves aspects of Darwin's original work.