Marine creatures look weird. We know what a fish looks like, and some people have seen a sea star, but some animals like the giant squid are 30 feet long and so elusive we have only caught them on video two or three times. Animals come in all different shapes, sizes and colors and each species has its own feeding behaviors and habitat they call home. Typically we categorize animals based on their morphology, if two creatures look similar they are probably related. But this doesn’t always work, if a creature has no features how can you determine who are its closest relatives?
 
We still don’t know very much about a lot of animals, for instance myxozoans are one creature in particular that has confused scientists for a long time. The name myxozoan is a combination of Greek words that mean slime animal! You have probably never heard of myxozoans because they are very small parasites that infect worms and fish in freshwater and marine habitats. These tiny parasites have confused scientists because there are very few morphological features to compare them to other types of life. A recent paper by Chang et al. uses advanced genetic techniques to show that myxozoans are a type of cnidarian, a phylum that includes jellyfish, anemones and corals.
 
So scientists found a creature that is slimy and related to jellyfish…who cares? Well probably the most exciting result of this paper is that these simple parasites have a very reduced genome, meaning they have lost many of the genes that are common to all animals between jellyfish and humans. A human genome consists of approximately 20,000 genes that are the basic blueprint for building a human. These genes are made up of strands of DNA that code for the material that runs every function in our body, and for the most part all animal genomes are made up of a very similar set of genes. Even though we look nothing like jellyfish we share 60% of the same genes with them.

There is a suite of genes that most animals share, and these serve basic functions that all animals need to survive. Think about how our body is laid out, we have limbs a trunk and a head, this body partitioning is created by a set of genes referred to as Hox genes. The genes Wnt program for cellular organization such as organs and the Sonic Hedgehog gene regulates the formation of limbs such as arms and legs as we develop from an embryo. These types of genes, the ones that are universally needed to make complex creatures are found in just about every animal. But what is exciting about myxozoans is that they have lost many of these ubiquitous genes. Humans have 20,000 coding genes and myxozoans only have 5,500. By reducing their size and complexity they can survive with a much smaller genome than even their most close relation, other cnidarians.
 
So we can start to ask some fundamental questions; what are the genes that are necessary for survival? The authors compared the gene functions to other cnidarians and found that the myxozoans had less genes for development, cell differentiation to make different tissues and cell to cell communication. This all makes sense, with less types of tissues to make and less cells in the body to communicate the myxozoans can survive even when these functions are removed from the genome. Basic genes that build complex body plans including HOX and Wnt and Hedgehog were all missing in the myxozoan genome.
 
This is really exciting; we are just beginning to understand what genes are absolutely necessary for complex life. Even more amazing is that we can sequence a genome at all! Twenty years ago it was impossible to look at the entire genome of an organism. In 2003 the first human genome took more than 10 years and 3 billion dollars to sequence. But now we can sequence a human genome for $1,000 and it takes about a week. Biology is going through a technological revolution, now we can sequence the genome of different organisms to really understand the building blocks of life even as we discover new wild and crazy creatures.

All photos from the website:
fishparasite.fs.a.u-tokyo.ac.jp

Original Article:
Chang, ES, Neuhof, M, Rubinstein, ND, Diamant A, Philippe, H, Huchon, D, Cartwright P (2015) Genomic insights into the evolutionary origin of Myxozoa within Cnidaria. Proceedings of the National Academy of Science, USA, 112:14912-14917.

Life has been on Earth for 3.5 billion years. But most of the familiar multicellular life related to modern plants and animals are 550 million years old. While we think there are currently close to 10 million species on our planet, over 5 billion species have existed but are now extinct. There are 99% more species that have died than are currently alive. When a species becomes extinct it is gone forever and no longer contributes to the maintenance of important ecosystem services including food webs. A mass extinction is when many species go extinct and entire ecosystems collapse.

So why do so many creatures go extinct? There is always a slow rate of extinction due to random events. The majority of species died during 5 mass extinction events that happened millions of years ago. The most recent event was 65 million years ago when dinosaurs were killed by an asteroid impact. This extinction probably killed 75% of the species that were on the planet. 200 million years ago an extinction event killed 50% of the living creatures and was caused by massive volcanic eruptions that produced toxic dust that blocked sunlight. The largest extinctions event on the planet was 252 million years ago and it caused more than 96% of marine and 70% of terrestrial species to go extinct. It is thought that high CO2 concentrations in the atmosphere caused widespread climate shift and increased ocean acidification enough to completely devastate food webs. The oldest known mass extinction event was 440 million years ago and also saw a huge reduction in species (60%, at that time there were no land species). This extinction event was caused by global cooling and a large-scale sea level drop. Common among all of these mass extinction events is that natural disasters caused a shift in the world’s climate. The species that exist on the planet are adapted to their current climate, species go extinct when climate changes.

Currently the rates of extinction are much greater than normal. There is evidence that we are in the middle of the 6th mass extinction. Huge numbers of amphibian species are dying due to warmer temperatures and a fungal disease. More than half of Hawaiian birds have gone extinct due to habitat loss and a disease related to malaria. An entire group of 18 predatory bird species similar to a small ostrich went extinct in South America. The list goes on and on.

So many salmon have been caught and their rivers dammed up that many species are close to extinction. We created the Endangered Species Act (ESA) to protect species that are on the brink of extinction, but this legal mechanism does not fix habitat degradation, the root cause of extinction. For instance we built hundreds of dams in the Pacific Northwest during the 40’s for cheep electricity, to control flooding and for water security. We built way more dams than we needed and now these dams are blocking ancient spawning grounds of endangered salmon. Restoring essential habitat for these fish is critical to avoid their extinction. Many people don’t want to pay to remove dams, they say it is just not worth the money.  This type of short sighted outlook will continue our current rapid pace of extinctions, which could be slowed by the restoration of land and ocean habitats.

But some people think that modern technology will save us from a mass extinction. You may have heard recent reports that we can use DNA to bring extinct animals back to life. Technically this is amazing, we can insert DNA in a cell and make it replicate itself to produce a living creature that has some of the genetic code of an ancient creature. This is basically cloning (remember dolly the sheep) but with DNA from a different species. Really a pretty major technical achievement, but fundamentally it does not help us, we need species that contribute to the larger complexity that makes the ecosystems we currently rely on for survival. We might be able to make a franken-creature that contains ancient DNA but we need healthy modern ecosystems, not to recreate ancient ecosystems.
 
So now we have caught ourselves in a pickle, we have created a changing climate that is causing mass extinctions! We are at a turning point in our history, if we wish to survive we must maintain ecosystem functions. We need clean air and water for survival, if we want to live on this planet we need the plants and animals that we have now. This life is what makes our planet habitable, makes it beautiful and richly complex. It would be a shame to cause a modern mass extinction that changes life as we know it. We need to actively protect the life on this planet to ensure that humans can continue to survive here.

What do lizards know, I mean really how could they teach us anything? A recent paper about lizards in Florida is a great illustration of evolution in action. The paper “Rapid evolution of a native species following invasion by a congener” written by Stuart et al., was recently published in the journal Science. This work is not only a great example of science done well but it highlights how fast selection can modify an organism’s body to survive a changing landscape.

A few quick definitions, I find that people don’t understand evolution because they don’t know the nuances of the definition. First evolution is change over time. No direction, no getting better, no superiority complex, just change. This is important because random mutations occur in populations and even if they aren’t beneficial they are still considered evolution (we call these random mutations genetic drift). Natural selection is a pressure that drives evolution in a direction (this is the heart of what Darwin and Wallace described in their research). When a habitat changes (gets dryer for example) an organism has to change (evolve) to survive the drought (a plant might have leaves with less evaporation). Any individual in the population that already has a random mutation that has less evaporation will be selected for, making that individual more successful and better able to pass its genes on to the next generation. So now you know that change (evolution) can happen by a pressure (natural selection) that makes an organism better able to survive and reproduce (the real currency in biology is reproduction because this is how genes persist) in a changing landscape.

Ok the lizards research, what did they discover? This study creatively used an occurring invasion by a novel lizard (Anolis sagrei) on a set of small spoil islands in Florida to test the impacts on a native lizard (Anolis carolinensis). These two species of Anolis eat the same thing, this creates a novel competition that the native lizards were never previously exposed to, a new selective pressure. This forced the native lizards to perch higher in the tree canopies. However, the features of these trees change as you go higher in the canopy, the branches are thinner and the bark is smoother. This makes it hard for the native lizards to hold on. The researchers found that over 10 years the native lizards changed their feet, they had larger toe pads and more lamellae (little scales on their feet that allow them to cling onto smooth surfaces better). This is incredible, in only 10 years these lizards evolved to take advantage of unoccupied space.

Ok into the nuts and bolts. So there are a few important considerations to understand when talking about natural selection. First, the absolute time is not really important, selection happens over generations, it is effectively selection of the fittest over many generations (this is because selection happens on heritable traits). So the researchers estimate that it took approximately 20 generations for these lizards to evolve these adaptive feet. This would only be a few weeks for some bacteria, or 500 years for humans (human generation time is somewhere between 20 and 30 years so for this estimation I’ve used 25 years). While this study is an impressive illustration of how fast evolution can work, the results are so striking because these researchers chose an animal with relatively short generation times.

There are some questions that I have that directly contribute to the speed of change in this system, but they aren’t mentioned in the study. What is the variation in toe size and lamellae number in a natural population? This gets at the very essence of how selection works. Natural variation is the raw material of selection.  If there were a wide variety of toe sizes in the lizards before the introduction of the invader, there would be a lot of raw material for natural selection to act on. However, if there was very low frequency of larger toes in the native lizards, you could imagine that it would take longer for these traits to develop. Also what is the selective pressure of competition? This is a very difficult thing to measure, how “powerful” is competition in driving change over time. In many ways this study shows that competition is a strong selective pressure. But we don’t know the relative effect in relation to other factors, novel diet, changing environment, etc. These questions don’t discount the importance of this published work, they just highlight important nuances that might have changed the rate of change in these lizards.

One of the reasons I really like this paper is the researchers’ approach to testing WHY the toes changed size. Instead of assuming that invasive lizards drive changing behaviors in the native lizards the researchers tested other potential explanations. First the authors tested whether toe size and lamellae number were variable during the life of a lizard but not necessarily heritable traits. To do this they took eggs from parents exposed to invaders and not exposed and raised newborn lizards in the same environment, called a common garden. Lizards from parents exposed to the invaders always had larger toes and more lamellae regardless of the environment they were raised in, showing a heritable difference among the lizards.

Next the researchers tested whether lizards with larger toes had migrated to the islands. Using genetic techniques that measure the number of mutations in gene sequences they found that each island contained lizards that were more related to each other than to lizards on any other island.  The researchers were able to measure 121,973 mutations in these lizards. This is an amazing amount of information, previously science relied on a handful of mutations to identify population level differences, but recent technology has allowed sequencing of many more mutations. This created a very strong test for lizard relatedness within and between islands. The researchers did find a few lizards that had migrated from different islands but overall there was a higher level of relatedness within an island and the islands were very distinct from each other. This is a very exciting result and this data could also be used to identify which genes influence toe size and lamellae number, I expect there will be some more interesting discoveries that come from this data.

Using multiple experiments this study is a powerful example of rapid evolution in response to an invasive species. This is an exciting study because it is actively occurring on a time scale that we can observe and in a real world situation. Many tests of evolution have been conducted in the laboratory but it is difficult to apply laboratory results to complex natural habitats. The invasion of lizards to the spoil islands in Florida offer a great natural experiment to study the mechanisms and speed of lizard evolution.

Here is the full citation for this paper:

Stuart, Y, Campbell, T., Hohenlohe, P., Reynolds, R., Revell, L., Losos, J. (2014) Rapid evolution of a native species following invasion by a congener. Science 346: 463-466.