#30 Where do they think snakes came from

This question comes in from Jacob.

The origin of snakes is relatively well researched and has some surprising, at least to me, members of the snake family. I think we should start by defining what makes a snake a snake and then look at the fossil record.

All snakes belong to a clade known as Ophidia which includes snakes and those extinct animals more closely related to snakes than other non-snake squamates (what most people would call lizards are non-snake squamates). Interestingly, more and more evidence is mounting to include members of the Mosasaur family within snakes as they share many characteristics with pythons, more on this later. The appearance of the Ophidia clade is suggested to lie in the early to mid Cretaceous period (145-65 million years ago). Characteristics of modern snakes, but not all snakes, includes scaled skin, limblessness (not entirely true for pythons, boas and extinct species), moderate to extreme extension of the thorax, and decrease in the number of cervical, lumbar and pelvic vertebrae.

Fossil evidence:

Tetrapodophis amplectus from http://www.sci-news.com

Snakes have a somewhat spotty fossil record. Delicate bones and the large number of ribs with a lack of more robust leg bones, or robust skulls leads to the rarity of snake fossils. However, many do exist and allow for significant, if not total, understanding of the origin of snakes. In fact, the earlier mentioned 145 million years ago is in the process of being disproven with a recent discovery in Alberta that may be as old as 167 million years. The skulls of these new species place them firmly in the snake classification. Another fossil, which is younger, Tetrapodophis amplectus, is a four legged animal that has all the body features of a snake in addition to having four limbs. The overlap between limbless fossils and those with two or four legs that all bear the derived anatomy of snakes makes a very tangled picture. However, it is not uncommon to have this kind of variability in the evolutionary process. Though two theories have been hotly debated, burrowing vs marine swimming, the burrowing theory is finding support from CT scans. These suggest snakes lost their limbs to burrow.

Mosasaur from the Oceans of Kansas collection: http://oceansofkansas.com/mus-mosa.html

When snakes have legs...:
Not all snakes, fossil or modern, are limbless. Modern members of the python and boa family have a structure called pelvic spurs. These spurs are remnants of their leg bones, referred to as vestigial structures. I discussed snake legs in a previous post about limbs. Cornell University has a short video about snake toes. Then we have the ancient marine reptiles of the Mosasaur family. Research is building to show that they at least share a more recent common ancestor with snakes than other reptiles while other research suggests that they might in fact be snakes.

So where does that leave us...
Research suggests that snakes evolved from burrowing lizards. Based in the fossil record there seem to be multiple species with legs that possess many of the derived characters that make a snake a snake. This information gives weight to the idea that snakes are evolved from a much earlier lizard which would have had four limbs. These limbs were later lost, mostly, but retained in full, perhaps, by the mosasours and partially until the modern time in the pythons and boids. In reality, scientist are still combing the fossil record and DNA, in the form of comparative genomes, to produce a complete family tree for snakes and their ancestors. Even venom origins are helping paint a better picture of snake evolution.

#24 What is Evolution Part 3 of ? What is meant by natural selection?

Evolution is change over time in its most basic description. The first question seems to be how does that change occur. This is where Darwin's Big Idea comes in... Natural Selection.

Natural selection has four main concepts that define how selection happens. Selection is just a fancy word for the species or varieties or populations that survive and continue on. Natural Selection means that those creatures that are best suited based on these four principles will be "selected" to survive and continue on.

Natural selection works on populations and species as well. It can be affected by interactions of members within the same species (say a cheetah who is faster than its neighboring cheetahs may catch more food and therefore more of its offspring survive) or between different species (say competition between two bird species for insects or the faster zebras leaving more offspring because they are better suited to outrunning predators).

1. Struggle for existence.

- More individuals are produced than can survive. All members of a population or species must compete for food, water, space, mates, etc.

This seems logical when you think about it since the same species is looking for the same food, same nest sites or same territories, etc. So which members survive? The members that are best suited for finding food, evading predators, raising young, catching prey, attracting mates or whatever it may be will leave the most offspring. If you leave more offspring more of your genes and genetic material make it to the next generation.

2. Variation

- Variation exists between individuals in a species or population and this variation plays an important role in the struggle for existence.

There are quite a few examples of how variation is helpful. The classic example is the Peppered Moth. Originally, this moth lived on tree that were mottled and greyish white. The white moths blended perfectly into the bark. However, sometimes black individuals were born. These black individuals were very visible on the white tree trunks and therefore eaten by birds. In this situation the white moths are better suited for the environment than the black... the white are better at winning the struggle for existence.

Enter man and the industrial revolution. With the introduction of coal burning soot began to cover and darken surfaces, including the tree bark. Now the black moths were better suited for hiding and the white moths were easy to find. Birds ate the white moths that they could see on the tree bark. The black moths, due to environmental change, had become better at the struggle for existence.

Currently, with clean air technology and laws the surfaces are once again lighter colored. Guess which moth color is better at surviving now? That's right, the white ones.

Prior to the industrial revolution.

During the Industrial Revolution

#3 Adaptation.

- This is a heritable trait, i.e. something in the genetic code, that increases an organisms ability to survive and reproduce.

Lets play a game... which snake pictured below is the venomous coral snake and which is the harmless milksnake?

Would you want to take the time to try and guess if you knew there was a red, black and white/yellow snake around that could kill you? This is a case of mimicry. The non-venomous snake which had a pattern similar to the venomous snake was avoided. That avoidance means it survived longer and its similarly patterned offspring could survive longer and produce more offspring. This adaptation of looking like a venomous or poisonous counterpart is very common in the animal kingdom. There are many kinds of adaptations mimicry is just one of them.

Oh and if you are curious the second one is the non-venomous milksnake. Would your guess have killed you?

#4 Survival of the Fittest

Before we get into survival of the fittest lets define some words:
Fitness is how well an organism survives and reproduces.
Survival is the ability to pass heritable traits to offspring.

So one way to think about Survival of the Fittest is... Individuals with the best adaptations and variations will be the most successful at the Struggle for Existence. The success will give those individuals the highest fitness and the best survival.

Does this mean that an animal is perfectly suited to its environment or perfect for its environment?  

NO. What it means is that of all the possible variations a species could possess the ones we see most commonly are the best for surviving in the current environment. If that environment changes the types of variations and adaptations needed to survive may change. Some people describe it as "just good enough to flourish". Which means the traits an individual, population or species has is just good enough for that species to flourish, continue surviving, in the current environment. The individual has just enough of an edge on the environment to survive.

So what is Natural Selection really?

Natural Selection occurs when there are heritable traits (variations and adaptations) that cause variations in a population that has more individuals born than are expected to survive. (And this survive here usually means surviving to reproductive maturity.)

This is one of the mechanisms for evolution.

What to try your luck at survival of the fittest? Go here and click on Play the Survival Game.