Community Change: Evolution

Hello everyone!

One of the biggest questions that floats around in a lot of ecology is “How, and why, do we see such a large variety of different species on the planet today?” With new species being discovered all the time – this is an ever present and relevant question and, therefore, a good one to theme my first few post on.

I am going to attempt to answer this question over the next three posts; with one on Evolution, one on Persistence and one on Interchange and Mass Extinction.

Now, the obvious answer is just to say “evolution”, but before we delve into this, let me define some terms;

  • SPECIES: There are many different definitions of the word “species” – the most common being the strict biological definition – If they can breed and produce offspring which are fertile, then they are the same species. Now, that isn’t much help when you are looking at a fossil – they are pretty incapable of breeding. In this case, we are more inclined to use a morphological definition – If they look pretty much identical (from the fossilised material) they are the same species. This, of course, has its own problems. Where do you draw the line? What changes denote actual species differences and what changes are just variations? For now, though, it’s all we can base it off.
  • EVOLUTION: Whole books have been written defining what evolution is, but I am going to summarise it as simply The gradual change in a population over time, both genetically and behaviourally. The important thing to note here is that evolution doesn’t have to be a physical change – but physical changes are pretty much the only thing we can (easily) get from fossil activities. (NOTE: We can infer some behavioural activities from fossils but, for now, I am going to ignore these. Sorry!)
  • FITNESS: When the term “survival of the fittest” is thrown around, it helps to define what biological “fitness” means. No, it doesn’t mean that an organism will survive if it goes for 10km runs every morning and then does 80 one handed press ups – biological fitness is How many genes from a particularorganism (or species) are passed down to the next generation or, alternatively, How well a particular organism can reproduce. Now, the term “survival of the fittest” makes sense using this definition – an organism that has 20 offspring is incredibly fit and the species as a whole is more likely to survive.
  • COMMUNITY: The palaeontologist DiMichele defined a community as A recurring, recognisable assemblage of organisms. Or, in other words, A group of organisms that both persists through time and can be traced through time.

    Credit to http://library.thinkquest.org for the image here.

    A simplified Phylogenetic tree showing the relationship between organisms. It should be noted that there is a slight error on this tree, and that the lineage for birds should branch out from the “dinosaur” lineage and not as close to the Pterosaurs as is suggested here.

Now, how and why do communities change over time?

Well, evolution plays a big role in this. Evolution “favours” (although, remember, that organisms don’t choose their own evolution) fitter organisms. Fitter organisms will be the ones who have many offspring, and many offspring results in plenty of your genes, the things that define who you are (and what species you are), being passed down.

But how does this work? Let’s take two theoretical examples that show this in action.

Imagine you are a rabbit living in a very cold environment. Now, let’s assume the colour of your coat is determined by one set of genes that give you either a brown or a white coat. We will assume that the gene for having a brown coat is dominant and the gene for having a white coat is recessive. All this means is that, at any given time, there will many more brown rabbits than white rabbits. (We will also assume all mating is random, and that brown/white rabbits will happily breed with each other as well as similarly-coated rabbits.)

Now, let’s say that the environment changes a bit and has a particularly snowy winter. When the foxes are out, any rabbit with a white coat will be much less likely to be caught and eaten. In this way, the community changes

A community that started off with 15 Brown rabbits and 5 white rabbits might now have 4 white rabbits and 4 brown rabbits – The proportion of brown:white rabbits is now much more in favour of white rabbits - what was a population where 1/4 were white rabbits is now a population where 1/2 are white rabbits. They breed, and the next winter comes. Over time, if the weather stays consistently snowy, white rabbits will tend to increase their numbers (relative to the brown rabbits) over time.

For another example, let’s look at something that focuses more on behaviour.

Imagine you are a peacock (male). Your genes (a number of them, this time) will determine how impressive your tail is – how long it is, how bright the colours are and so forth. We will mark your tail on a scale from 1 to 10, based on these aspects.

Now, the peahens (female) you are trying to impressive are very picky. They will only mate with a peacock with a tail which is at least 7/10 on the impressiveness scale – the mating is not random. As such, only the most impressive peacocks will mate, and pass their genes on – increasing their fitness. Their offspring will, on the whole, have more impressive tails (the “impressive” genes will be passed down).

Overtime, peacocks will grow to have more impressive tails. This is an example that shows evolution that, at first glance, may not make much sense. Having a larger tail makes you more of an obvious target to predators – it slows you down and makes you clumsy. Looking at change over time in a peafowl community would probably reflect this.

In our example, you needed a tail which scored at least 7/10 to have a chance of mating. However, let’s say that a score of 9 or 10 out of 10 resulted in you being too vulnerable to escape from predators. Overtime, the community would change to mainly have tails which score 7 or 8, with very few scoring above this (reduced chance to survive) and very few scoring below this (reduced chance to mate).

There are, therefore, two major things that affect change through evolution: the environment and behaviour. Both are equally important and both can radically change the outcome of a species. But how does evolution change a species over vast periods?

The idea to take into mind is the idea of separation and mutation. Let’s take the rabbit example again from above;
Let’s say that, at some point in time, the community of rabbits becomes separated. One group of rabbits, during their travels, goes down one route on a path and the other group goes another – and they end up completely separate – They are unable to interbreed. This separation could take place through a geographical separation (the type I have described – where there is a physical barrier such as a body of water or a mountain range in the way) but could also be a mechanical separation (where the two groups are in close proximity but unable to breed with each other, perhaps due to physical size differences).

Now, let’s say one group ends up where a white coat is more advantageous, and the other ends up where a brown coat is more advantageous. Give it a few million years and, with no other interference, one group will end up predominantly white coated and the other mainly brown coated.

The other thing that will happen in this time is genetic mutation - where a random mutation during the birth or conception of an individual rabbit results in a change to the organism. It could be anything from a slightly better coat colour (a green coat, perhaps?) or the eyes being slightly closer together (or anything more radical than that). If this is a significant enough advantage, that rabbit may pass it on to their offspring – resulting in (eventually) a change in the species. Your brown coated rabbit colony may end up being a green-brown colour if the environment favours it and the mutation happens.

If the changes are vast enough, and are given enough time, then in a few million years the two original rabbit colonies may be different enough that they now are unable to breed – they have become separate species.

Over time, colonies can change quite substantially – but it does take time. You can’t expect a fish to one day grow lungs and walk out onto land – it takes millions of years for things like that to develop. This is one of the reasons we see so much variety today – separation on different continents has resulted in a plethora of different organisms for us to look at and study.

But what happens if the mutations don’t happen, and the species don’t change? In my next post, I shall be discussing Persistence in the fossil record and how evolution can show apparent stasis – the lack of change – in a community over longer periods of time.

Until then, have a wonderful week.

~Mattchu

Additional Reading:

For those who are interested, there are many, many, books written about evolution. On the Origin of Species by Charles Darwin is a classic, and was the original book on the subject, but can be quite dry and has aged. Collin Paterson wrote an introduction to evolution just called Evolution which is a good starting material for evolution and genetics – and is written in a very clear way. A quick google search for books on evolution will show you several hundred results – and a trip to the local library will probably give you the same.

(Another good book to pick up is Dinosaur Hunters by Deborah Cadbury – which gives a bit of insight into early ideas about evolution and geology. it is written less as a science textbook and more as a story, and the evolution bits are quite hidden in there. The Selfish Gene by Dawkins is also meant to be quite good on the subject, but I have not read that yet so cannot comment on it’s accuracy.)

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