Faculty of Biological Sciences, University of Leeds

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Evolutionary Developmental Biology Lecture 6.

 

Speciation and Extinction (OVERHEAD)

 

Speciation

We now know about the basic shape of the evolutionary bush (OVERHEAD). We think that the phyla, the main branches of the bush are real, because they have different Bauplans, and different from each other. We are not so sure about the intermediate twigs, the orders, families etc., but we think that the species, the smallest twigs, are also real because we think that we know of a good Biological distinction between them - members of individual species do not interbreed.

 

The fact that the individual twigs join together at their bases implies that something has split, a pre-existing species divided into two: and we know that once split these two will not interbreed.

How could this come about?

The answer, of course, could be reduced to a few words, or it could be expanded to a whole series of lectures covering genetics and phenotypic variation. Because we are interested at a fairly superficial level lets just summarise speciation fairly briefly.

 

First of all Darwin’s theory of natural selection depends on four ideas:

  1. There are more than enough offspring (OVERHEAD). The population of any species nevertheless remains fairly constant because not all offspring survive to reproduce
  2. Whether or not you are the lucky ones may depend on the environment (OVERHEAD). If all offspring were identical survival would be pure chance
  3. If individuals are not all identical some might be more likely to survive than others. Colour might be important (OVERHEAD).
  4. Some differences are inherited. Inherited differences are passed on to the next generation. Proportions of different types may thus change with time. (OVERHEAD).

 

Darwin thought that species changed like this, gradually and over a long period of time. But other factors are also important: we recognise that species can be formed as a response to space as well as time: Geography is important too.

 

Forming new species.

We now think that three stages are important in speciation (OVERHEAD)

  1. a barrier to breeding.
  2. becoming different
  3. two populations become two species.

.

Lets look at these further.

1.A barrier to breeding.

One species may be split into separate populations by any barrier which prevents two populations interbreeding. This may be geographical, like a mountain range, sea, land, a river, a desert or a patch of ice (OVERHEAD). Quite a small barrier (in our terms) like a tarred road, may stop the progress of something like a butterfly. The yellow water lilly (OVERHEAD) is the same species throughout Europe, and is common. On the mainland lillies interbreed, but the English Channel prevents interbreeding between English and French lillies, although they could interbreed if they ever met (romantic scene on Sealink ferry?).

 

2. Becoming different

Each population is subject to natural selection. Once separated each has a (slightly) different environment. (OVERHEAD) Over many generations populations come to differ. (OVERHEAD)

 

3. Two populations become two species.

If the populations become so different that they can no longer interbreed they become two different species. Failure to interbreed comes from many different causes including (OVERHEAD) different habitats.

different appearance

different behaviour

different breeding seasons

incompatible sex cells

offspring don’t survive or are sterile.

Again the differences may seem trivial (OVERHEAD)(OVERHEAD)

 

The end result of this is often two species, but not necessarily.(OVERHEAD) If the geographical barrier is removed whilst the populations are still able to breed together the end result may differ, according to Darwinism. The resultant hybrid may now be better than the parents, and oust them as a new species, or be less fit and die out, leaving one or both parent species to survive.(OVERHEAD)

 

The number of species.

We now know about species formation and what species are. One point which may have escaped you is this: quite simply if one species becomes two species by whatever means then we have increased the number of species in the world. If this process has happened lots of times (as we believe) then there must have been an awful lot of species around at one time or another There are now millions of species of animals and plants - probably 40 million, (OVERHEAD) but 40 billion species have existed at some time or other - only about one in a thousand species is still alive - 99.9% failure. The average survival of a species is about 4 million years. Obviously extinction is as important as speculation if not more so.

 

Extinction

What is extinction for? Does it have a purpose? If we believe Darwin's theory of the survival of the fittest, then extinction is what happens to the unfit. Deep in our ideas of evolution and change is this idea, that extinction weeds out the less fit species. This means 99.9% of species are unfit or become unfit.

 

But just as we are unable to actually give an example of species formation we cannot find a solid biological explanation of any single extinction. We can suggest plausible reasons for the extinction of dinosaurs, or the Irish elk, but we can't prove that extinction is based on fitness, except by the circular argument that extinct animals are extinct because they are unfit.

 

Consider what would happen if extinction didn't occur though. Here (OVERHEAD) are two evolutionary trees. The present day line in the first, with extinction built in, has only three living representatives: the other, with evolution abolished so that species never die, many more.

 

Without extinction we would see an exponential increase in biodiversity - the more species there are the more potential for new species exists. Adaption by natural selection would continue to hone and refine existing species, because each would have infinite time to evolve, so we would expect better adaptation to the environment, more efficient predators etc. This suggests that environmental niches - the opportunities for new animals to try a new lifestyle would gradually diminish. We don't know for certain because this is a mind game, and extinction is real.

 

Extinction is real - so how do we explain it. Two explanations come to mind, bad luck and bad genes. We'll come back to bad genes, susceptibility to disease, failure to adapt fast enough to a changing environment etc. later, but first of all lets go gambling, and look at bad luck.

 

Lets make this very simple and find a game with a 50/50 chance of winning. Tossing a coin, drawing a red or a black card from a pack, playing red versus black at roulette all approximate to this. Lets suppose that the game works like this. I have a stake of ten dollars. For each toss of the coin I nominate heads or tails and stake a dollar. If the result is correct I get a dollar from the bank. If I am wrong I lose a dollar.

 

Doing this a number of times one of three things will happen.

1. I go broke

2. the bank goes broke

3. we get fed up and go down the pub.

 

If we look at a long series of sets of plays (OVERHEAD) we find that alternative 1. is the usual result (because the banker usually has more money in his stake), 2. almost never happens (for the same reason), and there is no information on 3.

 

These graphs confirm our intuitive feeling on gambling. Game theory (the study of this and other probability scenarios) has introduced specialised jargon which you may come across:

  • The fluctuation of the stake follows a random walk - it goes up and down following no particular path and does not tend to return to the original stake.
  • The stake = 0 axis is an absorbing boundary - when you hit this the game is over.

 

How does this work as a model of speciation and extinction? Lets suppose that the gambler is a genus, a group of 10 species, one for each dollar. The number of species (genetic diversity) follows a random walk - speciation is a win, extinction is a loss. Eventual extinction is a certainty, although having a bigger stake will prolong the game. There is no upper boundary (which would have to mean that your genus survives and all other genera in the world become extinct - as likely as you breaking the bank in a casino). Since there is no upper boundary to terminate the game it can only end in eventual loss of your stake or a global catastrophe (meteor or going down the pub).

 

One problem that you may see with this is that it relies on randomness, and that randomness may not occur. In fact the logic is sound for any odds, although the graphs change a little. And why not 50/50 odds anyway? This would imply that the number of speciation events = the number of extinctions in world history. I said that the approximate number of species ever was 40 billion, and the present number 40 million, so we are not far out (OVERHEAD AGAIN)

 

We made another assumption, that the number of species in a genus was 10, our $10 stake. But all genera must start out with one species, and we said that increasing the stake money gave us a better chance. The converse is also true , and if we look at the length of survival per genus (OVERHEAD) we find a very skewed distribution. Many more genera are short than long lived. This is due to a number of factors

number of species in a genus

life span of species

numbers of individuals per species

geographic range of species.

 

If we look at our extant species we find, for example, 4,000 living species of mammals, (OVERHEAD) grouped into 1,000 genera. About half these have only one species, 15% two. There are very few genera with 25 species, and the most speciose (a small insectivore) has 160.

 

So we can say (OVERHEAD)

 

Most species and genera are short lived (compared with the averages)

Most species have few individuals

Most genera have few species

Most species occupy small geographic areas.

 

A useful model of this sort of thing is the surname. Most new surnames arose by mutation of an existing name (Smythe from Smith) , or the coining of working name (smith because he makes things in iron: lots of Engineers in India under the Raj). They start with small numbers, and die out if no sons are born. Malthus, as part of his famous Essay on the Principle of Population looked at surnames in Bern from 1583-1783. Three quarters of the families extant at the beginning had gone by the end of the period. But everyone is aware of families which last, people descended from the Normans etc. They exist, but are very few. Most families are small, but there are a few, like the Smiths and Johnsons, which are huge.

 

This kind of skewed data is very common in biology, although we often pretend that it isn't. We like to think of data as being on a bell curve (OVERHEAD) normally distributed with as much above as below the average. Statisticians often transform data so that they can treat the results as if they were normally distributed, but in fact other forms are commoner. Examples of skewed data include incubation times of infectious diseases and life expectancy of cancer patients. In both the majority is lower than the average, with many short times and a few long ones.

 

The bell curve can often be replaced by the broken stick model. Suppose we take a stick 1m long and break it into 25 random pieces. The lengths of the bits (OVERHEAD) follow a skew distribution - this gives a better approximation of the sort of data we have been looking at.

 

 

 

Mass extinctions

 

The sort of extinction that hits the headlines is mass extinction, probably in connection with the Dinosaurs. Alvarez et al proposed in 1980 that a giant comet or asteroid collided with the earth 65 million years ago, triggering a huge mass extinction which accounted for the dinosaurs and a great deal of other life as well. In 1984 Sepkoski & Raup suggested that this was one of a series of mass extinctions, about 26 million years apart. The most popular astronomical explanation of this was a small companion star of the sun (Nemesis) which periodically disturbed its orbit.

 

If you ask a palaeontologist about mass extinction they will probably say that there have been five (OVERHEAD), punctuating what are seen as distinct periods dominated by different animal sets (Ordovician, Devonian, Permian, Triassic, Cretaceous) If asked about the periods in between they would probably opt for a continuous background level of extinction, perhaps with a few local peaks. In fact (OVERHEAD) there seem to be quite a lot of these irregularities. So we need to answer several questions (OVERHEAD).

  • How do we measure extinction?
  • Are there differences between large and small extinctions?
  • Is extinction short term enough to be called an event?

 

Lets start with a biggie the Cretaceous/Tertiary, dinosaur event (usually called the K/T because there are other periods beginning with C) (OVERHEAD). Virtually all plant and animal groups lost genera at or near the end of the Cretaceous. 38% of marine genera were lost, a bit more on land. This represents a lot of biomass, because remember that if a genus has gone all its species must have become extinct. Large groups went, marine reptiles and dinosaurs, one third of mammals, lots of amphibians. The pattern, in fact was of huge losses of a few very abundant species. Vegetation changed so that in a few mm of deposits fern spores jumped from 25 to 99% and there was a corresponding drop in flowering plant pollen. There were also large changes in marine plankton.

How can we quantify this dramatic event? One way is to look at the percentage loss in different groupings This percentage increases as we go down the list. Why? Because each category is a subset of the one above. Lets put it another way. Suppose that each species has 10 individuals, each genera has 10 species etc. The sum works out to 1 million individuals in one phylum. Now suppose that the extinction event kills 75% of all animals at random. No phyla will become extinct: 25% of its individuals survive. The probability of one of the ten classes becoming extinct is also very small, but the probability of the 10 individuals of one species being killed is high. The 10 fold increase per classification step is unrealistic, because of the skew we have already seen in group size, but that doesn't change the figures much.

 

The idea of killing may disturb you. An alternative scenario is that the birth rate fails to keep up with the death rate. This has an aesthetic appeal, but there is no reason to think that it is true. Recently several palaeontologists have suggested that extinction, similarly, is merely a decrease in the rate of speciation, or species birth. There is no reason to suppose that this is true either.

 

The duration of the extinction is also important. Did it happen in a few minutes ( as it might with a comet collision or a volcanic eruption, or did it take millions of years? Many published charts of the K/T extinction give the idea that it was instantaneous, because they draw a line at the K/T boundary. What this means, obviously, is that the species was there at the end of the Cretaceous and not thereat the beginning of the Tertiary. But the last period of the Cretaceous was 9 million years long, so we are only saying that the extinction occurred somewhere in that 9 million years. Supposing we try harder and collect samples centimetre by centimetre across t he K/T boundary? The problem here is which K/T boundary?. It isn't always visible for a start. Secondly (OVERHEAD) we know from the present that rocks erode away as well as get deposited. We look at the K/T boundary and assume no erosion: but the last 5 million years of the Cretaceous, say, could have been all, or partly, or in some places, erosive and we would then get a series of different versions of the truth. We can only say that the species was not extinct at the level where we see the most recent fossil individual.

 

Are mass extinctions really different from the general run of things? The Hurricane of 1989 in Southern Britain made all the papers, as do others in the Caribbean and North America. But what is the difference between a Hurricane and a tropical storm? The difference is in wind speed (OVERHEAD). We make differences appear in a continuum by defining arbitrary boundaries. Mass extinctions are a bit like that too. If we look at the number of extinctions in about 100 arbitrary , equallish, timed intervals we find (OVERHEAD) the familiar skewed distribution. There are some mass extinctions, but usually there are not many extinctions going on. So mass extinctions are there, infrequently, as part of the normal pattern of things - small events are common, large ones are rare, like hurricanes. You can express this another way as a kill curve (OVERHEAD): an event big enough to kill 65% of species happens about once in 100 million years.

 

Selectivity

 

So big extinctions happen sometimes. Who do they kill? Is species elimination random or not? The less random it is the more effect it will have on evolution.

The Spaniards introduced horses to the New World in the sixteenth century, with a devastating effect on the natives. Or reintroduced them rather. Horses have a long fossil record in America, but became extinct a few thousand years before the Spanish arrived. There were other large mammals too, mammoths, mastodons, sabre tooth cats, buffalo and giant sloths. There was a wave of extinctions in the Pleistocene which affected not only America but Australia, Madagascar, China, Britain and N. Europe. Because radiocarbon dating of these comparatively recent events is easy and accurate we can say that the extinctions occurred between 8,800 and 9,000 years BC. This coincides with the early human colonisation of N America. Also the various extinctions occurred at slightly different times in different places, and were less severe in Asia and Africa where human habitation was much older. Had all these Russians coming across the land bridge from Siberia killed off the food animals? This is the so called blitzkrieg theory. Does it hold up?

 

Well first of all the Pleistocene extinction was selective. Mammals died out, as did some large flightless birds, but nothing happened in the sea. Secondly the size of the mammal was important (OVERHEAD). Large animals were much more at risk than small ones. There are enough examples of extinction for us to test this difference statistically, and it is real. So being large was a risk. Perhaps man did exterminate many large mammals, in which case this is an atypical and hence poor example to study. Another theory is that this was a time of warming, melting of ice sheets, change in sea level and this was what did for the large mammals. Incidentally it also allowed man to colonise America.

 

If body size was a problem it also should have had an effect at the K/T extinction, which certainly wasn't man made (because there were no men, not even Raquel Welsh). We can find the following comments on this (OVERHEAD,OVERHEAD).

 

In fact both of these are true, because the goal posts are different - the Hurricane problem again. LaBarbera noticed that the largest vertebrates (large reptiles) went extinct, and this suggested that body size might be a factor. Clemens used a cut off of 25kg (not what LaBarbera may have had in mind, but what is large or small?) and noted at least some large survivors in each group (like large crocodiles and turtles).

 

As far as we are concerned how big you are is less important that what you are. Lets look at the K/T one last time (OVERHEAD) this time classifying taxonomically. Placental mammals did well, marsupials badly, but the numbers in each case are small and so unreliable. But the dinosaurs took a pasting.

 

So whatever the causes of extinction, a chance event that will happen to all species, human intervention, meteorites, volcanic eruption, ice ages, global warming we can isolate one relevant fact: what you are or who you are. Taxon, zoological kind of animal, Bauplan seems to be important..

   


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