Prof. D.D. Williams

[2005 Session]

Additional Notes from the end of  Lecture 1

Phylogenetic Classification: Linnaeus' purpose was to catalogue the species as they had been created. In doing so he believed that he had revealed the pattern of the Creation which, for the plants and animals, implied that their species had existed distinct and unchanging from the beginning of the world, without genetic connections between them. His divisions into hierarchical categories, therefore, represented structural differences and no more. With the acceptance of evolution by biologists in the latter part of the 19th century, it became clear that classification had phylogenetic significance and that morphological characteristics could be used to group organisms together rather than to separate them.  The emphasis, therefore, moved away from structural differences towards structural similarities.  But, as characters that separate some organisms are usually those that link others, surprisingly little change in classification followed this fundamental change in outlook. 

Today, biological classifications are phylogenetic in construction, that is they represent as nearly as possible current views on genetic relationships. To some extent, every classification is a compromise attempting to coordinate the different views of systematists on phylogeny and is thus subject to change as new information causes opinions to alter. A genus is thus a group of species placed together because they are very similar and of more or less immediate common anscestry, while a family is a group of similar related genera and so on.  Recognizing one genus or two is a judgement call, and higher taxonomic categories are similarly subjective.

Characters used in classification: Many of the structures used in classification are basic features of body organization, such as the coelom (body cavity) or the presence of body segmentation, but others are apparently trivial, such as the presence or absence of hairs on an insect's leg.

From what I've just said it will be evident to you that zoologists are not unanimous about the classification of invertebrates and some textbooks disagree with each other. In order to minimize confusion, we shall follow the scheme of classification outlined in the handout and on the CD.
In this course, we cannot look closely at all of the diversity of life, but in order to explore the processes that have led to diversity, we will study in the lab, the patterns found in some of the invertebrates. This makes sense from more than one point of view.  

First of all if we look at Wilson's distribution of the numbers of known animal species, we see again that invertebrates comprise about 95% of all animal species and insects about 75%. Invertebrates then are important simply because the greatest animal diversity exists there. 
In addition, the groups that we have chosen represent the early stages in evolution of life on earth and demonstrate many of the physiological, morphological and behavioural developments that have conferred evolutionary advantage on invertebrates and also on vertebrates, thereby leading to this great diversity.

The two most important steps towards increasing biological diversity (aside from the origin of life itself) have been:

(1) the origin of eukaryotic organisms about 1.8 billion years ago. These organisms had their DNA enveloped in membranes and the remainder of the cell contained mitochondria and other well-formed organelles. The first eukaryotes were single-celled in the manner of modern protozoans and the simpler forms of algae, but gave rise to more complex organisms composed of many eukaryotic cells organized into tissues and organs.

(2) "the Cambrian explosion", 540 to 500 million years ago. Abundant macroscopic animals large enough to be seen with the naked eye, evolved in a radiative pattern to create the major adaptive types that exist today.

We shall study the groups that are vital to this evolutionary "progress" – progress from simple to more complex and numerous animals. We shall begin with Protozoans as representatives of simple, single-celled organisms - looking at the extent to which diversity can be achieved through the physiology, morphology and behaviour available to single cells.

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Additional Notes from the end of Lecture 3

        Why has diversity continued to increase despite temporary declines and the almost complete turnover in species?
There are probably two major reasons:
The first was the creation of the earth's aerobic environment, which we have already discussed.
The second was the fragmentation of the continental land masses in a way that enhances species formation, and we will explore this topic in Lecture 4.

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Lecture 6  Notes   

Dispersal; biological realms: past and present patterns

        The development of a group of organisms, whether a species or family or order appears to consist of:
(1) a localized beginning,
(2) an expansive phase with adaptive radiation, and
(3) ultimately a regression with or without the formation of a new and more successful group.
        Of basic importance to the second phase and therefore to the study of the distribution of organisms is the concept of species dispersal or spread.  Dispersal can be divided into three main modes, all of which result in changes in the geographic range of a species:
(1) Jump-dispersal  is the movement of individual organisms across great distances, followed by the successful establishment of a population of the original disperser's descendents at the destination.  This usually takes place over a time period less than the life span of the individual and often over inhospitable terrain.  For example - spiders are carried across the open sea by air currents.
(2) Diffusion is the gradual movement of populations across hospitable terrain for a period of many generations.  Species that steadily expand their ranges can be said to be diffusing. For example - killer bees spread northward in South America and into North America following their introduction into Brazil.
(3) Secular migration is diffusion taking place so slowly that the diffusing species undergoes appreciable evolutionary change during the process.  The range of the species expands or shifts over long time intervals (thousands or millions of years).  The environments themselves may change and natural selection acts on the descendant populations For example, hte llamas and vicunas of South America are descended from now extinct North American members of the camel family that migrated during the pliocene over the Isthmus of Panama.
        Acting in conjunction with the spread mechanisms to effect changes in range is local extinction, so that ranges can expand, contract or "creep" (expand in one direction while contracting in another).  In practice spread often consists of a mixture of the above modes across terrain with various degrees of hospitableness.  Any terrain which is inhospitable enough to prevent or slow spread of the species can be termed a barrier.  For example, a body of water is a barrier to a terrestrial organism, just as dry land is a barrier to a marine organism.  In particular, Beringia, the area between Alaska and Siberia, has functioned simultaneously as a bridge for terrestrial animals and a barrier for marine ones during periods of marine regression and vice versa during periods of marine transgression.  Moreover, one terrain type may be a barrier to one mode of dispersal but not to another - for example, a desert may be an impassable barrier to slow diffusion of a mesophytic plant (plant with moderate moisture requirements) but offer no resistence to the rapid passage of wind-blown seeds across it (jump dispersal).   So the second principle is that barriers can cause long delays in dispersal.
        The third principle is that environments are constantly changing and affecting dispersal, colonization, speciation and extinction. Climate change, for example, also changes terrestrial or marine routes from barriers to bridges or vice versa for different organisms, either directly through the organisms’ temperature tolerance limits or indirectly through effects on vegetation that may serve as food or as barriers to the spread of seeds.
        The fourth principle is that new species which survive successfully are generally better adapted to their surroundings than their predecessors, although there are exceptions including many domesticated animals.
        The fifth principle is that the present-day distributions of species do not necessarily reflect the areas of origin.
        The sixth principle is that organisms with a previously wide distributions may become restricted eventually leaving a relict population, or become extinct in the face of increasing competition and/or adverse changes in the environment.
         In general, plants spread more rapidly than animals and as a result, the modern distributions of plant species tend to reflect current conditions of climate and soil, while those of animals are more likely to reflect the geographic and geologic history of a region.  
        Keep these 6 principles in mind as we go on to consider:

The effects of continental drift on species spread: 
        Continental drift affects all three modes of species spread. Of itself, it accomplishes (for terrestrial organisms) the slowest form of secular migration, by gradually rafting species - and even whole communities - across enormous distances.  Organisms whose observed distributions are directly attributable to this rafting are likely to be those for which jump dispersal is improbable, thus the evidence of slow drift has not been obscured. For example, large flightless vertebrates, freshwater fishes, or invertebrates that cannot survive drying, or exposure to salt water.
        The present-day distribution of earthworms is the obvious outcome of continental drift.  There are several Gondwanaland genera that occur in southern South America, South Africa and Australasia, whereas further north, another group is common to West Africa and Central America.  Further north still, the majority of earthworm species are the same on both sides of the Atlantic.  This preservation of past continental distribution in earthworm distributions is not surprising, since earthworms cannot survive submersion in salt water, they live in the soil where their eggs are unlikely to become attached to ducks feet, and they are particularly susceptible to desiccation. 
        Indirectly, continental drift has affected the spread of plants and animals by diffusion and secular migration, by breaking and creating land and marine bridges such as Beringia, or by shifting land masses into different climatic zones. For example, the mountains of Malaysia and New Guinea, uplifted in Pliocene-Pleistocene time, provided a cool, high altitude route for the diffusion of Northern Hemisphere plants into Australia. The dispersal of terrestrial organisms depends not only on the existence of land bridges or stepping stone routes, but also on those routes having climates that the dispersing organisms can endure. 
        Besides causing great change in the temperature and moisture regimes of an area, the rearrangement of the continents also has brought changes in the strength and direction of prevailing winds with important consequences for wind-dispersed organisms.
        For organisms capable of jump dispersal, as continents drift towards one another, the number of organisms that can jump disperse gradually increases as the gap narrows until contact is made and diffusion can also take place.  As land masses drift away from each other, jump dispersal becomes less frequent.  The clearest indication of non-occurrence of spread is obtained when sequences of fossil biotas are found that become steadily more different from each other the more recent they are, such as the post Cretaceous floras of South America and Africa.
        The effects of continental drift on species distribution (and diversity) are by no means straightforward and I want, now, to return to a historical approach to explore some of the theories about events and interactions after the formation of Pangea.

Evolution and dispersal during the Mesozoic
        At the end of the Paleozoic (that is at the Permian/Triassic boundary), a wave of extinctions is believed to have greatly reduced the diversity of the marine fauna of the continental shelves.  One theory to explain this, suggests that after the formation of Pangea, the crustal plates were for a time motionless - consequently, the formation of new oceanic crust at the mid-ocean ridges stopped and the ridges collapsed.  This caused a large drop in sea level, converting much of the continental shelf into dry land.  Reduction in extent of their environment therefore reduced the numbers of individuals and taxa of shallow water marine forms.
        An alternative explanation is that because the formation of Pangea resulted in one long unbroken and relatively uniform continental shelf, the spread of species resulted in competative exclusion and therefore extinctions.  Yet another explanation suggests that this decrease in diversity is simply an artifact of the fact that not much sediment was laid down in the many cool , dry areas, so that few fossil samples are available and those that are known do not therefore represent the entire fauna.
        Pangea remained intact for the major part of the Triassic Period, the first period of the Mesozoic Era, and throughout this period the landmass moved slowly northward and so continents remained elevated, and climates remained cool, dry and seasonal in much of their interiors.  The sea was still largely open and therefore relatively warm and therefore contributed to relatively warm climates in high latitudes.
        Triassic faunas and floras were markedly provincial. Terrestrial biotas are commonly divided into northern (Laurasian) and southern (Gondwanan) realms. The biotic character was probably linked to the dry or seasonal interior climates that we have just mentioned. The diversity of conifers, seed ferns and cycads increased. Conifers were dominant in Laurasia while seed ferns dominated the Gondwanan realm until late in the period when conifers became more common there as well.  The mammal-like reptiles (Pelycosaurs) that had diversified and spread throughout Pangea during the Permian eventually gave way to mammalian anscestors (the cynodonts and therapsids) and to the diapsids (including archosaurs) and, by the late Triassic, the archosaur descendants, the first dinosaurs. 
        It is believed that the diapsids were favoured at this time over the mammalian ancestors by their ability to excrete uric acid rather than urea, since this is more water efficient. The dominance and diversity of large herbivores increased during the middle and late Triassic and amongst these, the cynodonts, dicynodonts, and rhyncosaurs (thecodonts in the figure) were particularly common in Gondwana. The latter two groups became extinct by the end of the Triassic.
        During the late Triassic, Pangea began to split into fragments. In the Jurassic, North America was probably moving away from Africa and from South America. Eurasia also began to move away from North America and finally Africa began to split from South America and Antarctica. However, the process was very slow and separation by ocean basins probably did not occur until the late Jurassic. Global climates continued to be generally warm, but with strong latitudinal variation, particularly in rainfall.
        During the Jurassic Period, cycads, cycads, ferns, and conifers developed further in both the Laurasian and Gondwanan realms. Different groups of conifers were more important in Laurasia and Gondwana.  Many groups of herbivorous insects were arose during the Jurassic.
        The larger land fauna was dominated by large herbivorous reptiles, notably the dinosaurs. The dinosaurs diversified and spread throughout the world and to the air (pterosaurs) during the early and mid Jurassic before extensive continental separation had occurred. The route between North America and Asia was by way of the mild-climated Greenland and Europe.  These large animals reached Africa either directly, or via South America.
        Toothed birds (Archeopteryx) evolved from the thecodont reptiles.  By the late Jurassic, the Theria existed in North America - these were the first small, primitive mammal ancestors of monotremes, marsupials and placentals.
        During the early Jurassic, sea level rose and marine habitat diversity again increased.  In the shallow waters of tropical and low latitude continental shelves and slopes, fluctuations of temperature and salinity were relatively small and here in particular, marine invertebrates again increased in diversity and numbers. Ammonites were at their peak and the first scleractinian (hard) corals appeared.  Ichthyosaurs and plesiosaurs reached their peak in diversity and abundance.
            By the late Jurassic, North America/Greenland, Gondwana and northern Eurasia were separate continents.  On land, distinct floral provinces developed in temperate and tropical regions of both the northern and southern hemispheres. Conifers, cycads and other gymnosperms continued into the Cretaceous, but it is likely that, around this time, the first flowering plants (angiosperms) developed in tropical uplands with a warm, seasonally-dry climate - most likely in the interior of west Gondwana. The reason for this is thought to be that the enclosed ovules, characteristic of angiosperms, would have been selectively favoured in a region of seasonal drought.              Gradually, during the Cretaceous, angiosperms expanded their ecological tolerances and hence their geographic ranges. They migrated altitudinally downward and upward and, latitudinally, northward and southward.  However, primitive angiosperms have inefficient dispersal mechanisms and therefore cannot escape into new regions if the climate of their ancestral region deteriorates.  Therefore, today, they persist only in regions where the climate has remained much as it was when they first evolved - that is, southeast Asia, northeastern Australia and India. Their confinement to unchanging environments also means that they are not exposed to the rigours of strong selective forces - hence they evolve slowly and remain primitive.  More advanced angiosperms, on the other hand, with efficient long-distance dispersal mechanisms often find themselves in habitats markedly different from the ones they come from and will therefore be subject to intense natural selection.  Hence they evolve quickly.
        Along with the development of angiosperms went further radiation of the insects whose behaviour, life cycles and distributions are intimately connected with plant morphology and distributions - as is evident in the Lepidoptera (butterflies and moths) and advanced bees (Hymenoptera). They too had the advantage of efficient dispersal mechanisms including, by this time, flight.  In general, the angiosperms gradually became dominant over the gymnosperms and ferns - such that, by the end of the Cretaceous, flowering plants made up 50 to 80% of the species in middle to high latitudes.
        Although small mammals were present, the dinosaurs remained the dominant land vertebrates during the Cretaceous.  New types dispersed throughout the northern hemisphere while it was still undivided by seaways.  Those that evolved later had more restricted distribution patterns because sea floor spreading increased as the continents broke apart causing a rapid rise of the ocean ridges and a consequent rise in sea level.   As sea level rose towards its maximum, inland seas flooded low relief parts of the continents isolating new continents in both the northern and southern hemispheres. In addition, land barriers such as the Andes, Himalayas and Rocky Mountains were built at this time. 
        Vertebrate assemblages began to show pronounced differences between the northern and southern hemispheres. In the south, sauropods were the dominant herbivores while, in the north, ornithopods (circumboreal) and ceratopsians (North America only) were more abundant - although pachycephalosaurs developed at this time in the northern hemisphere.
        As the mammals evolved, the marsupials and monotremes probably first became established in the South America/Australia/Gondwana land mass. Marsupials dispersed to North America and throughout eastern Gondwana and were the dominant mammal group in North and South America for most of the Cretaceous.  Climates remained mild and the mammals therefore were able also to reach Europe via Greenland.  The placentals developed first in what is present-day Asia and reached North America via Beringia during the Cretaceous.  Some placentals reached South America but, at that time, neither group reached Africa or India since these continents had separated before the late Cretaceous.
        The raised sea level also provided an increased area of warm water for colonization and niche partition by marine invertebrates. The oxygen isotope record shows that the deeper waters of these oceans were 15˚C warmer than those of today.  As sea floor spreading progressed, deep ocean trenches became barriers causing genetic isolation and an accelerated rate of morphological divergence of shallow water marine invertebrates. 
        From the late Cretaceous onwards, as the continents moved further apart, the earth's climate became cooler and more seasonal. We will look at Cenozoic climates more thoroughly in a later lecture but at this point I want to jump to the present in order to better describe the surviving continental configurations.

Biological Realms
        The modern world can be divided up into regions on the basis of the present distribution of its flora and fauna, that is, the dominant and easily visible groups of organisms occurring there. These current assemblages of organisms are the result of the early history of the groups, together with continental drift and climate change during the Cenozoic Era.  By looking at their present distributions, we can see exactly how these distributions are related to factors such as space, climate, barriers, plant cover and each other.
        A biological subdivision of the earth's surface can take account of  either the terrestrial or the marine biosphere.  In a terrestrial subdivision, the oceans are treated as lifeless. Different systems of biogeographical classifications have been proposed based on different groups of organisms, and disagreement over the ranks to be assigned to the units and the exact locations of boundaries are inevitable.  A system may use up to four different ranks: the realm, region, subregion and province, but the region is the unit most often referred to. 
The regions that you are most likely to see referred to in the literature are:   
Nearctic - including North America, northern Mexico and Greenland
Palearctic - Europe, northernmost Africa and northern Asia
Neotropical - Central America, South America, tropical Mexico and the Caribbean
Ethiopian - the remainder of Africa, Madagascar and southern Arabia
Oriental - India, Indo-China, southern China, Malaya (tropical Asia and closely associated islands)
Australasian - Australia, New Guinea, New Zealand and associated islands
Antarctic - Antarctica
Oceanic - South Pacific islands, no large land masses
        The regions are sometimes grouped into 3 realms that reflect their physical relationship to one another:
(1) Megagea (Arctogea) - the great part of the world;
(2) Neogea - the Neotropical region alone; and
(3) Notogea - The Australasian, Antarctic and Oceanic regions. 
The regions can also be grouped in other ways that emphasise the type of boundary separating them from other regions - for example, climatic boundaries, or salt water boundaries.
        The designation of these regions represents an average pattern of the distributions of different major groups of terrestrial animals - and mammals and birds have been used most often.  The regions show, in a broad way, how animal distribution is fitted to the world and how climate and barriers affect them most deeply.  They can be used as a sort of meter stick by which the distributions of different animals can be measured, described and compared. They also allow special features to be determined and important things about the animals and their histories to be revealed.  Deviations from the standard patterns are expected and informative.  If, for example, a particular group of animals reaches the Australian region, but shows less than standard differentiation there, this suggests recent dispersal and the ability to cross bodies of water. 
        Faunal regions have other uses.  For example, they help zoologists in different parts of the world set natural limits to regional studies.  Also the names of the regions are useful terms. For example, to say "Oriental Region" is simpler, more exact and therefore more useful than "tropical Asia and certain closely associated continental islands".
        Regional faunas are not homogeneous assemblages of animals uniformly distributed, but instead represent animals that are:
(1) more or less concentrated in favourable places,
(2) vary in composition in different places, and
(3) enter into complex transitions with adjacent faunas. 
These transition zones are scientifically very interesting because they tend to have depauperate faunas with few faunal elements from either side. For example, Wallacea is the zone between the Oriental and Australasian regions. In this figure there are two major lines, Wallace’s line and Weber’s line - Wallacea is the area in between. True to transition zone characteristics, there are few mammals in this area.
        The regions constitute just one of the tools used in the study of biogeography.  Next week we will consider several others that contribute to the methods by which we can interpret the distributions of organisms over the earth.

Lecture 5 [Animal Diversity and Evolution] will be given on October 18th.

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Midterm Test Example Questions

Multiple Choice:

Which one of these factors does not contribute to sponge diversity?

    a.    folding of the body wall

    b.    polyp polymorphism

    c.    spicule structure

Short Answer:

What are considered to have been the two most important steps in increasing biological diversity on the earth (aside from the origin of life itself)?


Short Essay

Discuss the concept of Biological Realms, emphasising their utility.