Biology 284a : Patterns in Life's Diversity

a synoptic overview

©Paul Handford 2000 



1. BIOTIC DIVERSITY IS HUGE, COMPLEX, & AND MULTI- DIMENSIONAL

these dimensions include: These dimensions are explored in the various sections below.
 
 

Our key to comprehending all this complexity is the recognition that:
 

this biotic diversity has evolved across enormous spans of time, responding to, and interacting with, a changing world.

The evolutionary process involves both


Some of the conditions to which life adapts are:


2. what kinds of organisms are there?

the word "kinds" has distinct aspects:  it can mean form or function or type We expand on these aspects of "kinds" in 3. to 6. below
 


3. organismal form

involves both size (see immediately below) & structure (see 4.)

sizes range from:
 


4. the primary dimensions of organismal structureinclude:

 there is something of a relationship between SIZE and STRUCTURE, in that multicellular organisms tend to be larger, but there are many exceptions to this general rule.


5. how organisms live - their biological functioning -

        may be thought of as involving three primary issues:
 

BACTERIA CAN DO EVERYTHING EVERYWHERE

compared to them, all eukaryotes are metabolically very limited
see Lecture Notes chap. 3.


6. the major groupings of  types of organism - what is usually called biological classification -


are based on their inferred closeness of  EVOLUTIONARY RELATIONSHIPS - the recency of their common ancestors.

Traditionally, this involved consideration of characteristics of gross form & of function, and, not long ago, most organisms were considered to be either:

PLANTS or ANIMALS

 largely on the basis of whether they photosynthesised or not,

but now, through the use of tools such as the electron microscope and DNA/RNA sequence analysis, we have access to a more diverse set of characteristics, and their careful analysis has revealed a different & clearer picture of the evolutionary relationships among life's forms -

  so that now, all life is divided into several kingdoms rather than being forced into two artificial groups.

Exactly how many kingdoms there should be is a matter of much current debate, which need not concern us here;
the central point is that, regardless of how many kingdoms there are, these groups now much more closely reflect the true evolutionary relationships among organisms, rather than reflecting superficial similarities - such as being green.
 
 



 

In this course, we shall be looking at life with the framework provided by the

5-Kingdoms system
 

this system first separates life into:

Prokaryotes (Bacteria or Monera)
and
Eukaryotes(organisms with nuclei and other organelles)
 

and then further reduces the eukaryotes into

Protoctists

Fungi

Plants

Animals


Below is a figure showing the evolutionary relationships among all of earth's biota. Identified there are the five kingdoms (the Bacteria or Monera turn out to include two distinctive clusters of forms - here referred to as Bacteria and Archaea).

Note the diversity of UNfamiliar forms as compared to the closeness of the forms with which we are most familiar.

See also L.N. 1.5 showing the development of eukaryote structure from prokaryotes by endosymbiosis.



 
SO MUCH FOR ASPECTS OF TYPES OF ORGANISMS.
NOW ON TO THE QUESTION OF THEIR NUMBERS


7. what is the numerical composition of the biota?

how many of each of the kinds of organism are there?

the answer depends on what kinds you count: species; families; classes; phyla..............  most species are insects

 Here are some pie-diagrams which induicate the ways in which the various groups break down by sumbers of species.  For further detail, please see the Lecture notes, Chapter 8.
 

First , we have a representation of the relative numbers of species so far known to science in each of the five kingdoms.  The tiny number of described bacteria (Monera) and the small number of known fungi almost certainly is a serious underestimate of the numerical importance of these groups.

Note the apparent predominance of the Animal kingdom.


 
Here we see the species breakdown for the Animal Kingdom alone.  Within this kingdom, the overwhelmingly abundant group are the insects which number approximately one million species.

Note how numerically unimportant the vertebrates appear.


 
 
Focussing down onto the vertebrates, we find that nearly one half of all species are bony fish.  Next in importance are the birds, followed by reptiles.  Our own group, the mammals, are relatively insignificant numerically.

If we were to focus down closer, onto the species composition of the mammals, we would see that our own group, the Primates are really insignificant, with only 4% of species.  Mammals are strongly numerically dominated by rodents, bats and insectivores, in that order, which together provide over 70% of mammalian species.

Altogether, we see that the earth's biota is dominated by life forms with which most of us are not familiar.  Our own kind does not provide a good yardstick for taking the true measure of life on earth.



 
AND NOW TO PATTERNS IN TIME...............


8. how has organismal diversity changed through time ?

Below are two figures showing the manner in which the biota of earth has grown and changed in composition during the last 600 million years.

First we see the marine record, which includes a diversity of unfamiliar animals, many now extinct, along with a good number which are still around today.  Notice the rapid growth of the biota during the Cambrian and Ordovician periods, then a more or less steady-state until the end of the Permian (about 250 m.y.a.).  At that time there took  place the greatest extinction the world has eyt experienced.  SInce that time, the marine fauna has become progressively more diverse up to the present.


Next we see the vertebrate record, which largely includes forms familiar to most of us.  Until mid-Devonian, all vertebrates were aquatic, but slowly, terrestrial forms diversified, spectacularly so in the Mesozoic (Triassic, Jurassic & Cretaceous);  this was the time of the rise of the reptiles.  Birds, bony fish and mammals originated back in the Triassic and Jurassic, but began an enormous rise to their present prominence at the end of the Cretaceous, coinciding with the end of the dinosaur dominance of terrestrial habitats.


9. evolutionary time is vast

.....if one pace (about one yard or metre) were a century  (3-4 human generations).....................

a football field of paces takes us back to the start of agriculture;

a walk from Fairbanks, Alaska to Miami, Florida would take us to the origins of multicellular life;

a great-circle flight from N.Y. to Perth, Australia would take us to the origins of eukaryotes

TO REACH LIFE'S ORIGINS would take us past N.Y, and most of the way back to Perth again.
 

Another way to gain some impression of the vastness of time, especially as compared to the times with which we have some familarity, is to represent all the time since earth's origins in a circle, and break that circular pie into segments according to their durations:
 
 


To put this figure in some perspective, recall that the Age of Dinosaurs is the Mesozoic Era, lasting from about 250 million to 65 million years ago;  mammals did not come to dominate the terrestrial world until the Cenozoic, starting about 65 m.y.a.
Humans did not arise diverge from other apes until the last few m.y.a.;  modern Homo sapiens has only been around for 1-200,000 years. 
Such times could not be represented on this figure - the lines separating the periods shown are approximately 8 million years think!

 To expand upon this initial picture, see pp. 8 & 9 in Introductory materials and the detailed Time-charts.



 
AND FINALLY WE LOOK AT PATTERNS IN SPACE.....


10. how is life's diversity arrayed in space? (and why?)

again, we must distinguish the dimensions of diversity:  and again, the answers lie in evolutionary history. see L.N. 11


11. life's lineages are unique in space & time

That is to say, a given lineage - say the kangaroos - has an origin which is unique in time and unique in space (it didn't start off at several different times and points of the globe)

therefore, where they are now depends on where & when they originated  and how the global geography has changed since then ..............

Our understanding of the earth's past geography has changed radically in recent time, leading to a greatly-improved understanding of the geographical distribution of organisms.


12. over the last 30 years, we have learned in detail how the earth's crust has slowly drifted about the globe

at times plates are isolated; at other times, conjoined;

their passenger biotic lineages thus may evolve, depending on circumstances,

at times in isolation, when they have undisputed access to some restricted part of the globe;

at other times they may gain access to wider geographic areas and become exposed to other evolving lineages

these opportunities for protected diversification and/or expansion + competition lie behind some profound irregularities in the geographical distribution of kinds of organisms.  This is the topic of biogeography.

see L.N. 11.4, 5 & 6, showing continental drift and biogeographic realms.

For further description of the basics of organismal geography, click here.

On the net, see this excellent site, which provides a series of plate-tectonic reconstructions to show the broad patterns of Phanerozoic Earth history (the last 600 million years or so).


13. where most diversity is in the world

depends on what you count .............  species, families, phyla.............

in this course we shall look simply atpatterns in 'species richness' -the number of species in any given group in any specified region

species richness depends on both the geographical location and the size of the region.

In general, species richness increases towards the equator:

 species richness is also positively related to the area considered.

It is not simply a matter of latitude or area, however;  it is also profoundly affected by rainfall, among other things.........      see L.N. 8 & 9.


14. aspects of climate are implicated both in biotic richness and adaptive form

in addition to climate's impact upon the richness of regional biotas, we might also ask:

what are the qualitative, evolutionary-adaptive responses of the biota to climate?

we find that, to some extent, the different evolutionary lineages that are found in the different continental regions respond in similar adaptive ways to major features of global climate which may span several continents, generating  major adaptive assemblages recognised as biomes.

These biomes are recognisable as such because the general appearance of their prominent organisms is very much the same, even though those organisms often derive from very different evolutionary lineages.

Biomes include such major assemblages as arctic tundra, boreal forest, deserts, grasslands and rainforests.

This system of BIOMES is the final global biotic pattern which we shall illustrate.

 see L.N. 12 for global climate, and the rest of the Lecture Notes for the ways that the biota responds to it.


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