Evolution and Taxonomy
Dr Bill Sellers
Welcome to the "Human Evolution" course. We, the course organisers hope
that you will find it both interesting and enjoyable - but
unfortunately, before you can really get to grips with the fun stuff,
you will need some background theory. that's really what today's lecture
is all about.
The course is about human evolution. This means that at
some stage, preferably fairly early on, you need to have some idea about
evolution, and it's also helpful to have an idea what we mean by
"human". The first half of this lecture is an introduction to the
central ideas of evolution, and the second half is about how we group up
animals, including humans, so that we all know what we are talking about
when we talk about them. Discussion about how similar and how different
humans are from other animals makes up a lot of the subsequent course.
The word "evolution" merely means "change through time". It
doesn't imply a direction, nor, does it necessarily imply improvement,
merely change. In this context, evolution refers to the observation that
the animals in the world have not always been the same as those that we
see around us today. How do we know that this is the case?
We know that distributions of
animals change. We have historical records of bears in England 1000
years ago. We find bones in caves containing lemming (# slide of
lemming) and reindeer (#slide of reindeer) bones with no historical
record of either being present here. We have film sequences of the
Tasmanian wolf only fifty years old, and there are none around today (#
slide of Tasmanian wolf).
When we look at piles of
sediment that have accumulated in a cave for example, we find that the
bones present in the top layers are the most similar to what we expect
to find today: chicken and rabbit bones for example, which indicates
that this sediment has come in since Roman times when both the chicken
and rabbit were first introduced into Britain. These bones just look
like old bones. As you dig deeper, you find that the bones begin to
become "mineralized". The colour changes and the bone generally becomes
heavier, as the calcium and various other chemicals in the bone are
slowly dissolved away and replaced by other chemicals from the sediment.
These bones are fairly obviously older, and it is in these layers that
we start to find (in the UK) animals such as hippopotamuses and lions (#
slides of hippo). In a very large column of sediment, the bones found in
the lower layers becomes completely different from those found today:
very large cats and elephant like creatures, or whatever (# slides of
mammoth). Clearly, when these sediments were laid down, the fauna was
quite different from today: animals were similar, and the range of
animals was similar, but the actual animals themselves were noticeably
different in form.
In other places, there are rocks that look rather
like solidified sediment. We assume that these are columns of sediment
that have become solidified over a very long period of time due to
various geological processes. These rocks contain very bizarre animals
very unlike anything seen today. Sometimes, giant lizard like creatures
are common (# dinosaur slides). At other sites, only fish are found (#
early fish picture), even though the current location is miles from the
You may have noticed that I have avoided talking about ages.
I've used terms such as "old" and "very old". This is because techniques
for dating rocks, fossils and bones accurately are relatively recent.
earliest method of dating is "stratigraphic dating". This follows on
directly from the column of sediment. Anything nearer the top is younger
than anything further down. This gives you an idea of the relative ages
of rocks, and by estimating the rate of deposition of sediment you can
attempt to calculate an absolute age (# slide of early geological time
scale). It's not terribly accurate, but it is very intuitive. There
aren't any deposits of sediment that cover the whole age of the earth
continuously, so that you need to look at the change in the fossils in
one deposit, and match them with fossils in other deposits to attempt to
build up a full picture.
With the discovery of radioactive decay,
other more precise dating techniques have been possible. The best known
is radiocarbon dating which works well on organic material and relies on
the proportions of a radioactive and non-radioactive form of carbon. For
older materials, other radioactive forms can be used: potassium, uranium
etc. Each series covers a different time scale, and is useful in
particular geological circumstances. None are without their problems,
but they can give much better estimates of absolute ages than
There are also other techniques based on
thermo-luminescence, or magnetic field reversals that can be used. Used
together, these have provided a widely accepted set of dates for various
rock layers and fossil animals (# slide of geological age).
As I emphasized at the beginning if this lecture. Evolution
doesn't imply improvement, or any sort of direction itself. Merely
change. However, from our examination of the fossil record, it is clear
that animals and plants have become a great deal more sophisticated over
the years, and it certainly seems that more recent variations have a
tendency of replacing earlier versions. Very many varieties no longer
exist - indeed the average "life span" for a species seems to be of the
order of a few million years. From various mathematical considerations,
it seems highly unlikely that all this change can occur purely by
chance. Fortunately, Charles Darwin (# slide of C.D.) came up with a
mechanism that explains the apparent direction of evolution
The Darwinian argument, "Evolution by natural selection", is
extremely clever, and nowadays seems almost self-evident. It is based on
empirical observations of natural history and is supported by a wealth
[text and diagrams from the NHM book]
of the fittest'
However, it is now thought that although a large proportion of
genetic diversity can be ascribed to natural selection, it is now
realised that random genetic drift plays an important non-directed role
since certain features show variation without any fitness change. In
addition with the discovery of entities like retro-viruses that
incorporate their DNA into the host genome, it is clear that there are
other mechanisms that act directly at a genetic level.
simply the ordering of organisms into groups, and giving them names.
Before anyone was particularly bothered about evolution, this tended to
be a very simple exercise: we'll put all animals that swim in one group
& whales); flying animals in another (bats & birds); and the
ones that climb trees (monkeys & squirrels). Linnaeus expanded on
this a little by using more than one characteristic in his groupings,
but nevertheless, there was always dispute about how to produce a
When evolution became accepted, it became clear
that the obvious way of grouping organisms was by their evolutionary
relationship - a huge family tree, if you like, showing how the various
animals have descended from common ancestors and grouped accordingly.
This, then, is the goal. But how do we achieve it?
best/easiest way is to look at the fossil record and find all the
ancestral groups. (# diagram p.46 Evolution) This is precisely what has
been done for horses. Unfortunately, this is normally not possible. For
most animals, there are just not enough fossils available for this sort
There are other problems too. Although a family tree
is a natural way of grouping organisms, we still need to decide on what
we are going to use as the smallest group. The answer to this (usually),
is the species, but this begs the question: "What is a species?"
Generally, a species is defined as a sexually interbreeding
(or potentially interbreeding) group of individuals normally separated
from other species by the absence of genetic exchange. This is the
"biological species concept". This is fine, in theory, but in practice,
there are problems. (# diagram of closely related animals - Lemur
fulvus subspecies) Group A can mate with group B and produce
offspring. Group B can mate with group C and produce offspring, but
group A and group C can't mate. In addition, it doesn't help define what
a species is for fossil animals where mating can't be observed. And
finally, it is no help for defining species in organisms that don't
reproduce sexually (# slide of garlic plant). A number of other species
concepts have been postulated to overcome these problems, but none of
them are perfect.
My personal view is that the concept of a species
is a completely arbitrary construct that humans have created. Organisms
can be thought of as a continuum of genetic variation, and we use
species as a way of naming regions in that continuum for our own
convenience. The size of these regions is roughly consistent, but there
is definitely overlap at the edges, or even gaps. As long as we are
consistent in what we call these regions, then we can still use them for
practical purposes like conservation management, and it means that we
can stop bickering quite so much over whether a particular animal is or
isn't in the same species as another... The concept of a species is OK,
just remember that it tends to be fuzzy round the edges.
Firstly, evolution (change over time) doesn't have to lead to
a branching pattern. A single group can change gradually without
splitting into two distinct groups (# diagram horseshoe crabs, Evolution
P.209). This process is called Anagenesis. However, the much
more interesting problem is the reconstruction of the branching pattern,
where species split into two or more groups. This is called Cladogenesis
and is what gives us our family tree. (# diagram P.212)
of pylogenetic trees is difficult in practice because the common
ancestors are usually long extinct and the fossil record is inadequate.
However, the relationship can be inferred by looking at common,
inherited characteristics: the more morphological, embryological,
behavioural, physiological, biochemical, genetic and chromosomal
inherited characteristics that organisms have in common, the more likely
they are to have descended from a common ancestor.
common features is not enough since they may derive from different
This is what we want. The
feature is shared because it derives directly from a common ancestor.
For example the bony features of the forelimbs in vertebrates. (#
The similar feature occurs
in different species, but it is not present in their immediate common
ancestor. For example, anteater-like features in various different
mammalian lineages (# diagram P.211). These shared features are very
much functional adaptations.
Similar to parallelism,
but the ancestral lineages differed for a considerable period of time.
For example vertebrate and octopus eyes, or the hydrodynamic morphology
of marine predators from the widely separated fish, reptile and
mammalian classes (# diagram P.40).
Obviously, homologies are
what we need to consider to reconstruct phylogenies. However, they are
not always easily separated from the other 2. Consider the convergence
example: the shape of the pectoral fins in these animals is very similar
due to convergence. However, there is a great deal of homology there
two. Especially between the reptile and the mammal due to a common land
Taxonomy isn't only for
evolutionary reconstruction. We need fairly stable names and grouping
for practical purposes such as conservation. Groupings make animals
easier to remember and identify, and we don't want it all to change
every time someone decides that actually humans are closer related to
chimpanzees than chimps are related to gorillas. This means that
official naming schemes tend to lag somewhat behind the current thrust
of research. There is quite a bureaucracy preventing everyone from
renaming animals at a whim, and there are international efforts to try
and maintain some consistency. Even so, there are generally several
alternative classification schemes around for groups of animals that
seem to last about 5 years until the next big name in a particular field
writes the latest review paper on that specific taxonomy. A good text
books will tell you which scheme it is using, and a really good textbook
will list several alternatives so you can make up your own mind.
Evolution happens. There is
very good evidence for change in the life forms that inhabit the earth
over long periods of time.
Charles Darwin's theory about the origin
of species through natural selection explains the apparent direction of
evolutionary change extremely well. It is probably one of the most
widely accepted theories in modern biology.
purposes, we need to group animals. Animals that can interbreed are
grouped as species. Species are grouped in a tree structure that more or
less attempts to mimic the evolutionary process. We attempt to use the
interbreeding idea for extant animals, but for fossil forms we use
similarity in shape.
Unfortunately, every scientist has their own
personal preference when it comes to classification. That's life!
This page is maintained by Steve Paxton