Transcript ppt - Language Log

Biological Anthropology as
an Evolutionary Science
 Biological
anthropology, like other life
sciences, is founded on the mechanisms
and operation of evolution.
 Without
an appreciation of how evolution
operates, it is difficult to understand how
humans developed and appreciate the
diversity of modern human biology.
Evolutionary Background
 All
life is related, via evolution, to the first
life forms on earth. It is reasonable to
believe that the earth and its physicochemical makeup are responsible for the
origins and subsequent evolution of all
earthly life.
 Evolution represents change; though
there are many ways of characterizing this
process, here it will be viewed as descent
with modification.
Evolutionary Background
Although there is an abundance of
evidence supporting the reality of
evolution, there is still much debate about
the precise mechanisms responsible for
evolutionary change, and the relative
importance of these mechanisms in
explaining the diversity and patterning of
earthly life.
Charles Darwin: the Recognition
of Natural Selection
 Charles
Darwin (1809-1882), British natural
historian and co-developer, with Alfred
Russell Wallace (1823-1913), of a theory of
evolution based on the concept of Natural
Selection.
 From 1831 to 1836, Darwin participated in
a five year voyage of exploration during
which he made observations of the
natural world which led to his theory of
evolution.
The Theory of Evolution
 Darwin
presented his theory of evolution,
and the considerable evidence he had
amassed to support it, in The Origin of
Species in 1859.
 In
this book, Darwin avoided all mention of
humans, except for a cryptic sentence on
the last page: “Much light will be thrown
on the origin of man and his history”.
Humans in the Evolutionary World
In 1863, British anatomist Thomas Henry
Huxley (1825-1895) published his famous
defense of evolution: Evidence as to Man’s
Place in Nature. Using the comparative gross
anatomy of humans and the African apes
(chimpanzees and gorillas), plus the very few
human fossils known at the time, Huxley
documented the evidence for the placing
humans in an evolutionary world.
 In 1871, Darwin published The Descent of
Man, in which he too explored the meaning
and implications of human evolution.

Criticisms of Darwinian Evolution



By the time of Darwin’s death in 1882, The Origin
of Species had gone through six editions, in
which Darwin had attempted to answer the most
serious criticisms of his theory. In spite of this,
by 1900, Darwinian evolution was no longer
seriously considered by most biologists.
Ironically, the most important criticisms, those
dealing with the ways by which hereditary
material is transmitted across generations, and
the origins of new traits, were being answered by
discoveries in the emerging field of genetics.
One such critic was Richard Owen, one of the
founders of the British Museum, and the man who
coined the word ‘dinosaur’.
Genetics and Evolution




The first 30 years of the 20th Century were
marked by the accumulation of enormous
amounts of information about genetic processes.
It was recognized by a number of geneticists that
this information provided the missing answers
needed to support Darwinian evolution.
In 1942, Julian Huxley (grandson of Thomas
Henry) published: Evolution, The Modern
Synthesis, in which genetic research was utilized
with Darwinian Natural Selection to produce the
Synthetic Theory of Evolution, or Neo-Darwinism.
Thus, it was only in the middle of this century that
a viable theory of evolution to explain the origins
of life on the planet and its subsequent
diversitication was constructed.
Evolution Now
 In
1949, American scientist G. G. Simpson
published The Meaning of Evolution,
which presented the Synthetic Theory of
Evolution to a wide audience.
 In 1954, J.D. Watson and F. Crick
documented a model of the structure of
the genetic material: DNA, thus providing
the basis for the emerging understanding
of the molecular basis for genetic
processes.
 Genetic research continues to provide
more information about the mechanisms
underlying evolutionary change.
Challenges to Current Views of
Evolution




Biologists such as S. J. Gould argue that the
process of evolution is not slow and gradual, as
Darwin had indicated, but operates by rapid
bursts of evolutionary change punctuated by
longer periods of stasis, when little or no change
occurred.
Based on his examination of the fossil record of
shell fish in geolgoical deposits, the Scots
geologist, Charles Lyell said the same thing to
Charles Darwin in the 1840s when Darwin was
working on his theory.
Darwin rejected Lyell’s assertions, believing that
if the geological record of earth history were
complete, transitions would be found.
Over 150 years later, there are still very few
examples of the evolutionary change from one
animal species to the next in the fossil record.
The Study of Genetics
 Genetics
provides the information that
Darwin did not possess on the
underlying basis for variation and the
introduction new biological features.
 Genetic data answers fundamental
questions about how hereditary material
is transmitted across generations.
Genetics and Evolution
Darwin’s notion of gradual evolutionary
change over time, via the mechanism of
Natural Selection, is the dominant
model in modern biology to explain the
origins and development of life on the
planet, as well as modern human
variation and adaptation.
Let us briefly explore what underlies the
mechanism of Natural Selection, and
then turn our attention to the genetic
processes that produce variation.
Natural Selection
 Natural
Selection is usually viewed as
Differential Reproduction.
 Natural Selection is a relative process.
 Natural Selection is neither an absolute
positive or negative process, but works
relatively within environmental circumstances.
What may be adaptive in one environment at
one time may be maladaptive at other times
and places.
 Natural Selection always operates to
decrease variation in a population.
Evolution by Natural
Selection
Founded on three key concepts:
1. Adaptation
2. Reproduction and Mortality
3. Variation
Adaptation

Animals possess biological attributes that
permit them to follow a specific way of life.

Animal species differ in the sorts of
biological features they possess, and not
all herbivorous mammals for example,
have the same kinds of teeth.

These specific biological features permit
animals to be adapted to the way of life
they follow.
Reproduction and Mortality

British naturalist Thomas Malthus (17661834) had observed that although each
generation, most animal species produce
huge numbers of sex cells, fertilized eggs,
and young individuals, adult population
numbers remain more or less the same
generation after generation.

Something was killing off the vast majority
of young in these species before they
reached adulthood and sexual maturity.
Variation
 Darwin
had observed that animals in a
population are not identical in appearance
but vary in all their features.
 It is now known that variation is present in
all biological features, from genetic
materials to gross anatomical features like
skull shape, size of the teeth and limb
length.
 It is clear that biological variation is a
normal and natural part of the function of
genetic mechanisms.
The genetic origins of
Variation I

Random changes in the structure of the genetic
material (DNA) result in the appearance of new
variations. This is one of the crucial pieces of
information that Darwin lacked, and thus prevented
him from actually directly addressing the name of his
book: The Origin of Species.

It was an ongoing criticism of Darwinian evolution
until the discoveries of how genetic material can
randomly change provided the missing data.
The origins of genetic Variation II
 The
second major source of variation is the
recombination of genetic traits that results
from the transmission of genetic traits from
male and female parents.
 During the process of producing sex cells, the
genetic material carried in the parental
chromosomes assorts so that each offspring
receives a random selection of parent’s
genetic materials.
 Thus, each offspring confronts the
environment with a slightly different set of
genetic materials.
•Natural Selection

Darwin argued that in each generation of
an animal population, the immature
individuals are born with biological traits
that vary in their expression.

Those young whose variations permit
them to be better able to survive in the
way of life followed by their species will
have a better chance to survive adulthood,
thus reaching sexual maturity.
Natural Selection 2
 By
reproducing, these favored individuals
pass on their variations to their offspring.
In time, the species will be characterized
by animals with these traits.
 In
contrast, those animals whose
variations are not as adaptive will more
likely die before reproductive maturity,
and their variations will slowly be reduced
in number in the species.
Evolution by Natural
Selection
Although Darwin termed his theory
“Survival of the fittest”, most biologists
today prefer to term it “Reproduction of
the fittest” because it is clear that what is
important is not that an animal reaches
adulthood, but that once having achieved
reproductive maturity, that it does
reproduce, thus passing its genetically
based adaptive variations on to the next
generation.
Genetics and Evolution
A Neo-Darwinian View
 Thus,
genetic mechanisms produce
random changes (mutations) in the
genes. The changes are not in any way
dictated by the environment or the
needs of the animal.
 These mutations are acted upon by
natural selection via behavior.
 In time, the changes may come to
characterize the entire population.
The Genetic Basis for
Evolution
Next time, we will continue with our
discussions of evolution by exploring
the genetic basis for variation.
We will consider the case of Sickle cell
anemia as an example of the actions of
evolutionary processes on human
biology and variation.
These topics will provide basic
background perspectives for our
examination of human ‘races’.