Many ideas on evolution of species existed before Wallace and Darwin published
their papers in 1859. Their theory had the advantage of explaining how species
could evolve and go extinct by natural, rather than supernatural forces.
Their were four basic steps to their "descent with modification"
theory (see EHE 1.1 for more)
1.Organisms differ from each other in ways that are inherited.-- i.e. there
is variation.
2. More are born than can survive -- there is a "struggle"
for existence.
3.Certain inherited variations (traits, features...) increase the chances of
their possessors surviving and reproducing --natural selection.
4. Selection leads to the accumulation of favored variants, which over a long
period produce new forms of life -- the origins of species
(Of course, Wallace and Darwin didn't get everything right -- they knew nothing
about genes, they accepted some degree of Lamarckism and they differed on the
evolution of humans. Darwin went on to develop these ideas in great detail in
subsequent books on origins of species, emotions, and humans.)
Genes carry the the information of heredity in the structure of their
molecules. A given gene may have a variety of effects on the final form and
behavior of the organism it is in part responsible for constructing. A gene is
"adaptive" if the overall effects of it are advantageous in increasing the
relative frequency of that gene in succeeding generations.
Darwin observed that certain traits tended to go together, even though no
logical relationship linked them together. He called these "correlations of
growth" and "correlated variation." (Note that there is no necessary reason for
these type of correlations though Darwin remarks that these correlations may
not be distinguished from inherited characteristics.
Today these are called "pleiotropic effects."
The entire set of genes of an organism is its genotype. In sexually
reproducing species, the genetic material comes from both male and female
"parents."
The form and behaviors of the organism comprise its phenotype. The phenotype
arises from its genotype acting in a certain environment. Details of the
process are largely unknown today.
Genes are perpetuated when they successfully give rise to phenotypes that have
advantages over competing phenotypes in replicating copies of those genes.
These advantages may be viewed from somewhat different angles:
For example, swimming organisms e.g. fish, penguins, dolphins, tend to become
streamlined in similar ways despite different histories, because they use less
energy and may be more effective at finding prey and evading predators. Thus
different genes may converge on the same phenotypic form.
A variety of skills, including nest building, and human language acquisition
may be coded into the genes.
This is somewhat a controversial issue, but it seems reasonable to say that
while genes underlie characteristics of the organism, they only do so in a
specific range of environments. Hence to predict the effect of a set of genes,
one must specify the environment and how the genes in question interact with
that environment. While some genes can survive a wide range of environments,
others may not.
For example, parasites manipulate the behavior of their hosts, for the
parasites advantage and to the apparent disadvantage of the host. Note how
rabies organisms make dogs bite in order to spread the rabies genes. Sexual
reproduction may have evolved as a means to thwart parasites.
Prey and predators are engaged in a struggle to maintain their genes in
succeeding generations.
Altruistic (helping) behaviors including parental care ("parental investment")
may be costly to the individual phenotype but foster the perpetuation of its
genes through survival of kin--the offspring or "close" relations of the
phenotype that bear copies of many of its genes ("kin selection").
Contrast altruistic behavior with the consequences of infantcide--"non-kin
deselection." In a number of species, e.g. baboons and chimpanzees, males will
take the opportunity of welcoming a new female to their group by killing any
infant she is with--presumably to more quickly make her fertile and capable of
reproducing his genes.
Darwin suggested there were actually at least two selective forces--natural
selection and "sexual selection."
"This depends on the advantage which certain individuals have over other
individuals of the same sex and species, in exclusive relation to reproduction.
p.256...It is certain that with almost all animals there is a struggle between
the males for the possession of the female...hence the females, supposing that
their mental capacity sufficed for the exertion of a choice, could select one
out of several males. 259."
The notion of human neoteny has its roots in two obvious facts:
1. The striking resemblances between juvenile pongids and adult humans
and 2) the obliteration of this similarity during pongid ontogeny by the strong
negative allometry of the brain and positive allometry of the jaw. (see Fig. 61)
Thus as the body size increases the relative brain size decreases (negative
allometry)
As the body size increases the relative jaw size increases. (positive
allometry).
"In the head of the young orang, we find the childlike and gracious features of
man...We find the same correspondence of habits, the same gentleness and
sympathetic affection, also some traits of sulkiness and rebellion in response
to contradiction...on the contrary, if we consider the skull of the sdult, we
find truly frightening features of a revolting bestiality." (Geoffrey St.
Hilaire, 1836 p354 in Gould.)
The human adult stays close to the juvenile state; the brain continues to grow.
Human features, in contrast to those of other primates, have in comon they are
fetal conditions that have become permanent. Bolk (1866-1930) developed
Kollman's (1905) concept of "neoteny" to explain human evolution..
While Bolk had many insights into human evolution, he overly simplified the
processes involved.
"The evolutionary direction of each feature is controlled by natural selection;
the capacity for independent variation of characters is very great. An absence
of paedomorphosis in some characters is inevitable and poses no threat to the
notion that paedomorphosis played a central role in human evolution. If a
large suite of human features are paedomorphic,
"This mechanism [general retardation] was utilized again and again in human
evolution....prolongation of fetal growth rates leading to larger size and
retention of juvenile proportion....
Is not such a system the proximate cause for evolutionary increase of the human
brain? 365 "
Otis and Brent (1954) compared the stages of prenatal development of mouse and
human. The sequential order of stages is the same but early stages take two to
four times as long to develop in humans and later stages take five to fifteen
times as long. 366
"our birth is accelerated and we spend the first year as an extrauterine
embryo" (citing Portmann, 1945)
One is the retardation of genera growth giving us a large brain as the fetal
growth rate fails to slow down near birth.
The other is an acceleration of birth to counteract the anomalously large birth
weight produced by retardation.
There is also a corresponding delayed closure of sutures of skull. See
comparison of development of chimp (left) and child (right) below.
(These are largely taken from Gould (1977) with some modifications.)
A phyletic change in the onset or timing of development, so that the appearance
or rate of development of a feature in a descendant's ontogeny is either
accelerated or retarded relative to the appearance or rate of development of
the same feature in an ancestor's ontogeny. Either retardation or acceleration
may have similar consequences; see below. Provides a mechanism for mosaic
evolution and denies the necessity of allometry--though it is very
widespread.
Occurs by slowing somatic development. "our paedomorphic features are linked to
retardation in development."p.365 in Gould (1977 ch. 10)
hypermorphosis by retarding maturation (, i.e.phyletic extension of ontogeny
beyond its ancestral termination, usually to larger body sizes and increased
complexity of differentiating organs,-producing recapitulation as a result
because ancestral adult stages are now intermediate stages of a lengthened
descendant ontogeny. p.482 and ch. 10)
retention of formerly juvenile characters by adult descendants produced by
precocious sexual maturation of an organism still in a morphologically juvenile
state. Gould calls this "progenesis" which occurs in "r-selection life
strategies where early reproduction is favored.
This occurs here by acceleration of somatic organ growth
A selected set of adaptations to local environments, involving such
quantitative aspects of life history as fecundity (offspring number), the
timing of maturation, and the frequency of reproduction. Two extremes are K
and r selection strategies. Examples are primates and fruit flies,
respectively. (p. 483)
"K strategists might employ low reproductive effort, with late maturation,
longer life, and a tendency to invest a great deal of parental care in small
broods of late maturing offspring. p.291"
"Some attributes of r-selected organisms might include high fecundity, early
maturation, short life span, limited parental care, rapid development, and a
greater proportion of available resources committed to reproduction. p.291"
Paedomorphosis produced by retardation of somatic development: "probably the
major determinant of human evolution. p. 9 and ch. 10."
the life-history of an individual, both embryonic and post-natal.
the evolutionary history of a lineage, conventionally depicted as a
sequence of
adult stages. The term was coined by Haeckel (1866) who developed
phylogenetic "trees" for many groups of
living organisms.
change of shape correlated with increase or decrease in size
a similarity between organisms due to independent evolution of the similar
feature by each, e.g. fins, wings in various unrelated species Compare homology
and parallel evolution.
A similarity between organisms due to inheritance of the same feature from a
common ancestor (compare convergence (analogy) and parallel evolution. See
Passingham, HP text on origin of "smiling."
A similarity between organisms due to recent developments in parallel lines of
descent due to continued similiarity of evolutionary factors operating on
similar individuals, e.g. larger size of apes. This makes using existing apes
as models or prototypes of our ancestors very problemmatical (See p. 384)
Literally "short-headedness." Since the head increases in relative length
during ontogeny (development of individual), brachycephaly in adults is usually
interpreted as a paedomorphic feature. p. 480 and ch.10.
The notion that the individual is composed of a set of characteristics
("tiles"), any of which may be independently modified by evolutionary
processes--despite the evident correlation of parts ("allometry") within any
organism. "Correlations are no more immutable than species themselves. p.234"
Much interest and argument surrounds the issues of the number of independent
features involved in a structure like our complex head. See ch. 10, pp.390-397
and a general statement, p.399.-404 of the entire issue.
"I assume that major human adaptations acted synergistically throughout their
gradual development. The interaction system of delayed development--upright
posture--large brain is such a complex: delayed development has produced a
large brain by prolonging fetal growth rates and has supplied a set of cranial
proportions adapted to upright posture. Upright posture freed the hand for
tool use and set selection pressures for an expanded brain. A large brain may,
itself, entail a longer life span....This retardation interacted
synergistically with other hallmarks of hominization--with intelligence (by
enlarging the brain through prolongation of fetal growth tendencies and by
providing a longer period of childhood learning) and with socialization (by
cementing family units through increased parental care of slowly developing
offspring...Gould, 1977, pp.399-400"
New functions and behaviors typically arise in a species through the
recruitment of components previously evolved for some other purpose. Thus for
example the larynx evolved as a valve in amphibians to keep water out of the
lungs; that was a "preadaptation" for using the lungs and larynx as a noise
source in mammalian communication, which in turn was a preadaptation for
phonation in human speech.
A reference to a situation that restricts the variability of species genes
generally due to an extreme population reduction-- which may occur by chance or
natural selection. This is generally thought to be a bad situation for a
species; the greater the variability, the greater the resistance to parasites
and probability of adapting to changed circumstances. Humans have less genetic
variability than other large primates leading some to speculate that our human
ancestors experienced one or more bottlenecks: "...population biologists have
recently concluded that modern humans were indeed squeezed from a group of
about 100,000 to a population of about 10,000 breeding men and women sometime
during the last 400,000 years. p.35" (Gibbon,1995) Critics raised the
issue of whether 10,000 is really few enough to be called a "bottleneck" since
other animal populations may possibly be that small without corresponding loss
of genetic diversity.
Note how only a few "founding" members of population can have a dramatic effect
on the characteristics of that population --even if many of those
characteristics were not particularly adaptive. Imagine a scifi scenerio where
the only survivors of disaster were members of the "RedHads of America" club.
That subsequent generations of humans were primarily red-haired should not
imply red hair is adaptive and was selected for.
Gibbon, A. (1995) The mystery of humanity's missing mutations. Science,
267, 35-36.
Gould, S. J. (1977). Ontogeny and phylogeny . Cambridge: Harvard
University Press.
Passingham, R. (1982). The human primate . San Francisco: Freeman.
