evolution concepts

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.

gene survival

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:

optimizing performance

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.


encoding useful knowledge

A variety of skills, including nest building, and human language acquisition may be coded into the genes.

emergent characteristics

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.

behaviors relative to others behavior


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.

predatory behavior

Prey and predators are engaged in a struggle to maintain their genes in succeeding generations.

altruistic behavior

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.

sexual selection

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."

Gould ch. 10 Retardation and neoteny in human evolution


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).

parallels in both morphology and behavior noted by many in early 1830s

"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.)

neoteny and fetalization

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..

Gould seeks to save Bolk's observations and fix his errors

While Bolk had many insights into human evolution, he overly simplified the processes involved.

there is "no room for mosaic evolution in Bolk."

"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,

we must seek the adaptive significance of retarded development. 364"

"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 "

primates are retarded in development relative to other mammals

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

humans "should" require 21 months gestation!

"our birth is accelerated and we spend the first year as an extrauterine embryo" (citing Portmann, 1945)

thus we see two separate forces:

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.

comparisons with other species

a-most mammals brain formed at birth
b-macaques are at 65% of final cranial capacity
c-chimps at 40%
d-fossil australopithecus at from 37% to 25%
e-humans at 23% of final cranial capacity.

There is also a corresponding delayed closure of sutures of skull. See comparison of development of chimp (left) and child (right) below.

Glossary of terms

(These are largely taken from Gould (1977) with some modifications.)

1 heterochrony[1] ("different timing")

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.

a. retardation of features

1. paedomorphosis[2] ("child form")

Occurs by slowing somatic development. "our paedomorphic features are linked to retardation in development."p.365 in Gould (1977 ch. 10)

2. recapitulation[3]

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)

b. acceleration of features

1. paedomorphosis

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.

2. recapitulation

This occurs here by acceleration of somatic organ growth

2 life-history strategies

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)

a. K-selection

"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"

b. r-selection

"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"

3 neoteny

Paedomorphosis produced by retardation of somatic development: "probably the major determinant of human evolution. p. 9 and ch. 10."

4 ontogeny

the life-history of an individual, both embryonic and post-natal.

5 phylogeny

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.

6 allometry

change of shape correlated with increase or decrease in size

7 convergence or analogy

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.

8 homology

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."

9 parallel evolution

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)

10 brachycephaly

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.

11 mosaic evolution

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.

12. synergy

"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"

preadaptation (exaption)

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.

genetic bottleneck

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.

founder effects

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.