Evolution And History

Macroevolution and the Process of Speciation 125

these social barriers have no biological counterpart. For

humans, there are no sufficiently absolute or long-lasting

barriers to gene flow.

Because speciation is a process, it can occur at various

rates. Speciation through the process of adaptive change

to the environment as proposed in Darwin’s Origin of Spe-

cies is generally considered to occur at a slow rate. In this

model, speciation may occur as organisms become more

adapted to their environmental niche. Sometimes, however,

speciation can occur quite rapidly. For example, a genetic

mutation, such as one involving a key regulatory gene, can

lead to the formation of a new body plan. Such genetic ac-

cidents may involve material that is broken off, transposed,

or transferred from one chromosome to another.

Genes that regulate the growth and development of an

organism can have a major effect on its adult form. De-

velopmental change in the timing of events, a phenom-

enon known as heterochrony (from Latin for “different

time”), is often responsible for changes in the shape or size

of a body part. A kind of heterochrony called neotony, in

which juvenile traits are retained in the adult state, may

be responsible for some of the visible differences between

heterochrony Change in the timing of developmental events

that is often responsible for changes in the shape or size of a

body part.

Figure 6.1 Cladogenesis occurs as different populations of an

ancestral species become reproductively isolated. Through drift

and differential selection, the number of descendant species

increases. By contrast, anagenesis can occur through a process

of variational change that takes place as small differences

in traits that, by chance, are advantageous in a particular

environment accumulate in a species’ gene pool. Over time,

this may produce sufficient change to transform an old species

into a new one. Genetic drift may also account for anagenesis.

Time

Cladogenesis Anagenesis

Species

B

Species

B

Species

A or C

Species

A

Species

A

© David Bygott/Kybuyu Partners © Nishan Bingham

Regulatory genes turn other genes on and off. A mere change in their timing can cause signifi-

cant evolutionary change because these genes can alter the course of an individual organism’s

development. This may have played a role in differentiating chimps and humans; for example,

adult humans retain the flat facial profile of juvenile chimps. Within primate species in which

sexual dimorphism is high, females tend to retain the juvenile traits more than males.

Visual Counterpoint

Thus cuckoos that are physically capable of mating may

be isolated due to differences in courtship song behavior,

which effectively isolates them from other cuckoos singing

different tunes. Though social rules about marriage might

be said to impose reproductive isolation among humans,