Evolution, 4th Edition

EvoluTion And SoCiETy 591

they become apparent; furthermore, a cancer often has had multiple origins. Ani-

mals have evolved several mechanisms that reduce the incidence of cancer, such

as tumor suppressor genes and immune surveillance, but selection among cells

favors mutations in a cancer genome that help cells evade these mechanisms, just

as it does in pathogenic microbes. Several ideas about possible cancer therapies

that are based on evolutionary principles are being investigated. One of these,

adaptive therapy, aims to slow down the development of a cancer by using low

doses of drugs to control the cancer, thus allowing less aggressive clones within

the cancer to compete against more aggressive clones (FIGURE 22.5).

inFECTiouS diSEASES Some of the diverse bacteria, viruses, protists, and hel-

minths that cause infectious diseases have been associated with human ancestors

for millions of years; others (emerging diseases) are quite new. Pathogens that

cause infectious diseases in humans range from those that are mostly endemic to

other animals and only occasionally infect humans (e.g., rabies), through those

that are animal-borne but can be transmitted among humans to a greater or

lesser extent, to those that are specific to humans, such as the agents that cause

smallpox, syphilis, and measles (FIGURE 22.6).

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_22.05.ai Date 02-02-2017

1500

1000

500

2500

2000

8070 90 100 110120 130 140 150 160 170 180 190200

Days after cell injection

0

Control

Standard therapy

Adaptive therapy

Mean tumor burden (mm

3 )

FIGURE 22.5 Tumor development in mice that were

given a lower dose of therapeutic chemicals (adaptive

therapy) was slower than in mice given the standard

high-dose therapy. Adaptive therapy is based on the

hypothesis that within a tumor, competition among

clones may reduce the growth of aggressive clones

if less aggressive clones are allowed to survive. (From

[108], after [48].)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_22.06.ai Date 12-19-2016

Stage 1

Animal-exclusive

Pathogen present in

animals but has not

been detected in

humans.

Examples

Most malarial plasmodia

Feline leukemia virus

Stage 2

Primary infection

Under natural

conditions, pathogen

is transmitted from

animals to humans

but not among

humans.

Examples

Rabies

Anthrax

West Nile virus

Stage 3

Limited outbreak

Once acquired from

an animal pathogen,

infection can be

transmitted among

humans for a limited

number of cycles.

Examples

Ebola

Marburg virus

Monkeypox

Stage 4

Long outbreak

Infection by animal

pathogen can spread

among humans for

many cycles.

Examples

Yellow fever

Dengue fever

Cholera

Stage 5

Human-exclusive

Infection is exclusive

to humans.

Examples

Measles

Smallpox

Syphilis

FIGURE 22.6 The ecological relationships among pathogens, humans, and nonhuman hosts can be

expressed as five stages of increasing cycling within human populations. Whether or not all patho-

gens in the higher stages have evolved through the lower stages is not yet known. Some exemplar

pathogens are given for each stage. (After [122].)

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