Evolution, 4th Edition

CooPERATIoN AND CoNFlICT 313

Mitochondria are maternally inherited. Natural selection therefore favors any

mutation in mitochondria that increases the number of ovules that females pro-

duce. The effect on male reproduction does not matter in the slightest to the mito-

chondria, since they are not transmitted through pollen. When the CMS+ allele

knocks out male reproductive function, resources are diverted from making pol-

len to making more ovules. This gives the CMS+ allele a fitness advantage, and it

spreads in populations of thyme.

But selection on nuclear genes favors a very different outcome. Recall from

Chapter 10 that selection on those genes favors a 1:1 ratio of males to females

(or male to female gametes). The spread of the CMS+ allele leads to an excess of

females in the population. That in turn favors the spread of any mutation in a

nuclear gene that cancels the action of the CMS+ allele. The result is an evolution-

ary arms race between genes on the mitochondria and genes in the nucleus.

Genes that are inherited cytoplasmically often conflict with nuclear genes, as

thyme plants illustrate. Mitochondrial mutations that harm males are not selected

against [32, 42]. This explains why mitochondrial mutations that cause male-spe-

cific diseases are common in humans, fruit flies, and other species (see p. 19).

Group selection

In most situations, competition between individuals for survival and reproduc-

tion leads to the evolution of traits that increase each individual’s fitness. Under

the right conditions, however, selection operating on the phenotypes of groups of

individuals can lead to the evolution of traits that are not favored by selection act-

ing on differences between individuals within each group [84]. These traits can

include altruistic behaviors.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_12.15.ai Date 11-29-2016

Q: I have labeled (B) plant with “No stamen, larger ovary”. OK, or delete?

If the key point is this difference, perhaps a balloon text would work best that indicates the

absence of producing pollen (no stamen) makes plant increase seed production?

Stamen

Ovary

(A) Ancestor

Ovules:

Pollen: (None)

(B) Male sterile (C) Restored

CMS– R–

CMS– R– R–

CMS+

CMS+ R+

R– R+

CMS+

R– No stamen, CMS+ R+

more ovules

FIGURE 12.13 Cytoplasmic male sterility (CMS) in thyme plants

(Thymus vulgaris) illustrates genetic conflict. Next to each flower

is a schematic showing the mitochondrion (the circle) with its

genotype at the CMS locus, and a pair of nuclear chromosomes

that carry the R restorer locus. Mitochondria are transmitted only

through ovules. (A) Plants that are CMS– and R– produce both

ovules and pollen. The ovules transmit both the CMS and R loci,

while the pollen transmits only the R locus. (B) Plants that are CMS+

and R– are male sterile. Pollen production is eliminated, which

increases the ovule number because the plant reallocates energy

and resources from pollen to ovules. This increases the number of

copies of the CMS+ allele passed to the next generation, caus-

ing male sterility to spread. (C) Plants that carry the R+ allele have

their male fertility restored, which causes that allele to spread in

populations with the CMS+ allele. A population fixed for alleles

CMS+ and R+ may be phenotypically indistinguishable from the

ancestral population that was fixed for CMS– and R–.

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