Evolution, 4th Edition

PHylogENy: THE UNITy ANd dIvERSITy of lIfE 423

between polygamous and monogamous mating systems, and that testes size usually

also changed (Harvey and Harcourt 1984). The analysis supported their hypothesis.

Sometimes, evolutionary associations discovered by this method prompt fur-

ther research. The example in FIGURE 16.22 reveals a surprising evolutionary cor-

relation between sex determination and sex ratios. In all mammals, males have

two different sex chromosomes (the X and Y), while females have two of the same

(the X). Furthermore, in adults of most mammals, including humans, the sex ratio

is female-biased (with more females than males). In birds, the situation is reversed:

it is the females that have two different sex chromosomes (called Z and W), while

males have two of the same (the Z). In adults of most birds, the sex ratio is male-

biased. By themselves, these data are not convincing evidence for an evolution-

ary connection between sex chromosomes and sex ratio, for the same reason that

we rejected a connection between flight and reproductive mode. But the situa-

tion changes if we include lizards and amphibians. In those groups, phylogenetic

analysis shows that there have been many independent transitions in both the

chromosomal mechanism of sex determination and the adult sex ratio. Statistical

analyses that account for the phylogeny show there is a significant trend for XY

species to have female-biased adult sex ratios, and ZW species to have male-biased

sex ratios [26]. Why this correlation exists is uncertain. One of several hypotheses

is that genes on the Y and W chromosome tend to degenerate. This might increase

mortality of males in groups with XY sex determination (such as mammals), and

of females in groups with ZW sex determination (such as birds). If correct, this

hypothesis could help explain the shorter average life spans of human males.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_16.23.ai Date 01-25-2017

Note: Red and blue are in placed image so weren’t easily editable.

Red and blue have been used throughout the book for male/female coloring.

Snakes

Geckos,

skinks,

monitors

Frogs

Salamanders

Carnivores

ungulatesEven-toed

Primates

Galliformes

Waterfowl

Waders,

gulls

auks

Parrots

Passerines

Raptors

Penguins

Iguanas,

agamids, NewWorld lizards

Female

Male

Outer band:

ASR bias

XY

ZW

Inner band:

Sex determination

FIGURE 16.22 Comparative analysis

of the evolutionary relation between

sex determination and adult sex ratio

in tetrapods (mammals, birds, lizards,

snakes, and amphibians). The inner

circle shows the phylogeny that relates

the 344 species around the edge. The

inner colored band shows whether sex

in each species is determined by XY or

ZW chromosomes. The outer colored

band shows whether the adult sex

ratio is female-biased or male-biased.

Visually, we see that independent

origins of XY sex determination (in red)

tend to correlate with a female-biased

sex ratio (also in red). A statistical analy-

sis that accounts for the phylogeny

confirms that the trend is significant.

(From [26].)

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