Evolution, 4th Edition

62 CHAPTER 3

trees in a forest is important for many tree-nesting birds such as orioles, but almost

irrelevant for ground-nesting species such as partridges; the viscosity of water, which

varies with temperature, is much more important for a ciliate than for a fish. To some

extent, organisms construct their ecological niches [36], literally (as does a beaver)

or more metaphorically. Organisms “screen off” some aspects of their environment,

which may then cease to exert natural selection. Many species of ants, rodents, and

other animals have become so reliant on chemical signals that they have become

blind, because natural selection for sight has become reduced or even negative: well-

developed eyes may be disadvantageous if they conflict with other important func-

tions (FIGURE 3.8). Likewise, humans have lost the function of many olfactory recep-

tor genes, having become so much more reliant on vision than smell.

l evels of Selection

By “natural selection,” both Darwin and present-day evolutionary biologists usually

mean consistent differences in fitness among phenotypically and genetically differ-

ent individual organisms within populations. But our definition of natural selection

applies to any classes of variable entities that can change in number. Selection can

occur among genes, cell types, individual organisms, populations, or species, a hier-

archy of levels of selection.

Natural selection at the level of the gene (genic selection) is illustrated by trans-

posable elements, which replicate and proliferate within the genome, irrespective

of whether their proliferation affects the organisms for good or ill. Transposable

elements are among the many kinds of selfish genetic elements, which are trans-

mitted at a higher rate than the rest of an individual’s genome and may be detri-

mental (or at least not advantageous) to the organism [4, 27]. Some selfish alleles

exhibit segregation distortion, and are passed to a heterozygous individual’s

gametes more than 50 percent of the time. Segregation distortion can result from

meiotic drive (in which meiosis does not follow Mendel’s laws; see Chapter 12) and

from other processes that happen after the gametes are formed.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_03.02.ai Date 11-28-2016

(A) (B)

FIGURE 3.8 The evolutionary histories of some animals have made them less reliant

on vision, so selection for visual acuity has been relaxed. (A) Army ants (genus Eciton)

rely almost entirely on chemical information. In these ants, the compound eyes have

been highly reduced, consisting of a single unit (ommatidium) rather than the many

that compose most insects’ eyes. (B) Similarly, burrowing blind snakes (Typhlopidae)

have highly reduced eyes that perceive light but cannot form an image.

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