Biology Now, 2e

242 ■ CHAPTER 13 Adaptation and Species

EVOLUTION

continue reproducing until they are 60 years

old or more. So, while coral gametes and larvae

can and do disperse far from their parents

and may interbreed or take root at incorrect

depths, natural selection then has 15–20 years

to winnow out the less successful offspring. If

there are small differences in survival rates

between the two species at a particular depth—

if a deep-water coral fares slightly worse in a

shallow area than a shallow-water coral does—

those differences will become amplified over 15

years, and by the time the corals reach repro-

ductive age it is likely that only the shallow-

water species will still be alive in the shallow area.

New species form and adapt to their envi-

ronments in many different ways, from island-

bound lizards to depth-dependent corals. And

from those varied beginnings comes the vast

diversity of life that exists on our planet today,

the result of billions of years of evolution. In

healthy and fertile offspring are called post-

zygotic barriers, which act after the zygote is

formed.

A wide variety of cellular, anatomical, phys-

iological, and behavioral mechanisms generate

pre- and postzygotic reproductive barriers, but

they all have the same overall effect: little or no

mating occurs, and therefore few or no alleles

are exchanged between species (Table 13.1).

Hellberg suspects that some prezygotic barriers

exist between the two sea fan species—perhaps

the gametes of the two species don’t fuse success-

fully, as in some sea urchins (Figure 13.14)—but

there may also be a postzygotic barrier such as

generation time.

It seems likely that the reason hybrids

between the two species do not succeed is the

uniquely long generation time for these corals,

says Prada. Sea fans don’t reach reproductive

age until they are 15–20 years old, and they

Table 13.1

Reproductive Barriers That Isolate Two Species

in the Same Geographic Region

Type of Barrier Description Effect

Prezygotic

Ecological isolation

The two species breed in different portions

of their habitat, in different seasons, or at

different times of day.

Mating is prevented.

Behavioral isolation

The two species respond poorly to each other’s

courtship displays or other mating behaviors.

Mating is prevented.

Mechanical isolation The two species are physically unable to mate. Mating is prevented.

Gametic isolation

The gametes of the two species cannot fuse,

or they survive poorly in the reproductive tract

of the other species.

Fertilization is prevented.

Postzygotic

Zygote death

Zygotes fail to develop properly, and they die

before birth.

No offspring are produced.

Hybrid sterility

Hybrids survive but are unable to produce

viable offspring.

No offspring are produced.

Hybrid performance Hybrids survive poorly or reproduce poorly. Hybrids are not successful.