Biology Now, 2e

Fast Lizards, Slow Corals ■ 235

biological species concept, if individuals from

the two populations of lizards can still breed and

produce fertile offspring, they are not different

species. This same idea holds true for all sexually

reproducing organisms. (Incidentally, it’s not

obvious why sexual reproduction is so common;

see “Why Sex?”)

The definition of “species” is not a black-and-

white issue; it is a frequently discussed, multi-

faceted topic in evolution, and the biological

species concept doesn’t always apply. For exam-

ple, not all species can be defined by their ability

to interbreed, such as prokaryotes like bacte-

ria, which reproduce asexually. To handle these

cases and others, scientists may use biogeo-

graphic information, DNA sequence similar-

ity, and morphology—the organisms’ physical

characteristics—to identify and distinguish

species (Figure 13.5).

Figure 13.5

One species or two?

Though they look quite different, these tree frogs

are genetically similar enough to be considered

distinctly colored variations of the same species.

Breeding these frogs with each other would

determine their classification under the biological

species concept.

Q1: List three kinds of information that

scientists use to distinguish between

species.

Q2: What differences can you observe

between the individuals in the photos?

Why are these differences not enough to

confirm that they are from two different

species?

Q3: How is genetic divergence among

populations determined?

S

ex is ubiquitous in the animal and plant kingdoms. An estimated

99 percent of multicellular eukaryotes are capable of sexual

reproduction, which involves the joining of two haploid gametes

produced through meiosis. Yet sex is very costly for individuals, so

scientists have struggled to explain why it is so prevalent compared to

asexual reproduction. And no, it’s not because sex feels good; the first

eukaryotes to engage in sex were single-celled protists some 2 billion

years ago, long before animals developed neurons capable of giving an

individual a sense of pleasure.

Costs of Sex

1. Time and energy must be invested to find or attract a mate.


2. Parents pass on only 50 percent of their genetic material to
   offspring, as opposed to the 100 percent that is passed on through
   asexual reproduction.


3. Gene combinations that have benefited the parents may be shuffled
   and broken apart during meiosis and recombination.

Possible Benefits of Sex

1. The genetic diversity created by sexual reproduction is critical for
   adaptation to new environments.


2. Sexual reproduction can help a population eliminate detrimental
   alleles and generate new beneficial alleles.


3. Rapid genetic change that occurs through sexual recombination can
   help a population evolve resistance to parasitic infections.

Why Sex?