5 Steps to a 5 AP Biology, 2014-2015 Edition

Evolution ❮ 143

Speciation

Aspeciesis a group of interbreeding (or potentially interbreeding) organisms. Speciation,

the process by which new species evolve, can take one of several forms. You should be famil-

iar with the two main forms of speciation:

1. Allopatric speciation.Interbreeding ceases because some sort of barrier separates a single
   population into two (an area with no food, a mountain, etc.). The two populations
   evolve independently (by any of the four processes discussed earlier), and if they change
   enough, then even if the barrier is removed, they cannot interbreed.


2. Sympatric speciation.Interbreeding ceases even though no physical barrier prevents it.
   This may take several forms.
   Two other important terms are polyploidyandbalanced polymorphism:
   Polyploidy.A condition in which an individual has more than the normal number of sets of
   chromosomes. Although the individual may be healthy, it cannot reproduce with nonpoly-
   ploidic members of its species. This is unusual, but in some plants, it has resulted in new
   species because polyploidic individuals are only able to mate with each other.
   Balanced polymorphism.This condition (described above) can also lead to speciation if two
   variants diverge enough to no longer be able to interbreed (if, e.g., potential mates no
   longer recognize each other as possible partners).
   One more term to mention before moving on is adaptive radiation,which is a rapid series
   of speciation events that occur when one or more ancestral species invades a new environ-
   ment. This process was exemplified by Darwin’s finches. If there are many ecological niches
   (see Chapter 18, Ecology in Further Detail), several species will evolve because each can fill
   a different niche.

Mechanism Description Example

Heterozygote

advantage

The heterozygous condition has an

advantage over either homozygote,

so both alleles are maintained (AA

is worse off than Aa).

Sickle cell trait, a heterozygous

condition, gives people in malarial

environments an advantage because

they are resistant to this disease.

Hybrid vigor and

outbreeding

Two unrelated individuals are less

likely to have the same recessive,

deleterious allele than are relatives;

therefore, their offspring are less

likely to be homozygous for that

allele; in addition, outbreeding

increases the number of

heterozygous alleles, increasing

heterozygote advantage.

Artificially selected plants are

carefully outbred in order to

increase hybrid vigor; mating two

inbred strains of potato will

increase the number of

heterozygous loci and increase the

species’ resistance to disease.

Frequency-dependent

selection

The least common phenotype is

selected for, while common

phenotypes have a disadvantage.

In some fruit flies, females choose to

mate with males that have the rarer

phenotype, resulting in selection

against the more common variants.

Figure 12.2 How balanced polymorphism is maintained.

BIG IDEA 1.C.1
Speciation (and
extinction) have
occurred through-
out Earth’s history.

BIG IDEA 1.C.2
Speciation may
occur when two
populations
become reproduc-
tively isolated.

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