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

142 ❯ STEP 4. Review the Knowledge You Need to Score High

When humans become the agents of natural selection, we describe the process as

artificial selection.Instead of allowing individuals to survive and reproduce as they would

without human intervention, we may specifically select certain individuals to breed while

restraining others from doing so. Artificial selection has resulted in the domestication of a

wide range of plant and animal species and the selection of certain traits (e.g., cattle with

lean meat, flowers with particular color combinations, dogs with specific kinds of skill).

Evolution Patterns

There are four basic patterns of evolution:

Coevolution.The mutual evolution between two species, which is exemplified by predator–

prey relationships. The prey evolves in such a way that those remaining are able to escape

predator attack. Eventually, some of the predators survive that can overcome this evolutionary

adaptation in the prey population. This goes back and forth, over and over.

Convergent evolution.Two unrelated species evolve in a way that makes them moresimilar

(think of them as converging on a single point). They are both responding in the same way

to some environmental challenge, and this brings them closer together. We call two char-

actersconvergent charactersif they are similar in two species, even though the species do

notshare a common ancestor. For example, birds and insects both have wings in order to

fly, despite the fact that insects are not directly related to birds.

Divergent evolution.Two related species evolve in a way that makes them lesssimilar.

Divergent evolution can lead to speciation (allopatric or sympatric).

Parallel evolution.Similar evolutionary changes occurring in two species that can be related

or unrelated. They are simply responding in a similar manner to a similar environmental

condition.

Sources of Variation

Remember that one of the conditions for natural selection is variation. Where does this

variation within populations come from?

1. Mutation.We already discussed mutations as a mechanism by which evolution
   occurs. Random changes in the DNA of an individual can introduce new alleles into
   a population.


2. Sexual reproduction.Refer to Chapter 16, Human Reproduction, and the discussion of
   why offspring are not identical to their parents (crossover, independent assortment of
   homologous pairs, and the fact that all sperm and ova are unique and thus create a
   unique individual when joined).


3. Balanced polymorphism.Some characters are fixed, meaning that all individuals in a
   species or population have them: for example, all tulips develop from bulbs. However,
   other characters are polymorphic, meaning that there are two or more phenotypic vari-
   ants. For example, tulips come in a variety of colors. If one phenotypic variant leads to
   increased reproductive success, we expect directional selection to eventually eliminate
   all other varieties. However, we can find many examples in the natural world where
   variation is prominent and one allele is not uniformly better than the others. The var-
   ious ways in which balanced polymorphism is maintained are presented in Figure 12.2.

KEY IDEA

BIG IDEA 4.B.3
Interactions
between popula-
tions influence
patterns of species
distribution.

BIG IDEA 3.C.1
Heterozygote
advantage is one
way biological
systems can
increase genetic
variation.