Standard Set 8. Evolution (Speciation)
See Standards 7 above
8. Evolution is the result of genetic changes that occur in constantly changing
environments. As a basis for understanding this concept:
8. a. Students know how natural selection determines the differential survival of groups of
Genetic changes can result from gene recombination during gamete formation and from mutations. These
events are responsible for variety and diversity within each species. Natural selection favors the organisms
that are better suited to survive in a given environment. Those not well suited to the environment may die
before they can pass on their traits to the next generation. As the environment changes, selection for
adaptive traits is realigned with the change. Traits that were once adaptive may become disadvantageous
because of change.
Students can explore the process of natural selection further with an activity based on predator-prey
relationships. The main purpose of these activities is to simulate survival in predator or prey species as
they struggle to find food or to escape being consumed themselves. The traits of predator and prey
individuals can be varied to test their selective fitness in different environmental settings.
An example of natural selection is the effect of industrial â€œmelanism,â€� or darkness of pigmentation,
on the peppered moths of Manchester, England. These moths come in two varieties, one darker than the
other. Before the industrial revolution, the dark moth was rare; however, during the industrial revolution
the light moth seldom appeared. Throughout the industrial revolution, much coal was burned in the region,
emitting soot and sulfur dioxide. For reasons not completely understood, the light-colored moth had
successfully adapted to the cleaner air conditions that existed in preindustrial times and that exist in the
However, the light-colored moth appears to have lost its survival advantage during times of heavy
industrial air pollution. One early explanation is that when soot covered tree bark, light moths became
highly visible to predatory birds. Once this change happened, the dark-peppered moth had an inherited
survival advantage be-cause it was harder to see against the sooty background. This explanation may not
have been the cause, and an alternative one is that the white-peppered moth was more susceptible to the
sulfur dioxide emissions of the industrial revolution. In any case, in the evolution of the moth, mutations of
the genes produced light and dark moths. Through natural selection the light moth had an adaptive
advantage until environmental conditions changed, increasing the population of the dark moths and
depleting that of the light moths.
8. b. Students know a great diversity of species increases the chance that at least some
organisms survive major changes in the environment.
This standard is similar to the previous standard set on diversity within a species but takes student
understanding one step further by addressing diversity among and between species. For the same reasons
pertinent to those for intraspecies diversity, increased diversity among species increases the chances that
some species will adapt to survive future environmental changes.
8. c. Students know the effects of genetic drift on the diversity of organisms in a population.
If a small random sample of individuals is separated from a larger population, the gene frequencies in the
sample may differ significantly from those in the population as a whole. The shifts in frequency depend
only on which individuals fall in the sample (and so are themselves random). Because a random shift in
gene frequency is not guaranteed to make the next generation better adapted, the shiftâ€”or genetic driftâ
€”with respect to the original gene pool is not necessarily an adaptive change. The bottleneck effect (i.e.,
nonselective population reductions due to disasters) and the founder effect (i.e., the colonization of a new
habitat by a few individuals) de-scribe situations that can lead to genetic drift of small populations.
8. d. Students know reproductive or geographic isolation affects speciation.
Events that lead to reproductive isolation of populations of the same species cause new species to appear.
Barriers to reproduction that prevent mating between populations are called prezygotic (before
fertilization) if they involve such factors as the isolation of habitats, a difference in breeding season or
mating behavior, or an incompatibility of genitalia or gametes. Postzygotic (after fertilization) barriers that
prevent the development of viable, fertile hybrids exist because of genetic incompatibility between the
populations, hybrid sterility, and hybrid breakdown.
These isolation events can occur within the geographic range of a parent popuÂ¬lation (sympatric
speciation) or through the geographic isolation of a small population from its parent population (allopatric
speciation). Sympatric speciation is much more common in plants than in animals. Extra sets of
chromosomes, or polypoidy, that result from mistakes in cell division produce plants still capable of long-
term reproduction but animals that are incapable of that process because polypoidy interferes with sex
determination and because animals, unlike most plants, are usually of one sex or the other. Allopatric
speciation occurs in animal evolution when geographically isolated populations adapt to different
environmental conditions. In addition, the rate of allopatric speciation is faster in small populations than in
large ones because of greater genetic drift.
8. e. Students know how to analyze fossil evidence with regard to biological diversity, episodic
speciation, and mass extinction.
Analysis of the fossil record reveals the story of major events in the history of life on earth, sometimes
called macroevolution, as opposed to the small changes in genes and chromosomes that occur within a
single population, or microevolution. Explosive radiations of life following mass extinctions are marked by
the four eras in the geologic time scale: the Precambrian, Paleozoic, Mesozoic, and Cenozoic. The study
of biological diversity from the fossil record is generally limited to the study of the differences among
species instead of the differences within a species. Biological diversity within a species is difficult to study
because preserved organic material is rare as a source of DNA in fossils.
Episodes of speciation are the most dramatic after the appearance of novel characteristics, such as
feathers and wings, or in the aftermath of a mass extinction that has cleared the way for new species to
inhabit recently vacated adaptive zones. Extinction is inevitable in a changing world, but examples of mass
extinction from the fossil record coincide with rapid global environmental changes. During the formation of
the supercontinent Pangaea during the Permian period, most marine invertebrate species disappeared with
the loss of their coastal habitats. During the Cretaceous period a climatic shift to cooler temperatures
because of diminished solar energy coincided with the extinction of dinosaurs.
8. f.* Students know how to use comparative embryology, DNA or protein sequence
comparisons, and other independent sources of data to create a branching diagram (cladogram)
that shows probable evoluÂ¬tionary relationships.
The area of study that connects biological diversity to phylogeny, or the evolutionary history of a species,
is called systematics. Systematic classification is based on the degree of similarity between species. Thus,
comparisons of embryology, anatomy, proteins, and DNA are used to establish the extent of similarities.
Embryological studies reveal that ontogeny, development of the embryo, provides clues to phylogeny. In
contrast to the old assertion that â€œontogeny recapitulates phylogenyâ€� (i.e., that it replays the entire
evolutionary history of a species), new findings indicate that structures, such as gill pouches, that appear
during embryonic developÂ¬ment but are less obvious in many adult life forms may establish homologies
between species (similarities attributable to a common origin). These homologies are evidence of common
ancestry. Likewise, homologous anatomical structures, such as the forelimbs of humans, cats, whales, and
bats, are also evidence of a common ancestor. Similarity between species can be evaluated at the
molecular level by comparing the amino acid sequences of proteins or the nucleotide sequences of DNA
strands. DNA-DNA hybridization, restriction mapping, and DNA sequencing are powerful new tools in
Approaches for using comparison information to classify organisms on the basis of evolutionary history
differ greatly. Cladistics uses a branching pattern, or cladogram, based on shared derived characteristics
to map the sequence of evolutionary change. The cladogram is a dichotomous tree that branches to
separate those species that share a derived characteristic, such as hair or fur, from those species that lack
the characteristic. Each new branch of the cladogram helps to establish a sequence of evolutionary
history; however, the extent of divergence between species is unclear from the sequence alone.
Phenetics classifies species entirely on the basis of measurable similarities and differences with no attempt
to sort homology from analogy. In recent years phenetic studies have been helped by the use of computer
programs to compare species automatically across large numbers of traits. Striking a balance between
these two apÂ¬proaches to classification has often involved subjective judgments in the final decision of
taxonomic placement. Students can study examples of cladograms and create new ones to understand
how a sequence of evolutionary change based on shared derived characteristics is developed.
8. g.* Students know how several independent molecular clocks, calibrated against each other
and combined with evidence from the fossil record, can help to estimate how long ago various
groups of organ-isms diverged evolutionarily from one other.
Molecular clocks are another tool to establish phylogenetic sequences and the relative dates of
phylogenetic branching. Homologous proteins, such as cytochrome c, of different taxa (plants and animals
classified according to their presumed natural relationships) and the genes that produce those proteins are
assumed to evolve at relatively constant rates. On the basis of that assumption, the number of amino acid
or nucleotide substitutions provides a record of change proportional to the time between evolutionary
branches. The estimates of rate of change derived from these molecular clocks generally agree with
parallel data from the fossil record; however, the branching orders and times between branches are more
reliably determined by measuring the degree of molecular change than by comparing qualitative features of
morphology. When gaps in the fossil record exist, phylogenetic branching dates can be estimated by
calibrating molecular change against the timeline determined from the fossil record.