Chapter 18

Diversity and Variation

Natural Selection:• Not all members of a population

necessarily have an equal chance of surviving and reproducing.

• Some individuals are born with a mutation that gives them a survival advantage or adaptation

• The better adapted individuals are more "fit“, tend to survive and reproduce, passing on their adaptations to the next generation more so than the less "fit" members of the population

How does natural selection shape a population? • First, think of variation in terms of a bell shaped

curve:– what does the y-axis represent?

• The number of individuals– What does the x-axis represent?

• the value of a particular characteristic– like size, color, etc.

– What does it mean when a variable exhibits continuous variation?• It has a wide range over which it varies, like

human height

Stabilizing Selection:• Organisms with extreme characteristics die or fail

to reproduce, resulting in populations of individuals that possess intermediate characteristics.

•Infants weighing significantly less or more than 7.5 pounds have higher rates of infant mortality. Selection works against both extremes.

Lizards and Stabilizing Selection• http://wps.prenhall.com/wps/media/objects/487/499541/CDA22_2/CDA

22_2f/CDA22_2f.htm

Directional Selection• tends to favor

phenotypes at one extreme of the range of variation.

The graph shows the decrease in size of pink salmon caught in two rivers in British Columbia since 1950, driven by selective fishing for the large individuals.

Disruptive Selection• favors individuals

at both extremes of variation: selection is against the middle of the curve

http://wps.prenhall.com/wps/media/objects/487/499541/CDA22_2/CDA22_2d/CDA22_2d.htm

What’s taxonomy? The theories and techniques of describing, grouping and naming organisms

•Biologists do not think of species simply as a long alphabetical list. •Since Linnaeus, the father of modern taxonomy, species have been arranged in a taxonomic hierarchy: • Species are grouped in genera.

• The gray wolf species Canis lupus and the golden jackal Canis aureus , for example, are grouped in the genus Canis .

• Genera are grouped into families; • the genus containing dogs and wolves

combines with several other genera, such as the fox genus Vulpes , to make up the family Canidae.

• Several families combine to make up an Order• Carnivora, in this example.

• Orders make a Class (Mammalia).• Classes make a Phylum (Chordata).• Phyla make up one of the five Kingdoms

(Animalia).

Why do we classify organisms?

• To make sense of the diversity of organisms• Put them in order• To understand the relationships organisms have with

each other.• Morphological (form), physiological (structure),

ecological (environment), genetic and molecular characteristics can all be used to classify organisms.

• Nucleic acid structure is probably the best indicator of relatedness

Why else is classification important?

Practical and scientific reason—• identifying wild relatives of crops–

that provide breeders with useful alleles

• Benefits to public health--disease control

• Identifying unknown species that might help understand ecosystems

• Understanding the “unity of life”

What’s a Species?The basic taxonomic group is the species

All individuals and populations of a particular type of organism that can interbreed with one another.

Species may look different, but as long as they can produce FERTILE offspring they are still members of the same species.

The Species Concept

Species remain separate from one another in three basic ways: • potential mates do not meet; •potential mates meet but do not breed•potential mates meet and breed but do not produce fertile or viable offspring.

Species are interbreeding natural populations which are reproductively isolated from other such groups.

Appearance Isn’t Everything

•Organisms may appear to be alike and be different species.•For example, Western meadowlarks (Sturnella neglecta) and Eastern meadowlarks (Sturnella magna) look almost identical to one another, yet do not interbreed with each other•Thus, they are separate species according to this definition.

Evidence Used to Classify Organisms

• morphology• anatomy /development• the fossil record• molecular data

Placing Organisms in Groups

Ex. All animals with hair are classified as mammals

The Steller’s jay and the blue jay

Using Homologous Structures

Similarities of biological structures that results from evolution form a common ancestor.

Forearm of frogs, lizards, birds, humans, cats, etc.

Homologous structures are are also those that develop from similar embryological origins

Adult fish, salamanders, turtles, chickens, rabbits, and humans bear virtually no resemblance to one another. Yet these animals are virtually indistinguishable as embryos. Can you pick out which one is the human?

How about now? Which one is the human embryo?

Why should animals that have markedly different adult forms and function develop from such similar embryos?

Far back in vertebrate history, they all had a common ancestor, probably some type of primitive fish, that developed from a similar type of embryo.

As the various types of vertebrates evolved, they each retained this basic vertebrate embryo as part of their life cycle– even though its parts gave rise to different adult organs.

Analogous Structures Similarities in form or function that are not a result of evolution from a common ancestor, but is evidence of convergent evolution (similar environmental pressures).

Ex. wings of insect, bird, bat and pterosaur

Analogous vs. Homologous Structures

Chemical homologies

•The more similar the genetic code (DNA), the more closely related two species are (the more recently they evolved from one another)•So, the longer the period since two species have diverged from a common ancestor, the greater the number of differences in genes and proteins between the species.

Amino Acids as Evidence for Evolution Packet

The Linnean classification system

The system uses homologies to group species into larger and more general categories. Kingdom, phylum, class, order, family, genus, species

Binomial nomenclature?

The scientific naming of species where each species receives a Latin or Latinized name of two parts• the first part indicates the genus and the second is the specific epithet. •For example, Juglans regia is the English walnut; Juglans nigra, the black walnut.

How do you write a scientific name?•Capitalize genus•underline/italicize both names•lower case species name

Ex. American Black Bear

Ursus americanus Brown Bear

Ursus arctos •Subspecies Syrian (Brown) Bear (Ursus arctos syriacus)•Subspecies Grizzly Bear, (Ursus arctos horribilis)•Subspecies Kodiak Bear, (Ursus arctos middendorffi)

Ways to Classify Organisms

Current classification systems attempt to reflect the evolutionary relationships of organisms

Orthodox-

• Originally, organisms classified using the Linnaean system were grouped together based simply on physical similarities

• Ex. Dividing birds into orders based on their beaks and claws

• This method can be subjective

Cladistics•Assumes each group has an ancestor that other species do not share•Groups species according to ancestry and homologous characteristics.

Ex. All mammal species have mammary glands, but no other organism do. •Therefore all mammals must be descended from a species that has no other living descendants.•Mammals form a clade, or branch on this diagram of the history of the animals.

•Comparing the anatomical structures of many different organisms.

• homologous structures • for example, the front legs of

a horse, wings of a bird, flippers of a whale, and the arms of a person are all homologous to each other.

•When different organisms share a large number of homologous structures, it is considered strong evidence that they are related to each other.

Phylogeny

Making a Cladogram

SETS TRAITS Tuna Horse Human#1 Backbone

#2 Placenta

#3 Foramen magnum fwd.

TOTALS of X’s

x x

x

x

x

x

1 2 3

Step 1: determine which of the characteristics each animal has.

Step 2: Draw a cladogram to illustrate the ancestry of these animals. The diagram should reflect shared characteristics as time proceeds.

Notice how the different animals are all at the same time level (across the top) since they all live today.