© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 1 Populations may change through two major genetic forces: Natural Selection (includes artificial

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 1

Populations may change through two major genetic forces: Natural Selection (includes artificial seltn)

Genetic Drift Two other forces that change a population:

MutationsGene flow Not much change, mutation is too rare & gene flow

tends to equalize gene frequencies

Mechanisms of Population Change

2

Natural Selection Occurs because:

Organisms with favorable traits… chance at survival

The longer an organism lives, more offspring it may have

More of the next generation will have the favorable trait than the previous generation


Natural Selection

Individuals of one phenotype are favored

Peppered moths

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Genetic Drift

Differences in reproduction or survival between organisms due to random chance Environmental disturbances can cause genetic drift as

long as they act at random: Hurricanes Volcanic eruptions (Mt. St. Helens)

Can be from random events seeds from white flowers are blown

to gravel, some seeds from red flowers blown to good soil.


Genetic Drift


How Populations Change Mutation creates genetic variation (genes) Gene flow, genetic drift and natural selection

act on this genetic variation to change allele frequencies in populations Gene flow tends to equalize allele frequencies

between 2 pops. Genetic drift and natural selection tend to

diverge allele frequencies between 2 pops. (populations tend to diverge)


Adaptations

Adaptation –a characteristic that improves the survival of a population or individual


Adaptation

An improvement in a population over time, changing a population to better fit its environment

Only the result of natural selection acting on genetic variation, no other force adapts a population

Environments change and new genetic variation arises, so adaptation never reaches perfection


What are Species?

Members of a species generally look alike Bald eagles in Alaska (top) &

Colorado (bottom) But not always

Species –are reproductively isolated from other species, meaning that one can only reproduce within one’s own species


Walking Sticks

Adapting to match their environment (two mating pairs)

Striped form is well-hidden among the needle-like leaves w/ stripes, solid form among solid leaves


The striped form prefers needle-like leaves of the chemise (sheh-mēz) bush.

The solid-green form prefers the solid green leaves of the wild lilac.

The two forms are more likely to survive on their preferred plant. Predators (birds) more likely to catch them if they are

not on their preferred plant.

Walking Sticks


Walking Sticks

Is a complex adaptation, more than coloration. Lilac and chemise bush leaves have different

pulp, fiber, chemical contents, and plant toxins. The solid-green walking sticks have enzymes to

better digest the leaves of the wild lilac. The striped form has enzymes adapted to better

digest the leaves of the leaves of the chemise bush.


Walking Sticks

“Hybrid offspring” of a cross between solid and striped parents are less fit than offspring of a single form mating. Do not blend as well on either Lilac or chemise

bush leaves; predators more likely to eat them. Do not digest either plant as well as single forms.


Walking SticksBecause “Hybrid offspring” (a solid & striped cross) are less fit than “single form” offspring, the population is adapting in one other way: Individuals prefer to mate with others of the

same color pattern.


Walking Sticks

Population is diverging with respect to alleles for: coloration (once were all green), digestion (once were generalists), dealing with plant toxins, mating behavior.

Natural selection is better adapting each form to their preferred plant.


W/ time, natural selection will further diverge the two forms & further decrease their interbreeding.

When the two forms loose the ability to interbreed, they will become two different species.


Speciation

When one species splits to form two species Results from reproductive isolation Often, it is a secondary consequence of

changing populations Occasionally, is a direct consequence of a

diverging population The hybrid is less fit, walking sticks


Biological Evolution

Is a change in the genetic characteristics of a population over time

If any allele frequency in a population changes w/ time, the population is evolving

Most people think of evolution as Darwin’s speciation, but this is not correct


Only populations can evolve; individuals cannot(on molecular level, genes can evolve)

Small scale change = microevolution Large scale change = macroevolution




Macroevolutioncan lead to speciation,

but it does not have to has occurred in dogs

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Review Understanding evolution has

impacted our use of Antibiotics and our understanding the development of drug resistance

Doonesbury cartoon?

Understanding evolution has impacted our understanding of the world (shown at right)

Review of agents of change video

Documents

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 1 Populations may change through two major genetic forces: Natural Selection (includes artificial