Name My pts

Pts Poss. Tot “My Pts”

TPP Grade

3 Peppered Moth 10 1704 Bill Nye 10 1805 Bird Beak 15 195

Goal: • Summarize the effects of the different

types of natural selections on gene pools.

Section Objectives

• Summarize the effects of the different types of natural selections on gene pools.

• There are three different types of natural selection that act on variation: stabilizing, directional, and disruptive.

Natural selection acts on variations

•Variations increase or decrease an organism’s chance of survival in an environment.

• Stabilizing selection is a natural selection that favors average individuals in a population.

Selection for average size spiders

Normal variation

Natural selection acts on variations

Natural selection acts on variations• Directional selection occurs when natural

selection favors one of the extreme variations of a trait.

Normal variation

Selection for longer beaks

Natural selection acts on variations• In disruptive selection, individuals with either

extreme of a trait’s variation are selected for.

Selection for light limpets

Normal variation

Selection for dark limpets

Natural selection acts on variations

• Natural selection can significantly alter the genetic equilibrium of a population’s gene pool over time.

• Significant changes in the gene pool could lead to the evolution of a new species over time. (speciation)

Physical barriers can prevent interbreeding

• In nature, physical barriers can break large populations into smaller ones.

• Geographic isolation occurs whenever a physical barrier divides a population.

• A new species can evolve when a population has been geographically isolated.

The Evolution of Species

• When geographic isolation divides a population of tree frogs, the individuals no longer mate across populations.

The Evolution of Species

• The formation of a river may divide the frogs into two populations.

The Evolution of Species

• Over time, the divided populations may become two species that may no longer interbreed, even if reunited.

• Mistakes during mitosis or meiosis can result in polyploid individuals.

Parent plant (2n)

Meiosis begins

Nondisjunction

Normal meiosis

Normal gametes (n)

Fertilization

Zygote (3n)

Abnormal gametes (2n)

FertilizationZygote

(4n)

Sterile plant

New polyploidspecies

A change in chromosome numbers and speciation

Speciation rates

• Gradualism is the idea that species originate through a gradual change of adaptations.

Speciation rates

• In 1972, Niles Eldredge and Stephen J. Gould proposed a different hypothesis known as punctuated equilibrium.

• This hypothesis argues that speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between.

Speciation ratesLoxodonta africana

Elephas maximus

Mammuthusprimigenius

Mammuthus

Elephas

Loxodonta

Primelephas

about 55 million years agoAncestral species

6

5

4

3

2

1

0

Mill

ions

of Y

ears

Ago

Speciation rates

• Biologists generally agree that both gradualism and punctuated equilibrium can result in speciation, depending on the circumstances.

Patterns of Evolution

• Biologists have observed different patterns of evolution that occur throughout the world in different natural environments.

• These patterns support the idea that natural selection is an important agent for evolution.

Diversity in new environments• When an ancestral species evolves

into an array of species to fit a number of diverse habitats, the result is called adaptive radiation.

• Adaptive radiation in both plants and animals has occurred and

continues to occur throughout the world and is common on islands. • Adaptive radiation is a type of divergent

evolution, the pattern of evolution in which species that were once similar to an ancestral species diverge, or become increasingly distinct.

Diversity in new environments

Possible AncestralLasan finch

Amakihi Extinct mamo

Crestedhoneycreeper

AkialoaAkepa

Akiapolaau LiwiMaui parrotbill

Apapane

Ou

Grosbeak finch

PalilaAkikiki

Niihau

Kauai

Oahu

Lanai

Molokai

Maui

KahoolaweHawaii

Diversity in new environments

Diversity in new environments

• Divergent evolution occurs when populations change as they adapt to different environmental conditions, eventually resulting in new species.

Different species can look alike

• A pattern of evolution in which distantly related organisms evolve similar traits is called convergent evolution.

• Convergent evolution occurs when unrelated species occupy similar environments in different parts of the world.

The fur of an Arctic fox turns white in the winter. Is this an example of natural selection? Why or why not?

Question 1

IN: 1.32

The answer is no. An individual cannot evolve a new phenotype (in this case, changing the color of its fur) within its lifetime in response to its environment.

IN: 1.32

Which type of natural selection does NOT favor the evolution of new species?

Question 2

D. directional

C. stabilizing

B. disruptive

A. divergent

IN: 1.3

The answer is C. Stabilizing selection reduces variation in a population.

IN: 1.3

Which of the following rarely affects a population’s genetic equilibrium?

Question 3

D. disruptive selection

C. gene flow

B. lethal mutations

A. genetic drift

The answer is B. Organisms with lethal mutations do not survive. Therefore, organisms with lethal mutations cannot produce enough offspring to affect a population’s genetic equilibrium.

Why are the Galapagos Islands rich in unique species of organisms?

Question 4

D. The island species have been subjected to stabilizing selection.

C. The island species have been subjected to increased gene flow.

B. The islands are geographically isolated.

A. The islands are an area exhibiting an abnormal number of mutations.

IN: 1.36

The answer is B. Geographic isolation has helped to keep the islands’ species unique.

IN: 1.36