Natural Selection in Action

Holt - Chapter 7, Section 3

Foldable Format:

Natural Selection in Action

Changes in Populations

Forming a New

Species

Changes in Populations: • Changes in populations are sometimes

observed when a new force affects the survival of individuals.

1. Adaptation to Hunting• Ex: Elephant hunting in Uganda– 1930’s – 1% of male elephants did not have tusks– Now – 15% do not

Changes in Populations: 1. Adaptation to Hunting

• Why?– People hunt elephants for

their tusks.– Fewer survive to

reproduce, so more tuskless elephants are born.

– They pass this tuskless trait on to their offspring.

Changes in Populations: 2. Resistance to Chemicals

Reading for Meaning• Complete the “Before Reading” column.• Read the article, “Evolution of Antibiotic

Resistance.”– Find evidence for or against each of the

statements as you read.• Discuss your answers with a partner.• Discuss your answers with the class.

Antibiotic Resistance in Bacteria

• Just as insects develop resistance to pesticides, bacteria develop resistance to antibiotics.

• Bill Nye – Antibiotics (video clip)• PBS - Antibiotic Resistance

Changes in Populations:3. Competition for Mates

• Competition for mates can select for adaptations.

• Ex:– In many bird species, females prefer to mate with

colorful males.– Colorful males pass on their genes for color to the

next generation, so the proportion of colorful males is likely to increase.

Forming a New Species• Species – a group of organisms that can mate

with each other to produce fertile offspring.• A new species may form after a group becomes

separated from the original population.– Over time, the new population adapts to its new

environment.– Eventually, the two populations differ so much that

they can no longer mate successfully.• Speciation – the formation of a new species as

a result of evolution.

Forming a New Species

3 Steps in Forming a New Species

1. Separation– Population becomes separated from the rest– Ex: physical barriers such as canyons, mountains, or lakes

2. Adaptation– Populations constantly undergo natural selection– Adaptations in the separated populations differ

3. Division– The separated groups evolve and become different from

each other– No longer able to interbreed – two different species now

Summary• In Australia, many animals look like mammals

from other parts of the world. But most of the mammals in Australia are marsupials (mammals that carry their young in a pouch after birth). Few kinds of marsupials are found anywhere else in the world.

• What is a possible explanation for the presence of so many of these unique mammals in Australia?