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 Separation Population becomes separated from the rest Ex: physical barriers such as canyons, mountains, or lakes Adaptation Populations constantly undergo natural selection Adaptations in the separated populations differ 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?