How does Natural Selection affect the Genetics of

Populations?

How do different Species arise?

How do Species become Extinct?

The Mr. Potato head evolutionary tree… a model of descent with modification

• What characteristics are shared by all species?• What characteristics arose at A?• What characteristics arose at B?• What characteristics arose at C?

--A--B

--C

--D

Natural Selection Review• Natural selection is a cycle: • Replication: new individuals are produced through sexual

reproduction. • Mutations are introduced during Replication. This

produces variations in the population of individuals• Selection occurs: this favors some variations over others.

The variations that are most successful are copied• The overall fitness of the population increases over many

generations or cycles.

Natural Selection can shift allele frequencies in one of three ways• Stabilizing selection favors the average or mean value for a trait and

selects against the extremes.

Natural Selection can shift allele frequencies in one of three ways

• Directional selection favors one extreme over the other, can be either the lower or upper extreme.

Examples of Directional Selection:

Natural Selection can shift allele frequencies in one of three ways

• Diversifying selection (sometimes called disruptive selection) favors the extremes, and selects against the middle or average.

Natural History Museum Presentations start on Friday• All groups must be ready to present on Friday• I will ask for volunteer groups• Then call randomly for remaining groups

• Today’s Agenda:• Check/ collect Homework• Speciation and Extinction Notes• Group Work Time

How do new species arise? Reproductive Isolation and Time• A Species is a group of organisms that is so

genetically similar that they can reproduce together and have fertile offspring. • Individuals in a population that have the

potential to reproduce together make up the “gene pool” of the species. • If a population becomes separated into

two smaller isolated populations there is the potential for them to become separate species over many generations. • They will be considered separate species if

they can no longer reproduce together.

Below are different existing elephants and species related to elephants.

• The mammoths and mastodons were different cold climate adapted species.• Mammoths lived in N. America,

Europe and Africa.• Mastodons lived in N. America,

and went extinct around the same time that people first came to N. America from Asia about 11,000 years ago.

Routs and culture of the First Americans

What are Hybrids?• Hybrid organisms can be produced when two recently diverged species can

still breed, but their offspring are infertile, or have low fertility. • EX: Lions and Tigers will breed in captivity to produce a “Liger”, donkeys and

horses will breed to produce mules. • Recent genetic studies have shown that modern humans interbred with

Neanderthals, which were a closely related hominid species that lived in Europe around 30,000 to 40,000 years ago. Between 1-4% of your DNA could be Neanderthal.

One of the closest species to modern humans went extinct about 40,000 – 30,000 years ago. They had ceremonial burials, and made stone hand axes.

Other species of hominids, all extinct accept for modern humans

Extinction

• A species becomes extinct when no individuals of its type remains. • This can happen slowly or rapidly• Extinction can be caused by sudden changes to the environment. • Species that do not have enough variation to survive the change can no

longer reproduce and thus die off. • What caused the extinction of the dinosaurs 65 million years ago?• Probably a asteroid impact followed by climate change

• Humans have caused extinction of many species of animals, plants and choral through our activities in reshaping the world around us.

How does Natural selection change the genetics of populations over time?• Modern biology measures evolution as a change in

allele frequency over time. • What is allele frequency?

• It is the number of times an allele exists in a population, divided by the total number of alleles in the population (See Example)

• Yellow (Y) allele is dominant to red (y) allele. • What is the allele frequency of the y (red ) allele)

and Y allele in the example population?• Step 1: tally up number of yellow alleles and red

alleles• Number of Y= 6. Number of y= 4+20= 24• Y allele frequency = divide number of yellow alleles

by total: 6/30= 0.2 • Frequency of y allele is 1-0.2 = 0.8

Yy Yy

YYYY

YyYy

yy

Read the following scenario and calculate the change in allele frequency in the population:

• The lady bug population lives in a grass land where insect eating birds hunt. The birds have an aversion to eating red ladybugs, and tend to eat more yellow lady bugs. After several generations, the new population is shown below: • What is the new allele frequency of the Y and y alleles?• Number of Y = 1, Number of y= 29• Frequency of Y = 1/30 = 0.03• Frequency of y allele = 29/30 = 0.96

Yy