Mechanisms for Evolution

Allopatric speciation: The Grand Canyon is a geographical barrier separating the Albert and Kaibab

squirrels

Populations and Evolution Population – a group of organisms that interbreed Each population shares a gene pool (the different

alleles present in the population) Each population has a relative frequency of each

alleles, or the number of times the allele occurs in the gene pool.

The frequency of alleles in a population tend not to change unless there is an outside force causing it

1. Mutations

Mutations are inheritable changes to the genotype of an organism

Mutations occur randomly and spontaneously within a population

Most mutations are harmful, but some are useful

Mutations can affect allele frequency in a population by 1. Adding new alleles for a trait

2. Changing the amount of each allele present

It can take a long time to eliminate a mutation and a long time for a new mutation to become prevalent

Evolution occurs because of changes to the equilibrium

There are 5 mechanisms that can change the allele frequencies in a population

1. Mutation2. Migration3. Genetic Drift4. Non-random Mating5. Natural Selection

2. Migration Movement into and out of a population can

change the allele frequency in a population’s gene pool

Immigration can ADD individuals with variations to the population

Emigration can REMOVE individuals with variations from a population. Can you think of large human immigrations and emigrations within the last 150 years?

Many species encourage migration which can cause more gene flow which is the process of transferring genes among different populations

3. Genetic Drift Genetic drift is the random changes in

allele frequency due to population size.

In smaller populations the allele frequency can change more rapidly than in larger populations

3. Genetic Drift cont’d The founder effectoccurs when a fewindividuals from a largerpopulation colonize a new area.

• Ex. Amish community.

The allele frequency of this population maydiffer from the largerpopulation because of thelimited number of individuals

4. Non-Random Mating Having a limited number of

individuals can also impact mating.

Non random mating can influence allele frequencies because:• Mates can be limited by geography• Mates can be chosen for their traits• Mates can be more closely related to one

another

5. Natural Selection Natural selection is process by which

individual which are more fit for the environment survive and reproduce

The interaction of populations and the environment results in changing allele frequencies

There are 4 different types of natural selection:

a. Stabilizing Selectionb. Directional Selectionc. Disruptive Selectiond. Sexual Selection

5a. Stabilizing Selection Stabilizing selection occurs when

individuals with the average form of the trait are most fit for the environment and extreme traits are eliminated

This is the most common form of selection and works in all populations at all times

Example of stabilizing selection Lizard body size:

• Large lizards are easily seen by predators, but smaller lizards cannot run as fast to escape the predators

• Mid sized lizards are most fit in the environment, so they survive and reproduce more often, changing the allele frequencies in the population

5b. Directional Selection Directional selection occurs when

individuals with one extreme of variations are the most fit in the environment.

This causes a gradual shift in allele frequency to that extreme.

Example of Directional Selection Anteater tongue length:

• Anteaters with long tongues are most fit because of the depth of the nests of the termites they eat.

5c. Disruptive Selection Disruptive selection occurs when both extremes of

variations are the most fit There is selection against the middle variations Ex: Limpet shells

• Dark limpets blend with bare rocks• Light limpets blend with barnacle covered rocks• Tan limpets are visible in both situations and get

preyed upon by birds

5d. Sexual Selection Sexual selection is the competition

for mates within a population causing differences to occur in the allele frequencies of the 2 genders

Mates tend to be chosen for their phenotypes and females tend to choose the males.

Sexual Selection cont’d Ex: Peacocks

• Male peacocks have large tail feathers that make it difficult to fly and escape from predators

• Female peacocks choose males based on their tail feather length and fullness

• Over time males with larger tail feathers reproduce more causing large tails to be selected for

• Sexual Selection in Peacocks (3 min.)

Adaptive RadiationIn

evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage.

Coevolution

Coevolutionary relationships mainly occur between plants and herbivorous insects and between plants and pollinating insects.

Mutually beneficial relationship.

Covergent Evolution

When organisms evolve similar (analagous) structures or functions even though their evolutionary ancestors are very dissimilar or unrelated. Example:

The complex eyes of vertebrates, cephalopods (squid and octopus), cubozoan jellyfish, and arthropods (insects, spiders, crustaceans) evolved separately, but all perform the function of vision

http://jeffvrabel.files.wordpress.com/2011/08/wolf-spider-5-29-08-7eyes.jpg

http://images.sciencedaily.com/2009/06/090601182828-large.jpg

Cladograms Look at the

cladogram at the right. What conclusions can be drawn about the relationship between humans and chimps?

This diagram shows a relationship between 4 relatives. These relatives share a common ancestor at the root of the tree

Note that this diagram is also a timeline. The older organism are located on the bottom of the tree.

The four descendents at the top of the tree are DIFFERENT species. This is called SPECIATION.

Branches on the tree represent SPECIATION, the formation of a new species.

The event that causes the speciation is shown as the fork of the “V”.

Has your understanding of this diagram changed? How?

Did humans evolve from chimps? No

What familial relationship is a good description of the relationship between chimps and humans? Distant Cousins

Are humans more highly evolved than chimps?

NO- since the lineage is split, each species has evolved unique traits.

Anima: James Balogimages used with permission. http://www.jamesbalog.com/

With 98.4% similar genetic sequences chimpanzees are both a link to Kingdom Animalia and our kin.