Part 1: The Pace of Evolutionary...

Part 1: The Pace of Evolutionary Change

How fast is Evolution?

Evolution does not always occur at the same rate all the time.

Evidence: Looking at the fossil record, it shows some groups of organisms remaining unchanged for millions of years, while others are rapidly changing over time.

Gradualism

The pattern of slow and gradual evolutionary change over long periods of time.

Populations slowly diverge from one another due to different selective pressure:

Constant allele frequencies

Constant environmental conditions

Few chromosomal mutations

Gradualism

Some examples include:

Cockroaches

Sharks

Gradualism

Results from transitional forms that are seen in the fossil record.

Example: Trilobites

Punctuated Equilibrium

The pattern of long stable periods in which species stayed much the same.

Interrupted (punctuated) by short periods in which there was quick evolution, rapidly resulting in formations of new species.

Some reasons for it include:

Mass extinction of many life forms

Rapidly changing environmental conditions

Exploitation of new environments

Punctuated Equilibrium

Some examples include:

Galapagos finches

Cambrian (time period) explosion of animal types

Punctuated Equilibrium

The fossil record shows rapid bursts of evolution following mass extinctions.

Cretaceous extinction = Lots of mammals

Part 2: Genetic Variation

What causes a population to change?

1) Mutations constantly occur Mutations provide the source of new

alleles, or variation upon which natural selection can act.

Examples:

Butterfly colour, giraffe’s long necks, rabbits long ears

2) Immigration and Emigration Occur These both affect allele frequencies and

the gene flow in a population.

Examples:

Immigrants come to Canada, thus affecting the gene flow. Immigration affecting phenotype (animals in the wild)

3) Small Populations

Changes have more of an effect on a smaller population.

Examples:

Founder effect: when a small # of individuals from a population wander away to start (found) their own population.

Bottleneck effect: when a population is reduced to low number due to disease, predation, climate change, etc.

Founder Effect

See article on Ellis-van Crevald Syndrome

4) Non-Random Mating Occurs This often produces a change in alleles

within populations.

Examples:

Animals’ sexual selection is intrasexual (competition within 1 sex like deer) or intersexual (completion for a mate like peacocks).

5. Natural Selection Occurs

Only the most fit survive and pass their genes on to successive generations.

Therefore, change occurs because the most fit are always being selected for.

Examples:

Finches, peppered moths

Hardy-Weinberg Equation

This equation can be used to check if a population is changing.

If the equation is NOT EQUAL to 1, the population is CHANGING

If the equation is EQUAL to 1, the population is in equilibrium

The Hardy-Weinberg law states that the frequencies of alleles in a population will remain constant unless acted upon by outside agents or forces. In other words, the proportion of dominant to recessive genes remains the same. The Hardy-Weinberg law describes the genetics of non-evolving populations. A non-evolving population is said to be in Hardy-Weinberg equilibrium.

A population will remain in genetic equilibrium if, and only if, all of the following conditions are met:

-No mutations occur.-Individuals neither enter nor

leave the population through migration.-The population is large.

-Individuals mate randomly.-Natural selection does not occur.

If we use A and a as an example of alleles, the Hardy-Weinberg principle is described by the mathematical equation:

p represents the frequency of the dominant allele A

q represents the frequency of the recessive allele a

p2 represent the frequency of the homozygous dominant alleles: AA

2pq represent the frequency of the alleles heterozygous condition: Aa

q2 represent the frequency of the homozygous recessive pair of alleles: aa