1. Evolutionary Change without Selection
Objectives:
1. Explain how the genetic make up of a population can change by chance
2. Explain how ‘Genetic Drift’ can lead to evolutionary changes
3. Describe how the ‘Bottleneck and Founder Effects’ can lead to evolutionary changes.
4. Demonstrate how the Hardy-Weinberg Equation can be used to calculate allele frequencies in a population.

2. Quiz Name two things that Darwin observed as evidence for evolution.
What is the difference between and analogous structure and a homologous structure?
What is the difference between artificial and natural selection?
Genetic variation allows some organisms to have an advantage over others. What drives natural selection?
What is the phrase ‘survival of the fittest’ not an accurate way to describe natural selection?

3. Agents of Evolutionary Change
Changes in allele frequencies cause evolutionary change:
1. Natural Selection – produces adaptive evolutionary change
2. Mutation – only source for genetic change; beneficial mutations occur rarely
3. Migration
4. Genetic Drift
5. Non-Random Mating
6. Horizontal Gene Transfer
7. Human Influence

4. 2. Mutations Evolution requires genetic variation/diversity.
New genes being introduced into a population results in an increase in the genetic diversity of that population.
Mutations: A source of genetic variation
(Introduces new genes/alleles into a population)
Light and Dark Moths (mutation = dark allele for wing colour)
Dark Moths increased in frequency during industrial revolution because of dark soot in air
Dark Moths could hide from birds

5. 3. Migration
This is a change in allele frequency that doesn’t involve natural selection
Migration is the movement of alleles out of a population, reducing the population’s genetic variation
Alleles are moving into a population as other populations migrate in, bringing new alleles, and hence, diversity with them

6. Gene Flow The transfer of alleles from one population to another.
May be caused by migration (A bird with a particular mutation migrates into a population of other birds and mates. That mutation will be passed on and increase the genetic diversity of that population.)
An animal of a population that contains a different allele leaves a population or dies, that allele leaves the population and the genetic diversity decreases.)

7. 4. Genetic Drift
…changes to the allele frequency of a population as a result of chance.
Genetic makeup changes by chance
Mutation – causes variation in allele
The chance of passing on this is the same as the chance of passing on any other allele (random)
If this allele is passed on, it becomes more common.
If it is not passed on, it disappears
Genetic drift can cause dramatic changes in allele frequency in smaller population.

8. A. Bottleneck Effect
Genetic diversity is lost as a result of a drastic reduction in population size.
Alleles are lost
If population grows in size again, their genetic diversity will be limited.
Ie: Cheetahs
This population may be subject to
genetic drift that could create
differences in their gene pool that
weren’t present or as prominent in the
original population.

9. B. Founder Effect (Type of Genetic Drift)
Individuals of a species (founding population) start a new population somewhere else.
I.e.: Finches from S. Amer. to Galapagos
Some of these individuals may have a rare or uncommon allele.
The founding population may not contain an allele that was common in the original population

10. Consequence of Founder Effect
Amish woman holding her child with Ellis-van Crevold syndrome
Short limbs, extra fingers, dwarfed stature
Disorder originated from two founders who immigrated to Pennsylvania in 1774

11. 5. Non-random Mating:
individuals select mates based on certain phenotypes (choosing mates is not random)
ex: male caribou use their antlers to spar to compete for mates, those who win, are able to pass on their genes to the next generation
Results in:
Inbreeding - when closely related individuals breed together
results in faster change
can pose a risk as harmful recessive alleles are more likely expressed (some breeds of dog)
self fertilization in plants is an extreme example of inbreeding and has resulted in a domination

12. 5. Non-random Mating: Results in:
Assortative mating – individuals select partners that are like themselves in certain phenotypic character
Ex. Some toads only mate with toads that are the same size as themselves
This increases the number of homozygous genes in a population overtime which thus alters the genotype
Problematic when homozygous recessive, potentially having a genetic disorder; e.g., Tay-Sachs
The brutal truth eliminates all but the most favorable gene combinations. This is why Darwin referred to nature as:
“Red in Tooth and Claw”

13. 6. Horizontal Gene Transfer
Individual obtains DNA from another organism (e.g., bacteria) and then exhibit characteristics of that organism
New alleles are gained

14. 7. Human Influence
Humans impact all life on earth, creating evolutionary change in species
Fishing/Hunting
Habitat Loss
Climate Change
Insecticides/Herbicides

15. Calculating Genetic Change
A method of calculating the allele frequency in a large population (random mating and no external forces that could change the allele frequencies)
Shows change in allele frequencies over time
Independently developed in 1908 by G.H. Hardy (English mathematician) and G. Weinberg (German physician)

16. Hardy and Weinberg

17. Hardy-Weinberg Principle
The proportions of alleles in the gene pool remain constant from generation to generation, in populations not evolving
Dominant alleles do not replace recessive one because their proportions don’t change
(p q)2 = p2 + 2pq + q2
(Dominant + Recessive)
 Hardy-Weinberg equation

18. Problem
a) A population of trees is composed of 80% tall trees and 20% short trees. What is the frequency of each of the genotypes if tall (T) is the dominant trait and short (t) is the recessive trait, and what is the frequency of each allele?

19. Problem
b) There have been 10 generations of wind storms wreaking havoc on the tall trees. The population of tall trees is now at 75% and the short trees is at 25%. How did the alleles in the population change?

20. Plenary
Explain how genetic drift can lead to evolutionary change in the absence of natural selection.
Describe what a genetic bottle neck is.
Explain how the founder effect is an example of genetic drift.
Give an example of a founder population.
5. Explain why the mechanisms we discussed today tend to have a bigger impact on smaller populations?

21. H/W
Read pp
Answer # 1-11