Evolution of Populations

Vocabulary 1) Allele 2) Gene Pool 3) Genotype 4) Mutation 5) Phenotype 6) Polygenic Trait 7) Population

1. When Darwin developed his theory of evolution, he did not understand: how heredity worked. This left him unable to explain two things: a. source of variation b. how inheritable traits pass from one generation to the next

In the 1940’s, Mendel’s work on genetics was “rediscovered” and scientists began to combine the ideas of many branches of biology to develop a modern theory of evolution. When studying evolution today, biologists often focus on a particular population. This evolution of populations is called microevolution.

2. Vocabulary: population: group of individuals of the same species living in the same area that breed with each other.

2. gene pool: combined genetic info. for all members of a population

2. allele: one form of a gene

2. relative frequency of an allele: # times an allele occurs in the gene pool compared to other alleles (percent) Example Relative Frequency: 70% Allele B 30% Allele b

A population of cows has a codominant gene for fur color. RR is Red, RW is ‘roan’, and WW is white cows. The population has 140 Red cows, 220 roan and 50 all white cows.  What is the total number of cows in the population?  What is the total number of cows color alleles in the population?  What is the total number of R alleles?  What is the total number of W alleles?  What is the allele frequency (percent) of ‘R’ gene?  What is the allele frequency (percent) of ‘W’ gene?  (Yes or No) If after ten generations the R gene frequency is at 5% and the W gene is 95%: has microevolution occurred?

3. Sources of Variation: a. mutations: any change in DNA sequence ♦ Can occur because of: ♦ mistakes in replication ♦ environmental chemicals ♦ May or may not affect an organism’s phenotype

b. Gene Shuffling: recombination of genes that occurs during production of gametes ♦ Cause most inheritable differences between relatives ♦ Occurs during meiosis ♦ As a result, sexual reproduction is a major source of variation in organisms. ♦ Despite gene shuffling, the frequency of alleles does not change in a population, unless evolution is occuring

4.. Single gene trait: controlled by single gene with two alleles ♦ Examples: widow’s peak, hitchhiker’s thumb, tongue rolling

4. Polygenic trait: controlled by 2 or more genes, each with 2 or more alleles ♦ Examples: height, hair color, skin color, eye color Most human traits are polygenic.

Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits? type: single gene why? Only two phenotypes possible Example: tongue roller or non-tongue roller type: polygenic why? Multiple (many) phenotypes possible Example: height range 4 feet to 9 feet tall

5. Natural selection acts on phenotypes, not genotypes. Example: in a forest covered in brown leaves, dirt and rocks which mouse will survive better brown or white? Brown, more hidden.

5. If brown is dominant can the a predator tell the difference between: Mouse with highest fitness will have the most alleles passed on to the next generation. BB Bb ?

5. Which mouse will have the lowest fitness? Will the fitness of BB and Bb differ? Why? No, Both BB and Bb have the same fitness advantage of being brown BB Bb ?

6. Three ways in which natural selection affects polygenic traits.

Directional Selection Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness Directional Selection individuals at one end of the curve have higher fitness so evolution causes increase in individuals with that trait

Key Percentage of Population Birth Weight Selection against both extremes keep curve narrow and in same place. Low mortality, high fitness High mortality, low fitness Stabilizing Selection

22 lbs 8 oz baby born Indonesia  Smallest baby: 9.2 oz

Disruptive Selection Largest and smallest seeds become more common. Number of Birds in Population Beak Size Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Key Low mortality, high fitness High mortality, low fitness

Name the type of selection  Orangutans live in the trees of the Sumatran and Bornean rainforests. If they are too heavy, the branches can’t support their weight and they can’t get the food high up in the trees, if they are too small, they are easy prey for predators. Size of orangutans is a result of what type of selection?

Name the Type of Selection  There were rocks covering an island with predominantly gray lizards living on them, but there were a few lighter in color and a few darker. There was a volcanic explosion on part of the island. What type of selection on the surviving lizards after the explosion

Name the type of selection  Suppose there is a population of rabbits. The color of the rabbits is polygenic and ranges from white to black. If this population of rabbits occurred in an environment that had areas of black rocks as well as areas of white rocks, what type of selection would be exhibited?

Genetic Drift  In small populations, an allele can become more or less common simple by chance rather than because natural selection is acting on specific variations

Two phenomena that result in small populations and cause genetic drift 1. Founder Effect 2. Bottleneck Effect

Founder effect allele frequencies change due to migration of a small subgroup of a population

Founder effect  When a new population is started by only a small group of individuals  just by chance some rare alleles may be at high frequency; others may be missing  skew the gene pool of new population  human populations that started from small group of colonists  example: colonization of New World albino deer Seneca Army Depot Polydactyly in Amish populations

Sample of Original Population Founding Population A Founding Population B Descendants Fruit Flies on Hawaiian islands Founder Effect: : Fruit Flies on Hawaiian islands

2. Bottleneck effect major change in allele frequencies when population decreases dramatically due to catastrophe

Bottleneck effect

Bottleneck Effect: Northern Elephant Seal Population ♦ Hunted to near extinction ♦ Population decreased to 20 individuals in 1800’s, those 20 repopulated so today’s population is ~30,000 ♦ No genetic variation in 24 genes

3. Hardy-Weinberg principle: a.k.a: because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium:

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change a.k.a: because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium:

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change a.k.a: genetic equilibrium because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium:

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change a.k.a: genetic equilibrium because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium: a. Random mating b. c. d. e.

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change a.k.a: genetic equilibrium because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium: a. Random mating b. Large population c. No migration d. e.

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change a.k.a: genetic equilibrium because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium: a. Random mating b. Large population c. No migration d. No mutation e.

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change a.k.a: genetic equilibrium because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium: a. Random mating b. Large population c. No migration d. No mutation e. No natural selection

The Process of Speciation

Vocabulary 8) Equilibrium 9) Isolation 10) Probability 11) Species 12) Subspecies 13) Temporal

The Process of Speciation The formation of new biological species, usually by the division of a single species into two or more genetically distinct one. Species: Group of organisms that are able to reproduce with one another and produce viable offspring

How does speciation occur? First: Reproductive Isolation  Definition: Separation of populations so that they cannot interbreed and produce fertile offspring  What does this mean? The populations now have separate gene pools and respond to natural selection and genetic drift as separate units

Three Isolating Mechanisms : The gene pools must become separate so they can’t keep spreading similar alleles through population. Species can be isolated, forming subspecies and perhaps causing speciation by: 1. Geographic Isolation 2. Behavioral Isolation 3. Temporal Isolation

1. Geographic Isolation Two populations separated by a geographic barrier; river, lake, canyon, mountain range.

Example: 10,000 years ago the Colorado River separated two squirrel populations. Kaibab Squirrel Abert Squirrel

This resulted in a subspecies, but did not result in speciation because the two can still mate if brought together  Kaibab Squirrel Abert Squirrel

2. Behavioral Isolation  Two populations are capable of interbreeding but do not interbreed because they have different ‘courtship rituals’ or other lifestyle habits that differ.

Example: Eastern and Western Meadowlark Eastern and Western Meadowlark populations overlap in the middle of the US

Example: Eastern and Western Meadowlark Male birds sing a mating song that females like, East and West have different songs. Females only respond to their subspecies song.

3. Temporal Isolation Populations reproduce at different times

Example: Northern Leopard Frog & North American Bullfrog  Mates in: Mates in: April July

Conclusion:  Geographic, Behavioral and Temporal Isolation all lead to speciation.

However:  No examples ever observed in animals  A couple examples that may demonstrate speciation exist in plants and some insects.