Chapter 16: Evolution of Populations Students know both genetic variation and environmental factors are causes of evolution and diversity or organisms. Students know new mutations are constantly being generated in a gene pool.

Content Objectives:  Describe the main sources of heritable variation in a population?  Explain how is evolution defined in genetic terms.  Explain what determines the number of phenotypes for a given trait.  Describe the factors involved in the formation of new species?

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

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

Allele: one form of a gene

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

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

Sources of Variation 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. Explain why this is true. Similar to a deck of cards – no matter how many times you shuffle, same cards (alleles) are always there.

Gene Traits: A) Single gene trait: controlled by single gene with two alleles ♦Examples: widow’s peak, hitchhiker’s thumb, tongue rolling

Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits? Explain. 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 4feet to 9 feet all

Natural selection acts on phenotypes, not genotypes. 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.

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

a. Directional Selection: individuals at one end of the curve have higher fitness so evolution causes increase in individuals with that trait ♦Individuals with highest fitness: those at one end of the curve ♦Example: Galapagos finches – beak size Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness

b. Stabilizing Selection: individuals at the center of the curve have highest fitness; evolution keeps center in the same position but narrows the curve 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 Individuals with highest fitness: near the center of the curve (average phenotype) Example: human birth weight

c. Disruptive Selection: individuals at both ends of the curve survive better than the middle of the curve. ♦Individuals with highest fitness: both ends of curve ♦Example: birds where seeds are either large or small 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

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.

Three Isolating Mechanisms : Isolate species forming subspecies and perhaps causing speciation. 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 matting 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 January

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

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

Genetic Drift  random change in allele frequency that occurs in small populations

The results of genetic crosses can usually be predicted using the laws of probability. In small populations, however, these predictions are not always accurate. a. Founder effect: allele frequencies change due to migration of a small subgroup of a population ♦Example: fruit flies on Hawaiian islands

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

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

major change in allele frequencies when population decreases dramatically due to catastrophe ♦Example: northern elephant seals decreased to 20 individuals in 1800’s, now 30,000 no genetic variation in 24 genes 2. Bottleneck effect

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

Bottleneck Effect Original population Catastrophe Surviving population

Another picture to illustrate bottleneck effect