The Evolution of Populations

AP Chapter 23

What is microevolution?• Changes in the

allele frequencies of a population from generation to generation

Sources of genetic variation

• Mutations – more common in prokaryotes

• Sexual reproduction – more common in eukaryotes

• Can be discrete characters determined by a single gene locus

• Can be a quantitative character – varying along a continuum by more than one gene * most common in populations

Measures of genetic variability

• Average heterozygosity of a population

• Nucleotide variability

• Geographic variations (cline – graded variation in a character across a geographic area, may parallel an environmental gradient)

A cline

About mutations…

• Most occur in somatic cells• Point mutations that do not change the

aa or phenotype or harmless• Otherwise mostly harmful• Chromosomal mutations often

deleterious• Rates in animals and plants – 1/100,000

genes per generation• More rapid in prokaryotes and viruses

About variation in sexual reproduction

• Due to reshuffling of alleles in recombination, crossing-over, independent assortment of chromosomes during meiosis, random combination of gametes in fertilization

How to determine gene frequencies

• Hardy-Weinberg equation

• Gene pool – all alleles at all the loci present in a population

• If all individuals are homozygous for the same allele, the allele is fixed.

• Otherwise:

p = frequency of dominant alleles

q = frequency of recessive alleles

Therefore, p + q = 1

Hardy-Weinberg equation

• The gene pool of a population will remain constant (no evolving) from generation to generation IF

• No mutations

• Random mating

• Large population

• No natural selection

• No migration

• The probability that two gametes containing the same allele will come together is equal to (p + q)2.

• p2 + 2pq + q2 = 1

• P2 = homozygous dominants

• q2 = homozygous recessives

• 2pq = heterozygous ones

When doing problems:

• ALWAYS TAKE THE SQUARE ROOT OF THE PERCENTAGE OF RECESSIVE TRAITS FIRST

• That will equal “q”

• Subtract that from 1 to find “p”

• Plug in equation

How can allele frequencies be altered?

• Natural selection

• Genetic drift

• Gene flow - migration

Genetic Drift

• occurs in a small population when some members “drift” off and form a new colony – may not be representative of the original population

• Can lead to loss of alleles in a population or fixation of harmful alleles

Types of genetic drift

• Bottleneck Effect – some disaster affects the population

• Founder Effect – some members drift off voluntarily

Breast cancer in the Jewish women

• A disproportionate number of Jewish women have the BRCA1 and BRCA2 mutation: Where the odds in the general population are 1 in 450, for Jewish women, the likelihood that they have a mutation is 1 in 40.

• Geneticists attribute this to the founder effect, a theory suggesting that genes in certain isolated communities-Iceland and Finland are others-can be traced back to a small number of "founders" who marry only within the group. Intermarriage normally gets rid of unhealthy genetic mutations, since only the children who inherit the healthy genes survive. When the founders only marry each other, though, those unhealthy genes stick around, For Ashkenazi Jews, the founders were a few thousand people who lived in Eastern Europe 500 years ago.

A result of genetic drift

The Founder Effect in Action: Among the Amish, babies with Ellis-Van Creveld Syndrome are born with six fingers

Natural Selection

• Relative fitness – measure of an individual’s contribution to the gene pool

• Acts on the phenotype

Types of natural selection

• Directional selection – individuals on one end of a phenotypic range are favored

• Disruptive Selection – when environment selects individuals on both extremes

• Stabilizing Selection – favors more intermediate forms, tending to reduce phenotypic variation

Sexual selection

• Selection for a trait that enhances mating

• Can lead to sexual dimorphism (distinction of males and females by secondary sexual characteristics)

• Intrasexual selection – same sex competing for mates

• Intersexual selection – mate choice, females choose “sexier” male

Type of sexual selection?

Type of sexual selection?

Preservation of Genetic Variety

• Diploidy

• Balancing selection – maintaining two or more phenotypes in the population

• Heterozygote advantage

• Frequency-dependent selection – phenotype’s reproductive success declines if too many in the population

Balancing selection

A case of frequency-dependent selection

There’s too many

of us already!

Neutral variations

• Do not confer a selective advantage or disadvantage

• Ex in noncoding regions of DNA

Natural selection does not result in perfect organisms!

• Can act on variations in the population

• Many structures coopted for new situations

• Often many compromises

• Also chance events affects population’s evo history.