Download AP Biology Ch 21 Notes

Chapter 21
The Evolution of Populations
Individual organisms don’t evolve, populations do.
Darwin - lacked an understanding of inheritance that could explain natural selection
- it could explain how variations arise in a population and how they are passed on to offspring
population genetics
- emphasizes extensive genetic variation within populations
- recognizes importance of quantitative characters (Mendel recognized only “either-or”
traits)
modern synthesis
- comprehensive theory of evolution formed in the early 1940s
- combined discoveries and ideas from many different fields (paleontology, taxonomy,
biogeography, population genetics, etc.)
- emphasizes:
- populations as the units of evolution
- natural selection’s role as the most important mechanism of evolution
- the idea of gradualism
population
- localized group of individuals of the same species
- may be isolated from others of the same species, rarely exchanging genetic material
species - a group of populations whose individuals can interbreed and produce fertile offspring in nature
gene pool
-total collection of genes in a population at any one time
- all alleles at all gene loci in all individuals of a population
- if all members of a population are homozygous for the same allele, the allele is said to be fixed
in the gene pool
- usually, however, there are 2 or more alleles for a gene, each with a relative frequency in the
gene pool
calculating allele and genotype frequency
suppose:
wildflower population has 2 alleles for flower color (A = pink, a = white)
population size 500 (480 pink, 20 white)  320 AA, 160 Aa, 20 aa
In this population of 500, there are 1000 genes for flower color:
800 A’s  (320 x 2) + 160
200 a’s  160 + (20 x 2)
frequency of A allele = 800/1000 x 100 = 80%
frequency of a allele = 200/1000 x 100 = 20%
frequency of genotype AA = 320/500 x 100 = 64%
frequency of genotype Aa = 160/500 x 100 = 32%
frequency of genotype aa = 20/500 x 100 = 4%
genetic structure
evolution
- refers to a population’s allele frequencies and genotype frequencies
- change in allele frequencies over time
- frequencies are highly variable; subject to change as environment changes
Hardy-Weinberg theorem
- describes a nonevolving population
- named after 2 scientists who came up with principle independently (1908)
- states that the frequencies of alleles and genotypes in a population’s gene pool stay the same
over generations unless acted upon by agents other than sexual recombination
- shuffling of alleles by meiosis and recombination has no effect on genetic structure
Hardy-Weinberg equilibrium
– refers to a population that maintains the same allele and genotype frequencies
from generation to generation
Hardy-Weinberg equation
- allows us to calculate allele frequencies in a gene pool if we know the genotype
frequencies, and vice versa
first equation  p + q = 1
p - frequency of allele 1 (usually the dominant allele)
q - frequency of allele 2 (usually the recessive allele)
- the frequency of 2 alleles always adds up to 1 if the population is in Hardy-Weinberg equilibrium
- example:
if 60% of the alleles for a trait are dominant, then p = 0.6 and q (recessive allele) = 0.4
second equation  p2 + 2pq + q2 = 1
p2 – frequency of homozygous dominant condition (AA)
q2 – frequency of homozygous recessive condition (aa)
2pq – frequency of the heterozygous condition (Aa or aA)
- example:
A population of acacia trees is 16% short and 84% tall. Tall (A) is dominant to short (a)
What are the frequencies of the 2 alleles?
Determine the value of q (recessive) first  these have to be homozygous aa
(the tall ones could be homozygous AA or heterozygous Aa)
q2 = 0.16 so q = √0.16 or 0.40
p must be 0.60 since p + q = 1
What are the percentages of the homozygous dominant and heterozygous conditions?
(We know recessive condition is 16%  must be aa to express the trait)
Plug in what you know about p and q: p2 = (0.6)2 = 0.36  36%
2pq = 2(0.6)(0.4) = 0.48  48%
Check: 16% + 36% + 48% = 100%
Example:
Approximately 1 in 10,000 babies born in the US has phenylketonuria (PKU).
PKU is caused by a recessive allele. What % of the US population carries the allele?
q2 (frequency of homozygous recessive genotype) = 0.0001
so q = √0.0001 = 0.01
since p + q = 1, p = 1 – q  p = 1 – 0.01 = 0.99
frequency of carriers  2pq = 2(0.99)(0.01) = 0.0198 or ~ 2%
Hardy-Weinberg equation
- rarely applies to real populations
- used to determine whether a population is evolving or not (if allele
frequencies do not add up to 1)
microevolution - generation-to generation change in a population’s frequencies of alleles or genotypes
- a change in a population’s genetic structure
5 conditions necessary for Hardy-Weinberg equilibrium to be maintained in a population:
1. very large population size (no genetic drift)
2. isolation from other populations
3. no mutations
4. random mating
5. no natural selection
5 causes of microevolution:
1. genetic drift
2. gene flow (migration)
3. mutations
4. nonrandom mating
5. natural selection
genetic drift
- change in allele frequencies due to chance events
- can occur in small populations
- the smaller a sample, the greater the chance of deviations from the expected results
- the disproportion of results is called sampling error
- example:
flip a coin 10 times  7 heads & 3 tails is within reason
flip a coin 1000 times  700 heads & 300 tails would make you suspicious about the coin
bottleneck effect
- results when a population is drastically reduced due to a disaster (fire, flood,
earthquake, etc.)
- genetic makeup of the small surviving population is not likely to be
representative of the makeup of the original population
- genetic drift follows (reduces overall genetic variability)
founder effect
- genetic drift in a new colony (Darwin’s finches)
gene flow
- change in allele frequencies as genes from one population are incorporated into another
- sometimes known as migration
mutation
- change in an organism’s DNA
- always random with respect to which genes are affected
- a mutation transmitted in gametes can change the gene pool of a population by substituting 1
allele for another
- very rare
nonrandom mating
- individuals usually mate with close neighbors
- promotes inbreeding (mating between closely related partners)
other examples:
self-pollination
- extreme example
assertive mating
- individuals selects partners like themselves in
phenotype
natural selection
- only agent of microevolution to adapt a population to its environment
- based on 3 conditions:
variation
- differences in phenotypes must exist between individuals of a population
heritability
- parents must be able to pass on the traits
differential reproductive success
- there must be a variation in how many offspring
parents produce as a result of their different traits
Genetic Variation
- heritable variation
distinct characters
- “either-or”
- example:
pink vs. white flowers
- usually determined by a single gene locus
quantitative characters - most heritable variation
- vary along a continuum
- example:
height in humans
- usually polygenic inheritance
morph - contrasting forms of a character
polymorphic character - 2 or more distinct morphs are commonly found
polymorphism - existence of polymorphic characters
- example: human blood type has 4 morphs
geographic variation
cline
- difference in genetic structure between populations
- often due to natural selection
- type of geographic variation
- graded change in some trait along a geographic axis
Darwinian fitness
- relative contribution an individual makes to the gene pool of the next generation
sexual dimorphism
- distinction between males and females of a population
- examples:
difference in size (male usually larger)
colorful plumage in male birds
manes on male lions
antlers on male deer
- example: male with the most impressive features may be the most attractive to females
sexual selection