Ch. 23: The Evolution of Populations

I. IntroA. It is not individuals that evolve. It is the

population.B.Evolution on the smallest scale, called

microevolution is defined as a change in allele frequencies in a population.

Ex. Predators preying on snails with a certain shell pattern could change allele frequency.

II. Population Genetics: The study of genetic changes in a population.

A.Population:

B.Species:

C.Gene Pool:

1.Example: Flower population R = Red gene, r = white gene In an imaginary population of flowers:320 RR, 160 Rr, 20 rr

What is the frequency of the R allele in this population?

E.The Hardy-Weinberg Theorem:

D.Modern Synthesis: a comprehensive theoryof evolution that integrates paleontolgy, taxonomy, biogeography, and populationgenetics.

1.The Hardy-Weinberg Equation:

p2 + 2pq + q2 = 1

We can use the Hardy-Weinberg equation toestimate the percentage of human pop. That carries the allele for a particular inherited disease. Example: PKU

p = normal dominant alleleq = PKU recessive alleleGenotype of individuals with PKU = q2

There is one PKU birth per 10,000 birth.

2.The FIVE ASSUMPTIONS of the Hardy-Weinberg Theorem (if a population is to stay in Hardy-Weinberg equilibrium):

a.Very large pop. Sizeb.No migrationc.No net mutationd.Mating is randome.No natural selectionNo natural population is in Hardy- Weinberg equilibrium.

III.Microevolution: A change in the gene pool of a population over a succession of generations

A.Two main causes of microevolution:1. Genetic Drift 2. Natural Selection

B.Genetic Drift:

Two types of genetic drift:1.Bottleneck Effect:

a.By chance (natural disaster), some alleles can be eliminated or overlyrepresented.

-Bottleneck effect can drastically reduce variability and adaptability.

2.Founder Effect:

a.The gene pool of the newly developingpop. is limited to the alleles of the fewstarting individuals. b.An example of the founder effect is the

high frequency of certain inherited disorders among human pop.

C.Natural Selection: Some individuals in apopulation will leave more offspring than others. Some alleles are passed along to the next generation in disproportionate numbersto their frequencies in the present generation.

Violation of Hardy-Weinberg equillibrium.

D.Gene Flow can also alter allele frequenciesin a population.

1.Gene flow is genetic exchange due to migration of fertile individuals or gametes between populations.2.Gene flow tends to reduce differences

between populations.E.A mutation is a change in an organism’s DNA.

Mutations are the original source of geneticvariation; they are the raw materials fornatural selection.

IV.Genetic Variation is the raw material for natural selection.A.Genetic variation occurs within and between

populations.1.Individual variation occurs in populations

of all species of sexually reproducingorganisms.Variations you can seeVariations you can’t see (blood type)

2.Variations within populations are deter-mined by a combination of inheritable and non-heritable traits.

Heritable = GenesNon-heritable = Environment

These map butterflies are genetic identicalfor wing coloration. However, the differentphenotypes are due to seasonal hormones.

Spring

Late summer

3.There are two types of variations:a. Discrete characters:

b. Quantitative characters:

4.Polymorphism:

When there are two or more forms of a trait. Example: Red or white flowers 5.Measuring genetic variation:

a. Gene diversity:

b.Nucleotide diversity:

6.Geographic variations are differences in gene pools between populations or subgroups of populations that inhabit different areas.

a.A cline is a gradual change in a trait along a geographic axis.Ex. The average body size of many North American species of birds and mammals increases gradually with increasing latitude.

Yarrow plants is another example of acline. The environment affects growth rates directly, but some of the variationhas a genetic basis.

b.Unlike a cline, islands have isolated populations that demonstrate discrete differences.

Ex. When settlers came to the island ofMadeira, they broughtalong mice. Populationsof mice were separatedby mountains. Now youcan see distinct difference in karyotypesbetween populations. Genetic Drift

B.Mutations generate genetic variation.1.New alleles only originate by mutation.

2.Only mutations in cell lines that produce gametes are passed along to offspring.

3.As we have learned before, most pointmutations do not do any harm as most eukaryotic DNA do not code for any protein.

a.Exception:

4.A mutation changes the shape of a protein.a. A change is unlikely to improve the

genome.

5.Duplication of chromosome segments arenearly always harmful. However, if it is not harmful and persists, it can provide extra genome and may eventually take onnew functions by mutation.

6.Microorganisms have very short generation times and generate genetic variation rapidly.

a.In an AIDS patient, HIV generates 1010 new viruses per day. Each time it replicates, it provides another chance for errors. -Patients must receive a drug “cocktail” which is a combination of several drugs to treat the changing viruses.

C.Sexual reproduction also generates geneticvariation.

D.Diploidy and balanced polymorphism preserve variation.1.Diploidy prevents the elimination of

recessive alleles.

E.Natural selection maintains stable frequencies of two or more phenotypic formsin a population. This is called balanced polymorphism. Balanced polymorphism is preserved by twomechanisms:

1.Heterozygote advantage:

2.Frequency-dependent selection:

F.Neutral variations are types of genetic variations that have little or no impact on reproductive success.Example: human fingerprints

V. A closer look at natural selection as the mechanism of adaptive evolution:

A.Darwinian fitness is the contribution an individual makes to the gene pool of the next generation relative to the contributionsof other individuals.

B.Relative fitness is the contribution of a genotype to the next generation comparedto the contribution of alternative genotypes. 1.Example: If RR and Rr plants have red

flowers and produce more offspring thanrr white flowers, the relative fitness of RR and Rr is set as 1.

If plants with white flowers is only 80% asmany offspring, their relative fitness is .8.

C. Three modes of Natural Selection:1.Directional Selection:

a.An example of directional selection:Peppered Moth

Two variants: Dark and light

# of

Mot

hs

As pollution increased, the darker moth population increased.

Light Dark

# of

mot

hs

Light DarkAs pollution decreased, the light mothpopulation increased.

2.Diversifying Selection (or DisruptiveSelection):

a.An example of diversifying selection: Coho Salmon Male salmon will fight other males to get closest to a nest of eggs where a female has laid her eggs. The largest and smallest size males will get to the nests first.

# of

salm

on th

atRe

prod

uce

succ

essf

ully

Small Average Large

3.Stabilizing Selection:

D.Natural selection maintains sexual reproduction even though sexual reproductionis inferior to asexual reproduction (reproductively speaking).

Sex has a two-fold disadvantage.

E.Sexual Selection leads to pronounced secondary differences between the sexes.

1.Sexual dimorphism:

Male ProboscisMonkey

Female ProboscisMonkey

Elephant Seal

F.Intrasexual selection:

Male elephant sealssparring

1.Usually physical2.Strongest gains status3.Intersexual selection:

When mate choice occurs.Females usually chose themost masculine male which is a sign of health.

G.Natural selection does not produce perfectorganisms for 4 reasons: 1.Evolution is limited by historical constraints.2.Adaptations are often compromises.3.Not all evolution is adaptive.4.Selection can only edit existing variations.