AP Biology – Ch. 23 – The Evolution of Populations

The smallest unit of evolution? Microevolution 3 main mechanisms responsible for change in

allele frequency Which one of these consistently improves the

match between organisms and their environment?

Microevolution

Microevolution

Microevolution

Microevolution

Five Fingers of Evolution

https://www.youtube.com/watch?feature=player_embedded&v=5NdMnlt2keE

23.1

How do the variations that are the raw material for evolutionary change arise?

Mutation and sexual reproduction

Genetic Variation

A) Variation Within a Population

Discrete or quantitative Either-or-basis vs on a continuum Single gene vs two or more genes

Measuring Genetic Variation

Gene variability vs nucleotide variability

Average heterozygosity – gel electrophoresis, restriction fragment analysis

Nucleotide variability – compare DNA sequences

Why is this information important?

Variation between populations

Geographic variation-differences in the genetic composition of separate populations

Ex) mice populations separated by a mountain range, mummichog fish and a cold-adaptive allele(cline)

Mutation

The ultimate source of new alleles Change in the nucleotide sequence

of an organism’s DNA Point mutation Alter gene number or sequence

Mutation rates – low in plants and animals

- even lower in prokaryotes and viruses(but shorter generation time)

- RNA viruses – mutations accumulate faster – how does this impact treatment?

Sexual Reproduction

variation results from the unique combination of alleles that each individual receives due to:

- crossing over - independent assortment - fertilization

23.2 – Hardy-Weinberg

-population-gene pool/fixed allele-conditions for HW equilibrium? 1. no mutations 2. random mating 3. no natural selection 4. extremely large population 5. no gene flow

23.3

Natural selection, genetic drift, and gene flow – alter allele frequencies directly

1. natural selection – favoring some alleles over others can result in adaptive evolution

2. genetic drift – chance events causing allele frequencies to fluctuate

a. founder effect – Ex) Tristan da Cunha b. bottleneck effect – a severe drop in

population size due to a change in the environment

- case study – prairie chickens

Genetic drift: A Summary

1. significant in small populations2. can cause allele frequencies to

change at random3. can lead to loss of genetic

variation within populations4. can cause harmful alleles to

become fixed

Gene Flow

the transfer of alleles into or out of a population due to movement of fertile individuals or gametes

serves to reduce genetic differences between populations

Fig. 23.12 – Agrotis tenuis – What if?

Modes of selection

http://wps.pearsoncustom.com/wps/media/objects/3014/3087289/Web_Tutorials/17_A02.swf

Sexual Selection

A form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates

http://evolution.berkeley.edu/evosite/evo101/IIIE3Sexualselection.shtml

http://www.pbs.org/wgbh/evolution/library/01/6/l_016_09.html

The Preservation of Genetic Variation

1. Diploidy 2. Balancing Selection a. Heterozygote Advantage b. Frequency-dependent

selection 3. Neutral Variation

Frequency-Dependent Selection

Situation 1 – Butterfly mimics poisonous butterfly Situation 2 - Poisonous butterfly has several morphs Situation 1 – The fitness of the mimics is

positively/negatively frequency-dependent – The fitness of a genotype increases as it becomes more/less frequent.

Situation 2 – The fitness of each morph is positively/negatively frequency-dependent – Each morph gets fitter as it becomes more/less common.

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