chapter18

Mader: Biology 8th Ed.

Process of Evolution

Chapter 18


Evolution in a Genetic Context(Population Genetics)

• Microevolution– Change in gene frequency in a population

over timeGene pool = ALL alleles at ALL gene loci

in ALL individuals of the population.Allelic frequency = # specific alleles

total alleles in population Shown as fA or fa (f followed by subscript)

Indicates probability of allele


Microevolution

What could cause CHANGES in allele

frequencies?


Causes of Microevolution

• Genetic Mutations– Mutated alleles (or combinations of

alleles) may be more adaptive• Gene Flow

– Movement of alleles between populations by migration of breeding individuals.

Continual gene flow reduces variability (differences) between populations.


Gene Flow



• Nonrandom Mating– Individuals do not choose mates randomly.

Assortative mating - Individuals tend to mate with those with the same phenotype.

Sexual selection - Males compete for the right to reproduce and females choose to mate with males possessing a particular phenotype.



• Genetic Drift– Changes in allele

frequencies of a gene pool due to CHANCE.

EX: natural disaster, weather change

Larger effect in small populations.


Genetic Drift• Bottleneck Effect

– Event prevents majority of genotypes from entering the next generation


Genetic Drift• Founder Effect

– Subgroup starts new populationAlleles carried by population founders

are dictated by chanceFounding population does NOT reflect

original population


Founder effect

Sample of original population Descendants

Founding population

B

Founding population

A


Natural Selection

• Natural Selection results in adaptation of a population to the environment– Adaptation is result of new allele

frequencies


Hardy-Weinberg Principle

Hardy-Weinberg equilibrium = NO

microevolution

** Allele frequencies of gene pool will stay the SAME (equilibrium) IF



** Allele frequencies of gene pool will stay the same (equilibrium) IF

What could cause CHANGES in allele frequencies?



** Allele frequencies of gene pool will stay the same (equilibrium) IF– No Mutations– No Gene Flow– Random Mating– No Genetic Drift– No Selection


Hardy-Weinberg

• Under real conditions, these conditions are rarely, if ever, met, and allele frequencies in the gene pool of a population change between generations.– Evolution has occurred.


Hardy-Weinberg Math

• Consider a trait with 2 possible alleles…

• p = frequency of dominant allele– The probability that an allele chosen at

random is dominant• q = frequency of recessive allele

– The probability that an allele chosen at random is recessive

• p + q = 1; Why?


Hardy-Weinberg Math

• p = frequency of dominant allele– The probability that an allele chosen at

random is dominant• q = frequency of recessive allele

– The probability that an allele chosen at random is recessive

• p + q = 1; Why?• If there are only two allele possibilities, the

sum of their frequencies must be 1


Hardy-Weinberg Math

• What is the probability of an individual being homozygous dominant (AA) – the frequency of the AA genotype?

• What is the probability of an individual being homozygous recessive (aa)?


Hardy-Weinberg Math

• What is the probability of an individual being homozygous dominant (AA) – the frequency of the AA genotype?

• fAA = p x p = p2

• What is the probability of an individual being homozygous recessive (aa)?

• faa = q x q = q2


Hardy-Weinberg Math

• What is the probability of an individual being heterozygous (Aa) – the frequency of the Aa genotype?


Hardy-Weinberg Math

• What is the probability of an individual being heterozygous (Aa) – the frequency of the Aa genotype?

• fAa = (p x q) + (q x p) = 2pq

• There are 2 possible combinations: – allele 1 can be A and allele 2 can be a

OR – allele 1 can be a and allele 2 can be A


Hardy Weinberg Math

• Remember: p + q = 1• AND• (p + q)2 = p 2 + 2pq + q2 = 1

• q2 is usually known! (What does q2 refer to?)


H-W Practice: Practice Problems 18.1

1. In a certain population, 21% are homozygous dominant, 49% heterozygous, and 30% homozygous recessive. What percentage of the next generation is predicted to be homozygous dominant, assuming a Hardy-Weinberg equilibrium?




If it is in Hardy-Weinberg equilibrium, we would expect the same genotypic frequencies in the next generation.

21% homozygous dominant

Let’s do the math!




q2 = .30 so q = √.30 = 0.55

p + q = 1, so p = 1 – q = 1 – 0.55 = 0.45

p2 = (0.45)2 = 0.2025

Cheap example!



2. Of the members of a population of pea plants, 9% are short (recessive). What are the frequencies of the recessive allele t and the dominant allele T? What are the genotypic frequencies of the population?




q2 = .09 so q = √.09 = 0.30

ft = 0.3

p + q = 1, so p = 1 – q = 1 – 0.30 = 0.70

fT = 0.7




ft = 0.3 fT = 0.7

fTT = p2 = (0.7)2 = 0.49

fTt = 2pq = (2 x 0.3 x 0.7) = 0.42

ftt = q2 = (0.3)2 = 0.09

fTT = 0.49

fTt = 0.42

fTT = 0.09


Types of Selection

• Directional Selection– An extreme phenotype is favored and the

distribution curve shifts in that direction.Can occur when a population is

adapting to a changing environment.


Directional Selection


Types of Selection

• Stabilizing Selection– Occurs when an intermediate phenotype

is favored.Can improve adaptation of the

population to constant conditions.


Stabilizing Selection


Types of Selection

• Disruptive Selection– Two or more extreme phenotypes are

favored over any intermediate phenotype.Two distinctly different phenotypes are

found in the population.


Disruptive Selection


Maintenance of Variations

• Maintenance of variation is beneficial because populations with limited variation may not be able to adapt to new conditions.– Only exposed alleles are subject to natural

selection.Sickle-Cell Disease

Homozygote remains in equilibrium in some regions of Africa because the heterozygote is protected from sickle-cell and malaria.

• http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html

http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html


Speciation

• Speciation is the splitting of one species into two or more species, or the transformation of one species into a new species over time.– Species Definition

MorphologicalBiological

Reproductive IsolationPhylogenetic


Reproductive Isolating Mechanisms

• Prezygotic Isolating Mechanisms– Prevent reproduction attempts, and make

it unlikely fertilization will be successful.Habitat IsolationTemporal IsolationBehavioral IsolationMechanical IsolationGamete Isolation


Reproductive Isolating Mechanisms

• Postzygotic Isolating Mechanisms– Prevent hybrid offspring from developing

or breeding.Zygote MortalityHybrid SterilityF2 Fitness


Modes of Speciation

• Allopatric Speciation– Occurs when one population is

geographically isolated from other populations.

• Sympatric Speciation– A population develops into two or more

reproductively isolated groups without prior geographic isolation.

– Common in plants – polyploidy


Adaptive Radiation

• Adaptive Radiation is an example of allopatric speciation.– Many new species evolve from a single

ancestral species when members of the species become adapted to different environments.

chapter18

Education

probability of allele

dominant q

frequency of dominant

frequency of recessive

hardyweinberg math p

hardy weinberg math

recessive p

homozygous recessive