Remainder of Chapter 23

• Read the remaining materials; they address information specific to understanding evolution (e.g., variation and nature of changes)

• Always read the Featured Investigation and Genomes and Proteomes sections of each chapter (demonstrate tie organism and molecular levels of hierarchy

Gene - specific location of the genetic information for a given trait

Allele - The actual chemical composition of a gene. Determines how characteristic/ trait is expressed.

Polymorphism – two or more forms present

Allele Frequency - The frequency of occurrence of alleles in a population.

Genotypic Frequency - frequency of occurrence of genotypes in a population.

Population – group of individuals of the same species that live in the same area (can interbreed if reproduce sexually).

Gene Pool – All of the genes (more accurately all of the alleles) present in a population.

• Genotype - specific chemical composition of alleles defining a trait.– AA Homozygous Dominant– Aa Heterozygous– aa Homozygous Recessive

• Phenotype - physical expression of a trait– If the alleles for a trait are simple dominant and recessive,

then:• For AA and Aa, dominant trait is physically expressed• If aa, recessive trait is expressed

Evolution

• Is a genetic change in a population (not an individual) over time

• Scientists look at phenotypic (physical changes), in most cases, because that is how we recognize populations.

• It is, however, changes in the genotype, or more specifically, the gene pool.

Allele Frequencies

The frequency of occurrence of alleles in a population.

If we use the simple one dominant and one recessive allele model, this can be demonstrated by:

p = frequency of the dominant allele

q = frequency of the recessive allele

Example

AA - 30 individuals

Aa - 20 individuals

aa - 50 individuals

p = 2(# individuals AA) + # individuals Aa

2(Total # individuals in population)

p + q = 1; therefore q = 1 - p

Example

p = 2(30) + 20

2(100)

= 0.4

p + q = 1; therefore q = 1 - 0.4 = 0.6

With these values, we can calculate the probability of what genotypes would be present in the next generation if this population were to mate randomly

Genotypic Frequencies

p2 = probability of AA

q2 = probability of aa

2pq = probability of Aa

p2 + 2pq + q2 = 1

Mechanisms for Evolutionary Change

Mutation

Genetic Drift (small population size)

Gene Flow (immigration and emigration)

Non-Random Mating

Natural Selection

Hardy-Weinberg Equilibrium

In diploid, sexually reproducing organisms, phenotypes, genotypes and genes all tend to come to equilibrium in

populations in certain conditions are met

Hardy-Weinberg Equilibrium

No Mutation

Large Population Size

No immigration or emigration

Random Mating

No Selection for Traits

Hardy-Weinberg Equilibrium

Provides a means of experimentally demonstrating what happens to

populations in the absence of evolution.

How Natural Selection Works

• Variation occurs in every group of living organisms. Individuals are not identical in any population.

• Every population produces an excess of offspring.

• Competition will occur among these offspring for the resources they need to live.

How Natural Selection Works

• The offspring best adapted to survive and acquire resources will survive.

• If the characteristics of the most fit organisms are inherited, these traits will be passed on to the next generation.

Darwinian Fitness

Relative contribution an individual makes to the gene pool of the next

generation

Natural Selection

• The most fit genotypes will be more strongly represented in subsequent generations

• Less fit genotypes will remain in the population, but at low numbers

• If environmental conditions change, fitness will change

Figure 21.12

Figure 21.12

figure 21-12.jpg

*Disruptive selection also referred to as balancing selection

*

Stabilizing Selection

Figure 21.13

DirectionalSelection

Disruptive or balancingSelection

Example of maintainingBalancing selection – heterozygote advantage

Fig. 24.5a – Disruptive Selection

Maintenance of Variation

• Less fit alleles not completely eliminated

• Still reproduce, but do not produce as many offspring

• Also interbreed with more fit individuals

Properties of Fitness

• Fitness is a property of a genotype, not an individual or population.

• Fitness is specific to a particular environment. As the environment changes, so does the fitness of genotypes.

• Fitness is measured over one generation or more.

Sexual Selection

• Traits that infer greater fitness

• Sexual dimorphism– Secondary sex

characteristics• Intrasexual• Intersexual• Featured

Investigation

22.16 The Longer the Tail, the Better the Male (Part 1)

Genetic Drift

• Random events – missed opportunity, disturbance

Fixed A

llele

Bottle

neck

Fig. 24.10

Gene Flow

• Changes in gene pool resulting from immigration or emigration

• random

• Founder effect

Founder Effect – colonizersestablish genetic make up ofnew population

Mutation

• Changes in chemical composition of a gene

• Random

• Only evolutionary mechanism where new alleles can be added

• Most mutations are deleterious

• Neutral mutations add variation without changing phenotype

Nonrandom Mating

• Mating due to some attribute – Sexual selection

– Similar phenotype