Chapter 16

Objectives

• Identify traits that vary in populations and that may be studied.

• Explain the importance of the bell curve to population genetics.

• Compare three causes of genetic variation in a population.

• Calculate allele frequency and phenotype frequency.

• Explain Hardy-Weinberg genetic equilibrium.

Section 1 Genetic Equilibrium

Chapter 16

Variation of Traits Within a Population

• Population biologists study many different traits in populations, such as size and color.

• Population genetics – study of evolution from a genetic point of view

• For example: Studying dogwood trees in Middletown, Connecticut would be a way to describe a population

Section 1 Genetic Equilibrium

Chapter 16

Variation of Traits Within a Population, continued• Causes of Variation

– Traits vary and can be mapped along a bell curve, which shows that most individuals have average traits, whereas a few individuals have extreme traits.

– Variations in genotype arise by mutation, recombination, and the random pairing of gametes.

Section 1 Genetic Equilibrium

Chapter 16

The Gene Pool

• The total genetic information available in a population is called the gene pool.

Section 1 Genetic Equilibrium

Chapter 16

The Gene Pool, continued

• Allele frequency is determined by dividing the total number of a certain allele by the total number of alleles of all types in the population.

Section 1 Genetic Equilibrium

Chapter 16

The Gene Pool, continued

• Predicting Phenotype– Phenotype frequency is equal to the number of

individuals with a particular phenotype divided by the total number of individuals in the population.

• Allele frequencies in the gene pool do not change unless acted upon by certain forces.

Section 1 Genetic Equilibrium

Chapter 16

Phenotype Frequency

Section 1 Genetic Equilibrium

Chapter 16

The Hardy-Weinberg Genetic Equilibrium

• Hardy-Weinberg genetic equilibrium is a theoretical model of a population in which no evolution occurs and the gene pool of the population is stable.

• Certain conditions are needed: no mutations occur, the population is infinitely large, individuals neither leave nor enter the population

Section 1 Genetic Equilibrium

Chapter 16

Objectives

• Explain how migration can affect the genetics of equilibrium

• Explain how genetic drift can affect populations of different sizes.

Section 2 Disruption of Genetic Equilibrium

Chapter 16

Gene Flow

• Immigration – the movement of individuals into a population

• Emigration – movement of individuals out of a population

• If individuals are moving in or out a population the genetics of that population will change…if individuals move, genes move with them.

Section 2 Disruption of Genetic Equilibrium

Chapter 16

Genetic Drift

• Genetic Drift is the phenomenon by which allele frequencies in a population change as a result of random events, or chance.

• The larger the population the more stable the genetics will stay, the smaller the more things can change.

• Figure 16-5

Section 2 Disruption of Genetic Equilibrium

Chapter 16

Objectives

• Relate the biological species concept to the modern definition of species.

• Explain how the isolation of populations can lead to speciation.

• Compare two kinds of isolation and the pattern of speciation associated with each.

• Contrast the model of punctuated equilibrium with the model of gradual change.

Section 3 Formation of Species

Chapter 16

The Concept of Species

• Speciation - formation of new species as a result of evolution

• Morphology – study of the structure and form of an organism (this is the major way to classify organisms)

• Major limitations of the morphological concept:

• There may be a great deal of phenotypic variability in a species

• Organisms that actually can interbreed may have very different physical characteristics

• It does not consider whether individuals of a species can mate and produce viable offspring

Section 3 Formation of Species

Chapter 16

The Concept of Species

• According to the biological species concept, a species is a population of organisms that can successfully interbreed but cannot breed with other groups.

Section 3 Formation of Species

Chapter 16

Isolation and Speciation

• Geographic Isolation– Geographic isolation results from the separation

of population subgroups by geographic barriers– Ex. Canyon could form to divide a population

– Speciation can occur as a result of geographic isolation because populations that live in different environments may be exposed to different selection pressures

Section 3 Formation of Species

Chapter 16

Isolation and Speciation, continued

• Allopatric Speciation– Geographic isolation may lead to allopatric

speciation.– When this occurs new species arise as a result

of geographic isolation

Section 3 Formation of Species

Chapter 16

Isolation and Speciation, continued

• Reproductive Isolation– Reproductive isolation results from the

separation of population subgroups by barriers to successful breeding.

– Two types of reproductive isolation

– Prezytoic – occurs before fertilization

– Postzygotic – occurs after fertilization

– Reproductive isolation differs from geographic isolation in that members of the same species are not physically separated in repro. Isolation, whereas they are separated in geo. separation

Section 3 Formation of Species

Chapter 16

Isolation and Speciation, continued

• Sympatric Speciation– Sympatric speciation occurs when two

subpopulations become reproductively isolated within the same geographic area

– Ex. A population of insects might live on a single type of plant. If some of the individuals from this population began to live on another type of plant, they would no longer interbreed with the original population

Section 3 Formation of Species

Chapter 16

Rates of Speciation

• In the gradual model of speciation (gradualism), species undergo small changes at a constant rate.

• Under punctuated equilibrium, new species arise abruptly, differ greatly from their ancestors, and then change little over long periods.

Section 3 Formation of Species