Chapter 18 & 19

Evolutionary Change in Populations & Speciation

Ch. 18: Population Genetics

Evolution occurs in populations, not individuals

Evolutionary change is inherited from one generation to the next.

Darwin recognized that evolution occurs in populations, but did not understand how traits are passed on

Populations Population = all the individuals of the

same species that live in a particular place at the same time

Members of a population vary in many traits. Ex. p.390

Some variation is due to environment and some is due to heredity.

Population Genetics The study of genetic variability within a

population and of the forces that act on it. We will discuss the 5 factors responsible for

evolutionary change: Nonrandom mating Mutation Genetic Drift Gene Flow Natural Selection

Then, we consider genetic variation as the raw material for evolution.

Calculating Frequencies

Each population has a gene pool: All the alleles for all the genes present in

the population Genotype frequency: the proportion of

a particular genotype in the population Phenotype frequency: the proportion of

a particular phenotype in a population Allele frequency: the proportion of a

specific allele in a population

Hardy-Weinberg Principle

This law states an equilibrium of allele frequencies in a gene pool (using a formula p2 + 2pq + q2=1) remains in effect in each succeeding generation of a sexually reproducing population if five conditions are met.

5 conditions: No mutation: no allelic changes occur. No gene flow: migration of alleles into or out

of the population does not occur. Random mating: individuals pair by chance

and not according to their genotypes or phenotypes.

No genetic drift: the population is large so changes in allele frequencies due to chance are insignificant.

No selection: no selective force favors one genotype over another.

Hardy-Weinberg Principle

These conditions are rarely met, so allele frequencies in the gene pool of a population do change from one generation to the next, resulting in evolution.

Hardy-Weinberg Principle

p2 + 2pq + q2 = 1 p2 = Frequency of AA 2pq = Frequency of Aa q2 = Frequency of aa 1 = all the individuals in a population

Always begin by determining the frequency of the homozygous recessive genotype.

Microevolution Evolution within a population

Small changes over a few generations Result from 5 microevolutionary

processes:1. nonrandom mating 2. mutation3. genetic drift4. gene flow 5. natural selection

Nonrandom Mating

1. Random mating involves individuals pairing by chance, not according to genotype or phenotype.

2. Nonrandom mating involves inbreeding and assortative mating.

3. Inbreeding is mating between relatives to a greater extent than by chance.a. However, inbreeding decreases the proportion of heterozygotes.b. In human populations, inbreeding increases the frequency of recessive abnormalities.

Nonrandom Mating

4. Assortative mating occurs when individuals mate with those that have the same phenotype.

5. Sexual selection occurs when males compete for the right to reproduce and the female selects males of a particular phenotype. (guppies, lions)

Mutation Many traits in organisms are polymorphic, i.e.,

two or more distinct phenotypes are present in the population due to mutated genes.

Analysis of Drosophila enzymes indicates they have multiple alleles at least at 30% of their gene loci.

In humans, the ABO blood types are an example of polymorphism.

Mutations can be beneficial, neutral, or harmful; a seemingly harmful mutation that requires Daphnia to live at higher temperatures becomes advantageous when the environment changes.

Genetic Drift Genetic drift refers to changes in allele

frequencies of a gene pool due to chance, more often in small populations

Genetic drift occurs when founders start a new population, or after a genetic bottleneck with interbreeding.

The bottleneck effect prevents most genotypes from participating in production of the next generation.

1) The bottleneck effect is caused by a severe reduction in population size due to a natural disaster, predation, or habitat reduction. 2) The bottleneck effect causes a severe reduction in the total genetic diversity of the original gene pool.3) The cheetah bottleneck causes relative infertility because alleles were lost due to intense inbreeding when populations were reduced in earlier times.

Genetic Drift The founder effect is an example of genetic

drift where rare alleles or combinations occur in higher frequency in a population isolated from the general population.

1) This is due to founding individuals containing a fraction of total genetic diversity of the original population.2) Which particular alleles are carried by the founders is dictated by chance alone.3) As an example, dwarfism is much higher in a Pennsylvania Amish community due to a few German founders.

Gene Flow Gene flow (gene migration) is the

movement of alleles among populations by migration of breeding individuals.

Gene flow can increase variation within a population by introducing novel alleles

Continued gene flow decreases diversity among populations, causing gene pools to become similar.

Gene flow among populations can prevent speciation from occurring.

Natural Selection Natural selection requires

a. variation (i.e., the members of a population differ from one another),b. inheritance (i.e., many of the differences between individuals in a population are heritable genetic differences),c. differential adaptedness (i.e., some differences affect how well an organism is adapted to its environment), andd. differential reproduction (i.e., better adapted individuals are more likely to reproduce).

Fitness is the extent to which an individual contributes fertile offspring to the next generation.

Relative fitness compares the fitness of one phenotype to another.

Types of Selection Directional selection occurs when an extreme

phenotype is favored; the distribution curve shifts that direction.

a. A shift to dark-colored peppered moths from light-colored correlated with increasing pollution.

b. Drug-resistant strains of bacteria are a serious health threat and represent this type of selection.

c. Increases in insecticide-resistant mosquitoes and resistance of the malaria protozoan Plasmodium to medications are also examples of directional selection.

d. The gradual increase in the size of the modern horse, Equus, correlates with a change in the environment from forest-like conditions to grassland conditions.

Types of Selection Stabilizing selection occurs when

extreme phenotypes are eliminated and the intermediate phenotype is favored.

a. The average number of eggs laid by Swiss starlings is four or five.b. If the female lays more or less than this number, fewer survive.c. Genes determining the physiology of yolk production and behavior are involved in clutch size.

Types of Selection Disruptive selection occurs when

extreme phenotypes are favored and can lead to more than one distinct form.a. British snails (Cepaea nemoralis) vary because a wide range causes natural selection to vary. b. In forest areas, thrushes feed on snails with light bands.c. In low-vegetation areas, thrushes feed on snails with dark shells that lack light bands.

Chapter 19

Macroevolution and Speciation

Macroevolution

Macroevolution refers to any evolutionary change at or above the species level.

Speciation is the splitting of one species into: two or more species or the transformation of one species into a

new species over time

What is a species? Linnaeus separated species based on

morphology, i.e., their traits differed Darwin saw that similar species are related by

common descent. Ernst Mayr (1942) developed the biological

species concept: a species is a group of interbreeding populations that are reproductively isolated from other such groups.

*The biological definition of a species - members of one species interbreed and have a shared gene pool, and each species is reproductively isolated from every other species.

Species are based on interfertility, not physical similarity.

For example, the eastern and western meadowlarks may have similar shapes and coloration, but differences in song prevent interbreeding between the two species. In contrast, humans have

considerable diversity,but we all belong to thesame species because ofour capacity to interbreed.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 24.2

Speciation The key to speciation is reproductive isolation. Two types:

Anagenesis – small, progressive evolutionary changes over long periods

Cladogenesis – 2 or more populations of an ancestral species split and diverge, also called divergent speciation

Anagenesis

Anagenesis

Cladogenesis

Allopatric Speciation Allopatric speciation occurs when new species

result from populations being separated by a geographical barrier that prevents their members from reproducing with each other. While geographically isolated, variations accumulate

until the populations are reproductively isolated. Examples: rivers shifting their courses, glaciers

melting, mountain ranges forming, large lakes diminishing into several smaller, geographically separated pools

Ex. Kaibab squirrel – p.413

Sympatric Speciation Sympatric speciation would occur

when members of a single population develop a genetic difference (e.g., chromosome number) that prevents them from reproducing with the parent type. The main example of sympatric speciation

is in plants. Failure to reduce chromosome number

produces polyploid plants that reproduce successfully only with polyploids.

Evolutionary Change Can Occur Rapidly or Gradually

Punctuated Equilibrium – history of a species, long periods of stasis (little or no evolutionary change) are punctuated, or interrupted, by short periods of rapid speciation i.e. evolution proceeds in “spurts”

Gradualism – evolution proceeds continuously over long periods Difficult to observe because fossil record is not

always complete Maintains that populations evolve slowly by

accumulating adaptations due to selective pressures

Adaptive Radiation

The evolutionary diversification of many related species from one or a few ancestral species in a short time period. Ex. Hawaiian honeycreepers p.422 Ex. Finches of Galapagos Islands

13 species from 1 founder mainland species

Extinction

End of a lineage – occurs when the last individual of a species dies

Permanent Only 1 species is alive today for

every 2000 that have become extinct

Chapter 18 & 19 Review

Chapter 18: p. 404-405 Post-Test: 1, 4, 5, 7, 9-15 Review: 1, 2, 6

Chapter 19: p. 426 Post-Test: 1, 7, 9, 11, 15 Review: 4, 6, 9