Chapter 17: Processes of EvolutionUnit 6: Evolution

Case Study: Rise of the Super Rats

Rats are one of the most notorious pests of all time

Cost us about ____________ each year

Rodenticide warfarin: very effective when first used

Rats developed resistance – inherited gene that made the chemical ineffective

This is an example of: __________________________________

Variation in Populations _______________ : one group of

individuals of the same species in a specified area

Species share: Morphological traits

Physiological traits

Behavior traits

Populations show variation in traits: Qualitative: __________________________

Quantitative: _________________________

The Gene Pool The genes of a population comprise the

_______ ________, a pool of genetic resources

Alleles are the primary source of variation in appearance

Polymorphism: occurs when genes have three or more alleles that persist in a population with a frequency of at least 1% Example: _______________________

________________ : two distinct traits Example: male / female

Mutations Mutations are the source of new

alleles

We can predict average mutation rates

In humans: ________ mutations per person per generation

Many mutations give rise to structural, functional, or behavioral alterations that reduce an individual’s chance of surviving and reproducing

Mutations Mutations can be:

____________ : usually arise from drastic changes in the phenotype

_____________: alter the base sequence of DNA but have no effect on survival or reproduction

______________: enhances the survival or reproduction – occurs every so often

Natural selection will favor the transmission of beneficial mutations on to the next generations

Stability and Change in Allele Frequencies

Researchers typically track _____________ _________________ in populations (how often a certain allele occurs)

The relative abundance of alleles of a given gene among all individuals of a population

For a starting point, they use a reference called genetic equilibrium, when a population is not evolving with respect to a certain gene

Genetic equilibrium can ONLY be reached when five conditions are met

Genetic Equilibrium

Genetic equilibrium can occur only when: Mutations do not occur The population is infinitely large The population stays isolated from all

others of the same species Mating is random All members of the population survive

and produce the same number of offspring

In nature, _______________________________

Microevolution

If all 5 conditions are never met, change is occurring within a population

Results in small-scale changes in the population’s allele frequency; called __________________

Four processes of microevolution:

mutation

natural selection

genetic drift

gene flow

When is a population not evolving?

The Hardy-Weinberg Formula can be used to track whether a population is in genetic equilibrium or not

A mathematical formula which tracks allele frequency for a specific trait

Applied rules of probability to sexually reproducing populations and found that the gene pool can only be stable when __________________________

Researchers can use the formula to estimate the frequency of carriers of alleles that cause genetic traits and disorders

Hardy-Weinberg Genetic Equilibrium

Hardy-Weinberg formula:

p2 + 2 pq + q2 = 1

p + q = 1

where p and q are the frequencies of alleles A and a

You can draw this on a Punnett square:

Natural Selection

Natural selection: the differential survival and reproduction among individuals

Natural selection influences all levels of biological organization

Selection can be:

Directional Selection

Directional selection occurs when allele frequencies shift in a _____________________

Forms at one end of the range of phenotypic variations become more common than the intermediate forms

Examples:

Peppered Moth, pocket mice (predation)

Resistance to antibiotics

Directional Selection

Butterfly wing color: medium-blue is between two phenotypic extremes (white and dark purple)

Orange arrows identify which forms are being selected against over time

Figure 17.5, page 270

Peppered Moth

Peppered moth: a classic example of the directional selection process

Feed at night and rest on tree trunks during day

Light gray lichens grew on trees

In preindustrial England, light colored moths were the most common

Due to air pollution starting in 1850s, the dominant allele shifted to dark colored moths

Air pollution was _________________________

Peppered Moth

Since the advent of pollution controls starting in 1950s, allele frequency has begun to shift back to light colored moths

Figure 17.6, pg 271

Pocket Mice Rock pocket mice in Arizona are another

example of directional selection

Light brown granite; dark basalt (lava rock)

Figure 17.7, pg 271

Resistance to Antibiotics Human activity can also influence directional

selection

Antibiotics: toxins that kill bacteria by interfering in physiological processes

Since 1940s, have been widely prescribed in the U.S.

Overuse of antibiotics puts tremendous selection pressure on bacteria

Bacteria divide quickly and form huge populations with great genetic variation

E. coli can divide every _________________

Likely that some will survive antibiotic treatment

Resistant strains are becoming the norm

Selection Against or in Favor of Extreme Phenotypes

Stabilizing Selection: works against phenotypes at the fringes of a range

Disruptive Selection: favors phenotypes at the fringes of a range

Stabilizing Selection Stabilizing selection: intermediate forms

of a trait in a population are favored

Does NOT _______________________________

Examples:

Birth weight for babies – mid-sized babies selected for most often

Body mass size of sociable weaver birds

Body mass represents a trade-off between risks of starvation and predation

Stabilizing Selection

Stabilizing

eliminates the

______________

Figure 17.8a, pg 272

Disruptive Selection Disruptive selection: forms at both ends of

a range of variation are favored

Intermediate forms are ______________________

Example: selection for bill size in black-bellied seedcracker finch in Africa

Females and males have either large or small bills (no intermediate ones)

2 different types of plants: hard or soft seeds

Feeding performance maintains this dimorphism of beak size

Disruptive Selection

Disruptive selection

eliminates

_______________________

Figure 17.8b, pg 272

Modes of Natural Selection

Fig 17.4, page 269