Upload
ronald-russell
View
330
Download
3
Tags:
Embed Size (px)
Citation preview
AP Biology
Mechanisms of Evolution
AP Biology
Mechanisms of Change
Mutation Migration Genetic drift
Founder effect
Bottleneck Natural selection
AP Biology Minzenmayer
Genotype vs. Phenotype Genotype
Set of genes an organism carries Phenotype
Observable characteristicsInfluenced by both genotype and environment
In defining evolution we are concerned with genotypes that make up a population from generation to generation
AP Biology
Genetic Variation Evolution does not operate without variation being
present in population Three primary sources of genetic variation
MutationsChanges in DNAUsually requires accumulation of many mutations
Gene flowMovement of genes from one population to another
Sexual reproductionIntroducing new genes into a population through genetic shuffling
AP Biology
Gene Flow
Movement of genes from one population to another
Migration Examples
Pollen being blown to new destinationPeople moving to new cities or countriesImportant source of genetic variation
AP Biology
Gene Flow Amount depends on organismSedentary organisms more isolated than mobile ones
ExampleLower rate of gene flow
Corn—wind pollinatedUnlikely to fertilize individuals more than 50 ft away
Higher rate of gene flowFruit fliesReleased in death valley and recaptured 15 km away
Effects on evolutionWithin a population
Introduce or reintroduce genes to a populationIncreases genetic variation
Across populationsMakes distant populations genetically more similarReducing chances of speciation
Less gene flow between two populations, speciation more likely to occur
AP Biology Minzenmayer
Genetic Drift
Chance events changing frequency of traits in a populationnot adaptation to environmental conditions
not selection
founder effectsmall group splinters off & starts a new colony
bottleneck some factor (disaster) reduces population to small number & then population recovers & expands again but from a limited gene pool
AP Biology
Effects of Genetic Drift on Evolution Reduces genetic
variation in populations
potentially reducing population’s ability to evolve in response to new selective pressures.
AP Biology
Effects of Genetic Drift on Evolution
Can contribute to speciationsmall isolated population may diverge from the larger population through genetic drift.
AP Biology Minzenmayer
Types of Genetic Drift
Bottleneck event Founder effect
AP Biology
Genetic Drift—Bottlenecks Population bottlenecks
occur when a population’s size is reduced for at least one generationgenetic drift acts more quickly to reduce genetic variation in small populations
AP Biology
Example of Bottleneck Effect
Northern elephant sealsReduced genetic variation
Population bottleneck in 1890s due to overhunting
Population size reduced to around 20 individuals
now over 30,000
Very little variation left in this population
AP Biology Minzenmayer
Cheetahs All cheetahs share a small number of
allelesless than 1% genetic variation
as if all cheetahs are identical twins
2 bottlenecks10,000 years ago
Ice Age
last 100 yearspoaching & loss of habitat
AP Biology
Founder effect When a new population is started
by only a small group of individualsjust by chance some rare alleles may be at high frequency; others may be missing
Usually reduced genetic variation
skew the gene pool of new populationhuman populations that started from small group of colonists
example: colonization of New World
AP Biology
A group of birds are flying at sea. A storm strikes and only a few birds survive. They land on an island and are able to find enough resources to reproduce. A scientist studies what happened and determines that this is the founder effect.Which of the following assumptions is the scientist basing his conclusion on?
a) All birds had an equal chance of survival to colonize the island.
b) The birds underwent natural selection during the colonizing of the island.
c) The birds are able to adapt to their new environment.
d) The scientist made no assumptions.
AP Biology
Genetic drift as an evolutionary factor is:
A. greater in a population with small numbers than a population with large numbers.
B. greater in a population with much genetic variation than in a population with little genetic variation.
C. responsible for the selection of mutations.
D. connected to the movements of alleles between populations of a single species.
AP Biology
Natural Selection
Some individuals are able to reproduce more than others (FITNESS)
Selection acts on any trait that affects survival or reproduction
predation selection
physiological selection
sexual selection
AP Biology
Comparing Forces of evolutionary change
Natural selectiontraits that improve survival or reproduction will accumulate in the population
adaptive change
Genetic driftfrequency of traits can change in a population due to chance events
random change
AP Biology Minzenmayer
Predation Selection Predation selection
act on both predator & prey behaviorscamouflage & mimicryspeeddefenses (physical & chemical)
AP Biology Minzenmayer
Physiological Selection Acting on body functions
disease resistance
physiology efficiency (using oxygen, food, water)
biochemical versatility
protection from injuryHOT STUFF!Some fish had thevariation of producinganti-freeze protein
5.5 myaThe Antarctic Ocean freezes over
AP Biology Minzenmayer
Sexual Selection Acting on reproductive success
attractiveness to potential mate
fertility of gametes
successful rearing of offspring
Survival doesn’t matterif you don’t reproduce!
AP Biology
The lion’s mane… Females are attracted to males with larger, dark
manes Correlation with higher testosterone levels
better nutrition & healthmore muscle & aggressionbetter sperm count / fertilitylonger life
But imposes a cost to maleHOT! Is it worth it??
AP Biology Minzenmayer
Coevolution Two or more species reciprocally
affect each other’s evolutionpredator-prey
disease & host
competitive species
mutualismpollinators & flowers
AP Biology
Effects of Selection Changes in the average trait of a population
DIRECTIONALSELECTION
STABILIZINGSELECTION
DISRUPTIVESELECTION
giraffe neckhorse size
human birth weight
rock pocket mice
AP Biology
Fitness
Describes how good a particular genotype is at leaving offspring in the next generation relative to how good other genotypes are at it
Example:If brown beetles consistently leave more offspring than green beetles because of their color, you’d say that the brown beetles had a higher fitness
In a hypothetical population of beetles, there is a wide variety of color, matching the range of coloration of the tree trunks on which the beetles hide from predators. The graphs above illustrate four possible changes to the beetle population as a result of a change in the environment due to pollution that darkened the tree trunks
a) The coloration range shifted toward more light-colored beetles, as in diagram I. The pollution helped the predators find the darkened tree trunks.
b) The coloration in the population split into two extremes, as in diagram II. Both the lighter-colored and the darker-colored beetles were able to hide on the darker tree trunks.
c) The coloration range became narrower, as in diagram III. The predators selected beetles at the color extremes.
d) The coloration in the population shifted toward more darker-colored beetles, as in diagram IV. The lighter-colored beetles were found more easily by the predators than were the darker- colored beetles.
Which of the following includes the most likely change in the coloration of the beetle population after pollution and a correct rationale for the change?
AP Biology
Fitness Fitness is relative
A genotype may be very fit in one environment and not fit at all in another
Fitness lumps everything that matters to natural selection into one idea
SurvivalMate findingReproduction
Fittest individuals not always strongest, fastest or biggest
Fitness of a genotype depends onAbility to surviveFind a mateProduce successful offspring
AP Biology
Misconceptions about Natural Selection
Does not produce perfectionNo population or organism is perfectly adapted
Natural selection has no goalsNatural selection is the simple result of variation, differential reproduction and heredity
“need”, “try” and “want” are not very accurate words when it comes to explaining evolution
Natural selection selects whatever variations exit in the population—results in evolution
AP Biology
Misconceptions about Natural Selection
Sometimes interpreted as a random process
Genetic variation in a population is random
Selection that acts on the variation is
non-randomGenetic variation that aids survival and reproduction are more likely to become common that other variation
Natural selection is not random!
AP Biology
Misconceptions about Natural Selection
Natural selection acts on individualsDifferential survival
Survival of the fittest
Differential reproductive successBear more offspring
Populations evolvePopulations of organisms change over time
Traits which offer greater fitness become more frequent in the population