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Mechanisms of Evolution

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Mechanisms of Change

Mutation Migration Genetic drift

Founder effect

Bottleneck Natural selection

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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

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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

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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

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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

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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

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Effects of Genetic Drift on Evolution Reduces genetic

variation in populations

potentially reducing population’s ability to evolve in response to new selective pressures.

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Effects of Genetic Drift on Evolution

Can contribute to speciationsmall isolated population may diverge from the larger population through genetic drift.

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Types of Genetic Drift

Bottleneck event Founder effect

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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

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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

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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

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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

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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.

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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.

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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

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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

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Predation Selection Predation selection

act on both predator & prey behaviorscamouflage & mimicryspeeddefenses (physical & chemical)

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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

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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!

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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??

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Coevolution Two or more species reciprocally

affect each other’s evolutionpredator-prey

disease & host

competitive species

mutualismpollinators & flowers

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Effects of Selection Changes in the average trait of a population

DIRECTIONALSELECTION

STABILIZINGSELECTION

DISRUPTIVESELECTION

giraffe neckhorse size

human birth weight

rock pocket mice

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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?

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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

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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

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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!

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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