Natural Selection

Part 1 Evidence of Evolution

notes in red are for teacher versionanswer key

Artificial Selection

Humans have a long history of breeding plants and animals It is typically done to increase the efficiency of an animal (eg horses livestock dogs) or to maximize the output in agriculture (eg food crops ornamentals)

1) httpt2gstaticcom 2) httpwebidrccaopenebooks 3) httpwwwbulbsonlineorg

Can you describe how a human might intervene with breeding to artificially select for the

desired characteristics in a plant or animal

While humans can apply selective pressure to plants and animals so can the

environment Charles Darwin published On the Origin of Species by Means of Natural

Selection Natural selection explains how evolution can occur

There are four main principles to the theory of natural selection variation

overproduction adaptation and descent with modification

Variation-The heritable differences or variations that exist in every population are the

basis for natural selection The differences among individuals result from diffrences in

the genetic material of the organism whether inherited from a parent or resulting from a

genetic mutation

Overproduction- While having many offspring raises the chance that some will survive it

also results in competition between offspring for resources

Adaptation- Sometimes a certain variation allows an individual to survive better than

other individuals it competes against in its environment More successful individuals are

ldquonaturally selectedrdquo to live longer and to produce more offspring that share those

adaptations for their environment

Descent with modification- Over time natural selection will result in species with

adaptations that are well suited for survival and reproduction in an environment More

individuals will have the trait in every following generation as long as the environmental

conditions continue to remain beneficial for that trait

In order for selection to occur (artificial or natural) a trait must be heritable In addition a

population must contain genetic variability Individuals with favorable traits are more

likely to leave more offspring better suited for their environment increasing their fitness

Genetic traits that increase their survival will likely be passed onto their offspring--

natural selection

Explain why the phrase ldquosurvival of the fittestrdquo does not accurately reflect Darwinrsquos

concept of evolutionary fitness Answer Because it takes more than merely ldquosurvivingrdquo

for a creature to increase their fitness For natural selection to occur that individuals

must also successfully reproduce Therefore perhaps a better phrase to describe the

concept would be ldquosurvivals of the survivalsrdquo

Structural Adaptations

African Rock Rat Mole Rat

httpphotosmongabaycom httpwwwnewswisecom

Look at the above images The terrestrial ancestral African rock rat and the subterranean mole-rat have some very distinct differences Some of their differences proved to be beneficial to survival while other characteristics played no role in their survival

Can you name some of these structural adaptations and describe how these adaptations may have helped the animals to better survive and to inhabit their new subterranean habitat

Hints eyes claws teeth loss of hair

Structural Adaptations

African Rock Rat Mole Rat

httpphotosmongabaycom httpwwwnewswisecom

Look at the above images The terrestrial ancestral African rock rat and the subterranean mole-rat have some very distinct differences Some of their differences proved to be beneficial to survival while other characteristics played no role in their survival

Can you name some of these structural adaptations and describe how these adaptations may have helped the animals to better survive and to inhabit their new subterranean habitat

Hints eyes claws teeth loss of hair

Mimicry

Coral snakes (venomous) and king snakes (non-venomous) The harmless king snake mimics the poisonous coral snake causing would-be-predators to avoid them

http3bpblogspotcom

Monarchs (toxic to predators) and viceroys (non-toxic)

The viceroys mimics the coloration and pattern of the unpalatable monarch also avoiding would-be-predators

httpfishmongabaycom

Camouflage

A flounder will be less conspicuous to predators and to would-be prey if it blends in with its environment

httpconservationreportfileswordpresscom

Comparecontrast mimicry and camouflage

Pick an organism and explain the benefit of that animal or plant mimicking another Purpose how this leads to natural selection in that organism

Letrsquos put it all together with Peppered moths

The peppered moth population in England has had wide variability over the past two hundred years The original population composed of light colored moths with some speckling typically perched upon trees with lichen covered trees During the Industrial Evolution pollution decreased the lichen covering and deposited soot on the tree trunks Therefore the light colored moths were no longer camouflaged against the light colored bark Slowly the peppered moth population transitioned from one of a light color to one of a dark color With the increase in environmental awareness and the implementation of standards the pollution decreased and the light colored moths have again increased in number

httpiamyouasheismefileswordpresscom200803mothsjpg

Look at the diagram above Explain why the peppered moth population transitioned

from light to dark colored moths during the Industrial Revolution And how did cleaning

up the environment allow for a reversal in this trend

Peppered moth (website resource) httpwwwtechappsnetinteractivespepperMothsswf

Physiological adaptations

Antibiotic resistance

Can you name any other types of physiological adaptations that could become a problem for humans Hint think pests

Fossils Archeopteryx- The ldquoMissing Linkrdquo between reptiles and birds

1) httpwwwucmpberkeleyedu 2) httpwwwsearch4dinosaurscom

Anatomy

homologous structures The similarities in the bone structure of the forelimbs of mammals demonstrate evidence for evolution comparative anatomy and common ancestry

httpwwwindianaedu

analogous structures

Show similarities in function but are not evidence of a common ancestor--an example of convergent evolution

http2096813857lcarchivebiologyPublishingImages0345ljpg

vestigial structures The relic pelvic bones of a whale indicate that an ancestor may have walked on land with legs

httpwwwkirksvillek12mous

Evolution can be described as a change in allelic frequencies of a gene pool over time Natural selection can place pressure on specific phenotypes and cause a change in the frequency of the alleles that produce the phenotypes For example traits of prey species that either make them more difficult to detectidentify or to catch may be selected for within that prey population Think about the peppered moth example

What would happen to the allelic frequency within a prey population if the predator population declined Exploded

Embryology

It is easy to distinguish between an adult fish adult reptile or an adult mammal But can you do so for their respective embryos Take a look below

httpnitrobiosciarizonaedu

The embryos of fish reptiles and mammals all have a tail and pharyngeal pouches In fish these pouches develop into the supports for the gills while in the mammals and reptiles (birds too) they develop into parts of ears jaws and throat These shared features in young embryos suggest evolution from a distant common ancestor

Hypothesize the strengths and weaknesses of embryology as evidence for evolution

Part 2 Mechanisms of Evolution

Natural selection is a mechanism of evolution

Different types of natural selection act over the range of a traitrsquos variation In the following graphs the dotted bell-shaped curve indicates a traitrsquos variation in a population The solid blue bell-shaped curve indicates the effect of a natural selection

Stabilizing Selection

The intermediate phenotype is favored and becomes more common in the population Example Consider a population of spiders in which the average size is a survival advantage larger spiders are easily spotted and preyed upon by birds and smaller spiders have more difficulty located resources Therefore the average sized spider has a selective advantage---and is ldquoselected forrdquo while the extremes are ldquoselected againstrdquo

httpimgsparknotescom

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Degree of trait (eg spider size) Small medium large

To interpret this graph you should be able to say that the majority of individuals has approximately the mean (ie average) trait (ie intermediate spider size) There are few individuals at the ldquotailsrdquo of the curve representing small or large spiders

Hypothesize what might happen to the spider population if the bird population had a catastrophic decline due to invasion of a parasitic invasive species such as a cowbird Answer Hard to tell Might return to original population but might not if cowbirds eat them too

Divergent or Directional Selection

One extreme variation of a trait is favored and becomes more common in the population Example The peppered moth is a good example of directional selection as the changing environment selected for a relatively rare trait (conversely selecting against the more common trait) causing that trait to increase within the population

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mb

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f in

div

idu

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rait

httpimgsparknotescom

Degree of trait (eg amount of pigmentation) White with little speckling mostly dark

To interpret this graph you could say the majority of the population has had a increase in the mean amount of pigmentation

If a fishery only harvests large fish from the population how might that affect the population over time Answer It may select for smaller body size and slower growing individuals in the population

Degree of trait (eg plumage variation and color)

Disruptive Selection

Both extreme variations of a trait are favored and become more common in a population

Example Consider a population of lazuli buntings Males plumage ranges from bright blue to a mix of blue and brown to dullish brown Adult dominant males are those with the brightest blue feathers on their heads and backs These birds are more attractive to potential mates and therefore are typically more reproductively successful than say a male with a combination of brown and blue plumage (intermediate) The dominant males are very aggressive towards the intermediate plumage type but not so of the dullish brown plumage type Therefore the dullish brown plumage birds can affectively attract a mate due to less competition and harassment In this example the intermediate plumage type is selected against favoring individuals with either the brightest blue or the dullest brown plumage This is also an example of sexual selection where one sex favors a specific phenotypetrait in the other sex leading to differential reproductive success

httpimgsparknotescom

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mb

er o

f in

div

idu

als

wit

h t

rait

Dullish brown brown and blue bright blue

Interpret this graph What does it illustrate about the number of individuals within the populations with different degrees of this trait (ie plumage variation and color)

Other mechanisms of evolution are mutations migration and genetic drift

A population without selection pressure of a changing environment no mutations a gene pool that does not change due to movement of genes into or out of the population and not experiencing genetic drift due to changes in allelic frequency are said to be in genetic equilibrium These populations are not undergoing any form of evolution

Mutations

In the case of Down Syndrome

additional genetic material on

Chromosome 21 has detrimental

effects on the individual carrying it

httpwwwthetechorg

What can cause mutations

Are mutations good bad or neither for the organismspopulations in which they occur

Migration (gene flow)

Blue fin tuna are known to migrate

across the entire Atlantic Ocean Mainly

from east to west Understanding gene

flow in highly migratory species is a key

concept in fisheries biology and

management

httpwwwflmnhufledu

An animal may actually move from one location to another or it may be relocated due to human activity Give examples An animal may leave an area to find a mate to find food or other resources it may be relocated due to a tropical storm it may stow away on a ship to be transported to another geographical area humans may introduce an animal to a new geographical area

What about plants Can you name some ways that gene flow occurs in plants Seed dispersal by wind or by animals (sticking to fur or defication) importingexporting by humans

Genetic drift

Founder Effect When a small portion of a population becomes isolated whether it be due to geographical or socialreligious reasons genetic drift can occur

httpuploadwikimediaorg

In Lancaster PA there is a community of approximately 12000 Amish citizens They have a particular lifestyle and marryreproduce only within that group A recessive allele resulting in a trait where the individual has shortened limbs and an extra digit on hands and feet has increased over time due to the small gene pool Today the frequency of this allele is much higher in the Amish community than it is in the US population as a whole

Describe how the founder effect may occur in plants Seed dispersal

Bottleneck Effect When a catastrophic event drastically reduces the size of the population whether it be

natural disaster overharvestingoverhunting or disease genetic drift can occur

httpwwwbritannicacom

The northern elephant seal was nearly hunted to extinction in the 1800s By the turn of

the century there were only 20 individuals remaining Since then the population has

grown to over 30000 individuals but their genetic variability is still quite reducedmdashit is

much less variable than their counterpart the southern elephant seal that was not so

intensely hunted

Do you know any other examples of the bottleneck effect How can scientists identify

species that have gone through such a bottleneck Giant panda amp Galapagos Islands

Giant tortoises cheetahs Genetic research and comparison studies

How is genetic drift different from natural selection