processes evolution

8/4/2019 Processes Evolution

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Processes of EvolutionHow gene pools change across generations



The wisdom tooth question

• Remember this question?

• Some evidence exists that over several centuries, the number of people born with small wisdom teeth or no wisdom teeth has increased. Using your best

understanding of Natural Selection,explain how selection could cause this.



How selection works:

• Variation exists

• Traits are inheritable

• Some traits have more survival valuethan others (differential survival).

• Some individuals have a better chance

of reproduction than others and a betterchance of passing on their traits(differential reproduction).



Is this selection?

• “Humans in the past needed wisdomteeth, but since our way of eating isdifferent, we no longer need them.Because we no longer need them,

they’re getting smaller.”



Is this selection?

• “Our parents may have had genes for large wisdom teeth. Because they’re too

big for the child, the child may get amutation that causes him to have smaller

wisdom teeth.”



Is this selection?

• “Way in the past two people who hadsmall wisdom teeth probably mated andsince then people have had smallerwisdom teeth.”



Is this selection?

• “We inherited wisdom teeth from our ancestors even though they have nopurpose. Because they have no purpose,the genes are recessing from our

population.”



Is this selection?

• “People could be born with smaller or no

wisdom teeth because of naturalvariation that exists. Sometimes themouth is too small to contain the wisdomteeth without proper dentistry, so people

with large wisdom teeth may die fromimpactions and infections.”



Natural Selection

• There must be variation in the original population:

• Variation in size of wisdom teeth

• A few people are born without wisdom teeth (random

mutation)



Natural Selection

• There must bedifferential survival(selection) due to the

variation:

• People with largewisdom teeth are

more likely to getimpacted teeth, whichcan get infected.

Infections can be fatal.



Natural Selection

• There must be differential reproduction:

• Young people who have impactedwisdom teeth may die of an infectionbefore they reproduce. Their genes

that produced large wisdom teeth arenot passed on.



Thinking Question:

• People in Western culture often have access

to good dental care, and can have wisdom

teeth removed before they cause problems.

• How does this affect the process of natural

selection?

• Will wisdom teeth continue to get smaller inWestern nations? (The words “need” and

“purpose” should not be used in your

answer!)



Antibiotic Resistance

• We will watch a short video on the rise ofantibiotic resistance.

• At the end, you should be able to explainhow natural selection works has causedthe rise in antibiotic-resistant bacteria.(Note it is bacteria that become resistant,

not people . Bacteria do not become“immune” — they do not have immunesystems.)



Thinking Question:

• Use the principles of natural selection toexplain antibiotic resistance. Be sure toinclude these in your answers:

• Variation in the original population.

• Differential survival.

• Differential reproduction

• “Need,” “purpose,” and “immune” should

not be in your answer!



Genes and Evolution

• Genes are the units of heredity.

• Genes code for proteins, which result inour set of traits.

• Genes are passed from parent tooffspring through the sex cells.



Genes in Individuals

• Different “versions” of

genes are alleles.

• Dominant alleles areexpressed in thephenotype (expressed

trait) even if only onecopy is inherited.

• Recessive alleles areexpressed only if two

copies are inherited.

“Genotype” is a description of the

alleles for a given trait in an

individual: BB, Bb, or bb



The Gene Pool Concept

• The “gene pool” of a population is the

entire collection of alleles for a given traitthroughout a given population.

• The word for all genes for all traits in an

individual or population is genome .



• Allele ratios in a gene pool will not change

from generation to generation (that is, noevolution) only if all these things are true:

• No mutation

• Large population

• No migration

• No selection

• Random mating

Hardy-Weinberg Equilibrium



Mutations

• Small mutations appear randomly inpopulations.

• The appearance of mutations changesallele ratios by

• “breaking” functional alleles (as in

genetic disorders)

• adding new alleles



Population size

• In large populations, random eventshave a very small effect.

• In small populations, because fewerindividuals have any given trait, randomevents can have a larger effect. Changes

in gene ratios caused by random eventsare called “genetic drift.”



A population bottleneck is genetic drift



A population bottleneck is genetic drift.

F d ff i i d if



Founder effect is genetic drift.



• Migration into and out of a populationcan change gene ratios.

• Immigrants can bring in new mutations,or a different ratio of alleles.

• Emigrants may take away a high

proportion of a certain allele.

• Small population are more affected thanlarge populations.

Migration



Selection

• Selection may increase or decrease thefrequency of certain alleles:

• Directional selection: favors one end ofa range over another.

• Disruptive selection: disfavors the

midrange.• Stabilizing selection: favors the mid-

range.



• Mate choice among most organisms isselective, not random.

• Sexual selection may favor traits that arein conflict with natural selection. Forexample, bright-colored male guppies

attract more females, but are also morevisible to predators.

Mating Behavior



• Because perfect Hardy-Weinbergequilibrium is never met with in nature,all populations experience small shifts in

gene ratios with each generation.

• Gene ratio shifts may fluctuate withcyclical changes in climate. Long-term

changes in habitat (such as globalclimate change) can shift the gene ratiosfar enough to bring about speciation.

Evolution Happens

processes evolution

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