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Processes of Evolution

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Presentation on theme: "Processes of Evolution"— Presentation transcript:

1 Processes of Evolution
How gene pools change across generations

2 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.

3 How selection works: Variation exists Traits are inheritable
Some traits have more survival value than others (differential survival). Some individuals have a better chance of reproduction than others and a better chance of passing on their traits (differential reproduction).

4 Is this selection? “Humans in the past needed wisdom teeth, but since our way of eating is different, we no longer need them. Because we no longer need them, they’re getting smaller.”

5 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 a mutation that causes him to have smaller wisdom teeth.”

6 Is this selection? “Way in the past two people who had small wisdom teeth probably mated and since then people have had smaller wisdom teeth.”

7 Is this selection? “We inherited wisdom teeth from our ancestors even though they have no purpose. Because they have no purpose, the genes are recessing from our population.”

8 Is this selection? “People could be born with smaller or no wisdom teeth because of natural variation that exists. Sometimes the mouth is too small to contain the wisdom teeth without proper dentistry, so people with large wisdom teeth may die from impactions and infections.”

9 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)

10 Natural Selection There must be differential survival (selection) due to the variation: People with large wisdom teeth are more likely to get impacted teeth, which can get infected. Infections can be fatal.

11 Natural Selection There must be differential reproduction:
Young people who have impacted wisdom teeth may die of an infection before they reproduce. Their genes that produced large wisdom teeth are not passed on.

12 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 in Western nations? (The words “need” and “purpose” should not be used in your answer!)

13 Antibiotic Resistance
We will watch a short video on the rise of antibiotic resistance. At the end, you should be able to explain how natural selection works has caused the rise in antibiotic-resistant bacteria. (Note it is bacteria that become resistant, not people. Bacteria do not become “immune” — they do not have immune systems.)

14 Thinking Question: Use the principles of natural selection to explain antibiotic resistance. Be sure to include these in your answers: Variation in the original population. Differential survival. Differential reproduction “Need,” “purpose,” and “immune” should not be in your answer!


16 Genes and Evolution Genes are the units of heredity.
Genes code for proteins, which result in our set of traits. Genes are passed from parent to offspring through the sex cells.

17 Genes in Individuals Different “versions” of genes are alleles.
Dominant alleles are expressed in the phenotype (expressed trait) even if only one copy is inherited. Recessive alleles are expressed only if two copies are inherited. “Genotype” is a description of the alleles for a given trait in an individual: BB, Bb, or bb

18 The Gene Pool Concept The “gene pool” of a population is the entire collection of alleles for a given trait throughout a given population. The word for all genes for all traits in an individual or population is genome.


20 Hardy-Weinberg Equilibrium
Allele ratios in a gene pool will not change from generation to generation (that is, no evolution) only if all these things are true: No mutation Large population No migration No selection Random mating

21 Mutations Small mutations appear randomly in populations.
The appearance of mutations changes allele ratios by “breaking” functional alleles (as in genetic disorders) adding new alleles

22 Population size In large populations, random events have a very small effect. In small populations, because fewer individuals have any given trait, random events can have a larger effect. Changes in gene ratios caused by random events are called “genetic drift.”


24 A population bottleneck is genetic drift.

25 Founder effect is genetic drift.

26 Migration Migration into and out of a population can 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 than large populations.

27 Selection Selection may increase or decrease the frequency of certain alleles: Directional selection: favors one end of a range over another. Disruptive selection: disfavors the midrange. Stabilizing selection: favors the mid- range.


29 Mating Behavior Mate choice among most organisms is selective, not random. Sexual selection may favor traits that are in conflict with natural selection. For example, bright-colored male guppies attract more females, but are also more visible to predators.


31 Evolution Happens Because perfect Hardy-Weinberg equilibrium is never met with in nature, all populations experience small shifts in gene ratios with each generation. Gene ratio shifts may fluctuate with cyclical changes in climate. Long-term changes in habitat (such as global climate change) can shift the gene ratios far enough to bring about speciation.

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