15 Alleles Changing Essay

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Allele Frequencies:
Changing
Chapter 15

Changing Allele Frequencies
1. Mutation – introduces new alleles into population 2. Natural Selection – specific alleles are more likely to be passed down because they are somehow advantageous
3. Non-random Mating – individuals of one genotype are more likely to mate with individuals of same genotype
– Think of an example of this happening?

Changing Allele Frequencies
4. Migration – individuals with specific genotypes move in or out of a population
5. Genetic Drift – random changes in allele frequencies – Caused by random sampling of specific genotypes – Often seen in small, isolated populations
Can you think of why?
– Nothing to do with natural selection

Nonrandom Mating
Populations (not just humans either) often choose mates based on phenotypes
• Effectively this means certain genotypes are selected over others
• HWE requires populations mate at random
• What sort of genotypes are selected?
– Looks, intelligence, abilities
– Race, proximity, language

Nonrandom Mating
Also occurs when one male mates with a disproportion amount of females
• That person’s genotypes are based down to the next generation at a much higher rate than HWE would expect
• Can you think of examples of this?
– A man with many wives
– Genghis Khan or other leaders
– A prize stallion, bull, ram or tomato

Nonrandom Mating
• Other factors:
– Consanguineous marriages (between close relatives) – “Endogamy” – restricted marriages within a religion or community
– Support groups for a shared disorder lead to increases in meeting a spouse with same genetic disorder

• Traits lacking phenotypes often in HWE
– Blood types

Migration
When new alleles or genotypes move into a population, that population’s frequencies change • May see Clines – gradual changing in allele frequencies while moving from one geographical area to the next
• Or may see dramatic Borders – where allele frequencies are quite different because of some isolating factor – examples?

Genetic Drift
Random changes in allele frequencies
• Nothing to do with selection, natural or artificial • A sampling error of alleles:
– Imagine reaching into a bag of jelly beans with 10 colors of beans and pulling out a handful of one color – rare, but not impossible

• Occurs more often in small isolated populations – why?

Founder Effect
When a small group of people leave a large population and found a new population – their alleles are over represented in the new population
• More extreme effects:
– Smaller number of founders
– New population more isolated

• Think of distinct groups rather than a melting pot effect

Bottlenecks
When there is a large decrease in the numbers of the population – the small population left contributes to allele frequencies • Usually caused by dramatic environmental change • Majority of population does not survive
• Remaining population contributes entirely to genetics of population

Bottlenecks
• Because only a few are left the remaining population has a restricted gene pool
• Therefore some alleles will be lost and others will be amplified
100 Alleles
Environmental
Change

4

Increase in population – all from on

Genetic Drift
Random allele changes
1. Founder Effect
– When a small group moves and founds an entire new population

2. Bottleneck
– When some change in the environment ends up killing off a large segment of the population – leaving only a few to repopulate



Compare and contrast these

Mutation
Whenever one allele changes into another; introducing a new allele
• We covered the causes of mutations already: – Spontaneous
– Radiation, chemical or etc induced

• Although these do change HWE, this is a small effect compare to other changes we have learned

Natural Selection
When certain phenotypes are more likely to survive or have offspring – those alleles will increase in each generation
• Negative Selection:
– Removal of harmful alleles

• Positive Selection:
– Increase of protective alleles

• This is evolution…