Microevolution - Lecture notes 3 PDF

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review of exam 1 and type of microevolutoin...

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Microevolution In this section, we will take on evolution from a micro perspective, which will revolve around the concept of allele frequencies. Allele frequency = gene frequency (could be used interchangeably).

Recall from the Genetics chapter; we learned that alleles refer to different forms of a gene (yellow vs. green pea genes). Hence, allele frequencies basically indicate how often you can find a yellow allele vs. a green allele (gene variant) in a pea population.

Microevolution refers to the process when gene frequencies change within a population from generation to generation. Genes that translate into traits that best suit the environment will proliferate — they increase in frequency; whereas genes which become traits that suit the environment less optimally will die out — these unfavorable alleles decrease in frequency.

Sources of Genetic Variation: As humans, we have around 20,000 genes in our genome. And since we are diploid organisms, we will have about 40,000 alleles (two variations for each gene). Here, we will explain how each organism is unique, all thanks to the massive genetic diversity!

1. Mutation The most straightforward way to have a new allele is through genetic mutation. Note here that these mutations cannot be fatal!

https://commons.wikimedia.org/wiki/File:Antithrombin-gene-strand-switch.gif

2. Sexual Reproduction

This will create diversity in 3 ways, as we have seen in the cell division chapter: ● ● ●

Crossing over Independent assortment Random joining of gametes

https://commons.wikimedia.org/wiki/File:Meiosis_Overview_new.svg

3. Balanced polymorphism 'Poly' = ****many and 'morphism' = forms; hence, 'polymorphism' = many different forms. A balanced polymorphism means that different phenotypes within the members of a population can be maintained, through these advantages:

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Heterozygote advantage: ○ When a heterozygote form is more fitted to the environment than either homozygote forms ○ An example would be sickle cell anemia genes thriving in Africa. AA genotypes give normal hemoglobin, SS genotypes give sickle cell anemia (likely to die before puberty), whereas AS genotypes are beneficial because they offer resistance against malaria - a common killer in Africa - without causing sickle cell anemia.

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Minority advantage: ○ This is when a rare phenotype offers higher fitness than common phenotypes, just as we saw in disruptive selection! ○ However, as the rare allele increases in frequency, it then becomes common again, and will be selected against, leading to a decrease in frequency; hence, rare phenotypes cycle between low and high frequencies ○ Example: hunters usually develop a “search image” for their preys according to the most common appearance, and they hunt accordingly; prey that have the rare phenotype escape the predator, and are therefore more ‘fit’

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Hybrid advantage: ○ A hybrid is a result of breeding between two different strains of organisms. More breeding options = more variety! ○ The offspring is usually more superior due to the combination of different genes — avoiding deleterious homozygous diseases and maximizing heterozygous advantage ○ *Interesting side note: humans are very good at producing hybrid veggies and fruits through selecting the best traits of each parent

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Neutral variations: ○ These are variations that are passed down which do not cause any benefit or harm to the organism; one day they may come in handy if the environment changes.

4. Polyploidy Many animals are diploid, meaning that they have two copies of each chromosome, and therefore two alleles for each gene. Diploidy is beneficial because the dominant allele can mask the effect of the recessive allele, which is very helpful in cases where the recessive allele is harmful, such as sickle cell anemia. Imagine if we only had one gene for hemoglobin; people who happen to have one copy of the sickle cell allele would suffer from that disease. But since we are diploid, we would need two copies of the sickle cell gene to have the disease — greatly reducing the number of sickle cell patients! Some plants are polyploid, meaning that they actually have multiple alleles for a gene. This introduces more variety and preservation of different alleles in the genome. You never know, one day an allele may come in handy when the environment changes!

Finally, we will cover the last part of microevolution — the causes. Factors that Cause Microevolution: Let’s revisit our mnemonics ‘Large Random M&M’ for the conditions for Hardy-Weinberg equilibrium. Here, we will think in the opposite directions so that they become factors that cause changes.

1. Genetic Drift As we’ve mentioned above, genetic drift is a change in allele frequencies in a gene pool by chance. The fact that luck is involved differentiates genetic drift from natural selection, where allele frequencies are selected by the environment to increase or decrease. This is why genetic drift has a much bigger impact on small populations than big populations. There are two signature effects that result in genetic drift:

Bottleneck effect When there is a disaster that kills off most of the population. For example, a forest fire kills off all squirrels, and by chance two albino squirrels survive. The new population may be albino (if new squirrels don’t migrate to this area). What’s left is a handful of lucky individuals that survived, and a much smaller gene pool. Some alleles may be lost from this (by chance).

https://commons.wikimedia.org/wiki/File:Bottleneck_effect_Figure_19_02_03.jpg In this picture, we can assume that the colors of the marbles refer to the different alleles in a population. Inside the bottle, there are many green and red marbles. However, after passing through the bottleneck, we have lost all red marbles and only a few green ones remain. This shows the loss of alleles during a disaster.

Founder effect When there are a couple of individuals that migrate to and settle in a new location, these individuals will have a much smaller gene pool than their original population. The successive generations will descend from the founders, and their unique genetic makeup.

https://commons.wikimedia.org/wiki/File:Founder_effect_Illustration.jpg This shows a small group of marbles that “migrated” out from its original population. Since the group is small, it is prone to genetic drift. After a few more generations, all of the original red marbles (alleles) are lost.

2. Non-random mating This is when individuals choose who they want to mate with. This is a consequence of sexual selection, which we’ve covered beforehand. When certain traits are favored over others, they get passed onto offspring and become more represented within the allele frequencies of future generations. Outbreeding: breeding with individuals with no distinct family ties. Inbreeding: breeding with relatives.

3. Mutations Mutations (a heritable change in DNA) happen with varying damage to all organisms. Some mutations can happen and go into a ‘dormant’ phase until there is sudden environmental changes and the mutated traits suddenly become favorable and flourish.

4. Natural Selection As we’ve discussed, natural selection is the increase or decrease in allele frequency due to adaptations to the environment. No luck is involved and traits are selected for based on how they confer fitness within an ecosystem.

5. Gene Flow Though the name sounds pretty similar, gene flow is actually portraying a different concept than genetic drift.

Genetic drift is the result of a random change in allele frequency.

Gene flow is the process of moving alleles between populations through individuals’ migration. You can think of gene flow like how we are living in a global village nowadays — people emigrate and immigrate around the world and breed amongst different ethnicities. This causes alleles to mix and eventually make variations between populations smaller....