Week 7: Genetic Drift Flashcards
What was early 18thC opinion on creation? How did it change?
- Important shift in the classification of organic organisms came from Linnaeus in 1735
- Organized organisms by similar appearances
- Implied that similar species were not only visually similar, they had some relation
- However, Western theory was very Christian at the time
- All species were created in their proper form by God, and were thus stable
- Through the 18th century, intellectuals increasingly moved away from Christian beliefs
- i.e. species could change over time
- Evidence for this existed in the fossil record
- By the 19th evolution was a common belief – creationists were on the decline
What was Darwin’s life changing event? How did it happen?
How did Darwin come to find himself on the HMS Beagle? (1831-1836 voyage)
• Born into a wealthy family
o His father Erasmus was a well to do physician, and kept company like Wedgwood
• Initially studied medical school at Cambridge, but switched in favour of becoming a Christian Pastor
o Did not need to worry about job prospects due to his background
o Was much more interested in the study of Nature, which he felt was easier done as a Pastor
• HMS Beagle was looking for an onboard Naturalist (unpaid) for its journey
o 6-year journey to explore South America
o Darwin applies and is chosen
What did Darwin find in the Galapagos?
Makes his critical discovery while the Beagle passes through the Galapagos Islands (1830s)
• Islands contain many separated islands with unique ecosystems
• Noticed each Island had a slightly different Finches
o Especially visible in their beaks
• Darwin brought samples of the Finches back to England to study
o Experts suggested they were different enough to each be considered new species
• Darwin didn’t believe they could be different species
o Too many different species for such a tight Geological region
o Thought instead they were just variations of species
• Idea: Speciation is just extreme variations
o What he’d seen was just species in making
Variations were not yet large enough to be different species
o Galapagos was like the world’s lab, producing different species
What was Darwin’s mechanism of speciation? How was it received?
In 1859, Darwin publishes “On the Origin of Species”
Darwin proposes Natural Selection:
• In any population, there will be many variations, and all must compete for resources
o Some will be better adapted for the environment, and others worse
o Those who are better adapted are more likely to survive and reproduce
• Variations are passed on to the individual’s offspring
o As traits pass through generations, new species emerge
• For this to work, there needs to be enough diversity in the population
o More variation also provides better adaptability to environmental changes
• Public already aware of evolution; this wasn’t a new idea
• However, theory wasn’t accepted due to widespread doubt of Natural Selection
o Couldn’t satisfactorily explain mechanism of heredity
How would variations be passed on to offspring?
o Answer would come from Mendel
What was Mendel’s work? Why?
Mendel was a priest in Austria-Hungary, and from 1859-1863 was experimenting with peas
• Note that his work was going simultaneously with Darwin’s work
• However, he published his work in an obscure paper and it would go unappreciated for decades
Mendel analyzed how traits of peas are passed on
• Peas have clear binary traits; many traits are easily identified as present or not
o E.g. albumen is either yellow or green
What was Mendel’s theory? How did it differ from existing theory?
Mendel’s theory: The Gene
• Heritable traits are transmitted by¬ discrete units (“elements”)
o Others, such as Darwin, thought traits were continuous
o Elements were either dominant (e.g ‘A’) or recessive (e.g. ‘a’)
• One “element” (today: “allele”) would be passed from each parent
o One might have AA, Aa, or aa, depending on what was received
• Each parent randomly passes on one of their two elements equiprobably
o “Aa” has 50% chance to pass on A, 50% chance to pass on a
o So if both parents are “Aa”, child has 25% chance to be “AA”, 25% chance to be “aa”, and 50% chance to be “Aa”
• Each trait is passed on through its own gene (e.g. A, B, C…) and each is transmitted independently
What was the “Modern Synthesis”? What were its implications?
As mentioned, Mendel’s theory went largely unnoticed. But in the 1920-1940’s, it was rediscovered and merged combined with Darwin’s theories
• Combination of the two was non-trivial and required analysis, hence the delay
• Explains by stating that some of a population’s diversity comes from mutations
o But most due to recombination of genes through generations
• This provides sufficient variation to explain Darwinian Theory
o i.e. sufficient “gene pool”
Implied Genetic Drift
What was Genetic Drift? Who? What are its controversies?
By the 1930’s intellectuals such as Sewall Wright & Nikolai Dubinin realized that the modern integration introduced the possibility of “genetic drift” in small population pools:
• Given population dynamics, there are situations where gene pool can be driven by external conditions
• Not necessarily towards favourable evolution, just drifting randomly
Suppose: Asexual reproduction of Bacteria
• Randomly passes on an allele (assume both are equifavourable for life)
• Selection of one allele is like drawing a ball from one bin full of balls, and depositing in another bin (where bin you’re drawing from is the current generation, and the other bin is the next generation)
• With the Law of Large Numbers, you expect the second bin to look almost identical to the first bin after many balls are drawn from the first and put in the second
• But what if you only draw a few balls?
o Can have wildly different ratios in the next pool
• Then the population may move in a randomly selected direction
o Not necessarily a more favourable one – but for random reasons, the population may drift in that direction anyways
In such cases, the population is now driven by random variations, instead of natural selection
• Variations can’t be ignored and averaged away as before – they are the key driver of the population’s movement
Genetic drift was somewhat controversial in its time:
1. How can we determine something was caused by drift?
a. No direct evidence that “drift” (i.e. statistical fluctuations) exists and is responsible
2. Is drift a copout for our inability to see the evolutionary advantages of certain traits?
a. An epistemological question – but perhaps in cases we attribute to “drift”, we just need to look harder to see why the trait has advantages for survival
