Modern Synthesis Flashcards
3 things for Darwin’s theory
- variation in a trait
- variation needs to produce differential success in reproduction
- variation is inheritable
Darwinism
Natural selection acts on continuous variation
Jumps or mutation theory
Evolution proceeded by large leaps (rather than small variations over a long time) and o
Two competing explanations
Biometricians (Darwinian evolution) and mendelians (opposed Darwinian evolution)
Biometricians
Analysis of data from natural populations (height, shore crab breadth)
Found a normal distribution curve (observed continuous variation)
Mendelian’s
Controlled lab experiments
Observed discontinuous variation (saw really obvious difference in phenotypes eg. Flower of hamster colour)
Argued Darwin’s idea didn’t work: selective value on small variations ineffective
The mutation theory
Hugo de Vries early 1900s
Noticed different forms of evening primrose that produced new varieties, introduced term ‘mutation’, argued mutations led to very different forms (discontinuous variation) that could lead to evolution.
This was Mendelian opposition to darwinism
Up until 1906 who was winning
Mutation theory and Mendelism was winning versus Darwinian
William castle
Took rats at extreme ends of distribution
Showed that you could enlarge the distribution of variation available (huh?)
Shows the variation
Nilsson-Ehle
Found multiple loci can control colour
Experiments of wheat and oats
Lots of loci controlling something like colour = looks like continuous variation (Darwinian)
Thomas Morgan
Counted bristles drosophila have
Showed Mendelian factors could produce small variations in character
What did castle show?
Can select on continuous variation to outside original range
What did Nilsson-Ehle’s crossing experiments show?
Characters can be controlled by several loci (polygenic)
What did Morgan’s work on drosophila show?
Mendelian factors could produce small variations in character
What is population genetics theory
System of mathematic tools that allows you to predict what happens to genes over the time
Restating the theory of NS
Genes code for phenotypes
Phenotypes are heritable
Can have different forms of genes (alleles) which is the variation Darwin is talking about
Combine this with selection = restated theory of natural selection in genetic terms.
What does the mathematic framework of natural selection show
Can show how long natural selection will take
Why was the mathematical framework of NS good
It answered things Darwin hadn’t been able to answer
Example of use of mathematical framework of NS
Is it worth him trying to select out long horns?
- cattle with long horns (males)
- Farmer wants to know if it’s worth him trying to select our long horns and how long it will take
- can tell farmer how many generations he has to select to make sure the horns disappear from the generation
Explain history of natural selection theory after origin was published (2 things)
- After origin of species 60-70 years of controversy. (Darwinism - Biometricians VS mutation theory - Mendelism)
- Theory and data showed these competing ideas weren’t in conflict (actually two parts of the same idea).
How did Darwinism and Mendelism become one idea
(a) Mendelian inheritance can produce continuous variation
(b) natural selection can act via Mendelian genetics
Who was Mary Anning XX
Paleontologist
Discovery of several dînas use fossils
How could the genetics approach be viewed as reductionist
In reality natural selection is working on big organisms but we only look at gene frequencies etc and genetics
What’s the link between genetics + selection and the appearance of design ?
Process of gene level selection leads to the appearance of an organism as designed
(Increase in gene frequencies,as you collect best genes in population you think of the best adapted organism as the one that’s collects all the best genes)
What was the beauty of this genetic explanation as to why organisms become highly adapted ?
Means we can now explain why organisms look as if they’re designed without needing to invoke an actual designer
What does he say about fitness
Cant measure fitness directly because difficult
- so have to choose proxies for fitness
- eg. Weight correlates with survival which correlates with how many offspring you have which correlates with fitness
What are four things that can cause change in gene frequencies
Genetic drift
Migration
Mutation
Natural selection
What did the Biometricians think?
Natural selection acted on continuous variation
What did the Mendelians think?
They thought selecting small variations over time was ineffective
They observed so continuous variation and favoured the mutation theoryv
What was the mutation theory ?
That mutations led to variations which suddenly appeared (discontinuous forms) and these could then lead to evolution
This theory became the Mendelian opposition to Darwinism
How did data and theory help resolve the debate over Darwinism VS the mutation theory?
Castle - showed you could select on continuous variation to outside the original range
Nilsson-Ehle - used crossing experiments between wheats and oats to show that characters can be controlled by multiple loci
Thomas Morgan - work on drosophila showed Mendelian factors could produce small variations in character
Does NS act at a genetic or individual level? XXX
XXXX?
NS acts on a genetic level to produce an indivise with the best genes
Is natural selection evolution?
Natural selection is a mechanism of evolution
