Evolution in Populations (or How Natural Selection Works)

To take a step back: We have talked about 1) Historical views of organic change 2) Darwin’s views of organic change 3) Evidence for change through time (small and larger scale examples) Next few lectures : HOW this change through time happens.

Definitions Natural Selection Evolution (from previous slide show) Changes over time of the proportion of individuals differing genetically in one or more traits **PATTERN** Natural Selection Differential success in the reproduction of different phenotypes resulting from the interaction of organisms with their environment. **PROCESS**

A Little Basic Genetics Gregor Mendel (1822 - 1884)

A Little Basic Genetics Imagine a very simple creature in which every cell has one chromosome -

A Little Basic Genetics But every sexually reproducing creature has 2 of each chromosome In you, there are 23 pairs of chromosomes for a total of 46

Each chromosome contains a series of genes but for now we’ll imagine that there is just one on our chromosome Gene A

Now assume that there are two versions of each gene - represented with a capital and a small letter Gene A a

Another idea - Cell Division There are 2 kinds of cell division 1. Mitosis - (duplication) - every cell makes a copy of itself (this is how you grow and replace dead cells) 2. Meiosis - (reduction) - cells divide in such a way as to reduce the number of chromosomes by 50%

Meiosis In one of the stages of meiosis, the chromosomes line up on the midline of the cell and are separated from each other when the cell divides A A a A a a Chromosomes line up The cell divides

And just to inject a little bit of reality……. These are eggs or sperm a

And a little arithmetic……. What are the chances of getting each type of egg or sperm? A A a 50% chance of each a

A few more definitions and terms Trait - any physical characteristic of an organism (e.g. eye colour, hair colour, height) (for now you can assume that gene = trait) Allele - alternate versions of a gene or trait (e.g. blue, brown, green, or hazel eyes) Phenotype - the physical appearance of an organism Genotype - the genetic makeup of an organism

In our simple creature we have one trait A with two alleles of each A - the dominant allele for trait A a - the recessive allele for trait A Probability of getting either a or A = 0.5

What about producing offspring What about producing offspring? - all the possible combinations of eggs and sperm A a a A A A a a

Or to put this a little more formally - A Punnett square Female parent A a (0.5) (0.5) A (0.5) a AA Aa Male parent Aa aa The letters in each box are the GENOTYPE of the offspring

Or to put this a little more formally - A Punnett square Female parent A a (0.5) (0.5) A (0.5) a AA (.25) Aa (.25) Male parent Aa (.25) aa (.25) The letters in each box are the GENOTYPE of the offspring

So we have 3 possible genotypes A a (0.5) (0.5) A (0.5) a AA (.25) Aa (.25) Aa (.25) aa (.25) So we have 3 possible genotypes AA Aa (x2) aa Ratio 1 2 1

So we have 3 possible genotypes A a (0.5) (0.5) A (0.5) a AA (.25) Aa (.25) Aa (.25) aa (.25) So we have 3 possible genotypes AA Aa (x2) aa Ratio 1 2 1 ….and two possible phenotypes A_ aa Ratio 3 1

We can generalize this idea to be more inclusive If you say that the frequency of the dominant allele is ‘p’ and the frequency of the recessive allele is ‘q’ In our example, p = 0.5 and q = 0.5 And p + q = 1 (p + q always equals1)

And if you mate two organisms, you can mathematically determine the expected proportion of offspring of each type p + q p2 + 2pq + q2

In our simple organism, p = q = 0.5 and p2 + 2pq + q2 = (0.5)(0.5) + 2 (0.5)(0.5) + (0.5)(0.5) = .25 +.5 +.25 Which is back to our 1:2:1 genotypic ratio p2 2pq q2

This idea holds true for any value of p or q. For example: If p is very common - say 90% of the genes in the population Then p = .9 and q = .1 And p2 = .81 (the frequency of the AA genotype) 2pq = .18 (the frequency of the Aa genotype) q2 = .01 (the frequency of the aa genotype)

In the early 1900’s, Hardy and Weinberg used this idea to establish a fundamental idea in the genetic basis of natural selection

• In Generation 1 p2 + 2pq + q2 = .36 + .48 + .16 In Generation 2 The Hardy-Weinberg Equilibrium Assume that p = .6 and q = 0.4 In Generation 1 p2 + 2pq + q2 = .36 + .48 + .16 In Generation 2 p2 + 2pq + q2 = .36 + .48 + .16 In Generation 3 p2 + 2pq + q2 = .36 + .48 + .16 In Generation 4 p2 + 2pq + q2 = .36 + .48 + .16 •

The Hardy-Weinberg Equilibrium In any population, allelic and genotypic frequencies will remain the same if Mendelian inheritance patterns are the only factors at work Requires: Very large population size 2. No gene flow -no movement of genetic material between populations No mutations 4. Random mating 5. No natural selection

The Hardy-Weinberg Equilibrium In any population, allelic and genotypic frequencies will remain the same if Mendelian inheritance patterns are the only factors at work Requires: Very large population size No gene flow -no movement of genetic material between populations No mutations Random mating No natural selection

The Hardy-Weinberg Equilibrium 1. Large population sizes What happens if the population isn’t ‘large’? Genetic Drift - a statistic consequence of small populations

The Hardy-Weinberg Equilibrium 1. Large population sizes What happens if the population isn’t ‘large’? Genetic Drift - a statistic consequence of small populations

The Hardy-Weinberg Equilibrium 1. Large population sizes http://darwin.eeb.uconn.edu/simulations/drift.html

The Hardy-Weinberg Equilibrium 1. Large population sizes What happens if the population isn’t ‘large’? Bottlenecks

The Hardy-Weinberg Equilibrium 1. Large population sizes What happens if the population isn’t ‘large’? Bottlenecks Northern Elephant Seal

The Hardy-Weinberg Equilibrium 1. Large population sizes What happens if the population isn’t ‘large’? Bottlenecks 1700’s - ?? 1890 - 20 est. 2009 - 135,000 Northern Elephant Seal

The Hardy-Weinberg Equilibrium 1. Large population sizes What happens if the population isn’t ‘large’? Bottlenecks Assay of 16 genes in 159 individuals 0/16 had any variability Northern Elephant Seal

The Hardy-Weinberg Equilibrium 1. Large population sizes What happens if the population isn’t ‘large’? Founder effect

The Hardy-Weinberg Equilibrium 1. Large population sizes What happens if the population isn’t ‘large’? Founder effect 1740’s Amish

The Hardy-Weinberg Equilibrium 1. Large population sizes What happens if the population isn’t ‘large’? Founder effect Ellis-Van Creveld Syndrome Polydactyly congenital heart defects pre-natal tooth eruption short-limbed dwarfism, short ribs cleft palate, malformation of the wrist bones Amish

The Hardy-Weinberg Equilibrium 1. Large population sizes What happens if the population isn’t ‘large’? Founder effect U.S. Population Amish 700 600 500 400 300 200 100 500 Incidence in 100,000 Births 1.4

` The Hardy-Weinberg Equilibrium 2. Mutations - source of all new genetic variation Rates of mutation - typically about .00001 (may be as low as .0000001) OR 1 mutation/100,000 genes/generation (1 mutation in 1000000/generation)

2. Mutations - source of all new genetic variation How do we model this? Frequency of aa Frequency of AA Frequency of Aa Imagine that ‘A’ mutates to ‘a’ at a rate of m per generation Frequency of A after one generation of mutation Frequency of A after a second generation of mutation

For any number of generations (x) Imagine that ‘A’ mutates to ‘a’ at a rate of m per generation Frequency of A after one generation of mutation Substitute Frequency of A after a second generation of mutation For any number of generations (x)

The Hardy-Weinberg Equilibrium 3. Random mating This assumes no preferences in mates Humans: Preferences Height - we tend to mate with people closer to our own height

The Hardy-Weinberg Equilibrium 3. Random mating 1.0 .1 .01 .001 .0001 Humans - distance from home Probability of mating Distance from ‘home’

The Hardy-Weinberg Equilibrium 3. Random mating 1.0 .1 .01 .001 .0001 Humans - distance from home Probability of mating 40 km Distance from ‘home’

The Hardy-Weinberg Equilibrium 3. Natural Selection -depends on variability that is heritable -differences must be passed to the offspring Key idea : Fitness: The contribution an individual makes to the gene pool of the next generation relative to other individuals

The Hardy-Weinberg Equilibrium 3. Natural Selection -depends on variability that is heritable -differences must be passed to the offspring Key idea : Fitness: The contribution an individual makes to the gene pool of the next generation relative to other individuals Higher fitness Lower fitness

Types of Natural Selection Most traits have a normal (or bell curve distribution)

Types of Natural Selection Most traits have a normal (or bell curve distribution)

Types of Natural Selection 1. STABILIZING SELECTION

Types of Natural Selection 1. STABILIZING SELECTION Human birth weight

Types of Natural Selection 2. DIRECTIONAL SELECTION 

Types of Natural Selection 2. DIRECTIONAL SELECTION  Salmon fishing - largest fish are taken every year

Types of Natural Selection 3. DISRUPTIVE SELECTION 

Types of Natural Selection 3. DISRUPTIVE SELECTION • •  • • Breed together • • • Negatively geotactic • • Selection of positively and negatively geotactic Drosophila • Positively geotactic • • • • • • Breed together • • • • • • • • Artificial Selection - Drosophila geotaxis •

Types of Natural Selection 3. DISRUPTIVE SELECTION  Selection for bristle number in Drosophila

Modelling Natural Selection Select against recessive homozygote - aa AA Aa aa Total Initial Frequency p2 2pq q2 1 Fitness 1 1 1 - s Next Generation 1 1 q2(1 – s) 1 – sq2 Normalized