BCH 447 lecture 2 notes
Avoid thinking of evolution as a great chain of being or a ladder of
progress. There is no such thing as a higher eukaryote. All eukaryotes have
evolved for the same length of time. Paramecium is as modern and evolved as
tigers. Evolution is not linear.
With the explosion In molecular evolution we have more data allowing us to
create deep phylogeny. A new form of tree thinking arose in the 1970s. It was
called cladystics. A Clade is shown on the sheet. A Clade is a group of
organisms that share a common ancestor. The tree thinking based on Clades is
Today they believe that taxonomy of organisms should correspond to their
evolutionary relationship. Cladystics says that organisms should be grouped
and renamed according to their evolutionary relationships.
A tree of all life with luca at the bottom which would define one common
ancestor. Now, people are introducing the concept that there is no tree of
life. The genes are so confusing and so frought full of gene swapping... That
there may not be a tree of life. At least not in the way we depict them trees
There are no three domains of life. Archae, bacteria, and eukaryotes is
Look at handout. The way we name species now has to conform to thei
evolutionary relationship. It is not always easy to do this. An example of
this difficulty is that humans are apes. It's not that we are descended from
them or that we share a common ancestor with them. We actually cluster within
the ape Clade (chimp, gorilla). So therefore we are apes... If ape was a
valid taxonomic name.
The reptilian page. In green we see the reptiles. So, for us, the mammals. We
are both reptile and mammal. But there is a point on the reptilian tree where
reptiles started to look more like mammals than reptiles. The aves which are
birds... We always thought they had their own separate status. But in fact
they cluster in reptiles and descend from dinosaurs. So, birds are part of
the dinosaur and reptile Clade.
So... We are saying that mammals are not different from birds and reptiles.
What is evolution: same answers as before. The best definition is below.
The change in hereditary characteristics of populations over many
generations. It's heritable, it's in a population ,and it's occurring over
time. How this happens cannot be described in the definition, because we may
not know all the pathways.
If the change is in the germ-line and it's shared in the population than it
What is a population. A population is a group of organisms that are fairly
isolated and are more likely to breed within their own group. If a large
proportion of native Americans have the O blood type. That is evolution, but
it has nothing to do with natural selection. That's all random genetic drift.
Or the founder effect. Random genetic drift does not just effect allelles at the level of
alterations in codons, or neutral amino acid substitutions. It will effect
phenotypes... Like blood type. An example of this is that everyone in this
class looks different. Furthermore, we resemble our parents, so most of the
things that are characteristic of us therefore have a genetic component. Very
little variation is caused by natural selection... When we are talking about
how we look.
Evolutionary psychologists are very dumb because they create assumptions that
cannot be proven genetically. If we cannot prove it, then do not assume it.
Population genetics: population genetics didn't start until the 1900.
Remember genetics is a stochastic phenomenon... It is probabilistic
phenomenon. Then population genetics came in, and that is even more difficult
to predict because we are talking about changes in a population. People like
fisher, haldane, wright... These guys developed population genetics as a
theory in the 1930s.
First thing in population genetics is hardy Weinberg. Hardy describes the
distribution of two allelles in a population. One must be recessive and the
other dominant. If the frequencies in the population match this binomial
distribution then it means that we are looking at an area where no evolution
and no clear selection is taking place.
The above population equation doesn't work because of random genetic drift.
Because of this all populations are different. Hardy defines a perfect place.
Hardy defines no evolutionary change.
How can we measure change in a population then?
The things below are what we need to look at to define an equation for the
change in population. Firstly, population does not define species, usually,
there are many populations in one species. A species can be considered a
population when there are very few numbers in that species. In a population,
because breeding is relatively restricted it means that the population
evolves as a whole, separate from others.
How can we measure change in a population then? We want the criteria for
drift and selection.
1 - isolated population
2 - initial and final frequency of the allele, (in this class we will worry
about 2 alleles). The final frequency will be the answer to our equation.
3 - Fitness = selection coefficient, which will be 0 if neutral (the success
that an allele can confer to an individual and le population).
4 - generation time
5 - mutation rate
6 - population size = n
rate of mutation... it is represented by miu, 100 mutations per child.
7 - we need equal numbers of males and females for reproduction only under
the circumstances of having sexual reproduction. There are a few population
where that's not the case. In some populations the males mate with multiple
females. Elephant seals are an example.
3 - the fitness is defined thru the selection coefficient S. S=0 when the
mutation is neutral. If it is negative, it is detrimental, if it is positive
than it is selected for. So in an ideal population, all of the individual's in a population interbreed
or exchange alleles at random...note mating is random, don't worry.
In an ideal population there have to be discreet non overlapping populations.
But this is not real life. People don't mate collectively... And do not have
synchronized breeding which means the population generations are separate.
Furthermore, every individual in the population should have an equal chance
at breeding. Ex... People die in car accidents.
Furthermore, in order for our calculations to work, the population has to be
very large. This is so that the stochastic state of this works.
The probability of fixation of a new neutral allele, just by chance is 1/2n,
where n is the size of the population. It is 2n because we are dealing with a