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Unit 15 - Mechanisms of Evolution

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Nick Budziszewski

on 23 September 2015

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Transcript of Unit 15 - Mechanisms of Evolution

Mechanisms
of Evolution

Let's start off with a few basics...
#1
Evolution is a
genetic
change.
#2
Individuals do not evolve;
populations
do.
#3
The basic mechanism for genetic change is
mutation
.
#4
Selection
acting on these genetic differences leads to
new phenotypes
.
What types of selection are there?
Artificial Selection
Example
Selective breeding of plants/animals
Natural Selection
Example
Darwin's Finches
Artificial selection helps show the genetic possibilities present in a population, and not just the naturally beneficial ones.
Natural selection helps remove non-beneficial mutations from a population, as individuals carrying those mutations are less likely to survive and/or reproduce.
Can anything else cause genetic changes in a population?
Gene Flow
Populations are not usually isolated from one another.
What could happen if gene flow stopped?
Genetic Drift
Populations are susceptible to random changes.
Natural disasters are an unhappy fact of life.
Population Bottleneck
100 million --> 50
The Founder Effect
Let's review!
The Founder Effect - like a population bottleneck, only it's the first organisms to enter an area, not just the last ones left, that determine the genetic frequencies.
Nonrandom Mating
Mating patterns alter genetic frequencies.
Self-fertilization
Sexual Selection
Sometimes adaptations that help an organism attract a mate affect its chances of actually surviving long enough to mate in the first place.
How else can evolution be observed?
So far, we have focused on evolution at the phenotype level -- how organisms behave and what they look like.
We can also look at the molecular level to see the effects of evolution. Doesn't that sound exciting???
"I can't wait to learn about evolution at the level of the gene!" - You, right now
Molecular Evolution
Remember that mutation is the main way in which genes can change. A nucleotide substitution is one of the most common forms of mutation, and can take on two forms:
A. synonymous ("silent") substitutions
B. nonsynonymous ("missense") substitutions
Most mutations are
neutral
. In other words, they do not offer any sort of advantage or disadvantage on the population level, and tend to accumulate through random genetic drift (keep in mind that on an individual level, mutations can still have a huge effect!).

If you analyzed the genetics of a population, what would it mean if you found:
an equal number of synonymous and nonsynonymous mutations?
a high number of nonsynonymous mutations?
a high number of synyonymous mutations?
Let's talk about a specific example of the importance of mutations in evolution. Lysozyme is an enzyme that is normally used to kill bacteria by breaking down their cell walls. Sounds handy!



Some animals, however, use lysozyme for a completely different purpose: digesting their food. Pretty helpful if your diet consists mainly of tough-to-digest plant material!
The digestive system, however, is a pretty nasty place to be if you're an enzyme whose entire function depends on maintaining your shape (you do remember how enzymes work, right?).

As a result, changes to the surface of the lysozyme molecule must exist in order for it to last long enough to function. These molecular changes came about through nonsynonymous mutations.
The cool thing about all of this stuff? The exact same changes that allow lysozyme to function in the digestive system appeared in two
unrelated
groups of mammals, as well as certain birds (who haven't shared a common ancestor with these mammals for
hundred of millions
of years!).
How else can evolution be observed through genetics?
You should know by now that natural selection favors certain phenotypes. What you might not have known is that it also favors certain genotypes - usually those of the heterozygous variety. Sometimes heterozygotes have greater fitness due to the fact that they can often express both alleles of a particular gene at once (
heterozygote advantage
)

It's hard for these types of genotypes to get permanently fixed in a population's gene pool, however -- why?
Lastly, looking at the size of a genome can tell us something about evolution.

Consider the following: a lungfish has approximately 40 times more DNA than a human.
I am so awesome.
Most of the lungfish's genes, however, are noncoding. It possibly picked up a lot of its extra DNA through random genetic drift as its population numbers shrank over time.

To be perfectly honest, we just don't really know why some organisms have such huge genomes when compared to others. The more we understand about why there is so much diversity among genomes, however, the more we understand about the process of evolution
All joking aside,
molecular evolution is incredibly important
.

Nature selects for certain phenotypes, but where does the variation in those phenotypes come from in the first place?
New gene combinations can also come from:
Sexual Reproduction
- combination of genes from two different individuals
Lateral Gene Transfer
- influx of genes from another species (usually closely related)
Gene Duplication
- sometimes involves the duplicated gene evolving new roles
SO WHAT? Who cares about all this stuff?
Evolution has a lot of practical applications!

The more we know about the evolution of genes, the more we know about the proteins they help create. Knowing more about other organisms' DNA can tell us a lot about our own.
In the lab, new molecules can be created and selected artificially to produce new functions. Doing so can tell us a lot about how genes evolved to their present state.
Knowledge of evolution can also directly help human populations, as it can be used to boost agriculture and fight disease. The more we know about the organisms around us, the better!
Last but not least, to understand evolution is to understand biology.
Evolution drives the diversity and unity of life
, and that sounds pretty darn important to me.
Full transcript