Do you change during your lifetime? Do you evolve??5 things must be true of a population for its...

• Let’s try this again…

• Do you change during your lifetime?

• Do you evolve??

Introduction

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• What questions couldn’t Darwin answer?

• What if he could have called Mendel as a lifeline?


• Population genetics was born in the 1930’s.

• Darwin’s ideas + Mendel’s + a few others =

the Modern Synthesis, or Neo-Darwinism.

• This explained variation and natural selection in

terms of genes.


• The Modern Synthesis emphasizes:

(1) the importance of populations as the units of

evolution,

(2) the central role of natural selection as the

most important, but not the only, mechanism of

evolution, and

(3) the idea of gradualism to explain how large

changes can evolve as an accumulation of small

changes over long periods of time.


• Define population for me again…

• What’s a gene pool?

• Where did the Hapsburgs go swimming?

2. A population’s gene pool is defined by

its allele frequencies


• What would the term “gene frequency” mean?

Hint: it means we are going to do some more math.

• For example, Freq. Of R = 80%, or .80

» And Freq. Of r = 20%, or .20

We now define evolution simply as a change in gene

frequencies in a population over time.


• The Hardy-Weinberg theorem describes the gene pool of a nonevolving population.

• The shuffling of alleles during meiosis and random fertilization should have no effect, like shuffling a deck of cards doesn’t change the deck, just the combination of which cards are next to which, and which hands get dealt. We are all dealt a different hand when we are conceived.

• It shouldn’t change the gene frequencies or the genotype frequencies.

• So let’s do some math. Mind your p’s and q’s!!!!!

3. The Hardy-Weinberg Theorem

describes a nonevolving population


• Here is when a population is said to be in Hardy-

Weinberg equilibrium (in other words, it is not

evolving).

• Let’s say we look out and see a field of flowers and

count the red (dominant) and the white (recessive), and

get these numbers:

• Red – 960 White - 40


• p = ??? q = ????

• The combined frequencies must add to 100%; therefore

p + q = 1

• If p + q = 1, then p = 1 - q and q = 1 - p.

• Makes sense, yes? Now let’s take it the next level…


• What is the probability of generating an RR offspring?

• In our example, p = 0.8, so p2 = ???

• The probability of generating an rr offspring is?

• In our example, q = 0.2 and q2 =

• The probability of generating Rr offspring is ???.

• In our example, ????.


• The genotype frequencies should also add to 1:

p2 + 2pq + q2 = 1

• For the wildflowers, 0.64 + 0.32 + 0.04 = 1.

• This general formula is the Hardy-Weinberg

equation.

• So if we can only see or test for phenotypes, where

do we start to figure out these p’s and q’s???

• Let’s practice: How many of you can roll your

tongue?

• How about a worksheet?


Hardy and Weinberg described evolution like this:

5 things must be true of a population for its gene

frequencies to stay the same (for it to not evolve)

(1)Very large population size. Why???

(2) No migrations. Can genes flow???

(3) No net mutations. Simple math, yes?

(4) Random mating. Examples??

(5) No natural selection. What is different about this agent

of change compared to the other ones?


• Five total evolution causers:

genetic drift

natural selection

gene flow

Mutation

Non-random mating

These can cause the frequencies to change. It’s all math!

2. The two main causes of microevolution are

drift and natural selection


• Let’s emphasize this again:

• Natural selection is the only factor that

generally adapts a population to its

environment.

• Selection always favors the

disproportionate propagation of

favorable traits.

• The other three may effect populations in

positive, negative, or neutral ways.


Genetic Drift: Chance changes in small

populations

• Here’s some neat examples:

• The Founder Effect: American Amish

• The Bottleneck Effect: Ashkenazic Jews

• What causes variation?

• What causes evolution?

• Does anything cause both?

• Does anything that causes one play a role in the

other?

• Let’s compare lists of evolution “causers”

variation “causers” and steps in the natural

selection process.

Now let’s address some confusing lists


• So these are patterns we see caused by natural

selection.

directional selection,

diversifying selection, or

stabilizing selection.

2. The effect of selection on a varying

characteristic can be directional,

diversifying, or stabilizing


• Directional selection is the most common type.

Giraffe’s necks, Kettlewell’s moths, eg.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 23.12

• Diversifying selection might lead to …


Fig. 23.12

• Stabilizing selection reduces variation and

maintains the predominant phenotypes.

• Human birth weight is subject to stabilizing

selection.

• Babies much larger or smaller than 3-4 kg

have higher infant mortality.

• Let’s look at one more misconception…..


Evolution is limited by historical constraints.

• Evolution does not throw away old parts and build new

ones from scratch.

• Evolution “tinkers” with what already exists, and it ends

up doing a different job.

• Hence the arm bones in the fin of a whale and my aching

back. They aren’t perfect, but they survived selection.

• Ken Miller’s tie clip is a what? Here, chapter 24.

• Need to put an air conditioner somewhere? Tinker!

Natural selection cannot fashion perfect

organisms, even though some are really neat!


http://www.hhmi.org/biointeractive/fossils-genes-and-mousetraps

2. Adaptations are often compromises.

• For example, because the flippers of a

seal must not only allow it to walk on

land, but also swim efficiently, their

design is a compromise between these

two functions.

• Similarly, human limbs are flexible and

allow versatile movements, but at the

cost of injuries, such as sprains, torn

ligaments, and dislocations.


3. Not all evolution is adaptive.

• Chance affects the genetic

structure of populations to a

greater extent than was once

believed.

•For example, the Amish.


A small population of chimpanzees lives in a habitat that

undergoes no changes for a long period of time. How will

genetic drift probably affect this population?

A. It will accelerate the appearance of new traits

B. It will reduce genetic diversity

C. It will promote the survival of chimpanzees with

beneficial traits

D. It will increase the number of alleles for specific

traits

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