The Evolution of Populations

September 19th- AP Bio Lesson
The Evolution of Populations

First, it’s important to emphasize that individuals do NOT evolve, populations evolve.
When an individual is born with a variation beneficial for survival and aiding in their reproduction, that varying trait will get passed down from generation to generation. With that, it’s necessary to remember that variation is the raw material of evolution.

So what causes this change? Sure, by this point, we know all about selection, but what else? There must be other things, right? Right, there are 5 agents of evolutionary change.

1. MUTATION
a. Mutations can come about by both DNA mishaps in mitosis (cell division) and meiosis (gamete division) or by environmental influences such as UV light rays.
b. The likelihood of a particular mutation is not affected by natural selection. Mutations do not occur more frequently in situations in which they would help the organism survive and reproduce.
c. Here’s a pretty informative youtube video on mutations:



2. GENE FLOW
a. Gene flow is the movement of alleles from one population to another.

3. NONRANDOM MATING
a. individuals with certain genotypes sometimes mate with another more commonly than would be expected on a random basis
b. assortative mating: type of nonrandom mating in which phenotypically similar that causes the frequencies of particular genotypes to differ greatly from Hardy Weinberg principle (I’ll discuss that later)
disassortative mating: phenotypically different individuals mate producing a lot more heterozygotes

4. GENETIC DRIFT
a. frequencies of particular alleles may change drastically by chance alone as if the frequency was drifting.
b. two types: founder effect and bottleneck effect
Founder effect- the effect by which rare alleles and combinations of alleles may be enhanced in new populations
-Not rare in nature, new populations could be started with a single seed (albino deer)
Bottleneck effect- when a population size is dramatically reduced- may it be because of flooding, disease, the actions of human beings, etc.- and several alterations and losses of genetic variability in that population.

4. SELECTION
We know this one, super easy but to review:
a. Natural selection to occur: variation must exist, variation among individuals results in differences in the number of surviving offspring to the next generation, variation must be genetically inherited.
b. Selection passed on helps to: avoid predators, match climatic conditions, for pesticide resistance, for sexual advances, increase survival and in turn reproduction
So we understand all that stellar selection info but what makes a change due to natural selection favorable? Biologists measure success on quantity of surviving offspring or fitness. Good fitness is passed on from parent to offspring and those possessing the trait will have greater reproductive success.

Now for the tough part, math in biology, boooo.

THE HARDY WEINBERG EQUATION
p +q = 1
(p+q)² = p²+2pq+q²
P= homozygous dominant q= homozygous recessive 2pq= heterozygous
Used to calculate genetic frequencies in decimal form
Hardy and Weinberg pointed out tat the population will remain constant from generation to generation if:
1. the population size is very large
2. random mating is occurring
3. no mutation takes place
4. no genes are input from other sources (no immigration)
5. no selection occurs
If no other forces are working against it, the process of sexual reproduction (meiosis and fertilization) alone will not change the Hardy- Weinberg proportions.
If the proportions do not change, the genotypes are said to be in Hardy- Weinberg equilibrium.

although at first hard to grasp, the math thing is pretty simple if you look at examples, like the one on page 436 of the textbook and the one in our “Measuring Evolution of Populations” powerpoint