Natural selection is a major mechanism of evolution

ESSENTIAL KNOWLEDGE 1.A.1:

Natural selection is a major mechanismof evolution.

What is Evolution?

Evolution is a change in the genetic makeup of a population over time, with natural selection its major driving mechanism..

Evolution Natural selection:

populations of organisms can change over the generations if individuals having certain heritable traits leave more offspring than others (differential reproductive success)

Evolutionary adaptations:a prevalence of

inherited characteristics that enhance organisms’ survival and reproduction

November 24, 1859

Evolutionary history

Linnaeus: taxonomy Hutton: gradualism Lamarck: evolution Malthus: populations Cuvier: paleontology

Lyell: uniformitarianism Darwin: evolution Mendel: inheritance Wallace: evolution

Darwin’s Travels

Galapagos Finches

Factors that led to Darwin’s theory Overproduction Variation Artificial Selection

Descent with Modification

Descent with Modification 5 observations:

1- Exponential fertility 2- Stable population

size 3- Limited resources 4- Individuals vary 5- Heritable variation

Descent with Modification

3 Inferences:

1- Struggle for existence

2- Non-random survival

3- Natural selection (differential success in reproduction)

Darwin’s Theory

According to Darwin’s theory of natural selection, competition for limited resources results in differential survival.

Individuals with more favorable phenotypes are more likely to survive and produce more offspring, thus passing traits to subsequent generations.

Evolutionary Fitness Evolutionary fitness: the ability to

survive and Reproduce How is fitness measured? Evolutionary fitness is measured by

reproductive success.

Two driving forces behind evolution: Genetic variation

and mutations both play roles in natural selection.

A diverse gene pool is important for the survival of a species in a changing environment.

Environments

Environments can be more or less stable or fluctuating, and this affects evolutionary rate and direction;

different genetic variations can be selected in each generation.

Adaptations An adaptation is a genetic variation

that is favored by selection and is manifested as a trait that provides an advantage to an organism in a particular environment.

In addition to natural selection, chance and random events can influence the evolutionary process, especially for small populations.

Example 1: Rat Snakes

Rat snakes are found in wide variety of colors, from yellow striped to black to orange to greenish, because they adapted to their local environments.

http://timberrattlesnake89.tripod.com/ganonvenomous.html

Photos are courtesy of Phillip Higgins.

Example 2: Nylon Eating Bacteria

Since nylon wasn't invented until the 1940s, bacteria that can eat nylon can be nothing but new. The bacterium Pseudomonas is able to metabolize nylon thanks to certain enzymes it has. However, a surprising thing happens when you take a non-nylon eating variety of this bacterium and place it in an environment where the only type of food available is nylon. Every single time the experiment was tried, the bacteria would evolve until it was able to consume nylon [source: Michigan State University]. This is a very simple example of natural selection, where the most basic forms of life can adapt to whatever food the environment offers.

Example 3: Warrior Ants The warrior ants in Africa are probably one of the most

impressive examples of adaptation. Within any single colony, ants emit a chemical signal that lets the others know they all belong to the same compound. Or, put more simply, a signal that says "Don't attack me, we're all family." However, warrior ants have learned how to imitate the signal from a different colony. So if a group of warrior ants attacks a colony, they will be able to imitate that colony's signal. As a result, the workers in the colony will continue on, now under the direction of new masters, without ever realizing an invasion has taken place.

Warrior ants in Africa can imitate another ant colony's chemical signal so they can go undetected.

Law of Superposition

Hardy-Weinberg equilibrium Conditions for a population or an

allele to be in Hardy-Weinberg equilibrium are:

(1) a large population size, (2) absence of migration, (3) no net mutations, (4) random mating and (5) absence of selection. These conditions are seldom met.

5 Agents of evolutionary change

Hardy-Weinberg Equations

Problem 1 The allele for black coat is recessive. We can use the Hardy-Weinberg equation to determine the percent of the pig population that is heterozygous for white coat.

1. Calculate q2 :Count the individuals that are homozygous recessive in the illustration above. Calculate the percent of the total population they represent. This is q2.

2. Find q. Take the square root of q2 to obtain q, the frequency of the recessive allele.

3. Find p. The sum of the frequencies of both alleles = 100%, p + q = l. You know q, so what is p?

4. Find 2pq. The frequency of the heterozygotes is represented by 2pq. This gives you the percent of the population that is heterozygous for white coat:

Sample Problem 2

In a certain population of 1000 fruit flies, 640 have red eyes while the remainder have sepia eyes. The sepia eye trait is recessive to red eyes. How many individuals would you expect to be homozygous for red eye color?

Answer:160

Sample Problem 3

If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous(Ss) for the sickle-cell gene?

42% are heterozygous

Microevolution

A change in the gene pool of a population over a succession of generations

1- Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)

Microevolution

The Bottleneck Effect: type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

Microevolution

Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population

Microevolution

2- Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

Microevolution

3- Mutations: a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection)

Microevolution

4- Nonrandom mating: inbreeding and assortive mating (both shift frequencies of different genotypes)

Microevolution

5- Natural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment

Population variation

Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population

Geographical variation: differences in genetic structure between populations (cline)

Variation preservation

Prevention of natural selection’s reduction of variation

Diploidy 2nd set of chromosomes hides variation in the heterozygote

Balanced polymorphism 1- heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia); 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)

Natural selection

Fitness: contribution an individual makes to the gene pool of the next generation

3 types: A. Directional B. Diversifying C. Stabilizing

Sexual selection

Sexual dimorphism: secondary sex characteristic distinction

Intersexual and Intrasexual selection

Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

Homework:

Hardy Weinberg Practice Problems:

Natural Selection of Straw Fish“Strawfish” live in freshwater ponds. In this simulation, we will investigate

how different natural selection factors in the environment can influence the colors of Strawfish. We will also look “underneath the skin” and measure how these natural selection factors also affect the inheritance of the genes that code for the color of Strawfish. In Strawfish, there are three scale/skin colors (phenotypes)— blue, yellow, green. These three colors are controlled by a color gene that comes in two versions (two alleles) — the blue allele and the yellow allele. The blue and yellow alleles do not show a classical dominant / recessive interaction. Instead when they are inherited together they show an incomplete dominance interaction, therefore the heterozygote will be a green colored fish.

Each lab group (working in pairs) will be given a bag of alleles (straws) — 20 yellow and 20 blue straws. These represent the collection of genes in our population of fish — the fish gene pool. As in nature, Strawfish are diploid organisms — they have two copies of every gene. The color of each fish is always determined by the interaction of the two copies (the two straws).

Follow with College Board Lab: Mathmatical Modeling: Hardy-Weinberg

H-W Practice

In humans, albinism is a recessive condition caused by mutations in genes involved in the production of melanin, a skin pigment. In Americans of European ancestry, albino individuals occur at a frequency of about 1 in 10,000 (or 0.0001). If you assume Hardy-Weinberg equilibrium in this population, what percentage of them do you expect to be carriers (heterozygotes) for albinism?

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