Biology unit 5 Explanation & Notes

Section 10.1 Early Ideas About Evolution

Evolution is the process of biological change by which descendants differ from their ancestors.

A Species is a similar group of organisms that can reproduce and have fertile offspring.

Fossils are traces of organisms that had existed before. They are usually found in layers of rock and the deeper the layer, the older the fossil.

There was a main principle of geologic change that influenced Darwin. Uniformitarianism is the theory that geologic processes are happening everywhere and all the time at a slow and steady pace.

Section 10.2 Darwin’s Observations – The Galapagos Islands

Variation is the difference in the physical traits of an individual from those of other individuals in the same group. Darwin saw species on nearby islands that looked different from each other.

Questions: Why do these tortoises have different shell shapes?

Adaptation is a feature that allows an organism to better survive in its environment. In other words, a species can adapt to their surroundings and can lead to genetic change in a population over time.

Darwin’s observations led him to believe that the earth was much older than previously thought (which was 60,000 years) because he saw fossils that were similar to living species.

**PAIR UP! Complete pg 5 in your unit 5 SAW

Section 10.3 Theory of Natural Selection – how species change

Artificial selection is the process in which humans change a species by breeding it for certain traits. This is done by selecting favorable traits (color, size, etc) and then breeding individuals who have them. This can only happen with traits that are heritable, which is the ability of a trait being passed down from one generation to another.

Natural selection is a mechanism by which individuals that have inherited beneficial adaptations usually produce more offspring than others. What would happen if I turned on the air conditioner full blast? Who would have an advantage and disadvantage?

Question: If humans are the selective agent in artificial selection, then what is the selective agent in natural selection?

The selective agent in natural selection is: ________________________________.

Whatever traits or features you have gives you a survival advantage over others in the current environment, which will give you a better chance to produce offspring that will inherit those traits.

List some examples of selective agents in nature:

What example would the diagram to the right model?

A population is all the individuals of a species that live in an area. Darwin saw in the Galapagos Islands variation among the tortoise population.

Four main principles to natural selection

Variation – in every population, there are variations (heritable differences) that exists based on the differences in the genes

Overproduction – having many offspring will led to competition between them for resources (“early bird gets the worm”)

Adaptation – variations can give an individual an advantage over others when competing in an environment; those who are successful are “naturally selected” to live longer and pass on their traits to their offspring (Hunger Games analogy!)

Descent with modification – Over time, natural selection will create a species well adapted in the environment as long as the environment stays the same

Question: What traits can favor a high school student that is competing for entry into a university?

Fitness is a term that measures the ability to survive and produce more offspring compared to other members in a population.

An example of species changing due to fitness is in this diagram. What do you notice about the basic shape of the beaks? What happened to the original species?

Key point of Natural selection - Natural selection acts on phenotypes rather than genetic material. Only new alleles can be created by genetic mutation.

Section 10.4 Evidence of Evolution – several sources suggest a common past

Darwin’s argument for evolution mainly came from four sources:

EVIDENCE FOR EVOLUTIONFossils Geography Embryology Anatomy

Showed that organisms changed over time;

supported Darwin’s descent modification concept

Island species most closely resemble species on the

nearest mainland; Finches (birds) on Galapagos Isles

had different beak sizes and diets but were all related to mainland finches (refer to

drawings of finches)

The similar features of embryos in very different

organisms suggest evolution from a common ancestor; Darwin saw similarities in

crab and barnacle embryos (look at the Embryo resemblances chart)

Comparing body parts of different species –

Homologous structures are features that are similar in

structure but appear in different organisms and have different functions;

most common is forelimbs of four-limbed vertebrates (refer to diagram below)

Other anatomy structures

Analogous structures are structures that perform a similar function but not similar in origin. For example, bat wings have bones while insect wings have membranes. They evolved separately.

Vestigial structures are traces of an organ or structure that had a function in an earlier ancestor. An example is the wings of an ostrich that are only used for balance but not flying (but they have strong legs for running!). In humans, your appendix is a vestigial structure that came from a part of the large intestine

used for digesting plant-based cellulose. Since we are omnivores (eat everything) cellulose was not a main diet and humans gradually didn’t need the appendix.

**Pair up! pg. 14 in your unit 5 SAW

SKIP FOCUS READING on PAGE 17 and PAGE 25 in your unit 5 SAW

Section 11.1 Genetic Variation within Populations – common gene pools

In order for natural selection to act on different physical traits, there has to be a variety of genes to choose from. Genetic variation in a population is stored in a gene pool, which are the combined alleles of all of the individuals in a population. When organisms mate, they create offspring that may have different rates of alleles. The measure of how common a certain allele is in the population is called allele frequency.

Question: How do you calculate the allele frequency?

The sources of genetic variation

Mutation and recombination are two sources of genetic variation. Mutation is a random change in the DNA of a gene. Recombination is the mixing of parent’s alleles during meiosis.

Section 11.2 Natural Selection in Populations – changes in distribution

Every population has an average range of traits; height for example can be shown as a normal distribution with medium height as the most common and the shortest and tallest as rare. A bell-shaped curve can represent this distribution.

DRAW IT! – Draw a bell-shaped curve in the box to the right.

Natural selection can change the distribution of a trait in one of three ways below:

1.) Directional Selection – favoring phenotypes at one extreme of a trait’s range (really tall or really short height); it causes a shift in the distribution of the phenotypes where rare traits become more common (example would be drug-resistant bacteria due to antibiotics)

Normal Distribution

2.) Stabilizing Selection – The phenotype in the middle becomes favored and more common. Continued selective pressure on the extremes will decrease genetic diversity.

3.) Disruptive Selection – The extreme phenotypes are favored while the middle becomes less common. Eventually, disruptive selection can lead to the formation of new species. **PAIR UP! Complete pg. 27 in your unit 5 SAW

Section 11.3 Other Mechanisms of Evolution – genetic flow and drift

Species in one population can migrate (leaves to another) to other populations causing their alleles to join another population’s gene pool. When this happens, their alleles are withdrawn from its former population’s gene pool. Gene flow is the movement of alleles from one population to another. As gene flow increases, the more similar two populations become and as the gene flow decreases, the less similar two populations become.

Setting up genetic drift

Sometimes there will be a small sample size of a population that has been selected by chance to create a new population. The changes in allele frequencies that are due to chance are called genetic drift where some alleles increase in frequency and other are decreased or ultimately lost. There are two processes that can set up genetic drift:

1.) Bottleneck Effect – a destructive event greatly reduces the population to where only a few are left to repopulate. The surviving population will not have much genetic variation since only a small, random sample survived.

2.) Founder Effect – this occurs when a small number of individuals colonize a new area. The gene pools will be much different from the original, larger population. Example is the high rate of dwarfism in a particular Amish community in Pennsylvania which traces back to one of the original founding couples.

Question: What are two negative effects of genetic drift? *think genetic diversity and lethal alleles

Section 11.5 Speciation Through Isolation – how can they be separated?

One of the ways new species can be formed is when populations become isolated and there is no longer gene flow occurring between them. From there, speciation can occur which is the rise of two or more species from one existing species. Here are the different types of isolation that can lead to speciation:

Geographic isolation involves physical barriers that divide a population into two or more groups. Examples are rivers, mountains, or dried lakebeds.

Reproduction isolation occurs when members of different populations can no longer mate successfully with one another. This is the final step before two populations become separate species.

Behaviorial isolation is isolation caused by differences in courtship or mating behaviors.

Temporal isolation exists when timing prevents reproduction between populations. Certain individuals in a population may be ready to mate at a different time then others, which can lead to speciation. **Pair up! Complete pg. 35 with your partner in your unit 5 SAW

Section 11.6 Patterns in Evolution – natural selection is not random!

Genetic drift and mutations are random and unpredictable but natural selection which acts on these is not. When the environment changes, it is up to a species to respond by adapting.

Convergent evolution is evolution toward similar characteristics in unrelated species. Analogous structures, such as wings on birds and insects or fins on sharks and dolphins (fish and mammal), are common examples.

Divergent evolution is when closely related species evolve in different directions. For example, the red fox lives in forested areas (red coat helps it blend) while the kit fox lives in desert regions (sandy coat helps it blend) and they both came from a common ancestor.

Coevolution is the process in which two or more species evolve in response to changes in each other. For example, the bull-thorn acacia plant had co-evolved with stinging ants by providing a hallow-out thorn as a house in return for the ant’s stinging defense.

Extinction is the elimination of a species from Earth. This usually occurs when a species is unable to adapt to changes in the environment. Background extinctions occur all the time but at a low rate and affect only one or a few species. Mass extinctions are rare but more intense and can destroy many species (an estimated 5 have occurred in last 600 million years).

Question: Should humans allow extinctions to occur or should humans intervene to save the species? Explain your answer.

Mutation Lab! Turn to page 43 in your unit 5 SAW to begin your mutation lab activity. Find a partner and gather your necessary lab materials.