Evolution Test Review Session!!

Review: Natural Selection

• Natural selection is the process by which random evolutionary changes are selected for by nature in a consistent, orderly, non-random way

• Natural selection works on phenotypes because phenotypes interact with the environment. A bird doesn’t care if a green beetle is a heterozygote or a homozygote, it just cares what color (phenotype) the beetle is

More Natural Selection

• Darwin came up with natural selection based on observations from his travels

• Natural selection does not always remove lethal recessive alleles from the gene pool because heterozygotes can act as carriers – they are not affected by the one recessive allele, but they can pass it on to their offspring

Competition and the Struggle for Existence

• Competition - when there are not enough resources (food, water, space) to support all of the organisms in an environment, the organisms have to compete with each other to get enough resources to survive.

• The struggle for existence is how members of each species compete to obtain food, living space, and other necessities of life

Patterns of Natural Selection

• Directional: selection favors one extreme version of the trait, over time, the range of phenotypes shifts on one direction

• Stabilizing: selection favors traits close to the middle, over time the range of phenotypes narrows

• Disruptive: selection favors both extreme forms of a trait, over time phenotypes in the middle are selected against

Conditions Necessary for Natural Selection to happen

Natural selection is the mechanism for evolution that Darwin proposed. Darwin wrote about three conditions that must be met for natural selection to happen.

1. Variation in characteristics: different individuals in a population must have different characteristics. For example, beetles of the same species may come in green or brown.2. Differences in fitness: the differences in characteristics between individuals must contribute to their probability of surviving and reproducing. For example, the green color beetles might help them survive by making them harder to see and therefore less likely to get eaten.3. Heritability of characteristics: the different characteristics that affect fitness must be heritable (able to be passed down to offspring)

More on Natural Selection

• Adaptations are traits that give organisms an advantage, making them more likely to survive and reproduce

• In natural selection, traits selected for or against. Traits that are selected for are seen in organisms that survive and reproduce, traits that are selected against start to disappear as organisms who have them die without reproducing OR they reproduce less

• Successful reproduction means that organisms have offspring, and that those offspring are able to reproduce. It does not really matter how many babies they have at a time or how quickly they can make babies – what matters is whether or not they survive

Review on Mutations & Genetics

• Mutations are changes in the DNA code• Mutations cause genetic diversity• Dominant & recessive alleles – dominant

alleles are always expressed – they always show up I the phenotype if they are present. Recessive alleles only show up in the phenotype if there are no dominant alleles present

More on Genetics

• Punnett squares• Heterozygotes have two

different alleles for the trait! (Aa)

• Homozygous dominant organisms have two dominant alleles for the trait (AA)

• Homozygous recessive organisms have two recessive alleles for the trait (Aa)

Antibiotic ResistanceIn any population of bacteria, there may be a few that have a random mutation that makes allows them to survive even in the presence of an antibiotic. The antibiotics then kill off all the remaining bacteria, so only the resistant bacteria survive and reproduce. The offspring of these bacteria will also be resistant (remember that bacteria reproduce asexually, so all offspring are identical to the parents unless there was a mutation).

Genetic Variation & Biodiversity

• Genetic variation increases chances that some members of a species will survive environmental change

• Lots of biodiversity (many different species) increases chances that some species will survive great change to environment

Change in Allele Frequency

• At first, we defined evolution as any change in the heritable traits within a population across generations

• This definition is still true, but we added another way to define evolution

• Evolution is also any change in the allele frequency in a population.

Isolation leads to speciation• Geographic – population is separated by a mountain

range, river, island, etc• Behavioral – some members of the population start

acting differently, such as doing different mating dances

• Temporal – some members of the population prefer to mate at different times of day or during different seasons

• Reproductive – members of the population cannot mate with each other and produce fertile offspring. Horses and donkeys can mate to produce mules, but mules are sterile

Genetic drift

• Genetic drift is a change in allele frequencycaused by a random event• The Founder effect happens when a new population

is started by a group that randomly has different allele frequencies than the rest of the population

• A bottleneck is when many members of a species die, leaving only a few survivors to live on and reproduce

• IN ALL TYPES OF GENETIC DRIFT, THERE IS A LOSS OF VARIATION!!!

Calculating Allele FrequencyThe allele frequency is the percent of dominant or recessive alleles in a gene pool. The total alleles in a gene pool always adds up to 100%

To calculate allele frequency, add half the percent of heterozygotes to the total percent of whichever homozygotes you are looking at.

Example:A population is 10% homozygous dominant (BB)70% heterozygous (Bb)20% homozygous recessive

Frequency of dominant allele = 10% + ½ 70% =10% + 35% = 45%

Frequency of recessive allele = 20% = ½ 70% = 20% + 35% = 55%

Mass Extinctions

• Mass extinctions have a profound effect on speciation and on the variation of life on earth. After a mass extinction, there are periods when a many new species form (speciation) and evolution happens quickly because there are so many niches that are now available for the survivors.

• There have been 5 mass extinctions • The last mass extinction (besides the one we are probably in the

middle of now) was 65 million years ago and it was probably caused by a meteor that hit the Yucatan peninsula, caused a big dust cloud, and changed weather patterns

• All of the previous mass extinctions were not from human-made causes because humans had not evolved yet!!!!

Fossil Evidence

• What it tells us about life on earth:– Life has changed over time– The total number of species that have ever lived

on earth is FAR greater than the number of species alive today

Good Luck!!!