ap bio unit 7

Evolution

The gradual change of a species over time to better suit their environment.

• The current accepted theory for evolution was created by Charles Darwin during the 1880s

Adaptation

An inherited trait that makes an organism more fit for its environment.

• “Fitness” refers to an organism’s ability to reproduce

• Example: large beaks are an adaptation of some finches, which allows them to crack open seeds

Niche

Refers to the role or function of an organism or species in an ecosystem.

• Example: an organism’s food source, where it lives, what services it provides to an ecosystem, etc.

• Organisms or species with overlapping niches leads to competition

Natural Selection

A process in which individuals with favorable inherited traits are more likely to survive and reproduce.

• Natural selection is the mechanism, or driving force, of evolution

Artificial Selection

Where humans modify other species by selecting and breeding individuals with desired traits.

Key Features of Natural Selection

• Members of a population often vary in their inherited traits

• All species can produce more offspring than the environment can support, and many of these offspring fail to survive and reproduce.

• Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to have more offspring than other individuals.

• This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations.

• Note that populations, not individuals, evolve over time

• Natural selection can only increase or decrease existing heritable traits that vary in a population. It does not create new traits.

• The traits that are adaptive will vary with different environments.
  

Requirements for natural selection to occur

• Competition: More offspring must be produced than can be supported by the environment.

• Variation: Offspring must be produced with traits that differ from one another.

• Adaptation: Certain variations must be more advantageous than other variations. Some individuals must be more likely to survive and reproduce.

• Heritability: The favorable trait, or adaptation, must be genetic and capable of being inherited by the next generation.

Homologies

Similarities between species resulting from common ancestry.

Homologous structures

Anatomical similarities between species that share a similar structure, but not necessarily the same function.

SIMILAR STRUCTURE, DIFFERENT FUNCTION

• Example: The image shows these animals share the same basic structure of bones. However, the function of them are different.

Vestigial structure

Structures that once have served important functions in ancestral species, but have lost most, or all, of their function in living species.

Examples: Humans, dogs, and cats all have similar pelvises, which are homologous structures (similar structure, different function) to a vestigial pair of bones that snakes and whales have. These bones are the last remains of a pelvis, with no legs to attach.

Analogous structures

Similar features found in different species that are NOT the result of recent common ancestry. These structures may serve similar functions but have different anatomical structures.

DIFFERENT STRUCTURE, SIMILAR FUNCTION

Example: A shark fin, penguin wing, and dolphin flipper have the same function but differ in their anatomical structure.

Comparative embryology

Reveals anatomical homologies (similarities between species resulting from common ancestry) not visible in adult organisms.

Example: Gill slits and tail bones found in the embryos of humans, reptiles, birds, and fish.

Fossil record

Provides evidence of:

• The extinction of species

• The origin of new groups

• Changes within groups over time

Fossils can document important transitions.

Ex: the transition from land to sea in the ancestors of cetaceans

DNA Sequencing

• Modern technological advances now allow scientists to sequence nucleotides in DNA and amino acids in proteins

• By sequencing genes in different species, scientists can figure out what type of homologies exist between species on a molecular level.

Population

A localized group of individuals capable of interbreeding and producing fertile offspring.

• Gene variation is required for a population to evolve, but does not guarantee that it will. One or more factors that cause evolution must be at work for a population to evolve.

Gene pool

Refers to all the alleles, for every gene, present in individuals of a population.

• Represents all the possible alleles that could be inherited by the next generation

Allele Frequencies

• The frequency of an allele in a population can be calculated. 

  • For diploid organisms, the total number of alleles at a locus (gene location on a chromosome) is the total number of individuals times 2 

  • The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles.

• By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies.

  • p  usually represents the frequency of dominant alleles, while q represents the frequency of recessive alleles. 

• The frequency of all alleles in a population will add up to 1

  • p + q = 1

The Hardy-Weinberg Equation

• If p and q represent the frequencies of the only two possible alleles in a population at a particular locus, then p² + 2pq +q² = 1

  • p = frequency of dominant allele (usually)

  • q = frequency of recessive allele

  • p² = frequency of homozygous dominant genotype

  • q² = frequency of homozygous recessive genotype

  • 2pq = frequency of heterozygous genotype

• This is called the Hardy-Weinberg equation. It describes the genetic makeup expected for a population that is not evolving.

If the population is not evolving we say that the population is in Hardy-Weinberg equilibrium.

Hardy-Weinberg Equilibrium

• The Hardy-Weinberg equation describes a hypothetical population that is not evolving.

  • Allele and genotype frequencies remain constant from generation to generation for a population in Hardy-Weinberg equilibrium.

• If the data observed for the population differ from the expected values, then the population may be evolving.

  • In real populations, allele and genotype frequencies usually change over time.

(If Hardy-Weinberg equation equals to 1, the population is not evolving. If it doesn’t equal to 1, the population is evolving because something is causing a change in allele frequencies like natural selection or a mutation).

Conditions needed for a population to be in Hardy-Weinberg equilibrium:

1. No mutations

2. Random mating (individuals do not pick mates based on traits)

3. No natural selection

4. Extremely large population size

5. No gene flow (no migration)

PKU

The occurrence of PKU (a recessive genetic disorder) is said to be in Hardy-Weinberg Equilibrium for these reasons:

1. The PKU gene mutation rate is very low

2. Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

3. Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions.

4. The population is large.

5. Migration has no effect, as many other populations have similar allele frequencies.

Microevolution

The change in allele frequencies in a population over many generations.

• Three major factors alter allele frequencies and bring about most evolutionary change:

  • Genetic drift

  • Gene flow

  • Natural selection

Genetic Drift

Occurs when allele frequencies in a population change by random chance, not because of natural selection.

• The smaller a population, the more likely it is that chance alone will cause deviation from a predicted result.

• Genetic drift tends to reduce genetic variation through losses of alleles, especially in small populations. 

• Two examples of genetic drift:

  • The founder effect

  • The bottleneck effect