Evolution of Populations

Variation and Gene pools Gene Pool – All genes, including all the different alleles that are present in a population. Relative Frequency –the number of times a particular allele is present in a population compared to other alleles for the same gene. (%)

Variation and Gene Pools Evolution involves changes in populations over time. Evolution is any change in the relative frequency of alleles in a population.

Sources of genetic variation Mutations – any change in a DNA sequence Remember mutations don’t always lead to changes in an organisms phenotype (the change could result in the same amino acid being coded). Gene Shuffling – Remember that homologous pairs separate and your alleles get shuffled around, crossing over also occurs all to INCREASE genetic variation.

Variation and Traits Single-gene trait: controlled by a single gene that has two alleles. Mendel saw a ton of examples of these in his pea plants! Widow’s peak.

Natural Selection – Single-Gene Traits Natural selection on single-gene traits can lead to changes in allele frequencies. Example: The introduced black color mutation becomes more common over time, leading to a change in allele frequencies and therefore, evolution.

Polygenic traits Most traits are controlled by more than one gene (Polygenic) This means that one polygenic trait can have many possible phenotypes and genotypes, forming the shape of a bell curve. Examples Height Skin Color What’s your height in inches? Graph students heights 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

Directional Selection Directional Selection: when individuals at one end of the curve have a higher fitness than individuals in the middle or at the other end of the curve.

Stabilizing Selection Stabilizing Selection: When individuals near the center of the curve have higher fitness than individuals at either end of the curve.

Disruptive Selection Disruptive Selection: When individuals at the upper and lower ends of the curve have higher fitness than individuals in the middle.

5 Minute Break

Other Sources of Evolutionary Change Natural selection isn’t the only source of evolutionary change. In small populations it’s much harder to predict allele frequency. Genetic Drift: Random change in allele frequency due to chance in small populations.

Founder Effect Founder Effect: Change in allele frequencies as a result of the migration of a small subgroup of a population. This is Genetic Drift – a chance event The migrating population may have different allele frequencies from the original population, which would alter the population allele frequencies.

Genetic Equilibrium – No evolution Are there any conditions under which evolution will not occur? Hardy-Weinberg Principle: allele frequencies in a population will remain constant unless one or more factors causes the frequency to change. Genetic Equilibrium: allele frequencies remain constant in a population.

What conditions are required for genetic equilibrium? 1. There must be random mating All members have an equal opportunity to reproduce. 2. The population must be really large. Less affected by genetic drift. 3. There can be no movement in or out of the population. Gene pool is isolated. 4. No mutations. So no new alleles are introduced into the population. 5. No natural selection. All genotypes have an equal probability of survival in the environment.

Genetic Equilibrium If any of these conditions are not met, EVOLUTION will take place.

Speciation Natural selection and chance events can alter the allele frequencies drastically over time. When this results in a new species it is known as speciation

What leads to speciation? Reproductive Isolation: When the members of two populations cannot interbreed and produce fertile offspring. Respond to Natural selection as separate units. This can occur in 3 ways: Behavioral Isolation Geographic Isolation Temporal Isolation

Isolating mechanisms Behavioral Isolation: members of two populations are capable of interbreeding but don’t because they differ in mating rituals or reproductive strategies. Examples: birds with different songs, frogs with different mating calls.

Isolating Mechanisms Geographic Isolation: members of different populations don’t interbreed because they are separated by a geographic barrier (river, mountain etc.). This results in the development of two different gene pools and natural selection acting on them separately.

Isolating Mechanisms Temporal Isolation: members of different populations don’t interbreed because they reproduce at different times.

1. Founders Effect/Geographic Isolation. 2. Geographic isolation, finches adapt independently on new island. 3. Finches fly to other islands in the Galapagos (geographic isolation again). 4. Natural selection acts separately on different islands (different gene pools). 5. Finches fly back and forth between islands but at this point have different behaviors and can no longer interbreed.

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