Evolution of Populations Chapter 16 Dr. Donna Howell Biology I Blacksburg High School

Genes and Variation In the 1930’s, experts finally connected the work of Gregor Mendel and Charles Darwin. They realized for the first time that changes in genes produced variation in offspring. They then said that natural selection works on this variation.

Variation and Gene Pools One way scientists study genetic variation is through the study of populations – both animal and human. Because members of a population interbreed, they share a common group of genes called a “gene pool.” The relative frequency of an allele is the number of times that the allele occurs in a gene pool.

Variation and Gene Pools Gene pools are important in the field of evolution because evolution is the change in relative frequency of alleles in a population.

Sources of Genetic Variation One major source of genetic variation is mutations in DNA. These can occur due to mistakes in DNA replication or due to environmental factors such as chemicals, radiation, etc. Can affect an organisms “fitness.”

Sources of Genetic Variation Another source of genetic variation is the shuffling of genes. This occurs during the production of eggs and sperm. The 23 pairs of chromosomes can produce 8.4 million different genetic combinations.

Single-Gene and Polygenic Traits The number of phenotypes produced for a given trait depends on how many genes control the trait. Some traits are controlled by a single gene. Others are controlled by two or more genes.

Single-Gene and Polygenic Traits An example of a single-gene trait is the gene that codes for the widow’s peak in your hair. An example of a polygenic trait is skin color in humans. Because multiple genes code for this, there is a very wide range of possible skin colors.

Natural Selection on Single-Gene Traits Natural selection doesn’t act on genes – it acts on whole organisms because either the organism lives or dies with all of its genes. Natural selection on single-gene traits results in changes in allele frequencies and thus evolution.

Natural Selection on Single-Gene Traits An example is the population of lizards to the right. Mutations have occurred that produced red and black lizards. The red ones are more visible to predators, so disappear. The black ones can get warmer in the sun, eat more, and thus survive.

Natural Selection on Polygenic Traits When there is more than one gene that controls a trait, natural selection is more complex. Can affect in three ways: Directional selection Stabilizing selection Disruptive selection

Natural Selection on Polygenic Traits Directional selection is when individuals at one end of the curve have higher fitness than individuals at the middle or other end of the curve. Example: a food shortage causes the supply of small seeds to run low, and beak sizes get bigger as a result because only big seeds left.

Natural Selection on Polygenic Traits Stabilizing selection is when individuals near the center of a curve have higher fitness that those at either end. Example: human babies born at average mass are more likely to survive than babies born either much smaller or much larger than average.

Natural Selection on Polygenic Traits Disruptive selection is when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle Example: average-sized seeds become less common, and larger and smaller seeds become more common.

Genetic Drift Genetic drift is a random change in allele frequency due to a smaller population. Can occur when a small group of individuals colonizes a new habitat. The founder effect occurs when a small subgroup of a population migrates away from the rest.

Genetic Equilibrium Genetic equilibrium is when allele frequencies in a population remain constant. If allele frequencies remain constant, evolution does not occur. The Hardy-Weinberg Principle says that allele frequencies will remain constant unless something causes it to change.

Genetic Equilibrium Five conditions are required for genetic equilibrium to take place: Random mating Population must be large No movement into or out of population No mutations No natural selection

Genetic Equilibrium Five conditions are required for genetic equilibrium to take place: Random mating Population must be large No movement into or out of population No mutations No natural selection

Speciation Natural selection can lead to new allele frequencies in a population. But how does this lead to the formation of whole new species? The formation of new species is called speciation.

Speciation – Reproductive Isolation As new species evolve, populations exhibit reproductive isolation.

Speciation – Reproductive Isolation One way reproductive isolation can occur is through behavioral isolation. Ex: different courtship rituals or mating songs

Speciation – Reproductive Isolation Another way reproductive isolation can occur is geographic isolation. Ex: get stuck on two different sides of a river

Speciation – Reproductive Isolation Another way reproductive isolation can occur is by temporal (time) isolation. Ex: reproduce at different times of year

Speciation of Darwin’s Finches Darwin studied a group of finches in the Galapagos, and documented the process of speciation.

Speciation of Darwin’s Finches Darwin found that speciation occurred by: Founding of new population Geographic isolation Changes in gene pool Reproductive isolation Ecological competition

1. Founders Arrive To begin, a few species of finches either flew or were blown to another island. They survived and reproduced.

2. Geographic Isolation Later, some birds crossed to another island and stayed there. They survived and reproduced and no longer shared the same gene pool with birds on the original island.

3. Gene Pool Changes Over time, populations on the different islands became adapted to their local environments. Natural selection caused beaks to change according to what birds ate.

4. Reproductive Isolation Once the gene pool changes, different species can no longer mate with one another and produce offspring. Example: mate at different times of the year, different courtship rituals, etc.

5. Ecological Competition Now that different species are living together, they must compete for available food (seeds). Their beaks will evolve to eat whatever kinds of food they can find.

The End