Evolution of Populations Chapter 16

Evolution of PopulationsChapter 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

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

Evolution of Populations Chapter 16