Mechanisms of Population Evolution

Rebecca Hite

Chapel Hill High School

Chapter 16

The History of Evolutionary Biology

• When Darwin developed his theory of evolution, he did not understand how heredity worked! – He didn’t know how traits were passed onto the next generation.– He didn’t know how variation in populations appeared.

• Since the 1930s, we know now that traits are passed via DNA from parents to offspring and these traits that are passed are dependent on natural selection.

Populations Evolve, not Individuals• An individual organism

cannot evolve its phenotype in response to its environment.

• Each individual has genes that characterize the traits of their species, and they exist as pairs of alleles on a chromosome.

• Evolution occurs as a population’s genes and their frequencies change over time.

How do a population’s genes change?• Picture all the alleles of a population

together in a pool, called a gene pool.• The percentage of any specific alleles

in the gene pool is the allelic frequency.– Relative frequency is the number of times

that an allele occurs in a gene pool.

• A population in which the frequency of alleles remains the same over generations is in genetic equilibrium.

• Therefore, Evolution is defined as the change in the relative frequency of alleles in a population over time. – Think of a change in the environment in

which the recessive allele is favored, what would eventually happen to the entire population?

2 Main Sources of Genetic Variation• Mutations. There is any change in the

sequence of DNA. – This can occur naturally though mistakes in DNA

replication or as a result of unnatural radiation or chemicals in the environment.

• Can be positive – Increase fitness and propagate into the population.

• Can be negative - Those individuals cannot survive or reproduce.

• Can be neutral - Sometimes mutations have no affect on the phenotype.

Changes in Genetic Equilibrium: A.k.a. Evolution

• A population in genetic equilibrium is NOT evolving.

• One mechanism for evolution is mutation.

• Genetic Drift can occur where rare events can alter allelic frequencies.

• Can be prominent in very small populations.

• Gene Flow is the input or output of genes due to individuals moving around.

Genetic Drift in Action

2 Main Sources of Genetic Variation• Gene Shuffling – variation in offspring due to

gene shuffling in the production of gametes. – Remember during Meiosis a process called crossing over

(Chiasmata) occurs during prophase I causing genes to get mixed up.

– As a result, the 23 pairs of chromosomes found in humans can produce 8.4 million different combinations of genes!

– Sexual reproduction can produce many different phenotypes but does not change the relative frequency of alleles (or genotypes) in a population.

• Think a card game, you can get different hands, but it’s always the same deck of cards. The likelihood of pulling an ace is still 4/52 or 1/13 no many times you shuffle the deck.

Calculating Phenotypic/Allelic Frequencies

• w3.dwm.ks.edu.tw/bio/ activelearner/18/ch18c5.html

5 Conditions to Maintain Genetic Equilibrium

• In order for a population to not evolve, the following must happen.

1. Mating must be random. (No sexual selection.)

2. There is a very large population size. (No bottlenecks.)

3. There is no movement into or out of the population. (No immigration or emigration.)

4. No mutations. (No changes in the DNA.)

5. No natural selection. (No changes in the environment or presence of predators.)

• But as you know of geologic time and the reality of the world these do not always occur.

Natural Selection, the major driving force of Evolution!

• Recall variations can increase OR decrease an organism’s chance of survival in its environment. (Fitness)

• The factor that lends to particular success or death is called the selection factor. – What was the selecting factor for

Darwin’s Finches?

• There are 3 outcomes to these selection factors.

Stabilizing Selection• Favors average

individuals in the population.

• Those average in the population survive and reproduce, flooding the gene pool.

• Reduces variation of individuals in a population.

• Keeps all individuals towards the middle and stabilizes the species.

Directional Selection

• Favors 1 extreme of individuals in the population.

• Those extreme in the population survive and reproduce, flooding the gene pool.

• Can lead to rapid evolution of a population.

• What can you think of that would cause this?

Disruptive Selection• Favors either extreme of

individuals in the population.

• Those 2 extremes of the population survive and reproduce.

• Leads to no intermediate forms of the trait and the evolution of 2 new species.

• How could the breakup of Pangaea cause disruptive selection?

Outcomes to Selection• Speciation occurs when

members of similar populations no longer interbreed and produce fertile offspring within their natural environment.– Are mules a species?

• Three ways to cause speciation:– Geographic Isolation– Reproductive Isolation– Temporal Isolation

Models of Evolution

• Gradualism – species originate through a gradual change of adaptations. – Fossil record– Slowly and steadily

throughout time

• Punctuated Equilibrium – speciation occurs quickly, in rapid bursts, with long periods of genetic equilibrium in between – About 10,000 years or less– Fossil Record

• Both end up with Speciation depending on the circumstances

Patterns of Evolution• Adaptive Radiation – an

ancient species radiated out into many species adapted to their environment.

• Divergent Evolution – One species diverged to become distinct species.

• Convergent Evolution – unrelated species occupy the similar environments in a different part of the world.– Because of similar environmental

pressures, they hare similar pressures of natural selection despite their common ancestry.