Mechanisms of Evolution An individual organism cannot evolve its phenotype in response to its environment. Review Question: What determines the phenotype in an organism? Genes or Alleles! Lamarckism

How do the frequencies of genes in a population change…? Evolution occurs as a population’s genes and their frequencies change over time. How do the frequencies of genes in a population change…?

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 allele or gene frequency. What is the frequency of the red (R ) allele in the F1 generation? 5 R / 12 total alleles = 42%

A population in which the frequency of alleles remains the same over generations is in genetic equilibrium. 2 R / 4 total = 50% Is this population in genetic equilibrium? Between the generation 1 & 2? 5 R / 12 total = 42% NO Between the generation 2 & 3? 5 R / 12 total = 42% YES

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?

What causes changes in the allele frequencies of a population? Natural Selection: Mutations and gene shuffling create variation in a population’s alleles. A pressure from the environment “selects” one variation over another, allowing one variety to survive and reproduce better than another.

What causes changes in the allele frequencies of a population? 2. Genetic Drift: - a rare event changes the allele frequency in a population - small populations are more likely to be effected by genetic drift.

Genetic Drift Genetic drift can cause a bottleneck event.

What causes changes in the allele frequencies of a population? 3. Gene Flow: - Gene frequencies change due to the movement of individuals into or out of a population. - also known as migration

Cheetahs experienced a severe bottleneck around 10,000 years ago. Bottleneck: Loss of genetic variation due to sudden drastic reduction in population size. Cheetahs experienced a severe bottleneck around 10,000 years ago. Cheetahs are now so similar to one another genetically, that veterinarians can perform skin grafts between cheetahs without the immune system rejecting the skin!

Do you think all of these happen often? In order for genetic equilibrium to occur, the following requirements must be met: 1. Mating must be random. (No sexual selection.) 4. No mutations. (No changes in the DNA.) 2. There is a very large population size. (No bottlenecks.) 5. No natural selection. (No changes in the environment or presence of predators.) 3. There is no movement into or out of the population. (No immigration or emigration.) 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.) Do you think all of these happen often?

There are 3 major patterns that selection follows… The biggest cause of changes in gene frequencies, or evolution, is natural selection. Recall variations can increase OR decrease an organism’s chance of survival in its environment. (Fitness) There are 3 major patterns that selection follows…

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 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? Favors 1 extreme of individuals in the population.

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. 15

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.

Patterns of Evolution 2. Divergent Evolution: One species diverged to become distinct species.

Patterns of Evolution 3. Convergent Evolution: – unrelated species occupy the similar environments in a different part of the world, so they evolve similarly (Selective pressures that drive evolution are similar)