Microevolution & Macroevolution

From observing to extrapolating

Gene Pools and Variation

• Gene Pool: All the alleles in all the individuals that make up that population i.e. the reservoir from which the next generation draws its genes

• This is where the genetic variation is stored

• For some traits, there can be a range of phenotypes

Sources of Variation• 2 main sources within a

population:• Mutation: change in

sequence of DNA. This may be caused by DNA replication errors or due to exposure to radiation /chemicals in the environment. Most are silent, but some change the phenotype

• Sexual Reproduction: Due to the scrambling of existing alleles. Differences as a result of sexual recombination. Each offspring gets a new combination of alleles

Hardy-Weinberg equilibrium

• Populations that do not undergo change to their gene pools are not currently evolving, this condition is known as the Hardy-Weinberg equilibrium.

• i.e. Frequencies of alleles in that gene pool are constant over time. Expressed as a percentage/decimal. Five conditions:• There must be random mating• There is a large population• There is no movement in or out of the population• There are no mutations• There is no natural selection

• Rarely for long in nature

Microevolution

• The generation-to-generation change in frequencies of alleles within a population is called microevolution. Its evolution on the smallest scale.

• Mechanisms of microevolution:• Natural Selection• Sexual Selection• Artificial Selection• Genetic Drift• Gene Flow

Natural Selection

• Not a random process

• The environment increases the frequency of the alleles that provide a reproductive advantage to individuals and thus leads to the evolution of adaptations• Directional Selection• Stabilizing Selection• Disruptive Selection

Stabilizing Selection• Occurs when individuals

near the centre of the phenotype range have a higher fitness than individuals at either end of the range e.g. human babies at birth• Those extremely underweight

or overweight are unlikely to survive

Directional Selection• Occurs when individuals at one

end of the phenotype range have higher fitness than individuals in middle or the other end of the range.

• E.g Medium ground finches during El Niño years.

• E.g. Pesticide Resistance

Disruptive Selection

• Occurs when individuals at the upper and lower ends of the range of phenotypes have higher fitness than individuals near the middle.

• This may lead to the rise of two distinct phenotypes within a population

• E.g

Sexual Selection• Sexual selection is a form

of natural selection in which certain individuals with certain inherited traits are more likely to obtain mates than other individuals.

• It is not a random process

• e.g. peacocks and birds of paradise

Artificial Selection• Farmers breed cows that excel

in milk production

• Not a random process because the farmers leave nothing to chance.

• In selective breeding, microevolution can occur at a very fast pace

• BUT also reduce the genetic variability within the population leading to genetic disorders

• E.g. Pedigree dogs

Genetic Drift• This is a random process

• Genetic drift is a change in the gene pool of a population due to chance. All populations are subject to some genetic drift.

• The smaller the population the more impact genetic drift has on it.• Bottleneck Effect: Disasters like

earthquakes, floods etc. may drastically reduce a population, reducing its gene pool and genetic variation e.g. cheetah

• Founder Effect: Likely when few individuals colonize an isolated habitat like an island e.g. Galapagos finches

Gene Flow• This is a random process

• Gene flow involves the movement of genes with another population i.e when fertile individuals or their gametes migrate between populations

• This movement disturbs the genetic equilibrium

• Tends to reduce genetic differences between populations

• Can eventually mix neighboring populations into a single population with a common gene pool

Species• Biological Species

Concept: A species is a population or group of populations whose members have the ability to breed with one another in nature and produce fertile offspring.

• One species cannot interbreed with members of another species

• Limitations: Asexually reproducing organisms

Speciation• The origin of new species is

known as speciation.

• This is the main focus of the study of macroevolution

• Formation of new species depends on some type of reproductive isolation from similar species

• Reproductive isolation refers to the inability of two organisms to reproduce due to some kind of physical or behavioral barrier

Geographic Isolation• Geological Processes can

separate different populations of one species and lead to geographic isolation e.g. Mountain range which individuals on either side cannot cross

• In other cases, populations may become separated when a small group disperses from the main population and colonizes an isolated location like an island

• This type of isolation is dependent on the ability of organisms to move about and is different for different organisms e.g. a canyon for squirrels

“Splinter” population• The small population that gets separated

from the main population is known as a “splinter population”

• This population may follow its own evolutionary course

• Changes in allele frequencies caused by genetic drift and natural selection can accumulate, making it less and less similar to the main population

• For each small, isolated population that becomes a new species, many more die because a lot of colonizing populations are unable to survive in the new locations

• Even if they survive, they don’t necessarily develop into a new species

• Speciation would occur only if the population can no longer breed with the main population, even if they come back into contact

Adaptive Radiation• Oceanic archipelagos serve

as living laboratories to study speciation.

• They often have species found nowhere else because the isolation and diverse habitats of some islands create conditions that seem to favor speciation

• Small populations of colonizing species may undergo evolutionary change

• Evolution from a common ancestor that results in diverse species adapted to different environments is called Adaptive radiation or Divergent Evolution

Convergent EvolutionDistantly related species that live in similar environments develop similar adaptations (analogous structures) e.g. All ant eating mammals have long pointed noses and jaws and long sticky tongues

The Rate of Speciation• Gradualism is the evolution of a species

by gradual accumulation of small genetic changes over a long period of time

• However in the fossil record species suddenly emerge. In 1972 paleontologists Niles Eldredge and Steven Jay Gould developed the Punctuated Equilibrium Model

• It suggests that species often diverge in spurs of relatively rapid evolutionary change i.e. long periods of little evolutionary change are broken, or punctuated, by shorter times of rapid speciation

• By the time the species grew in number and became widespread enough that it might leave a fossil record, its distinctive features would have already evolved

Evolution as an Ongoing Process

Species have a unique ability to adapt to their environment. Two traits in a species develop to help the species survive e.g. poison-dart frogs: color and venom and predator awareness

Co-Evolution

• When one species evolves in response to the evolutionary changes in another species.

• This is really evident in the evolution of flowering plants and insects. They have a mutualistic relationship with their pollinators.

• Symbiotic Relationship: Both organism mutually benefit from each other. Pollen is food for insects and necessary for plant reproduction

Plants and Herbivores• Flowering plants have

evolved toxic chemicals to prevent herbivores from eating them.

• But then natural selection favored the animals that could alter, inactivate or eliminate those toxins

• E.g. Monarch caterpillars feed on toxic milkweed plants and mange to eat them and use them to make themselves poisonous

Mimicry• Evolution of one species

can affect evolution of another species, but it doesn’t work the other way around.

• Mimicry: When one species, the mimic, resembles another species, the model, to gain survival advantage e.g. Hoverfly looks like a wasp but can’t sting

Evolution of Complex Features

• Complex features evolve as a result of small steps of adaptation

• e,.g. eye evolution: Patch of light sensitive cells Eyecup Simple Pinhole camera-type eye Eye with primitive lens Complex camera-type eye