Evolution (Unit 7)

Four major mechanisms of microevolution

Genetic drift, mutation, gene flow and natural selection (including sexual selection).

Genetic drift

A change in the gene pool of a population due to chance, bottleneck effect (event such as a natural disaster decreases population size, random who survives), and founder effect (event where organisms leave their population and start a new population, with smaller random population size) both decrease genetic diversity.

Mutation

Specific gene mutations, or larger chromosomal mutations which is the only way to create new alleles, increasing genetic diversity by lowering existing allele frequency in a population and increasing the allele frequency of the mutation in a population.

Gene flow

Movement of individuals or gametes/spores between populations, key difference with genetic drift is that it joins an already existing population (most of the time).

Natural selection (including sexual selection)

Individuals differ in their survival & reproductive success, generally increases one allele frequency and thus decreases another.

Evolving population?

Must be a change in allele frequencies (percentages, not just numbers per population).

Evolution is a change in the allele frequencies of a population over generations.

Biological species concept strengths

A species is a group of organisms that can actually or potentially interbreed in nature to produce viable fertile offspring. Clear, widely accepted definition, direct link to evolutionary mechanisms.

Biological species concept limitations

Does not apply to asexual organisms, can not apply to fossils.

Genetic isolation → speciation BARRIER

Creates barriers to gene flow, such as

PREZYGOTIC: Habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gametic isolation.

POSTZYGOTIC: Reduced hybrid viability, Reduced hybrid fertility, hybrid breakdown (Future offspring of hybrids feeble or sterile).

which lead to genetic divergence and diversity due to the other 3 mechanisms, and then more barriers can occur making interbreeding impossible!

Genetic divergence → speciation PROCESS

Accumulation of different mutations and adaptations in the isolated groups, eventually leading to distinct species/reproductive isolation.

Microevolution vs Macroevolution

Small scale genetic changes within a population over a few generations, ie. pesticide resistance, versus large scale evolutionary changes occurring above the species level over geological time, such as the origin of new species. Mainly scale, as rely on the same processes.

Natural Selection

1. Does the trait vary among individuals in the population?

2. Is the trait influenced by genes that pass from parents to offspring?

3. Are individuals with a certain trait variation more successful at reproducing than others?

Frequency change (result, not checkbox), variability, heritability, reproductive advantage required.

Misconceptions

Individuals do not evolve: POPULATIONS EVOLVE.

Acquired characteristics can face natural selection: NATURAL SELECTION CAN ONLY INCREASE OR DECREASE HERITABLE TRAITS; ACQUIRED CHARACTERISTICS CANNOT BE PASSED ON TO OFFSPRING.

Environmental challenges cause advantageous mutations: ADVANTAGEOUS MUTATIONS ARE NOT “CAUSED.“

Evolution is goal directed and leads to perfection: EVOLUTION IS NOT GOAL DIRECTED AND DOES NOT LEAD TO PERFECTION; FAVORABLE TRAITS VARY AS ENVIRONMENTS CHANGE.

Evolution

A change in allele frequencies of a population over generations.

Evolution results in existing variations that provide advantages to environmental challenges becoming more common in a population. Circumstantial, and not perfect since circumstances change.

Evidence for evolution

Fossil Record (Compare fossils, track similarities, anatomical similarities), Biogeography (Studying geographic distribution provides info about how and when species evolved), Direct observations (Case studies, ie. evolution of antibiotic resistant bacteria), and Homologies (Embryos, Molecular data, Anatomical similarities, and vestigial structures).

Anatomical homologies

Anatomical similarities can imply a common ancestor between diverse animals, the more similar they are, the more similar their DNA sequences must be. Also implied not uniquely engineered since different functions but same skeletal elements, variations → signs of adaption.

Vestigial structures

Most often seen with a smaller version of Anatomical Homologies, but it explains how we have these things that are no longer needed and unnecessary, and the reason why we still have it is because it was passed down by ancestors who did need it.

Molecular homologies

By comparing the same sequences of DNA/amino acid monomers between different species, the differences can be more easily seen, and the more similarities = closer common ancestor, more differences = farther common ancestor, as DNA is inherited, so essentially passed down, so similarities imply connection. Useful to compare necessary sequences with functions that most organisms require to survive/be fit.

Embryonic Homologies

These reveal similarities that are not visible in adult organisms, as it is present during embryonic development but it differs into different functions for adults. The similarities imply a common ancestor, as the genes appear similar due to these similarities.

Phylogenetic trees.

Closer branch off point, more closely related, since had less time to develop differences. More similarities = more closely related, see which ones share more versus less. Reasoning for more similarities, had to have less time to develop differences, more closely related. However, the trees are works in progress as new evidence emerges, since we find out more and more about the species and their similarities, their differences, new evidence of who is more closely related to who, who is diverging.

Rapid development of adaptations and high reproduction rate of viruses and bacteria

Viruses and bacteria reproduce very quickly, so mutations are more likely for them to occur more frequently (just due to reproducing more in a given period of time), and so they rapidly develop adaptations.