Evolution and Natural Selection I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection Charles Darwin The Origin of Species

Evolution and Natural Selection We wish to ask: What are the main points of Darwin’s theory of evolution? How did observations of nature lead to the formulation of the theory of evolution? How does the process of natural selection work? What evidence do we have for local adaptation? How can natural selection affect the frequency of traits over successive generations?

The Intellectual Climate in Darwin’s Time The age of the earth was uncertain, but studies of strata suggested a time sequence. Uniformitarianism -- the concept that present conditions and processes are the key to the past -- was becoming influential. Fossil discoveries revealed unknown life forms, suggesting extinctions had occurred. Similarities among organisms were beginning to be seen as evidence of relatedness.

Jean Baptiste Lamarck A predecessor of Darwin, Lamarck argued that animals adapted to their environment His advocacy of gradual evolution change is remembered mainly because he got the mechanism wrong.

Summary of the history of evolutionary thought

Darwin’s Theory of Evolution Organisms have changed over time. Those living today are different from those that lived in the past. All organisms are derived from common ancestors by a process of branching. Similarities of traits are evidence of a recent, common ancestor. Change is gradual and slow, requiring a very long time. Natural selection is the mechanism of evolutionary change.

The Process of Natural Selection All populations have the capacity for geometric increase, yet do not. There must be a “struggle” to survive and reproduce, in which only a few are successful. Organisms vary in traits that influence their likelihood of success in this “struggle”. Organisms whose traits enable them to survive and reproduce will contribute a greater number of offspring to the next generation. Offspring resemble their parents, including in these traits that influence survival. Therefore the population in the next generation will include a greater fraction of individuals with whatever traits enabled their parents to survive and reproduce.

Natural selection requires….. For natural selection to occur, two requirements are essential: 1. There must be inherited variation for some trait. Examples: beak size, color pattern, fleetness. 2. There must be a greater or lesser probability of survival and reproduction due to possession of that trait. Examples: peppered moth, Galapagos finches.

The peppered moth, Biston betularia. Light and dark morphs on (a) a light background, (b) a dark background

Distribution of the two moth morphotypes during the industrial era

The Link between Evolution and Genetics “Evolution is the change in the genetic composition of a population over time” Where does genetic variation come from? - Mutation (raw material, ultimate source) - sex, meiosis, and exchange of chromosome segments - other chromosomal re-arrangements What are other causes of changes in the genetic composition of a population over time? –Non random mating- migration –Mutation - natural selection –“Sampling error or genetic drift”

The Hardy-Weinberg Equilibrium “In a large population, with random mating, and in the absence of forces that would change their proportions, the proportions of alleles at a given locust will remain constant” Allele:Aa Frequency:pq The H-W Equilibrium is significant because: 1.It tells us that as long as mating is random and nothing else happens, gene frequencies will stay unchanged indefinitely 2.It gives us a tool to calculate allele frequencies if we know genotype frequencies, and vice-versa

Populations can become small enough for genetic drift to be an important influence on evolution. Bottleneck effects Catastrophic mortality leading to only a few survivors Flood, earthquake…etc. Who survives is random--few green and no orange--

Genetic drift Loss of habitat Hunting Populations with little genetic variation have a reduced ability to adapt to environmental change. Cheetahs have less variation than some inbred strains of laboratory mice. The Cheetah shows evidence of some past bottleneck event – homozygous at many loci, and skin grafts “take” between different individuals

How does natural selection change populations? Are there different types of natural selection?

The “fit” individuals are the ones that survive and continue to reproduce. They pass on their genes and leave offspring that look like them.

The way populations change depends on what is being selected for.

In this case both extremes do well but the intermediate ones do not.

Three Types of Selection Under stabilizing selection, extreme varieties from both “tails” of the frequency distribution are selected against; the mean stays constant. Under directional selection, individuals at one tail of the frequency distribution are favored (“selected for”), the other tail is selected against. The mean value shifts in the direction favored Under diversifying (disruptive) selection, both extremes are favored at the expense of intermediate varieties. The result is a bimodal distribution.

Stabilizing Selection Babies of intermediate birth weight have higher survivorship than very small and very large babies

A Compromise Between Opposing Pressures (a) A male giraffe with a long neck is at a definite advantage in combat to establish dominance. (b) But a giraffe's long neck forces it to assume an extremely awkward and vulnerable position when drinking. Thus, drinking and male–male contests place opposing evolutionary pressures on neck length.

Directional selection in horse evolution. Over time, the horse grew taller and its hoof evolved as the nail of the middle digit. Its molars grew larger as the horse became a bigger animal, and its diet became primarily grasses. A drier climate resulted in changes in habitat (open savannahs instead of forests) and food (grass instead of leafy plants).

Disruptive Selection In the finch, Pyrenestes ostrinus both very large and very small bills are beneficial for eating large and small seeds, respectively

Trinidadian guppies and escape ability Guppies were introduced into both high and low predation habitats. Twenty years later, 6 guppies were removed from a low-predation and 6 from a high predation habitat. They were placed in an enclosure with their natural predator, a cihlid. When the cichlid had consumed 1/2 of the guppies, the remaining guppies were removed and scored as to whether they came from high or low predation habitats. Guppies from high-predation populations had a much higher survival rate. Thus, they had improved escape ability.

Trinidadian guppies and escape ability BUT… Is escape ability a heritable trait? F2 generation were tested from low and high predation habitats (bred and raised in predator-free enclosures). F2s of guppies from high predation populations had significantly greater escape ability - Escape ability is a heritable trait.

A phylogenetic tree constructed from similarities between cytochrome c molecules in various organisms.

"nothing in biology makes sense except in the light of evolution" -Theodosius Dobzhansky ( )