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.

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: A aFrequency: p q

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

Three Types of Selection

Envision a frequency distribution for a trait such as beak size, much like a grade distribution.

• 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.

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Stabilizing directional diversifying

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Birth weight appears to be under stabilizing selection. Babies that are too large or too small have higher mortality

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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).

Darwin’s finches evolved from a common ancestor, similar to a species now found on the mainland of Ecuador.

We’ll look at this adaptive radiation again when we discuss how species compete for food.

Sub-species of the rat snake Elaphe obsoleta, referred to as geographical “races”. They interbreed where their ranges meet, so are not distinct species.

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

Summary• The core of Darwin’s theory of evolution is its

mechanism -- the theory of natural selection.• Natural selection requires inherited variation in a

trait, and differential survival and reproduction associated with possession of that trait.

• Examples of natural selection are well documented, both by observation and from the fossil record.

• Selection acts on the frequency distribution of traits, and can take the form of stabilizing, directional, or diversifying selection.