Chapter 3: Evolutionary biology

What are Darwin's four postulates?

1) Individuals vary

2) Some variations are heritable

3) More offspring are produced that can survive

4) Individuals with traits that can confer an advantage are more likely to survive and reproduce.

How does HIV fit into Darwin's postulates?

1) There is variation amongst virions in their resistance to AZT

2) Ability to distinguish between AZT and thymidine is inherited

3) Not all virions are able to infect T cells and reproduce

4) Virions that distinguish AZT the best produce more offspring and are naturally selected for

Alred Wallace

Independently developed the theory of natural selection before Darwin published his book

The four postulates

Can be independently verified because natural selection is testable

Domestication

Domesticated crops (genetically modified) can look very different but descend from the same common ancestor, this also applies to the many dog species that descended with modification from wolves

Fitness and adaptation

- Individuals who win the competition and survive to reproduce are more fit

- Darwinian fitness is ability of an individual to survive and reproduce in its environment

- An adaptation is a trait that increases an individual's fitness in its environment

- Adaptations are enhanced by natural selection

Old field mouse (Peromyscus polionotus)

Beach mice some are poorly camouflaged on white sand and some are well camouflaged the variation is inherited as survivors reproduce the more camouflage coloring becomes more common

Finch beak evolution in the galapagos

Peter and Rosemary grant studied darwin's 14 finch species with various beak dimensions and characteristics, variation in beak morphology was tied to feeding habits ( insects, seeds, nectar, parasites, leaves, fruit, etc)

Study of ground finches on daphne major

A very small and distant island that is a good natural laboratory where finches cannot easily disperse to or from

Geospiza fortis medium ground finch

1200 finch individuals on the island that were all marked they eat seeds and crack them by grasping at base of their bills and applying force, beak size varies between species. Bigger beaks eat big seeds and smaller beaks eat smaller seeds.

Postulate 1: Are populations variable? (finches)

Varied by weight, wing length, beak width, beak depth, beak length etc, beak depth was variable and focused on

Postulate 2: Is variable heritable? (finches)

Beak depth variation could be genetic or environmental comparison of parents and offspring revealed break depth was heritable, variable is heritable

Measuring heritability

Difficult process, parent and offspring correlations are influenced by misidentified paternity, nest parasitism ( in birds), maternal effects, and shared environments

Postulate 3: Is there an excess of offspring so that only some live to reproduce? (finches)

There was a severe drought and 84% of the finch population died but in normal times 89% of finches die before they can reproduce so more individuals are born than will survive to reproductive age

Postulate 4: Are survival and reproduction nonrandom? (finches)

After drought the grants measured surviving birds again and found the finches with the deepest beaks survived more than the others they were favorable during the drought because only one plant produced fruit Tribulus cistoides has large hard fruits and beaks must be deep and narrow to crack them

El nino bird beaks

El nino caused a very wet year which caused many small soft seeds to be produced in this environment small birds with shallow beaks were selected for

Natural selection

Acts on individuals but evolution occurs in populations ex: HIV virions change in individuals

Acts on phenotypes but evolution changes allele frequencies

Natural selection can only act on traits that have a genetic basis and for evolution to occur the trait must be passed on to the offspring

What is evolution?

the process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the earth.

The nature of natural selection

Organisms cannot adapt to future conditions only those in the past, if there is a new environmental change they may not be well adapted to it, each generation if a product of selection by the environmental conditions from the generation before

Selection looks backwards not forwards

Production of new traits

Natural selection can produce new traits even though it acts on existing traits, over time natural selection will produce new traits there are new mutations every generation for natural selection to act upon

Artificial selection experiment

Ears of corn oil content went up over 60 years there was no overlap with original population and evolution produced a new trait

Panda thumbs

Natural selection can repurpose traits for new functions, panda thumbs are wrist bones that is modified to act as a thumb and is used to strip bamboo stalks it must have been a variation in the size of the wrist bone which was selected for the trait to evolve

Why is natural selection not perfect?

It can only work with variation present, finches with deep narrow beaks can break hard seeds the best but most birds have wide beaks, genes for wide and deep beaks must be linked, natural selection cannot make the perfect beak because the genes for it were not present

Natural selection is...

extremely nonrandom but not progressive it increases adaptation but is not directional some organisms have an increase in complexity and some a decrease in complexity

More on natural selection

Acts on individuals not groups, organisms are never self sacrificing or altruistic and only endanger themselves for selfish gain, group selection does not exist

Evolution is...

the unifying theory of biology became standard around 70 years after Darwins publication

Evolution of Darwinism

Darwin didn't know how variability was created and thought the variation would be used up eventually but really it was created continuously by mutation

He didn't know traits were inherited and thought traits would be lost when merged with other traits but after mendel was rediscovered we know alleles are inherited independently

The modern synthesis

Between 1932 and 1953 modern synthesis answered Darwins original problems with the data

Propositions of the modern synthesis

- Gradual evolution results from small genetic changes that are acted upon by natural selection

- The origin of species and higher taxa, macroevolution, can be explained in terms of natural selection acting on individuals, micro evolutions

What is microevolution?

Change in allele frequencies in a population over generations.

What is macroevolution?

Evolutionary changes that result in the formation of new species.

Restatement of Darwin's postulates with modern synthesis

1) Mutation creates new alleles and independent assortment shuffles alleles into new combinations causing individuals in populations to be variable for nearly all traits

2) Individuals pass their alleles to their offspring

3) In most generations more offspring are produced than can survive

4) Individuals that survive and reproduce or who produce the most are those with alleles and allelic combinations that best adapt them to the environment

Is natural selection the only cause of evolution?

No it is the only means of adaptive evolution but not the only mechanism for evolution