Darwin and modern synthsis

Transmutation

• pre-Darwin

• Ideas for the altering of one species into another

• Promotes by Lamarck and Darwins grandpa

Darwin aboard the Beagle

1831→ Beagle set sail from Plymouth

Visited Galápagos Islands

Collected finch species on different islands

• found that different species had adapted to different habitats

Darwins insights from finches

• confirmed they were different species, not variations of mainland birds

• But, the islands were recently volcanic

• 1845→ seeing the diversity of structure in one small, intimately related group, concluded that they evolved from a common ancestor and isolating different islands gave way to different species

Malthus populations

→ huge growth potential, but resources are limited = a struggle for existence

1798→ “a population unchecked increases exponentially, whereas food supply grows only arithmetically”

Other ideas that lead to the theory of origin of species

• pigeon fanciers used artificial selection to make many kinds of pigeon

• Wallace- had a idea called ‘evolution by natural selection’

Wallace/Darwins paper = 1858/59

Natural selection

→ organisms with advantageous traits survive and reproduce, passing traits onto offspring, so evolution occurs over time

• ‘survival of the fittest’

Fitness = differential survival and reproduction of individuals due to differences in phenotype e.g. bigger tails

Darwins 4 postulates of natural selection

1. Individuals within species are variable

2. Some variables are passed onto offspring

3. In most generations, more offspring are produced than can survive

4. Survival and reproduction are not random, individuals with the highest reproductive success have the most favourable variations

• produces ‘descent with modification’

Moth example of natural selection

1811→ 100% ‘typica’ peppered moths

1895→ 98% ‘melanic’

• due to tree bark becoming darker, so moths with darker wings survived and reproduced = phenotypic change

Blending inheritance

Darwin believed in ‘blended inheritance’

• cells that derived from all other parts of parents bodied (soma cells) could not transmit from parents to offspring

• Mendel published his pea studies in 1865, discovered in 1900

• It was found (in animals only) to be true, the only hereditary substance was in germ cells

Modern synthesis

Neo-Darwinism evolution:

→an understanding of mechanisms of inheritance combined with Darwin’s concept of Natural Selection

•Evolution = in “terms of changes in allele and gene frequencies over time” and the average action of selection on genotypes.

•Individuals are the units of selection that survive and reproduce, or don’t…

•The only continuity is therefore the transmission of alleles

Altruism

Altruistic behavior = no survival or reproductive advantage to the adult

• selection operates on alleles in relation to their average contribution to their own transmission through their action on the individual

• Same alleles are present in close relatives

• ‘Lay down my life fro 2 brothers or 8 cousins’

4 postulates updated

HW equation applications

→ expected relationship between allele frequencies and genotype frequencies

Applications:

• if we know allele frequencies, HW equilibrium can be used to calculate the expected phenotype frequencies

• If we know phenotypic frequencies, HW can be used to calculate expected allele frequencies/genotypes

Genetic drift in large/small populations

Small (18 people)

• by chance an allele could be transmitted to a high proportion of offspring

• If another allele is ‘lost’/extinguished, the allele becomes fixed

Large (100 people)

• Allele frequencies change more slowly

Founder effect & bottlenecks

Founder effect→ loss of genetic variation that occurs when a new population is established by a small no. Of individuals

Population bottleneck→ sharp reduction in the size of a population over several generations = loss of genetic diversity

Spread of an allele with selective advantage

• an allele has a selective advantage when it makes the organims more likely to survive/reproduce

• There must always exist another allele at the same locus which gives a selective disadvantage

Evolution in prokaryotes

• many pathogenic bacteria have evolved resistance to the main classes of antibiotics

• Multi-drug resistant bacteria have abused untreatable infections

Conjunction→ plasmids can pass between different species by horizontal gene transfer through a sex pilus (attachment)

Transformation→ bacteria can take up free DNA and integrate it

Transduction→ transfer DNA through phases