Organic Evolution

Evolution

Change in frequency of alleles in a population over multiple generations

Evolution by Natural Selection

Bright red ladybird beetles are distasteful, and avoided by predators thus more survive and reproduce

The alleles that code for the red coloration increases in frequency within the population

Why study evolution?

Provides a unifying theory: the unity and diversity of life; why living things function the way they do; how living things came to be; to understand how the diversity of life has resulted in multiple ways to solve the same problems; helps catalog diversity; insight into conservation, inform medicine, aid pandemics, hinder or help agricultural innovations

Essentialism

The physical world and its life forms are fixed, all members of a class share unchanging properties that define that class

Aristotle

Ideas before Darwin:

Essentialism

The earth was young

• 1664; Archbishop Ussher estimated formateion of October 26th 4004 BC

• Issac Newton Estimated 3998 BC

Georges-Louis Leclerc, Comte de Buffon

1707-1788

Earth was between 75,000 and 3 million years old using laws of physics

James Hutton (1726-1797)

Used geological evidence to determine the earth was inconceivably old

Georges Cuvier (1769-1832)

First to suggest the idea of extinction, and its ties to catastrophic events

Thought of bones as something that used to exist, and a new form developed since then

Since organisms were perfectly adapted to their environment, any change would result in death

so fossils weren’t ancestors of current organisms

Social status allowed these ideas to remain dominant

Wrote eulogies for other scientists and started beef Lamarck

What did James Hutton and Charles Lyell advocate for?

gradualism and uniformitarianism, these theories didn’t explain changes in organisms

gradualism

earths physical features gradually changed us to slow geological processes

Uniformitarianism

historical changes result from uniform geological processes that still occur today (erosion, sedimentation, volcanism)

Erasmus Darwin (1731-1802)

Charles Darwins grandfather

Zoonomia

“The strongest and most active animals should propagate the species, which should thence become improved”

Jean-Baptiste Lamarck (1744-1829)

organisms change in response to their environment, changes are due to changes in behavior

Complex species are more complex because they’ve been around longer (microbes were new species)

Some traits become less prominent with disuse

Lamarck’s ideas on an evolutionary tree

lineages persist forever, but change in form

No extinction or branching of lineages

Lineages arise at different times, accounting for differences in complexity

Al Jahiz (781-869)

”animals engage in a struggle for existence, and for resources, to avoid being eaten, and to breed”

Charles Darwin (1809-1882)

Went to med school at 16, neglected studies, didn’t like blood

Father sent him to school to become a preacher

After graduation, signed up to be naturalist for the HMS Beagle

Didnt publish for 20 years to build strongest case possible, until received manuscript from Alfred Wallace

HMS Beagle excursion

surveyed the coast of South America (1831-1837)

Found fossils with many similarities with extant species

Reach Galapagos in 1835, collected many species, but mislabeled them

Darwins Pigeon experiments

domesticated pigeons resemble only one wild species

Phenotypic effects of selective breeding (artificial selection)

How does nature select for certain traits?

failure to survive

Population size shows exponential growth

Food productions follows a linear path

Thomas Malthus’s population assay

if population size exceeds food sources, there are two alternatives: find new resources, fail to survive

Galapagos finches and the Malthusian dilemma

in periods of drought, favorable resources (soft seeds and buds) are quickly depleted, leaving only less conventional food (tough seeds, hard-to-reach foods)

Only those birds with beaks capable of accessing these food items can survive and reproduce

The rest fail to survive and/or reproduce through natural selection

Adaptive radiation

diversification of a common ancestor to fill various niches

Darwins 4 Postulates

variation - individuals within species are variable

Inheritance - some of these variations are passed on to offspring

Differential survival - in every generation, more offspring are produced than can survive

Extinction - the survival and reproduction of individuals is not random. Individuals are survive and reproduce are those with the most favorable variation

What did Darwin propose other than natural selection

common descent

Gradualness of change over time

• Differences amount organisms have accumulated in small increments over a long time

Influenced by Lyell’s theory of uniformitarianism

Common Descent

all living things share a common ancestor, organisms that are more closely related (have more recent common ancestor) are more similar than those that are more distantly related

Weakness of Darwin’s Theory

had no mechanism of inheritance, favored blended inheritance

Contained speculation on the natural selection of the natural selection of wild species (including his finches)

Did not realize how fast evolution can occur

Without variation there can be no evolution

Natural selection is a sorting process of differential survival and reproduction

Darwins finches

didn’t observe ground finches behaving differently or eating different food types, nor another ornithologist over the next 140 years

Only evidence for differences in ability to utilize different food sources was David Lack in 1938, and found that similar species outcompeted one another

Peter and Rosemary Grant’s Galapagos expedition in 1973

found ground finches eating the same seeds, during the dry season

Went during the wet season and found finches specializing on seeds according to beak type

Modern synthesis of Darwins Postulates

• Only gene-determined traits are involved in evolution

• Mutation and genetic recombination provide raw materials for evolution

• Evolution is usually a gradual process at the population level in response to environmental challenges

• Evolution arises from changes in allele frequency within a population caused by various mechanisms

  • Natural/sexual selection, gene flow, genetic drift, polyploidy

Gregor Mendel

Independently discovered a mechanism of transmission

Joined the Augustinian Order of St. Thomas in Brno (Czech Republic) at 21

Initially worked with mice

Published in 1866, ignored, and rediscovered in the 1900s (unknown to Darwin)

Mendel’s Experimental Setup

2 homozygous individuals within species contrasting traits were cross-fertilized

Traits of F1 offspring were measured; all purple

F2 offspring were 3:1 purple to white (75%:25%)

Results similar in other characteristics

Mendel’s Contributions

Dominance

Law of Segregation

Law of Independent Assortment

Dominance

alternative versions of genes (alleles) account for variations in inheritance characteristics

Dominant alleles determines an organisms appearance, recessive alleles have no noticeable effects

Law of Segregation

homologous chromosomes separate during meiosis to create different alleles

Law of Independent Assortment

unlinked characters separate during meiosis (leading to different combinations of traits)

9:3:3:1 dihybrid ration

Blending

rare variants are blended out of the population

Mendelian

rare variants can establish and persist in the population

Mutation

ultimate source of genetic variation

Occur randomly due to DNA damage, replication errors, transposable elements

Substitution mutations

change of a single nucleotide

Somatic mutation

affect the body cells of an individual

somatic mutation

passed on to offspring

Important for evolution

silent mutations

based on redundancy of genetic code

Usually neutral but not always

Nonsense mutation

substitution resulting in a premature stop codon

Missense mutation

substitution codes for a different amino acid

insertions and deletions

add/subtract codons, alter the reading frame

Chromosomal duplications

results in polyploidy, most are rare and lethal, with the exception of plants and amphibians