SBI3U1 Unit 3: Evolution

Contextualizing discoveries

Understand the social, political, and economic conditions of that time

Microevolution

Change in allele frequencies over time within a population of one species

Mechanisms of microevolution

Natural selection, mutations, non-random mating, gene flow, genetic drift

Natural selection

When an allele or combination of genes makes an organism more/less fit to survive and reproduce in an environment

Patterns of natural selection

Directional, stabilizing, disruptive

Directional selection example

During the Industrial Revolution in England, when soot covered buildings, population of pepper moths shifted from light coloured, which were better camouflaged in clean environments, to dark coloured to camouflage

Directional selection

Phenotype shifts over time

Stabilizing selection

Average phenotype is favoured, extreme phenotype is not

Stabilizing selection example

Robins typically lay 4 eggs as larger clutches may leave malnourished chicks but smaller can result in no/low viable offspring

Disruptive selection

Extreme phenotypes favoured, average phenotypes not

Disruptive selection example

Light and dark morph oysters are more favourable than medium morphs as they blend in better with light sand or dark waters, whereas medium blend in with neither

Mutations

Mutation rate is low because they’re random, so they only impact one organism in the first gen, and mutations in gametes often result in non-viable offspring

Non-random mating

Organisms prefer to mate with another with a same/different phenotype which can alter genotype frequencies. Although there are no new alleles reproductive isolation may result in new species

Gene flow

Movement of genes in/out of population which can occur cause of gametes moving (pollen) or if organisms are introduced to an isolated population (migration)

Genetic drift

When allele frequencies in a population change due to chance events with a stronger effect in a small population

Bottleneck effect

Disasters can drastically reduce population and gene pool with a random reduction in alleles possibly eliminating one which may reduce genetic variation in a population.

Founder effect

Occurs when a small group migrates from a larger population, changing the allele frequencies in the new colony

Reproductive isolation

When two populations of a species no longer interbreed, meaning gene flow between these 2 species stops and they become more genetically different

Geographic isolation

Two groups of the same population are physically separated with no gene flow, so mutations occur in each location and the groups become too different to reproduce if they ever meet again

Habitat isolation

Different phenotypes allow groups of species to occupy different habitats, and since they live in different habitats they’re less likely to reproduce and eventually become too different to reproduce if they ever meet again

Temporal isolation

A group of organisms may spontaneously develop different breeding seasons and are less likely to reproduce with each other resulting in isolated gene pools

Behavioural isolation

Groups may be very similar but remain separate as they don’t interbreed due to different mating rituals

Mechanical isolation

Similar species are unable to mate due to incompatible reproductive organs. This is also present in plants which have different pollinators

Gametic isolation

Sperm and eggs from different species are unable to recognize each other by their molecular markers

Hybrid inviability

Two species may mate and their gametes fuse, but the fetus will be inviable and die before it develops fully

Hybrid infertility

Some very close related species can produce hybrid offspring, but they are infertile

Forms of reproductive isolation

Geographic, habitat, temporal, behavioural, mechanical, gametic, hybrid inviability, hybrid infertility

Speciation

Requires many many instances of microevolution as it occurs through accumulation of barriers to gene exchange between organisms

Allopatric speciaition

Occurs when two groups of a species are geographically isolated

Peripatric speciation

Occurs when a small group of individuals break off from the larger group of one species and, due to geographic separation, become separate species.

Parapatric speciation

When a species is spread out over a large area and only mate with members of their geographic region. This is considered habitat isolation as there is no separation from a physical barrier.

Sympatric speciation

Occurs when there are no physical barriers between 2 groups in the same region, but they spontaneously begin differently eating, sheltering, or mating at different times. They could still reproduce, but don’t.

Artificial selection

Results from interference from humans by intentionally separating members of a species or intentionally breeding individuals

Forms of speciation

Allopatric, peripatric, parapatric, sympatric, artificial selection

Divergent evolution

Result of accumulated microevolution events. Close evolutionary relatedness results in similar features.

Convergent evolution

Result of accumulated microevolution events. Even though species are not closely related, they share an environment and therefore have experience similar natural selection resulting in features with the same function.

Adaptive radiation

Result of a rapid increase of species with a common ancestor and occurs when organisms are adapting to new ecological conditions (new resources/habitats)

Coevolution

When one species evolves in response to another, which is most pronounced in symbiotic relationships where the extinction of one species threatens the other

Forms of macroevolution

Divergent, convergent, adaptive radiation, coevolution

Evidence for evolution

Fossil evidence, biogeography, comparative anatomy, molecular biology

Fossil evidence

Fossil records show us what species looked like millions of years ago and can be used to reconstruct the evolutionary history of present day species

Strata

Layers of Earth that fossils are found in, which act as a timeline

Radiometric dating

Measures the radioactive decay of certain elements over time, formerly referred to as carbon dating

Carbon dating

Organisms incorporate carbon into their bodies but stop when they die, so they have the same c12:c14 ratio as the atmosphere. Since c14 decays, we can measure the c12:c14 ratio in dead organisms to determine when it died.

Limitations of fossil evidence

Since they don’t contain soft tissue, there are limits to the information they provide

Best fossils

Hard corals, vertebrates, echinoderms crustaceans, belemnites because hard parts of them are best preserved. They are formed where there is deposition and sedimentation which eventually leads to rock formation

Biogeography

The study of the distribution of species/ecosystems in geographic space/geological time. It can provide insight into evolutionary history of species

Geographic factors that influence organisms

Latitude (poles/equator), elevation, climate, isolation, habitat area

Continental drift

Wegener saw fossils of the same species on different continents and concluded with other evidence that they were once the same continent and had separated due to tectonic plate movement

Comparitive anatomy

A tool that shows how bodies of animal groups are similar to help understand evolutionary relationships

Homologous structures

Share common recent ancestors and have the same internal components but are differently shaped and likely have different functions. Appear via divergent evolution

Analogous structures

Have similar functions but differ in structure and don’t share a recent ancestor. Appear via convergent evolution

Vestigial structures

Remnant structures that held important functions for ancestral species but no function in modern descendants. E.g. goosebumps, wisdom teeth, tailbone

Human appendix

Darwin believed this was a vestigial structure used to help us digest cellulose but we now believe it is a reservoir for beneficial gut bacteria

Molecular biology

Allows scientists to compare the molecular structure of organisms to gain insight into their ancestral origins, most often comparing genetics. This provides the most accurate information for evolutionary trees.

Cytochrome C

Essential protein for human cellular respiration. In humans and chimps its nearly identical, but spider monkeys and macaques do not, showing that they are less related

Hemoglobin

Protein inside red blood cells that carries oxygen found in all vertebrates, showing they all share a common ancestor

Universal genetic code

All life uses the same genetic code to produce proteins, meaning it all evolved from an organism using RNA and amino acids. This common ancestor is called LUCA

LUCA

Estimated to have appeared 3.5 billion years ago and was anaerobic, likely dependent on hydrogen and CO2 from geochemical sources. Like modern cells it had a membrane and was likely a thermophile.

Phylogeny

Study of relationships between species and their evolutionary history

Phylogenetic relationships

The more homologous structures two species share, the more related they are. But analogous structures can sometimes be mistaken for homologous structures

Molecular data

Now used in combination with other sources to help determine evolutionary relationships, which is very helpful for determining if a structure is homologous/analogous

Cladistics

In a phylogenetic diagram, organisms within each branch are grouped into clades. This is the study of determining the sequence of the branching

Derived characteristic

A shared trait that is common to all the organisms in a specific clade

Ancestral primates

First evolved 50 million years ago and resembled modern-day lemurs

Primate phylogeny

Lemurs, tarsiers, lorises, old and new world monkeys, and hominoids all share a common ancestor

Hominoids phylogeny

Hominoids/apes have relatively large brains, lack tails, and have swinging arms

Bipedalism

This separates humans from other hominoids as we are the only member that is always bipedal as our anatomy is evolved for it

Ardipithecus ramidus

Believed to be the first bipedal species, existing 4.6 million years ago and known as the missing link between ancestral humans and chimpanzees

Human vs chimpanzee/bonobo genomes

They are 98.7% identical

Ape karyotypes

Typically have 24 chromosomes

Human chromosome 2

Believed to have formed from the fusion of two smaller chromosomes

Hominins

Category of human-like organisms that evolved after the split from ancestral chimpanzees and bonobos, homo sapiens being the only remaining species

Darwin’s Finches

He observed significant species variety in local communities, observing that finches on different islands of the Galapagos region had very different beak shapes

Fossil records

Based on fossils he found, he concluded that species varied over time and that they evolve from ancient ancestors

Species variability

On the HMS Beagle Darwin concluded that some areas had unique organisms not found anywhere else, and that species living in similar habitats in different parts of the world looked/acted similarly

Local species diversity

On the HMS Beagle Darwin observed significant species variety in local communities and related species that live in different habitats in an area have different features

Contemporary influences

On the HMS Beagle Darwin read Lyell’s Principles of Geology and observed gradualism with marine fossils that were formerly underwater 12,000 feet above sea level due to gradual movements in Earth’s crust

Alfred Russel Wallace

In 1858, Darwin got a letter form Wallace, who wanted Darwin’s advice on his discovery of natural selection. They published a joint letter briefly explaining their evidence-based theory of how organisms evolved

Postulates in On the Origin of Species

1.All species of organisms living on Earth today are descended from ancestral species and 2.The mechanism that causes species to change over time is natural selection

Descent with modification

The original name for natural selection

Survival of the fittest

In natural selection, the few that survive in each generation have the best traits for survival and pass them on to their offspring. Since those with worse traits don’t reproduce, each new generation has a higher proportion of advantageous traits

Competition - Thomas Malthus

Organisms produce more offspring than can survive so those offspring must compete for the limited resources

Uniformitarianism - James Hutton

Processes and natural laws of the present have always operated through the past

Gradualism - James Hutton

Geological features of Earth were formed by slow process like erosion, deposition, and uplift, meaning the Earth was very old

Gradualism - Charles Lyell

Slow geological changes had a stronger impact on Earth’s crust than catastrophic events

Catastrophism example

Volcanoes, floods, and earthquakes are catastrophic events once believed to be responsible for mass extinctions and formation of landforms

Gradualism example

Canyons carved by rivers show gradual change over long periods

Uniformitarianism

Rock strata show that geological processes add up over long periods to cause big changes

Paleontology - Georges Cuvier

Compared skeletons of current day elephants with fossilized elephants and concluded that they were two different species and established extinction

Paleontology in Linnaean taxonomy

Cuvier incorporated fossils into Linnaeus’s classification system

Species Adaptation - Jean-Baptiste Lamarck

Proposed that species adapt and change over time and identified adaptation as an inherited trait. He theorized that giraffes got long necks by reaching towards tall trees and throughout their lives they would get slightly longer

Inheritance of acquired characteristics

Inaccurate theory that using a particular feature repeatedly, the body part will adapt and be passed to the offspring

Flaws in Lamarck’s theory

No evidence, traits are predetermined by genes received from parents, organisms cannot change their own DNA by adapting to their environment