Biology 1M03: Evolution + Ecology + Population Dynamics

Speciation

A species diverging into 2 distinct lineages, acts as a barrier to reproduction

What does lack of gene flow cause

allele frequencies in isolated populations can diverge = speciation

Why do populations diverge

Natural selection, genetic drift, mutations

Phylogeny

The evolutionary history of a lineage/lineages, similar to a family tree

Species

An evolutionarily independent population or group of populations

Biological species concept

Reproductive isolation is the method of identifying species

Mechanisms that stop gene flow

Pre and Postzygotic isolation

Prezygotic isolation

Individuals of different species are prevented from mating successfully

Postzygotic isolation

Hybrid offspring do not survive/are sterile

Pollinator isolation

Floral traits are adapted to specific pollinators, ensuring only conspecific pollen (from the same species) is transferred

Hybridization

Some closely related species are able to interbreed and reproduce

Is hybridization favoured?

No, natural selection acts against hybrids

What does hybridization signify

Reproductive isolation has not completely evolved between the 2 species since they are able to reproduce

Advantages of Biological species concept

Reproductive isolation signifies evolutionary independence

Disadvantages of Biological Species Concept

Cannot be evaluated in fossils species or species that reproduce asexually and difficult to assess if populations do not overlap geographically

Morphological species concept

Species are those that look alike

Advantages of Morphological Species Concept

Useful for lack of data on extent of gene flow, applicable to sexual, asexual and fossil species

Disadvantages of Morphological Species Concept

A single polymorphic species may be incorrectly classified as more than 1 species, states that members of the same species all look alike, can be disproved (Ex. butterflies), and traits used to distinguish species are subjective

Ecological Species Concept

Species are organisms that exploit the same resources, have similar environmental tolerances and face the same predators/parasites

Advantages of Ecological Species Concept

Useful for identifying asexually reproducing species

Disadvantages of Ecological Species Concept

Cannot be defined by reproductive isolation from other species - explained by adaptations to ecological conditions

Phylogenetic species concept

Smallest monophyletic group on a phylogenetic tree indicates a species, based on synapomorphies

Advantages of Phylogenetic species concept

Applicable to any type of population (Ex. fossil, asexual, sexual) and logical

Disadvantages of Phylogenetic Species concept

Phylogenies are only available for a small amount of populations, Independent Lineage Sorting, and leads to recognition of many more species compared to other species concepts

What assumptions does the Phylogenetic Species Concept make

All species are related by common ancestry

Independent Lineage Sorting

When ancestral genetic variation is randomly distributed into descendant species, causing gene trees to differ from the actual species tree (Ex. A -> B, C can turn into B -> A, C: makes AC look more closely related than BC)

Gene tree discordance

Gene trees do not always match species trees

Subspecies

Populations that live in discrete geographical areas, have distinguishing features, can interbreed if geographical barriers are removed

Synapomorphies

Homologous traits found in common ancestors and descendants but missing in distantly related ancestors (Ex. Genetic synapomorphy)

Dusky Seaside Sparrow

Had 6 subspecies that formed 2 monophyletic groups 

• 1. Atlantic Coast: Genetically indistinguishable from Seaside Sparrow’s

• 2. Gulf Coast: Genetically distinguishable from Seaside Sparrow’s

Eventually the Seaside Sparrow went extinct as revitalization attempts failed

Allopatry

Populations that live in different areas

Allopatric Speciation

Speciation through geographic isolation

Types of geographic isolation

Dispersal & Vicariance

Dispersal

Individuals colonize a new area (Ex. Genetic drift)

Vicariance

A geographic barrier arises and splits a population into 2 (Ex. Uplifting mountain range)

Symaptry

Populations that live in the same geographic area

Sympatric speciation

Speciation among species inhabiting same geographic area

What can cause sympatric speciation

External & Internal events, Polyploidy

External events

Sympatry: Disruptive selection based on ecological niches or mate preferences

Internal events

Chromosomal mutations

Polyploidy

An error in meiosis or mitosis results in more than 2 sets of chromosomes, common in plants

Why is polyploidy common in plants

Many diploid plants are closely related to polyploids, polyploids have higher levels of heterozygosity and can tolerate higher levels of self-fertilization (not as affected by inbreeding depression as diploids), and genes on duplicated chromosomes can diverge independently = genetic variation

Autopolyploid

Offspring with a mutation that doubles the chromosome number = all chromosomes are from the same species

Autopolyploid Tetraploids

Arise from self-fertilization of gametes that were produced due to chromosomal error

Allopolyploid

Offspring when parents are of different species and mating results in non-sterile offspring = 2 copies of chromosomes from each parent

Allopolyploid Tetraploids

Arise due to self-fertilization of gametes that were produced as a result of hybridization between 2 species

Tetraploid reproduction

can reproduce with other tetraploids but not diploids

How does polyploidy lead to speciation

Through immediate post-zygotic reproductive isolation from their diploid ancestors

Instantaneous speciation

When 2 species hybridize and the offspring are reproductively isolated from both parental species, may be able to reproduce with other hybrid species

Biogeography

How species and populations are geographically distributed

Homology

Traits shared due to similar ancestry

Homoplasy

Traits that are similar for reasons other than common ancestry (Ex. Convergent evolution)

Evolution of eyes

Originally thought to be convergent due to vast differences in function and morphology between animals

Hypotheses of Eumetazoa eyes

• 1. Originally eyeless and descendents convergently developed eyes (convergence)

• 2. Had simple photoreceptor eye spots which were developed into complex eyes (parallelism)

• 3. Had the primitive makings of an eye and ancestors derived from this (homology)

Adaptive radiation

Rapid production of many descendant species from a single lineage

Criteria for adaptive radiation

Part of a monophyletic group, speciated rapidly, diversified ecologically into many niches

How does adaptive radiation occur

Extrinsic & Intrinsic factors, Ecological opportunity

Extrinsic factors

New favourable conditions or access to new environment (colonization)

Intrinsic factors

Adaptive radiation: Evolution of morphological, physiological or behavioural traits

How does ecological opportunity drive adaptive radiation

The availability of more or new types of resource makes it more likely for species to diverge

Escape-and-radiate coevolution hypothesis

A plant lineage evolves a new defense to escape herbivores and radiates into new species and a herbivore evolves to overcome the defense, radiating into new niches = coevolution

Cambrian explosion

All organisms were unicellular until animals became larger and more complex

Hypotheses of what triggered the Cambrian explosion

• 1. Higher Oxygen levels: efficient aerobic respiration

• 2. Evolution of predation; drove morphological divergence among prey (selection for shells, rapid movement, other defenses)

• 3. New niches beget more: Mass migration off of ocean floor = exploit land resources = new niches

• 4. New genes, new bodies: Development of Hox genes in animals made it possible for larger more complex bodies to evolve

Early animal fossils (before cambrian explosion)

Microfossils, less than 1mm, first macroscopic fossils included sponges, jellyfish, fossilized tracks, most likely burrowers, floating or immobile feeders

Cambrian fossils

Included wormlike animals, anthropods, mollusks, swam, walked, ran, more mobility overall, filled many ecological niches (ex. Predator, scavenger)

The Burgess Shale Reef

Slit sifted into buried animals preventing animal scavenging and bacterial decomposition = well preserved fossils (Ex. antennae, eyes, digestive tracts)

How did the Burgess Shale Reef provide evidence of predation

Contained fossils with gut contents containing fragments of shelled prey + fossils with healed damage resembling modern predators

Hominids

Great apes, large bodied, long arms, short legs, no tail, monophyletic group that comprise Homo Sapiens + 20 extinct bipedal relatives (Ex. neanderthals)

WHO DA ONLY BIPEDAL APES

Humans! Bipedalism is the synapomorphy that defines hominins

4 general groups of hominid species

• Oldest known hominin: Ardipithecus ramidus

  • Comibation of tree climbing and bidepal activity

• 1. Gracile australopithecines

• 2. Robust australopithecines

  • 1&2 small bipeds with small teeth

  • Evolved big teeth to eat plants

• 3. Early Homo

  • Larger braincase and reduced cheek teeth

• 4. Recent Homo

  • Even larger braincase 

  • Started to see the use of fire to cook food, used stone utensils

Neanderthals

Last hominin species to live with our species, heavily built, large brains, low foreheads, powerful jaws, large browridges, small chins

First ancient-human hybrid

small piece of bone found in Siberia indicating 40% Neanderthal DNA and 40% Denisovan

Homo sapiens

Reduced browridges, flat faces, largest braincases, larger brain advantage = efficient hunting, tool use, communication, social lives (Trade-off: Energy expensive)

Hominin family tree

no linear progression from one species to another, branches off each other

WHO HAS DA LARGEST BRAINS?

Species in Homo genus have extremely large brains relative to overall body size!

What does the evolution of larger brains indicate

Support a natural selection hypothesis for the ability to reason and communicate, triggered by increased tool and language use = larger brains

Out-of-Africa hypothesis (relative to H. sapiens)

H. sapiens evolved its distinctive traits in Africa and dispersed throughout the world

Mechanism of Out-of-Africa hypothesis

First lineage to branch off led to descendant populations that live in Africa today (highest genetic diversity), subsequent branching = residents of central Asia, Europe, etc. 2 waves of migration: Wave 1 = Australia, Wave 2 = Europe and Mainland Asia

Evidence of H. sapiens interbreeding

1 - 4% of genome of indigenous Europeans and Asians are derived from Neanderthals & 5% of genome of Aboriginal Australians is Denisovian

Why did we evolve bipedalism?

• 1. Lower surface area exposed to sun, risk of heat stress reduced

• 2. Hands free to carry food or tools to forage more efficiently

• 3. Long-distance migration, follow migrating herds

Major modifications to H. sapiens

• Position of foramen magnum (spine connection): enters skull from below

• Shape of pelvis: Bowl-shaped to support upper body and provide balance

• Feet: Lost opposable great toe, larger heel, arched food to propel forward

• Large brain: Modern brain development continues until age 20

Evolution of language

No fossil evidence, potential link between toolmaking and language development, related to FOXP2 (language gene)

FOXP2

Essential for human speech, linked to vocalization in mice, song for bids, other verbal comm, Hyoid bone position allowed tongue movements for speech

Evolution of human altruism and cooperation

Evolved through fitness feedbacks in small groups, punishment of cheaters, cooperative hunting

How does the fossil record support the evolution of human cooperation

Visible injuries due to hunting, healing from wounds that required care from others

Do we evolve today?

Yas, coevolution with pathogens, spread of lactose tolerance mutation

Genetic variation in humans

Greater genetic variation within groups than between groups, any 2 humans differ by ~0.5% of genome on average

Examples of human evolutionary traits

Skin pigmentation, Skin cancer, Resistance to malaria, & Lactase Persistence

Skin pigmentation evolution

Over 350 associated genes, natural selection favours more melanin in regions with high UV radiation (Ex. Equator), Light pigmentation = Vitamin D synthesis in regions with lower UV Darker pigment = Protective in regions with high UV

Fitness trade-off of pigmentation

UV breaks down folate and damages DNA but is essential for vitamin D production

Skin cancer types

Melanoma and Non-melanoma

Melanoma

High mortality rate, rare

Non-melanoma

Common, slow progression, rarely metastasizes, low mortality rate

Folate

An essential nutrient that must be obtained by diet, necessary for DNA repair and synthesis, deficiencies decrease chances of successful reproduction. UV radiation causes breakdown in blood folate

Vitamin D

Regulates immune function, protects from pathogens, can be both obtained and synthesized

Resistance to malaria

Deadly infection, targets young children, pregnant women, selection favours modifications of RBC’s that reduce success of parasite (Heterozygote advantage = protection against malaria)

Lactase persistance (pre-evolution)

Milk is a primary food source for young mammals. Nutrient rich: Lipids, proteins, minerals, etc…Digested via lactase during weaning, lactase production is halted post-weaning, lose ability to digest lactose by age 2

Lactase persistance (post-evolution)

Pastoralism: Domestication of cattle, goats, sheep, other milk-producing farm animals. Selection on these populations caused lactase production to be maintained into adulthood

Population

Group of individuals of the same species living in the same area at the same time

Features of a population

Size, density and range