Graphical summary

Species Concepts, Modes of Speciation, and phylogenetics

Anthropology 2200

Primate classification

What is a Species?

Definition of a Species • Biological species

concept

• Ecological species concept

• Morphological species concept

Biological Species Concept

• “Groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups” – Ernst Mayr (1942)

• Complications • So what do we mean by “reproductively

isolated”?

Lion

Tiger

Liger or Tigon

Reproductive Isolating

Mechanisms

Any factor that prevents a male and female of two different “species” from hybridizing

Reproductive isolation causes “species” to be recognized as distinct biological species

• Example: Lions and tigers do not naturally mate with one another at present, but their territories overlapped in the past

• Are they separate species today? In the past?

So, what exactly do we mean by Reproductive

Isolating Mechanisms?

Pre-Mating RIMs

Post-Mating RIMs

1. Habitat Isolation 1. Sperm-egg incompatibility

2. Temporal Isolation 2. Zygote inviability

3. Behavioral Isolation 3. Embryonic inviability

4. Mechanical Incompatibility

4. Offspring inviability

5. Offspring sterility

Reproductive Isolating Mechanisms

• Natural selection favors pre-mating reproductive isolating mechanisms

• Hybrid mating in most cases would be a wasted reproductive effort (due to inviability or sterility) and natural selection promotes mechanisms to prevent such matings.

• Not mating uses less energy than mating, wasting reproductive resources (eggs/sperm), and producing Non-viable offspring

Ecological Species Concept

• Defines species based on the uniqueness of their ecological niche • Formation of discrete species

because of adaptation to exploit the resources in nature

• Based on the competitive exclusion principle

• Two species competing for the same resources can not both survive

Morphological Species Concept

“Classic” way of defining species

Linnaeus

Descriptive rather than theoretical concept

Defines species based on shared phenotypic characteristics

If one group of organisms consistently differs from other organisms, it will be defined as a separate species

Ring Species – defined as separate based on Morphology .

Species Concepts

• Biological, environmental, and morphological species concepts will generally recognize many of the same “species”, although not always.

How do we study extinct species? Paleospecies

• Species defined using fossil evidence • What concept of species do you think applies

best to fossils?

Morphological? Biological?

How do we study extinct species? Paleospecies

• Use morphological species concept to infer biological species concept

gene exchange species similarity CAUSE CAUSE

gene exchange species similarity INFERENCE INFERENCE

Paleospecies: Problems

• Allometry  Study of the change in proportion of various parts of an organism as a consequence of growth

Paleospecies: Problems • Allometry: Humans

Paleospecies: Problems

• Allometry: Orangutans

Growth comparison

H um

an

Ch im

pa nz

ee

Paleospecies: Problems • Sexual dimorphism • Pronounced

morphological differences between Males and Females

• Must differentiate between Sex. Dimorphism and morphological differences between species in the fossil record

Gorilla - Male Gorilla - Female

Paleospecies: Problems

• Sexual dimorphism: Mandrills

Paleospecies: Problems

• Variation • Intraspecific (within species) • Interspecific (between species) • Pathological conditions • At what point do we define a new

species? • Lumpers vs. splitters

How Are Species

Formed?

• Two general modes of evolutionary change • Anagenesis

• Evolution of a trait or a species into another over a period of time

– Cladogenesis • Evolution through the branching of a species or a

lineage (ancestor species may/may not = new array)

Evolution does not need to be gradual

What is Speciation?

• Speciation = radiations that occur through cladogenesis

• Results in one species diverging into multiple species

• Ancestor species may or may not persist over the long term

What is Speciation?

• 4 geographic modes of speciation • Allopatric • Peripatric • Parapatric • Sympatric

Allopatric Speciation

• Speciation occurring via geographic isolation • Common in nature • Geographic barriers = mountains, rivers, etc. • Divergence = drift, dif. Mutations, dif. Selective pressures

Allopatric Speciation • Geographical features divide groups over time • Ex. Grand Canyon Squirrels • Albert Squirrel

• South rim • White bellies • Dark tails

• Kaibab squirrel • north rim • black bellies • White Tails

• Warming after last Ice age = isolated high elevation Ponderosa Pine forests

• Allopatric speciation occurred

Has been different species, now subspecies!

Peripatric Speciation

Subform of Allopatric Speciation

Subpopulations can migrate to a new location outside the range of the

original population

e.g., a few individuals colonize an island away

from the mainland populations

Population at the edge of the main range of a species = peripheral

isolate

Related to Founder’s effect

Parapatric Speciation • Speciation occurring when two populations are

geographically contiguous • “Stepped cline” pattern of geographic variation

• Caused by abrupt environmental changes • Species A adapted to one side of boundary,

species B adapted to the other side • Hybrid Zone in the middle • Conditions for parapatric speciation are particularly

great if the hybrid zone is a tension zone (exists when the hybrids between the forms on either side of the boundary are selectively disadvantageous)

Sympatric Speciation

• Speciation occurring in the same geographic location • Polymorphism in the

population does not depend on space (in contrast to the stepped cline of parapatric speciation)

Sympatric Speciation

• Ex. Speciation of Cichlids of Lakes Nabugabo, Victoria, Tanganyika, in East Africa

• Rapidly evolved (1 million years) • Over 500 very different species • Thought to be due to sexual selection

Speciation Comparison

Speciation Events

• Adaptive radiation – The diversification of an ancestral

group of organisms into new forms that are adapted to specific ecological niches

– Ex: • The radiation of mammals

following the extinction of the dinosaurs

• Darwin’s finches

Phylogenetics and Cladistics

Phylogeny

• Phylogeny – The study of the

branching relationships of populations as they give rise to multiple descendant populations over evolutionary time

– Aimed at reconstructing evolutionary histories

What is Phylogenetics?

– Practice of creating “phylogenetic trees” – Shows the pattern of evolutionary relationships

among species – Can be created using living or fossil species

• This is a HYPOTHESIS!

How do we develop phylogenies?

• Rests on our observations of traits displayed by organisms

• Traits: • Any observable characteristics of

an organism • Anatomical features • Developmental or

embryological processes • Behavioral patterns • Genetic sequences

How do we read a phylogenetic tree?

• Traits are used to determine the relationship between species

• Taxon Group of related organisms that share one common ancestor

• Node Point where tree splits

• Outgroup Taxon that is related to the groups of interest but that branched off earlier in evolutionary history

• Root Base of tree Common lineage from which all species in the tree are derived

What is Cladistics?

• A phylogenetic method that uses shared derived traits to create ancestor-descendant trees – Shared derived traits

(synapomorphies) • Trait that is shared by two

or more taxa and their most recent common ancestor

• Species of organisms are grouped into clades

• Clade: group that consists of an ancestor organism and all its descendants

How Do We Determine Clades?

• To determine clades we need to determine which traits are shared derived traits (synapomorphies)

• Shared derived trait (synapomorphy) – Homologous trait that is shared

by two or more taxa and their most recent common ancestor but not ancestors that existed prior to this

• Homoplasy (analogous trait) – Same trait – Trait not present in a

common ancestor

How Do We Determine

Synapomorphies? • Try to determine whether a trait is homologous

or analogous – Homologous Trait

• In two or more species • Inherited from a common ancestor

– Analogous Trait • In two or more species • Evolution independently fashioned

similar traits in each species • Not from a common ancestor

• Homologous traits are then used to determine clades

Homology

Analogy

Divergent vs.

Convergent Evolution

Divergent vs. Convergent Evolution

Issues for reconstructing

phylogenetics of past species

What features are variation within a species? What features distinct to species?

How do you know a trait is homologous?

Are sample sizes large enough?