Fish Taxonomy and Systematics

Lecture 4:

Cladograms & Speciation

Cladograms

Phylogenetic relationships expressed in cladograms - branching representation of the evolutionary relationships among taxa based on shared common traits and shared unique traits

Constructing a Cladogram

Listing of traits Coding of each taxon by presence/absence of

each trait Assemble groupings based on trait conditions Use the simplest branching structure possible:

principle of parsimonyWhich states that “cladogram (tree) having the fewest number of “steps” (evolutionary

changes) is the one accepted”

TAXA

Lanc

elet

(out

grou

p)

Lam

prey

Sala

ma n

der

Leop

ard

Turt

le

Tuna

Vertebral column(backbone)

Hinged jaws

Four walking legs

Amniotic (shelled) egg

CH

AR

AC

TER

S

Hair

(a) Character table

Hair

Hinged jaws

Vertebralcolumn

Four walking legs

Amniotic egg

(b) Phylogenetic tree

Salamander

Leopard

Turtle

Lamprey

Tuna

Lancelet(outgroup)

0

0 0

0

0

0

0 0

0

0

0 0

0 0 0 1

11

111

1

11

1

1

11

11

Sequentially group taxa by shared derived character states (apomorphies)

More on traits...

• Meritic-count it!• Morphometric-measurable shape

fin length eye shape head lengthratios between such measures...anatomical characteristicsmolecular characteristics

Which traits do I use?

Relationship

Recency of common ancestry

i.e., taxa sharing a common ancestor more recent in time are more closely related than those sharing common ancestors more distant in time.

Ingroup – group studied

Outgroup – group not part of ingroup, used to “root” tree

The out-group serves as a base line for comparisons with the other organisms being evaluated, the in-group

Sistertaxa

ANCESTRALLINEAGE

Taxon A

PolytomyCommon ancestor oftaxa A–F

Branch point(node)

Taxon B

Taxon C

Taxon D

Taxon E

Taxon F

Apomorphy (derived trait) = a new, derived feature e.g., for this

evolutionary transformation

scales --------> feathers(ancestral feature) (derived feature)

Presence of feathers is an apomorphy for birds.

Taxa are grouped by apomorphiesApomorphies are the result of evolution.

Taxa sharing apomorphies underwent same evolutionary history should be grouped together.

Shark Fish Humans

TIME

Example: Are fish more closely related to sharks or to humans?

Shark Fish Humans

TIME

common ancestor of Fish and Humans

common ancestor of Sharks, Fish, and Humans

monophyletic group

Osteichthyes Vertebrata

Shark Fish Humans

TIME

common ancestor of Fish and Humans

common ancestor of Sharks, Fish, and Humans

Constructing a Cladogram

This example shows the evolutionary relationships among plants.

1. In the table below, the traits in the row for the out-group are marked with a zero.

When a plant has a trait not found in the out-group, the trait is considered a derived trait and is marked with a one.

Next the numbers of shared derived traits are totaled.

Constructing a Cladogram

Derived Traits

Organisms Vascular tissue Seeds Flowers

Mosses (Out-group) 0 0 0

Pine trees 1 1 0

Flowering plants 1 1 1

Ferns 1 0 0

Total 3 2 1

Constructing a Cladogram

2. Starting with a diagonal line, as shown on the next slide, the out-group (mosses) is placed on the first branch of the cladogram.

Just past this first branch, the most common derived trait is listed—vascular tissue.

Vascular tissue is a series of tubes and vessels within a plant.

Constructing a Cladogram

3. Next the second most common derived trait is determined, which in this case is seeds.The ferns lack seeds and so are placed in the second branch of the cladogram.

4. The third most common derived trait is flowers. Conifers do not have flowers and so are placed in the third branch above the second branch on the cladogram.

The flowering plants are placed at the end of the cladogram.

Constructing a Cladogram

Mosses

Ferns

Pine trees

Flowering plants

Vasculartissue

Seeds

Flowers

Speciation

Q. How do populations become distinct species?Speciation - process by which a new species originates.

Involves the creation of a population of organisms that are novel enough to be classified in their own group.

The process whereby gene flow is reduced sufficiently between sister populations to allow each to become different evolutionary lineagesAllopatric (with geographic isolation): speciation resulting

from divergent evolution of populations that are geographically isolated from each other.

Non-allopatric (without geographic isolation)

SpeciationAllopatric (with geographic isolation)

speciation:Vicariant - large populations geographically isolated

(little inbreeding) (United States)

Founder - small population becomes geographically isolated and then reproductively isolated via inbreeding, selection, drift (Gilligan’s Island)

Reinforcement - early isolation followed by sympatry, but selection against hybrids

Speciation

Non-allopatric (without geographic isolation)

Sympatric - sister species evolve within the dispersal range of each other, but adapt to different habitats - habitat-dependent assortive mating (tribes)

Parapatric - sister species evolve in segregated habitats across a narrow contact zone - little mixing in spite of proximity

Speciation Based on Gene Flow

(a) Allopatric speciation (b) Sympatric speciation

AllopatricIn allopatric speciation, gene flow is

interrupted or reduced when a population is divided into geographically isolated subpopulations

The definition of barrier depends on the ability of a population to disperse

Separate populations may evolve independently through mutation, natural selection, and genetic drift

Allopatric speciation in the Grand Canyon

A. harrisi A. leucurus

Examples of Allopatric speciation:            Allopatric speciation of squirrels in the Grand Canyon. Animals like birds do not show speciation like those animals that are barred from breeding by the canyon.

Key question about allopatric species is whether they are indeed different enough that viable, fertile offspring would not be produced by mating. This can be tested sometimes as in…

Allopatric speciation

Reproductive isolation between populations generally increases as the distance between them increases

Barriers to reproduction are intrinsic; separation itself is not a biological barrier

Mantellinae(Madagascar only):100 species

Rhacophorinae(India/SoutheastAsia): 310 species

Other Indian/Southeast Asianfrogs

Millions of years ago (mya)1 2 3

1 2 3

100 80 60 40 20 0

88 mya 65 mya 56 mya

India

Madagascar

Sympatric Speciation

Sympatric means “same country”Speciation takes place in geographically

overlapping populationsOccurs because of

Polyploidism Habitat differentiation Sexual selection

Polyploidism

Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division1. Autopolyploidy2. Allopolyploidy

Autopolyploidy in plants

2n=6 4n=12

Failure of cell division after chromosome duplication gives

rise to tetrapliod cells which

may be viable and self-fertile

Allopolyploid

An allopolyploid is a species with multiple sets of chromosomes derived from different species

Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids

Ancestral species:

Triticummonococcum(2n = 14)

AA BB

WildTriticum(2n = 14)

Product:

AA BB DD

T. aestivum(bread wheat)(2n = 42)

WildT. tauschii(2n = 14)

DD

Habitat differentiation

Sympatric speciation can also result from the appearance of new ecological niches

North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees, so food preference may isolate the two populations

Sexual selection Sexual selection can drive sympatric speciation

Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake VictoriaExample: Lake Victoria has 200 closely related species of

Cichlids (fish) which probably all arose from one ancestor with the driving force for speciation being:Competition for a limited resource (food) within the lake, and

adaptation to new food sources. This gave rise to different species that are kept from breeding with each other by distinctive coloration pattern.