Speciation

Chapter 13

Evolution

• Microevolution—Relatively short term changes in ALLELE FREQUENCIES within a population or species

• Macroevolution—Large scale evolutionary changes– Over long periods

– Small scale microevolution leads to macroevolution

Biological Species Concept

• Species—Group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but do not produce viable offspring with members of other groups.

• What maintains species? – Gene flow

– Reproductive isolation (biological)

Other Species Definitions

• Morphological Species Concept

– Characterizes by body shape and other structures

• Ecological Species Concept

– Characterizes in terms of ecological niche

• Phylogenetic Species Concept

– Smallest group of organisms that share a common ancestor

Reproductive Isolation

• Prezygotic Barriers—block fertilization from happening– Prevent different species from attempting to mate

– Prevent an attempted mating from being successful

– Hindering fertilization

• Postzygotic Barriers—occur after zygote is formed– Developmental errors

– Birth defects cause death

– Infertile offspring

Prezygotic Reproductive Isolating Barriers

• Behavioral isolation—Absence of cross-attraction between individuals of separate species inhibiting any courtship initiation or individual behavior during copulation does not allow normal fertilization to occur.

• Ecological isolation—Variations in the ecology of species give rise to barriers:– Habitat isolation—Even when living in the same common locality, species

occupy diverse habitats due to different biological or genetic tendencies thereby limiting gene flow during breeding seasons.

– Temporal isolation—Species living in the same area have different breeding seasons preventing gene flow.

– Pollinator isolation—Flowering plants have varying interactions with pollinators thereby reducing gene flow.

• Mechanical isolation—Reproductive structures are incompatible between species inhibiting copulation or pollination.

• Gametic isolation–Gametes that are transferred during copulation or pollination are ineffectual for fertilization due to problems with storage or transfer of gametes or because of conspecific pollen or sperm partiality.

Prezygotic Barriers—Habitat Isolation

Prezygotic Barriers—Temporal Isolation

Prezygotic Barriers—Behavioral Isolation

• http://www.fiddlercrab.info/video/u_heteropleura02.html• http://www.fiddlercrab.info/video/u_ornata01.html

Prezygotic Barriers—Mechanical Isolation

Prezygotic Barriers—Gametic Isolation

Postzygotic Reproductive Isolating Barriers

• Ecological unviability—Although normal development occurs, hybrids cannot find a suitable ecological niche thereby lowering viability.

• Hybrid unviability—Hybrid species have developmental issues causing complete or incomplete lethality.

• Behavioral sterility—Although normal gametogenesis occurs, hybrids are less fertile and typically exhibit phenotypes or courtship behaviors that make them less desirable mates.

• Hybrid sterility—Hybrids can have developmental problems of the reproductive organs or gametes, or can suffer from physiological or neurological issues that prevent effective courtship.

Postzygotic Barriers—Reduced Fertility

Speciation

• Development of a new species through a variety of factors

• Rate of speciation depends on generation time, environmental conditions, etc.

• Can be caused by a change in just 1 gene or a set of genes causing some sort of isolation

Types of Speciation

• Allopatric Speciation—gene flow is interrupted when a population is divided into geographically isolated subpopulations

• Parapatric Speciation—occurs when part of a population enters a new habitat bordering the range of the parent species– Some gene flow may occur between populations in border zone

• Sympatric Speciation—occurs in populations that live in the same geographic area– Less common than allopatric speciation– Happens when gene flow is diminished by:

• Polyploidy• Habitat differentiation• Sexual selection

Types of Speciation

Allopatric Speciation

ParapatricSpeciation

Ephedra californica

Ephedra trifurca

Sympatric Speciation

Apple maggot flies (Rhagoletis pomonella) on domestic apple (left) and on much smaller native hawthorn fruits (right). Flies that emerge from a given host generally return to mate and lay eggs on the same type of fruit.

How Does Speciation Occur?

• Gradualism—one species gradually transforming into another through a series of intermediate forms– Evolutions occurs in small, incremental changes

over MANY generations

– Should be able to see in fossil record but we do not – intermediate forms NOT present in fossils• Fossil record incomplete or;

• Missing links too rare in fossils or;

• See next slide

Gradualism

How Does Speciation Occur?

• Punctuated Equilibrium—relatively brief bursts of rapid evolution interrupting long periods of little change

– Fits with allopatric speciation

– Can occur during adaptive radiation—population inhabiting a patchy environment gives rise to multiple specialized forms in short time period

• Common in island groups

Punctuated Equilibrium

Adaptive Radiation A. loysiana

GULF

OF

MEXICO

Florida

ATLANTIC OCEAN

CUBA

JAMAICAHISPANIOLA

PUERTO

RICO

Upper trunk/canopy

A. evermanni

Midtrunk

Lower trunk/ground

A. cristatellus

Grass/bush

A. pulchellus

Adaptive Radiation

Extinction

• A species goes extinct when ALL of its members die.

• Many factors can cause—climate, reproductive barriers, disease.

• No matter what it is, it is a FAILURE of that species to adapt to the new conditions

• More prevalent in small, less genetically diverse populations

Types of Extinction

• Background Extinction—gradual loss of species over time.– Loss of habitat, small climate change, etc.

• Mass Extinction—many number of species disappeared over a relatively short period of time.– Open up new habitats for adaptive radiation

– Two theories:• Impact Theory—meteor, comet crashes into earth making it

inhabitable for the life there

• Movement of Earth’s Crust—causes dramatic environmental changes

Taxonomy

• How we name and classify organisms

• Binomial Nomenclature

– First name—Genus

– Second name—Species

– Homo sapiens

• Organisms are classified based on a hierarchy of more specific categories

Phylogeny

• Phylogeny—Depiction of species relationships based on descent from shared ancestors

• Phylogenetic Trees—pictures of these relationships

– Anatomical features of fossils and living creatures

– Behaviors

– Physiological adaptations

– Molecular sequences

Cladistics

• Phylogenetic system based on ancestral and derived characteristics– Ancestral Characteristics—inherited attributes and

RESEMBLE those of ancestor– Derived Characteristics—features that are different

from ancestors group– Monophyletic—Group of organisms with 1 common

ancestor and ALL the descendants– Paraphyletic—Group of organisms with 1 common

ancestor and some, but not all descendants– Polyphyletic—Group of organisms that EXCLUDES the

most recent common ancestor

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Clades

Taxa at branch

tips

Birds

Nonavian

dinosaurs

Crocodiles

Lizards

and snakes

Amphibians

Mammals

Turtles

Time

Node

(common

ancestor)

Last

common

ancestorRoot

of tree

Cladogram

Cladogram

Cladogram

Constructing Cladograms

• Light bones

• 3-toed foot

• Wishbone

• Breastbone

• Loss of 4th

and 5th digits

• Down like

feathers

• Longer arms

and hands

• Complex

feathers

• Arms as long or

longer than legs

• Feathers

support flight

1

2

3

4

5

Coelophysis

Allosaurus Modern birdsProtarchaeopteryx

SinosauropteryxArchaeopteryx

Last

common

ancestor of

theropods

Coelophysis

Allosaurus

Sinosauropteryx

Protarchaeopteryx

Archaeopteryx

Modern birds

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1

2

3

4

5

Constructing Cladograms

Cladistics

Phylogenetics and Cladistics Cheat Sheet

Term Definition

Ancestral characters a trait inherited from the ancestor of a clade

Clade a group consisting of an ancestor and all its descendants—a single "branch" on the "tree of life”

Cladistics the science that tries to reconstruct phylogenetic trees and thus discover clades

Cladogram tree-shaped diagrams; the result of cladistic analyses

Derived characters a trait that has evolved—not from the common ancestor of the clade

Monophyletic group a taxon (group of organisms) which forms a clade

Outgroup a monophyletic group of organisms that serve as a reference group when determining the evolutionary relationship among three or more monophyletic groups of organisms

Paraphyletic group a group that consists of all the descendants of the last common ancestor of the group's members minus a small number of monophyletic groups of descendants

Phylogenetic tree a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities—their phylogeny—based upon similarities and differences in their physical or genetic characteristics

Polyphyletic group a group characterized by one or more homoplasies: character states which have converged or reverted so as to appear to be the same but which have not been inherited from common ancestors

Systemactics the study of the diversification of living forms, both past and present, and the relationships among living things through time

The World Around Us