Ch 24: origin of species• What is a species?• Species Concepts

– Morphological– Biological

• Isolating mechanisms and hybridization– Other Species Concepts

• How do new species arise?– Allopatric vs. sympatric speciation

• What is the tempo of speciation?– Punctuated equilibrium model and fossil record

• How can new, complex structures evolve?

What is a species?

Species

• A Latin word meaning “kind” or “type”

• There are many kinds of living thing!

• Biologists mean something special by the word species

One species or two?

Species Concepts

• Our ideas about how best to define a species have changed over time.

• If speciation is occurring, no species concept will “work” all the time - you should always be able to find arguable forms.

Morphological Species Concept

• morphology = shape or appearance– Collect a type specimen– Others that are similar to the type are the

same species– Very widely used (e.g., insects)– Problem - doesn’t take intraspecific

variation into account - how similar is similar enough? Are the similarities we notice the important ones?

Biological Species Concept

• Formulated by Ernst Mayr: Ornithologist; one of the contributors to the Modern Evolutionary Synthesis

• Species are populations or groups of populations whose members have the potential to interbreed in the wild and produce viable, fertile offspring. (1942)

• Rationale: hybridization implies mixing - species are distinct because they don’t mix

• Advantage: the organisms decide themselves how different is different enough

Reproductive isolation

• What keeps different species from interbreeding? (not counting geography)– Prezygotic barriers

• Before mating or fertilization

– Postzygotic barriers• After mating or fertilization

• What’s a zygote?

Prezygotic barriers (1)• Habitat isolation: species occupy different

habitats in the same geographic area

Three-spined stickleback speciesdiffer by lake habitat: benthic vs.pelagic zones

Prezygotic barriers (2)

• Breeding behavior ->

Prezygotic barriers (3)• Temporal isolation: species reproduce at

different times in the same geographic area

Prezygotic barriers (4)• Mechanical isolation: species have

incompatible anatomy

Prezygotic barriers (5)

• Gametic isolation: species have gametes that recognize only their own species

Postzygotic barriers• Reduced viablility: embryos die or

fail to develop• Hybrid offspring born, but have

reduced viability or fertility• Hybrid breakdown: offspring of

hybrids have reduced survival or fertility

Horse + donkey = sterile mule

mom

dad

Reproductive isolation

• Prezygotic barriers

• Postzygotic barriers

Problems with Biological Species Concept

• The Biological Species Concept works very well with some taxa (birds, for instance) but..

• Information on hybridization lacking for most species

X

?

Problems with Biological Species Concept (2)

• In some groups (oak trees, for instance, and many other plants) hybridization between what seem to be quite different species is common.

Problems with Biological Species Concept (3)

• In other groups (some birds), hybridization at least happens occasionally - a little mixing doesn’t always break down differences between species

Alternative species concepts

• Phylogenetic species concept - minimum diagnosible unit

• Ecological Species Concept - a set of organisms adapted to a particular set of resources, called a niche, in the environment

• All these species concepts agree most of the time - they differ mostly when applied to borderline or poorly understood cases.

Speciation

• Speciation - when one species splits into two (aka cladogenesis)

• How does it happen?• Allopatric vs. sympatric speciation

Modes of speciation

Fig. 24.6

Allopatric Speciation

• Allopatry - “different fatherlands”– 1. One species is divided into two isolated

populations in two geographic regions– 2. The two populations evolve in different

directions while apart– 3. Isolating mechanisms evolve in the two

groups– 4. The two populations are unable to breed

with each other when they reconnect - they are now two separate species

Allopatric speciation in ground squirrels

Sympatric Speciation

• Sympatry = living in the same place• How could individuals evolve

reproductive barriers to others in a local, interbreeding population?

• Theoretically possible, but evidence suggests it’s rare at best

• Organisms can undergo sympatric speciation by producing polyploid offspring

Speciation by production of tetraploids

Sympatric Speciation (1) Tree Frogs

Hyla chrysocelis Hyla versicolor

Sympatric speciation (2)• Mate choice by cichlids

Normal light Monochromatic lightFig. 24.16

Pundamilia pundamilia

Pundamilia nyererei

Adaptive radiation

• Example?

Fig. 24.11

Is speciation gradual or sudden?

• Paleontologists have a hard time seeing gradual change in the fossil record. More often, a form appears suddenly, then, after persisting for some eons, disappears from the fossil record just as suddenly.

• Why is this?

• Niles Eldredge and Stephen Gould proposed that formation of new species happened rather quickly, and that species, once formed, changed little until they went extinct.

• This model was called punctuated equilibrium by Eldredge and Gould.

Earlier

later

So are Gould and Eldredge right?

• The evidence for equilibria comes from the fossil record only - no living species has shown any resistance to change under artificial selection.

• The nature of the fossil record: small slices of time are preserved. Huge slices are lost.

• Nobody believes that the tempo of evolution is absolutely constant.

earlier

later

Earlier

later

Evolutionary Novelties

• If natural selection is essentially a “weeding” process, eliminating the less fit, how does a complex new structure like an eye develop?

• This is one of the arguments of anti-evolution religious fundamentalists:– it’s impossible that natural selection acting on

chance mutations could gradually produce something so complex and perfect as an eye

• Is it really impossible, though? What would have to happen?

Question: How do complex structures evolve?

• Answer: gradually

• Important principles:– Natural selection is not goal-directed, and cannot

anticipate future needs– Evolutionary novelties arise by modification of pre-

existing structures – All intermediate steps must be beneficial to the

organism - like working on an engine while it is running.

• So for novel complex structures to arise by natural selection– 1. Each step must be a plausible

small minor modification of the one before it

– 2. Each step must be an improvement on the one before it

A simple patch of pigmented cells detects

light

If a depression forms with the pigment cells in it, can detect

direction of light source

A depression with a small opening allows formation of

images on the light-sensing cells

A lens within the eye allows better focussing and allows more light to

enter

A lens that can change shape allows near and far

focussing

Is it plausible?

• Could these different stages have existed?

• Would they each give their owners an advantage?