chapter 24: the origin of species

Chapter 24:The Origin of

Species

Essential Knowledge 1.c.1 – Speciation and extinction have occurred

throughout the Earth’s history (24.3 & 24.4). 1.c.2 – Speciation may occur when two

populations become reproductively isolated from each other (24.1).

1.c.3 – Populations of organisms continue to evolve (24.2).

2.e.2 – Timing and coordination of physiological events are regulated by multiple mechanisms (24.1).

Two Concepts of Species1. Morphospecies 2. Biological Species

Morphospecies Organisms with very similar

morphology

Two Schools1. Splitters - Break apart species

into new ones on the basis of small phenotype changes

2. Lumpers - Group many phenotype variants into one species

Biological Species A group of organisms that could

interbreed in nature and produce fertile offspring

Don’t identify species based upon similar likeness ALONE!

Key Points Could interbreed Fertile offspring May or may not

be similar in phenotype

Morphospecies & Biological Species Often overlap Serve different purposes

Other Concepts of Species1. Recognition Species Concept2. Cohesion Species Concept3. Ecological Species Concept4. Evolutionary Species Concept

Ecological Species Concept Emphasizes a specie’s role or

function in the environment.Called an organism’s niche

(Added Slide ) Lions and tigers are ecologically isolated

Live in open

grassland

Live in forest

Hybridization has been successful in captivity But it does not occur in the wild

Tiglon

Evolutionary Species Concept

Emphasizes evolutionary lineages and ecological roles

Used extensively in comparative biology

Popular among paleontologists

Speciation Speciation = formation of a NEW

speciesCaused by disruptive selection (see

Ch 23) 3 Requirements for speciation:

1. Variation in the population2. Selection3. Isolation

Reproductive Barriers Serve to isolate a populations from

other gene pools Create and maintain “species”

Main Types of Reproductive Barriers

Prezygotic – Prevent mating or fertilization

Postzygotic – Prevent viable, fertile offspring

Prezygotic - Types1. Habitat Isolation2. Behavioral Isolation 3. Temporal Isolation4. Mechanical Isolation5. Gametic Isolation

Habitat Isolation Populations live in different habitats or

ecological niches Ex – mountains vs. lowlands

Behavioral Isolation

Mating or courtship behaviors different

Different sexual attractions operating

Ex – songs and dances in birds

Temporal Isolation Breeding

seasons or time of day different

Ex – flowers open in morning or evening

Mechanical Isolation Structural differences that prevent

gamete transfer Ex – anthers of flower not positioned to

put pollen on a bee, but will put pollen on a bird

Gametic Isolation Gametes fail to attract each other and

fuse Ex – chemical markers on egg and

sperm fail to match

Postzygotic - Types1. Reduced Hybrid Viability2. Reduced Hybrid Fertility3. Hybrid Breakdown

Reduced Hybrid Viability Zygote fails to mature or develop Ex:

When two different species of frogs hybridize○ Egg is fertilized (zygote IS formed)○ Zygote never develops properly (if at

all)

Reduced Hybrid Fertility Hybrids/offspring are living, viable Offspring cannot reproduce sexually

(nonfertile) Chromosome count often “odd”

Meiosis doesn’t work out Ex: mules, ligers

Hybrid Breakdown Hybrid/offspring are living, viable AND

fertile Offspring cannot compete

successfully with the “pure breeds” Ex:

Most plant hybridsPlants are “born” and can produce seedsBlooms/plants may lack certain color or

size of petal

• Hybrids between them produced defective embryos in the lab

• Their mating calls also differ substantially

Modes of Speciation1. Allopatric Speciation2. Sympatric Speciation Both work through a block of

gene flow between two populations

Allopatric Speciation Allopatric = other homeland Ancestral population split by a

geographical feature. Comment – the size of the

geographical feature may be very large or small

Example Pupfish populations in Death Valley Generally happens when a species

range shrinks for some reason

Another Example

Conditions Favoring Allopatric Speciation1. Founder's Effect - with the peripheral

isolate2. Genetic Drift – gives the isolate

population variation as compared to the original population

3. Selection pressure on the isolate differs from the parent population

Result Gene pool of isolate changes

from the parent population New species can form

Comment Populations separated by geographical

barriers may not evolve much Ex - Pacific and Atlantic Ocean

populations separated by the Panama IsthmusFish - 72 identical kindsCrabs - 25 identical kindsEchinoderms - 25 identical kinds

Adaptive Radiation Rapid emergence of several

species from a common ancestor Common in island and mountain

top populations or other “empty” environments

Ex – Galapagos Finches

Sympatric Speciation Sympatric = same homeland New species arise within the range of

parent populations Can occur In a single generation

Plants Polyploids may cause new

species because the change in chromosome number creates postzygotic barriers

Animals: Don't form polyploids and will use other mechanisms

Polyploid Types1. Autopolyploid - when a species

doubles its chromosome number from 2N to 4N

2. Allopolyploid - formed as a polyploid hybrid between two speciesEx: wheat

Autopolyploid

Allopolyploid

Rate of Evolution??

Gradualism Evolution Darwinian style of evolution Small gradual changes over long

periods of time

Gradualism Predicts: Long periods of time are needed

for evolution Fossils should show continuous

links Problem: Gaps in fossil record

Punctuated Evolution New theory on rate/pace of

evolution

Elridge and Gould – 1972

Punctuated Equilibrium Evolution has two speeds of

change:1. Gradualism or slow change2. Rapid bursts of change/speciation

Predictions Speciation can occur over a very

short period of time Fossil record will have gaps or

missing links New species will appear in the

fossil record Established species will show

Comment Punctuated Equilibrium is the

newest, most widely accepted view

Best explanation of timing

Summary Identify several concepts of "species". Recognize reproductive barriers that keep

two species from reproducing with each other.

Recognize various mechanisms of speciation and hybrid zones.

Identify the Gradual and Punctuated interpretations of speciation.

Identify genetic mechanisms of speciation.