Chapter 24

The Origin of

Species

What You Need to Know:• The difference between microevolution and

macroevolution.

• The biological concept of a species.

• Prezygotic and postzygotic barriers that maintain

reproductive isolation in natural populations.

• How allopatric and sympatric speciation are similar

and different.

• How autopolyploid or an allopolyploid

chromosomal change can lead to sympatric

speciation.

• How punctuated equilibrium and gradualism

describe two different tempos of speciation.

Microevolution vs macroevolution

• Microevolution – changes in gene

frequencies within population

• Macroevolution – origin of new

taxonomic groups, ie – species

level and above

• Speciation is at the boundary

between microevolution and

macroevolution.

What is a species?

•Species is a Latin word meaning “kind”

or “appearance.”

•Traditionally, morphological differences

have been used to distinguish species.

•Today, differences in body function,

biochemistry, behavior, and genetic

makeup are also used to differentiate

species.

Biological Species Concept

• Species = population or group of

populations whose members have the

potential to interbreed in nature and

produce viable, fertile offspring

–Reproductively compatible

• Reproductive isolation = barriers that

prevent members of 2 species from

producing viable, fertile hybrids

Other definitions of species:

• Morphological – by body

shape, size, and other

structural features

• Ecological – niche/role in

community

• Phylogenetic – share common

ancestry, branch on tree of life

4 approaches to species concept

1. Biological – reproductive isolation

2. Morphological – anatomical

differences

3. Ecological – unique roles in

environment

4. Phylogenetic – based on evolutionary

lineage with distinct morphology and

molecular sequences

Speciation = Reproductive Isolation• In the 1980s, Diane Dodd experimented with fruit flies

– One group fed starch, the other maltose

– After several generations, groups of individuals that originally interbred now ignored each other even if in same jar

– Created reproductive isolation

• Species are created by a series of evolutionary processes that result in populations becoming isolated– Geographically & reproductively

Figure 24.2b The biological species concept is based on interfertility

rather than physical similarity

Interfertility

concept does

not apply to

• Asexually

reproducing

organisms

• Extinct species

Barriers to speciation

• Prezygotic – prevent mating or

successful fertilization

• Postzygotic - prevent the

hybrid zygote from developing

into a viable, fertile adult.

Types of Reproductive Barriers

Prezygotic Barriers:

– Impede

mating/fertilization

Types:

– Habitat isolation

– Temporal isolation

– Behavioral isolation

– Mechanical isolation

– Gametic isolation

Postzygotic Barriers:

– Prevent hybrid

zygote from

developing into

viable adult

Types:

– Reduced hybrid

viability

– Reduced hybrid

fertility

– Hybrid breakdown

REDUCED HYBRID

VIABILITY

REDUCED HYBRID

FERTILITYHYBRID BREAKDOWN

Types of Reproductive Barriers

Prezygotic barriers

• Habitat isolation

• Temporal isolation – different

breeding times

• Behavioral isolation

• Mechanical isolation

• Gametic isolation

Fig. 24-4h

(f)

Bradybaena with shellsspiraling in oppositedirections

Mechanical Isolation

Fig. 24-4e

(c)

Eastern spotted skunk(Spilogale putorius)

Western spotted skunk

(Spilogale gracilis)

Temporal Isolation

These skunks have different breeding times.

Fig. 24-4g

(e)

Courtship ritual of blue-footed boobies

Behavioral Isolation

Fig. 24-4k

(g)

Gametic isolation in sea urchins• Sperm of one species may not

be able to fertilize eggs of

another species

– Biochemical barrier =

sperm cannot penetrate egg;

receptor recognition: lock &

key between egg & sperm

– Chemical incompatibility =

sperm cannot survive in

female reproductive tract

• Example: Sea urchins release

sperm & eggs into

surrounding waters where

they fuse & form zygotes.

Gametes of different species

are unable to fuse.

Postzygotic Barriers

•Reduced hybrid viability - Genetic

incompatibility between the two species may

abort the development of the hybrid at some

embryonic stage or produce frail offspring.

•Reduced hybrid fertility - Even if the hybrid

offspring are vigorous, the hybrids may be

infertile and the hybrid cannot backbreed with

either parental species.

Reduced hybrid breakdown – In some cases, first

generation hybrids are viable and

fertile.However, when they mate with either

parent species or with each other, the next

generation is feeble or sterile.

Reduced Hybrid Viability

• Sometimes referred to as zygote mortality

• Genetic incompatibility between the two

species may abort the development of

the hybrid at some embryonic stage or produce

frail offspring.

– Ex. Species of salamander and Rana; coral

Reduced Hybrid Fertility• Even if hybrids are

vigorous they may be

sterile

– Chromosomes of

parents may differ

in number or

structure & meiosis

in hybrids may fail

to produce normal

gametes

• Ex. Mule

Hybrid Breakdown• First generation hybrids are

viable, but second

generation offspring are

feeble or sterile

– Caused by incompatibility

between interacting

genes.

• Ex. Cotton, rice plants

– In strains of cultivated

rice, hybrids are vigorous

but plants in next

generation are small &

sterile.

REDUCED HYBRID

VIABILITY

REDUCED HYBRID

FERTILITYHYBRID BREAKDOWN

Types of Reproductive Barriers

Geographic Isolation• Isolation in population due to

where they exist– Patric = homeland

• Allopatric Speciation –geographic separation– Ex. Squirrels separated by a

mountain range

• Sympatric Speciation – still live in the same area– Ex. Resident and transient

orcas

• Peripatric speciation– Related to founder effect; ex.

African elephant, pygmy elephant

• Parapatric speciation – Similar to sympatric; ex.

Tennessee cave salamander

Modes of speciation1) Allopatric – geographical separation

Figure 24.7 Allopatric speciation of squirrels in the Grand Canyon

Allopatric speciation of antelope squirrels on

opposite rims of the Grand Canyon

Modes of speciation2. Sympatric – biological barriers prevent

gene flow in overlapping populations as in

autopolyploidy, allopolyploidy, mate

preference, etc.

Two main modes of speciation

Two main modes of speciation:

Allopatric Speciation“other” “homeland”

Geographically isolatedpopulations

• Caused by geologic events or processes

• Evolves by natural selection & genetic drift

Eg. Squirrels on N/S rims of Grand Canyon

Sympatric Speciation“together” “homeland”

Overlapping populations within same geographic area

Gene flow between subpopulations blocked by:

• polyploidy• habitat differentiation• sexual selection

Eg. polyploidy in 80% of plants (oats, cotton, potatoes, wheat)

Figure 24.8 Has speciation occurred during geographic isolation?

Examples of Sympatric Speciation involving

polyploidy (extra sets of chromosomes) which can

lead to new species. • Autopolyploidy – more than two sets of chromosomes;

meiotic failure of chromosomes to separate, common in self-pollination in plants

• Allopolyploidy * – interspecific hybrid; may become fertile due to nondisjunction in formation of gametes

• * more commonNotice that the chrom

do not separate!

One mechanism for allopolyploid speciation in plants

“allo” means coming

from another place, in

this case another

species.

• Around 1870, a new species of grass turned up at the salt marches near the coast of the English Channel: Spartinatownsendii . It was taller than the indigenous Spartinaalternifolia.

• Another relative, Spartina stricta, inhabits the North-American east coast. It was brought in to Europe and began to occupy the sites of Spartina alternifolia.

• It was now suspected that Spartina townsendii was a hybrid of the two original species. The fact that Spartinatownsendii has 2n = 126 chromosomes, Spartinaalternifolia has 2n = 70 and Spartina stricta has 2n = 56 chromosomes makes this suggestion seem likely.

Modes of Speciation

3. Parapatric Speciation

• Involves both time and space

• Is speciation at the perimeter of the

ancestral species range where the

environment changes in a qualitative way

• Local environment or resources available at

the margin of the species range are

sufficiently different that natural selection

selects for different adaptations

• Natural selection would be against the

hybrids

Hybrid zones• Where divergent allopatric and parapatric

populations come back and interbreed

“Grolar” or

“Pizzly”

Grizzly Polar

Three outcomes…With renewed or continued contact between two populations, there are three possible outcomes:

1. Individuals can hybridize readily.

2. Individuals do not hybridize at all (reinforcement)

3. Individuals hybridize but offspring have reduced fitness.

No speciation

Full speciation

Speciation in progress. Selection for evolution of strongreproductive barriers.

Fig. 24-14-4

Gene flow

Population(five individualsare shown)

Barrier togene flow

Isolated populationdiverges

Hybridzone

Hybrid

Possibleoutcomes:

Reinforcement

OR

OR

Fusion

Stability

Strengthening of

reproductive

barriers

If gene flow is great enough, the parent species can fuse into a single species

Hybrids continue to be produced between

the two species in the area of their

overlap, but the gene pools of both parent

species remain distinct.

Fig. 24-16

Pundamilia nyererei Pundamilia pundamilia

Pundamilia “turbid water,”hybrid offspring from a location

with turbid water

Gene pools

have fused.

Gradualism

• Common ancestor

• Slow, constant change

Punctuated Equilibium

• Eldridge & Gould

• Long periods of stasispunctuated by sudden change seen in fossil record

Time Course of Speciation

Rate of Speciation• Speciation rates can vary,

especially when considering adaptive radiation

• Two schools of thought– Gradualism – Gradual

divergence over long spans of time; assume big changes occur with accumulation of many small ones

– Punctuated equilibrium –rapid bursts of change with long periods of little or no change; species undergo rapid change when first diverge from parent population

• Ultimately depends on changes in the environment

Species undergo

most morphological

modifications when

they first bud from

their parent

population.

After establishing

themselves as

separate species,

they

remain static for the

vast majority of

their existence.

Speciation Rates• The punctuated pattern in the fossil

record and evidence from lab studies

suggests that speciation can be rapid

• The interval between speciation events

can range from 4,000 years (some

cichlids) to 40,000,000 years (some

beetles), with an average of 6,500,000

years

5 primary forces affect genetic

composition of populations and

cause evolution• Natural selection

• Mutation

• Gene flow

• Genetic drift

• Mate choice

What You Should Know…According to the Curriculum Framework ☺

• Speciation and extinction have occurred throughout the Earth’s

history.

– Speciation rates can vary, especially when adaptive radiation

occurs when new habitats become available

– Species extinction rates are rapid at times of ecological stress

• The level of variation in a population affects population

dynamics – species/populations with little genetic diversity are

at risk for extinction

To demonstrate understanding, make sure you can explain one of

the following:

• Five major extinctions

• Human impact on ecosystems and species extinction rates

– Note: Names and dates of extinctions are beyond the

scope of this course/AP Exam

• Speciation may occur when two populations become reproductively isolated from each other

– Speciation results in diversity of life forms. Species can be physically separated by a geographic barrier such as an ocean or a mountain range, or various pre-and post-zygotic mechanisms can maintain reproductive isolation and prevent gene flow.

– New species arise from reproductive isolation over time, which can involve scales of hundreds of thousands or even millions of years, or speciation can occur rapidly through mechanisms such as polyploidy in plants