The Origin of Species

Chapter 24

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.

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

Mayr’s concept of species

• 1942

• Potential to interbreed and produce fertile offspring

Figure 24.2a The biological species concept is based on interfertility rather than physical similarity

Figure 24.2b The biological species concept is based on interfertility rather than physical similarity

Barriers to speciation

• Prezygotic – prevent mating or successful fertilization

• Postzygotic - prevent the hybrid zygote from developing into a viable, fertile adult.

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

Fig. 24-4g

(e)

Courtship ritual of blue-footed boobies

Behavioral Isolation

Fig. 24-4k

(g)

Sea urchins

Gametic isolation in sea urchins

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.

Fig. 24-4m

(i)

Donkey

Reduced Hybrid Fertility

Interfertility concept doesnot apply to

• Asexually reproducing organisms

• Extinct species

Modes of speciation

• Allopatric – geographical separation

• Sympatric – biological barriers prevent gene flow in overlapping populations as in autopolyploidy, allopolyploidy, mate preference, etc.

Figure 24.6 Two modes of speciation

Figure 24.7 Allopatric speciation of squirrels in the Grand Canyon

Figure 24.8 Has speciation occurred during geographic isolation?

what is?

• Autopolyploidy – more than two sets of chromosomes; meiotic failure, in self-pollination in plants

• Allopolyploidy * – interspecific hybrid may become fertile due to nondisjunction

• * more common

Sympatric speciation by autopolyploidy in plants

Figure 24.15 One mechanism for allopolyploid speciation in plants

Fig. 24-UN2Ancestral species:

Triticummonococcum(2n = 14)

AA BB

WildTriticum(2n = 14)

Product:

AA BB DD

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

WildT. tauschii(2n = 14)

DD

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

• 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 Spartina townsendii has 2n = 126 chromosomes, Spartina alternifolia has 2n = 70 and Spartina stricta has 2n = 56 chromosomes makes this suggestion seem likely.

Hybrid zones

• Where divergent allopatric populations come back and interbreed

• Biologists look for patterns to study reproductive isolation

Hybrid Zones over Time

When closely related species meet in a hybrid zone, there are three possible outcomes:– Strengthening of reproductive barriers

– Weakening of reproductive barriers

– Continued formation of hybrid individuals

Fig. 24-13

EUROPE

Fire-belliedtoad range

Hybrid zone

Yellow-belliedtoad rangeYellow-bellied toad,

Bombina variegata

Fire-bellied toad,Bombina bombina

Alle

le f

req

uen

cy (

log

sca

le)

Distance from hybrid zone center (km)

40 30 20 2010 100

0.01

0.1

0.5

0.9

0.99

Reinforcement: Strengthening Reproductive Barriers

• The reinforcement of barriers occurs when hybrids are less fit than the parent species that is reproduce less successfully.

• Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species

Fig. 24-15

Sympatric malepied flycatcher

Allopatric malepied flycatcher

Pied flycatchers

Collared flycatchers

Nu

mb

er o

f fe

mal

es

(none)

Females matingwith males from:

Ownspecies

Otherspecies

Sympatric males

Ownspecies

Otherspecies

Allopatric males

0

4

8

12

16

20

24

28

Fusion: Weakening Reproductive Barriers

• If hybrids are as fit as parents, there can be substantial gene flow between species

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

Fig. 24-16

Pundamilia nyererei Pundamilia pundamilia

Pundamilia “turbid water,”hybrid offspring from a locationwith turbid water

Stability: Continued Formation of Hybrid Individuals

• 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. 24-14-4

Gene flow

Population(five individualsare shown)

Barrier togene flow

Isolated populationdiverges

Hybridzone

Hybrid

Possibleoutcomes:

Reinforcement

OR

OR

Fusion

Stability

Timing of evolution?

• Gradual (Lyell, Darwin)

• Punctuated equilibrium (Gould) – evolution occurs in spurts and interspersed within long periods of stasis

Species undergo most morpholo-gical 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