The Origin of Species

AP 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

Interfertility concept doesnot 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.

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)

Sea urchins – Their gametes will not fuse with other gametes of other species.

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)

Reduced Hybrid Fertility

Mules are infertile.

Modes of speciation

1) Allopatric – geographical separation

Figure 24.7 Allopatric speciation of squirrels in the Grand Canyon

Modes of speciation

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

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 common

Sympatric speciation by autopolyploidy in plants

Notice that the chrom do not separate!

Figure 24.15 One mechanism for allopolyploid speciation in plants

“allo” means coming from another place, in this case another species.

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.

Modes of Speciation3. 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

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.

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Fig. 24-14-4

Gene flow

Population(five individualsare shown)

Barrier togene flow

Isolated populationdiverges

Hybridzone

Hybrid

Possibleoutcomes:

Reinforcement

OR

OR

Fusion

Stability

Strengthening ofreproductive 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. 24-15

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

Reinforcementof barriers to reproduction,has to bestrong in sympatric species

Fig. 24-16

Pundamilia nyererei Pundamilia pundamilia

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

Gene poolshave fused.

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.

Anagenesis vs cladogenesis

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

Case StudyAs the Worm Turns: Are

apple maggot flies and hawthorn maggot flies different species?

5 primary forces affect genetic composition of populations and cause

evolution

• Natural selection

• Mutation

• Gene flow

• Genetic drift

• Mate choice

Species

• Species contain groups of interbreeding populations connected by gene flow.

• The species is a natural and fundamental unit of evolution

• Each species is the product of a unique and independent evolutionary pathway.

How to define species

• Biological concept – reproductively isolation

• Phylogenetic concept – if a phylogenetic tree, each species uniquely occupies the tip of a tree branch

• Morphological concept – consistent, distinguishing physical differences

• Ecological concept

How species form:

AllopatricSympatricParapatricFounder Effect

Three outcomes if hybridization

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.

3. Individuals hybridize but offspring have reduced fitness.

No speciation

Full speciation

Speciation in progress. Selection for evolution of strongreproductive barriers.

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How hybrids determine species

• Are they reproductively isolated?

• Can they interbreed back with parent groups?

• Are they a separate but “weak” group that can interbreed?

1. Are they different species?

2. What species definition might apply here?

3. If they are (or are you thinking they might be), how did the speciation occur? Develop this idea.

4. Which evidences are the most important for your choice? Star those on your sheet.

5. What else do you need to know to validate your conclusion? Go online and research what you can find out. Think of a specific question you are trying to answer. (HW)

Test Review• About Darwin• Evolution as defined by Darwin and concept today• Scientists/theories influenced development of the theory• Points of Darwin’s Theory• Evidences (4) for evolution• Homologous vs analogous structures• Vestigial structures• Microevolution vs macroevolution• Origins of genetic variation• Average heterozygosity• Hardy-Weinberg – how to do problems, conditions that

maintain genetic equilibrium

• How gene frequencies can be altered • Difference in types of genetic drift – bottleneck, founder

effect• Relative fitness • Types of natural selection• Sexual selection – types• How to preserve genetic variation• 4 concepts of species• Types of reproductive isolation (prezygotic and

postzygotic, types of each)• 3 main types of speciation, how differ• Autoploidy and allopolyploidy in sympatric speciation• Role of hybrids in determining speciation• Punctuated equilibrium vs gradual change in timing of

evolution