The Origin of Species

the origin of species

Species is a Latin word meaning “kind” or “appearance.”

Speciation is the process by which new species arise.

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.

two areas of evolutionary study

Microevolution describes the details of how populations of organisms change from generation to generation and how new species originate. (confined to a single gene pool)

Macroevolution describes patterns of changes in groups of related species over broad periods of geologic time. (change above the species level) The patterns determine phylogeny, the evolutionary relationships among species or groups of species. (like the appearance of feathers)

Biological Species Concept

1942 Ernst Mayr Population or group of populations members interbreed with one another to produce viable,

fertile offspring

Members interbreed with each other but are unable to

produce viable fertile offspring with members of other

populations. is based on infertility rather than physical similarity emphasizes reproductive isolation

Biological barriers that impede members of two species from producing viable, fertile hybrids

Pre-zygotic and post-zygotic barriers prevent members of different species from producing offspring that can also successfully reproduce.

Species are based on interfertility, not physical similarity.

For example, the eastern and western meadowlarks may have similar shapes and coloration, but differences in song help prevent interbreeding between the two species.

In contrast, humans haveconsiderable diversity,but we all belong to thesame species because ofour capacity to interbreed.

Pre-zygotic reproductive barriers:impede mating between species or hinder the fertilization of ova.

Post-zygotic reproductive barriers: prevent hybrid zygote from developing

into a viable, fertile adult. If subspecies occur together, but remain

reproductively isolated, these subspecies may eventually become two distinct species.

A species can be divided into subspecies, if they become reproductively isolated

Factors that lead to Reproductive Isolation (pre-zygotic barriers)

Habitat Isolation: two species live in different habitats within the same geographic area.

Garter snakes- one aquatic, one terrestrial Behavioral Isolation: signals to attract mates,

elaborate behaviors, courtship rituals differ between species. Don’t recognize the behavior therefore don’t breed

Eastern & Western Meadowlark songs differ Seasonal/Temporal Isolation: two species that

breed during different times of the day, seasons, or years cannot mix their gametes

Skunks: S. gracilis mates in late summer; S. putorius mates in late winter.

Factors that lead to Reproductive Isolation (pre-zygotic barriers)

Mechanical Isolation: anatomical incompatibility.

Insect copulatory organs don’t fit together; floral anatomy specialized to one pollinator

Gamete Isolation: incompatibility between sperm/egg.

Sperm of one species may not be able to survive in the environment of the female reproductive tract of another species; gamete recognition based on complementary molecules found on sperm/egg surfaces.

Ecographic Isolation: geographic isolation Asian and African elephants

Factors that lead to Reproductive Isolation (post-zygotic barriers)

Reduced Hybrid Viability: genetically incompatible hybrid zygotes abort development at some embryonic stage.

frogs in genus Rana Reduced Hybrid Fertility: results in completely

or largely sterile hybrids. Chromosomal differences (structure or number) results in malformed gametes during meiosis.

Mule – a sterile but robust hybrid of a horse and donkey Hybrid Breakdown: first generation hybrids are

viable but second generation offspring are feeble or sterile

Cotton

Courtship ritual as a behavioral barrier between species

Prezygotic barriers impede mating or hinder fertilization if mating does occur

Individualsof differentspecies

Matingattempt

Habitat isolation

Temporal isolation

Behavioral isolation

Mechanical isolation

HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION

(b)

(a)(c)

(d)

(e)

(f)

(g)

Prezygotic and postzygotic barriers

Prezygotic and postzygotic barriers

Viablefertile

offspring

Reducehybrid

viability

Reducehybridfertility

Hybridbreakdown

Fertilization

Gameticisolation

GAMETIC ISOLATION REDUCED HYBRID VIABILITY

REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

(h) (i)

(j)

(k)

(l)

(m)

Summary of reproductive

barriers

No single barrier may be completely impenetrable to genetic exchange, but many species are genetically sequestered by multiple barriers.

How do new species arise?

How do new species arise?

1. By geographic isolation:

1. This is the way the flora and fauna of the Galapagos Islands evolved.

2. The barrier prevents gene flow.

3. When two different species arise this way, it is called allopatric speciation.

2. Greek: allos = other patria = homeland

Factors that lead to divergence

1. Size of population (small)1. The founder effect- genetic drift attributed to

colonization by a limited number of individuals from a parent population.

2. However, very few small, isolated populations will develop into new species; most will simply perish in their new environment.

2. Ability of organism to move about (isolation)

3. Harshness/differences of new environment.

Allopatric speciation of squirrels in the Grand Canyon

The valley of the Grand Canyon is a significant barrier for ground squirrelsthat have speciated on opposite sides,but birds that can move freely have no barrier.

Another Allopatric Example

Ring species provide examples of what seem to be various stages in the gradual divergence of new species from common ancestors. In ring species,

populations are distributed around some geographic barrier, with divergence increasing as they move.

Another Allopatric Example

The salamander, Ensatina escholtzii, probably expanded south from Oregon to California, USA.

One chain moved along the coastal mountains and another along the inland mountains forming a ring around the central valley

Another Allopatric Example

Salamanders of the different populations contrast in coloration and exhibit more and more genetic differences the farther south the comparison is made.

Another Allopatric Example

At the northern end of the ring, the coastal and inland populations interbreed and produce viable offspring.

In this area they appear to be a single biological species.

At the southern end of the ring, the coastal and inland populations do not interbreed even when they overlap.

In this area they appear to be two separate species.

How do new species arise?

By adaptive radiation evolution of many diversely adapted species from a

common ancestor.

Darwin’s FinchesThe 14 species of Finch evolved from one species of

ancestral finch.They have adapted to exploit different food sources

with differently shaped beaks and feeding behaviors.They exhibit character displacement -

evolutionary change driven by competition among species for a limited resource (Food)

Gause’s Law- competitive exclusion principle.

A model for adaptive radiation on island chains

Sympatric Speciation

sympatric speciation –

requires the emergence of some reproductive barrier that isolates a subset of the population without geographic separation from the parent population.

Sympatric Speciation

Polyploidy is having more than the diploid number of chromosomes

This condition is common in plants and quite uncommon in animals

Non-disjunction in meiosis (autopolyploidy) It can make offspring reproductively

isolated from their parental species. (post-zygotic barrier is created in one generation)

Cannot breed with diploid members and produce fertile offspring

Polyploid population can self-pollinate, mate with other polyploids, or reproduce by asexual propagation.

Sympatric speciation by autopolyploidy in plants

One mechanism for allopolyploid speciation in plants

Causes of Polyploidy

1. Accidents during meiosis, autopolyploidy, results in the wrong number of sets of chromosomes in the gametes

2. The contribution of two different species to a polyploid hybrid, allopolyploidy, non-homologous chromosomes can’t align during meiosis.

3. The chemical colchicine induces polyploidy.

In the early 1900s, botanist Hugo de Vries produced a new primrose species, the tetraploid Oenotheria gigas, from the diploid Oenothera lamarckiana. This plant could not interbreed with the diploid

species.

Sympatric Speciation

Many plants important for agriculture are the products of polyploidy. For example, oats, cotton, potatoes, tobacco, and

wheat are polyploid. Plant geneticists now hybridize plants and use

chemicals that induce meiotic and mitotic errors to create new polyploids with special qualities.

Example: artificial hybrids combine the high yield of wheat with the ability of rye to resist disease.

Sympatric Speciation

While polyploid speciation does occur in animals, it is generally the result of Genetic factors cause individuals to be fixed on

resources not used by the parent. These may include

New habitat New food source Change in mating preference Change in nesting behavior

An Example of Sympatric Speciation

Sympatric speciation is one mechanism that has been proposed for the explosive adaptive radiation of almost 200 species of cichlid fishes in Lake Victoria, Africa. While these species are clearly specialized for

exploiting different food resources and other resources, non-random mating in which females select males based on a certain appearance has probably contributed too.

An Example of Sympatric Speciation

Individuals of two closely related sympatric cichlid species will not mate under normal light because females have specific color preferences and males differ in color. However, under light conditions that de-emphasize color

differences, females will mate with males of the other species and this results in viable, fertile offspring.

The lack ofpostzygoticbarriers wouldindicate thatspeciationoccurredrelatively recently.

summary

Allopatric speciation, a new species forms while geographically isolated from its ancestor.

Sympatric speciation requires the emergence of some type of reproductive barrier that isolates the gene pool of a subset of a population without geographic separation from the parent population.

Punctuated Equilibrium Stephen J. Gould

A catastrophic event or major genetic change occurs, rapid evolution and speciation occurs.

The new population works back toward a long period of no evolution (few or no transitional forms.)

http://brembs.net/gould.html

The Cambrian Explosion represents a period in time(560 MYA) where we see diversification of animal phyla.

Patterns of macroevolution

Gradualism- evolution occurs by the gradual accumulation of small changes. The intermediate stages of evolution not represented by fossils merely testifies to the incompleteness of the fossil record.

Punctuated Equilibrium- evolutionary history consists of geologically long periods of stasis with little or no evolution, interrupted or “punctuated” by geologically short periods of rapid evolution.

REMEMBER!!!

Individuals do not evolve.They do not “struggle to survive.”They cannot change their genetic makeup in

response to a catastrophe.The individual lives or dies.Those that live reproduce and pass of

adaptive heritable variations.INDIVIDUALS DO NOT EVOLVE! ONLY

POPULATIONS CAN EVOLVE!