Models of Speciation& Macroevolution

Mark McGinleyAssociate Professor

Honors College and Department of Biological SciencesTexas Tech University

Allopatric Speciation

Allopatric Speciation in Squirrels

Peripatric Speciation

Special Case of Allopatric Speciation happens when one of the isolated populations has very

few individuals.

Peripatric Speciation

Imagine fruit flies float to a new island on a banana bunch 1. Fruit Flies are isolated & population size small 2. Rare genes survive:

If these few survivors just by chance carry some genes that are rare in the mainland population & one of these rare genes happens to cause a slight variation in the mating dance. Another causes a slight difference in the shape of male genitalia.

Peripatric Speciation

3. Gene frequencies drift: These small differences, which are rare on the mainland, drift to fixation in the

small population on the island over the course of a few generations (i.e., the entire island population ends up having these genes).

4. More changes: Over time natural selection improves the fit of male and female genitalia to one another and female sensitivity to nuances of the mating ritual. Flies also experience natural selection that favors individuals better suited to the climate and food of the island.

Peripatric Speciation

Speciation: After some generations, the island flies become reproductively isolated from the mainland flies. Peripatric speciation has occurred.

Parapatric Speciation In parapatric speciation there is no specific extrinsic barrier to gene flow.

the population does not mate randomly. Individuals are more likely to mate with their geographic neighbors than with individuals in a different part of the population’s range.

In this mode, divergence may happen because of reduced gene flow within the population and varying selection pressures across the population’s range.

Parapatric Speciation Anthoxanthum odoratum

Some of these plants live near mines where the soil has become contaminated with heavy metals. The plants around the

mines have experienced natural selection for genotypes that are tolerant of heavy metals.

Meanwhile, neighboring plants that don’t live in polluted soil have not undergone selection for this trait.

Parapatric Speciation Anthoxanthum odoratum

The two types of plants are close enough that tolerant and non-tolerant individuals could potentially fertilize each However, the two types of plants have evolved different flowering times. This change could be the first step in cutting off gene flow entirely between the two groups.

Although continuously distributed, different flowering times have begun to reduce gene flow between metal-tolerant plants and metal-intolerant plants.

Symaptric Speciation

Unlike the previous modes, sympatric speciation does not require large-scale geographic distance to reduce gene flow between parts of a population.

How could a randomly mating population reduce gene flow and speciate? Merely exploiting a new niche may

automatically reduce gene flow with individuals exploiting the other niche.

Sympatric Speciation

This may occasionally happen when, for example, herbivorous insects try out a new host plant.

Apple Maggot Fly Speciation

Apple Maggot Fly

Apples

Hawthorn

Apple Maggot Fly Speciation 200 years ago, the ancestors of apple maggot flies laid their eggs only

on hawthorns but today, these flies lay eggs on hawthorns (which are native to

America) and domestic apples (which were introduced to America by immigrants and bred).

Females generally choose to lay their eggs on the type of fruit they grew up in, and males tend to look for mates on the type of fruit they grew up in. So hawthorn flies generally end up mating with other hawthorn flies and

apple flies generally end up mating with other apple flies. This means that gene flow between parts of the population that mate on

different types of fruit is reduced. This host shift from hawthorns to apples may be the first step toward

sympatric speciation—in fewer than 200 years, some genetic differences between these two groups of flies have evolved.

Sympatric Speciation by Pollinator Switching

Pollinator switching in species with highly specialized pollination mechanism may also lead to sympatric speciation.

pollinators are attracted by the flowers' species-specific chemical attractants that mimic the female pheromones of the targeted insects.

Yellow-bee Orchid (Oprys lutea) Ophrys sicula.

Sympatric Speciation

However, biologists question whether this type of speciation happens very often. In general, selection for specialization would have to be extremely strong in order to cause the population to diverge.

- gene flow operating in a randomly-mating population would tend to break down differences between the incipient species.

Hybridization

Speciation by hybridization: Often the hybrid offspring are sterile, but

occasionally they are fertile and are reproductively isolated from their “parent” species.

In the latter case, a new species is formed. The sunflower species was produced by the hybridization of two other sunflower species.

Hybridization in Sunflowers- Helianthus

Sympatric Speciation in Gilia

Speciation by ploidy changes a ploidy change generally

means multiplying the number of chromosomes the species has by some number.

Ploidy changes are common in plants and often produce a species that is reproductively isolated and distinct from the “parent” species.

Here we see two species of anemone flower and their chromosomes. Changes in number of chromosomes, as has occurred in this genus, can cause speciation.

Cospeciation

If the association between two species is very close, they may speciate in parallel. This is called cospeciation. It is especially likely to happen between parasites and their hosts.

Cospeciation

For example, a species of louse living on a species of gopher.

When the gophers get together to mate, the lice get an opportunity to switch gophers and perhaps mate with lice on another gopher.

Gopher-switching allows genes to flow through the louse species.

Cospeciation

Consider what happens to the lice if the gopher lineage splits into lineages A and B:

Lice have few opportunities for gopher-switching, and lice on gopher lineage A don’t mate with lice living on gopher lineage B.

This “geographic” isolation of the louse lineages may cause them to become reproductively isolated as well, and hence, separate species.

Cospeciation

parallel host and parasite phylogenies is evidence of cospeciation.

This example is somewhat idealized—rarely do scientists find hosts and parasites with exactly matching phylogenies.

However, sometimes the phylogenies indicate that cospeciation did happen along with some host-switching.

Allopatric (allo = other, patric = place)                      

     

geographically isolated populations                 

          

                                           

Peripatric (peri = near, patric = place)                      

     

a small population isolated at the edge of a larger populatin                 

          

                                           

Parapatric (para = beside, patric = place)                      

     

a continuously distributed population                 

          

                                           

Sympatric (sym = same, patric = place)                      

     

within the range of the ancestral population                 

          

                                           

Modes of Speciation Review

Macroevolution

Macroevolution means evolution on the grand scale, and it is mainly studied in the fossil record. It is contrasted with microevolution, the study of evolution over short time periods., such as that of a human lifetime or less. Microevolution therefore refers to changes in gene frequency within a population .... Macroevolutionary events are more likely to take millions, probably tens of millions of years. Macroevolution refers to things like the trends in horse evolution described by Simpson, and occurring over tens of millions of years, or the origin of major groups, or mass extinctions, or the Cambrian explosion described by Conway Morris. Speciation is the traditional dividing line between micro- and macroevolution.

Mark Ridley (1997) p. 227

Patterns of Macroevolution

Geologic Timescale

Geologic Time Scale

Earth formed about 4.6 billion years ago. That's 4,600,000,000 years which is a long long time (even older than my MOM)!

First life appeared 3.6-3.8 billion years ago.

How can we come to grips with that length of time?

Geologic Time Scale

A football field is 100 yards long If a football field equaled the age of the earth then

each yard equals 46,000,000 years each first down equals 460,000,000 years.

Geologic Time Scales

Example, we have used the football-field model for our time line. first appearance of

dinosaurs (225 mya) the disappearance of

dinosaurs (65 mya), the first appearance of

homo sapiens (1/2 mya).

Geologic Time Scale

first microscopic life (3.6 bya) 78 yards

first multicellular life (900 mya) 19.5 yards

first land plants appear (450 mya) 9.8 yards

first appearance of dinosaurs (225 mya) 4.9 yards

the first appearance of Homo sapiens (1/2 mya). 0.39 inch

Domestication of dogs (~14,000 years ago) 0.01 inch

Radiometric Dating

RADIOMETRIC DATING  -   Radiometric dating is based on the fact that unstable 'parent' isotopes decay at a constant rate, yielding a stable 'daughter' isotope. 

Because the rate of decay is constant, you can measure the amount of parent material and the amount of daughter material (the proportion of parent to daughter) and determine the amount of time necessary to obtain the observed ratio. 

Age of earth determined using Pottassium-Argon dating

Radiocarbon Dating

all living organisms absorb Carbon 14 (an unstable carbon isotope) during the period when they are alive

  As soon as the organism dies, the Carbon 14 begins decaying to Nitrogen 14 (stable). 

The half-life of this reaction is 5730 years. 

This dating technique is very useful for dating of (relatively) recent events (less than 55,000 years ago) 

Patterns in Macroevolution- Stasis

Morphological characteristics remain unchanged for long periods of time

Stasis- Coelocanth

Until 1938, scientists thought that coelacanths went extinct 80 million years ago. But in 1938, scientists discovered a living coelacanth from a population in the Indian Ocean that looked very similar to its fossil ancestors.

Character Change

Morphological characteristics change over time.

Character Change

Lineage Splitting (Speciation)

Extinction

over 99% of the species that have ever lived on Earth have gone extinct.

In this diagram, a mass extinction cuts short the lifetimes of many species, and only three survive

Convergent Evolution

Similar Adaptations are produced in similar environments from distantly related taxa Cactus and Euphorbia

Convergent Evolution

Ocotillo (Southwestern US) vs allauidia (Madagascar)

Convergent Evolution

Convergent Evolution

Parallel Evolution

Two species that share a common ancestor but are raised in similar environments in different places have similar adaptations

Long-term Patterns of Evolution

Punctuated Equilibrium (proposed by paleontologists) Rapid change associated with speciation followed by long periods of stasis

Gradualism Gradual change over time Changes not limited to period of speciation

Long-term Trends in Biodiversity

Long-term Trends in Biodiversity

Bivalvles

Mammals

Long-term Trends in Biodiversity

Most taxa have limited longevity

Long-term Trends in Biodiversity

Long-term Trends in Biodiversity

Extinctions

Extinction is a natural process natural extinction rate is 1 species per

1,000,000 per year.

Mass Extinctions

Mass Extinctions

Recent Extinctions

Birds worldwide. 1/5 of birds worldwide have been eliminated in the last 2000 years. 11% or 1,029 of surviving species are threatened.

Fishes Worldwide. 20 % of the worlds fresh water fishes are threatened. Of 266 species in Malaysia only 122 remain.

Coral Reefs: Warming during 1982-83 El Niño killed up to 30% world wide. To what extent is the El Niño Southern Oscillation

(ENSO) increasing in intensity due to human-generated climatic change?

Recent Extinctions

Lake Victoria – Africa 300 or more species of

Cichlid fishes eliminated by competition or predation from the Nile Perch.

In order to eat this 2 meter foul-tasting fish, the natives smoked it. This resulted in the destruction of the forest around the lake.

Recent Extinctions- Passenger Pigeons

It is estimated that there were 3 billion to 5 billion passenger pigeons at the time Europeans discovered America. 25 to 40 per cent of the total bird

population Samuel de Champlain in 1605

reported "countless numbers," Gabriel Sagard-Theodat wrote of

"infinite multitudes," Cotton Mather described a flight as

being about a mile in width and taking several hours to pass overhead.

Recent Extinctions- Passenger Pigeons

Sign from Cincinnati Zoo

MARTHA

Last of her species

died at 1 p.m.,

1 September 1914,

age 29,

in the Cincinnati Zoological Garden.

EXTINCT.

Martha

Recent Extinctions

Birds in North America. From 1940 to 1980 the

populations of migratory birds in North America dropped 50% and many local extinctions ensued.

Ivory-billed Woodpecker

Recent Extinctions

In US, Canada, Mexico there are 1,033 species of freshwater fishes. 3 % extinct within the

past 100 years 26% are likely to go

extinct.Pecos Gambusia

Comanche Springs Pupfish

THE SIXTH GREAT EXTINCTION: The Homogeocene Causes:

Habitat destruction 73% lntroduction of new species 68% Chemical pollutants 38% Hybridization 38% Over-harvesting 15%

> 100% because some species face more than one type of threat.

Rainforests In 1979, the rainforest stood at 56%

of its original cover. By 1989, there was less than 50%. During the last decade, the rate of

deforestation has accelerated from 1.0 to 1.8% per year.

The surviving rainforest is equivalent to the size of the United States.

At the current rate of loss, an area the size of Florida is cleared every year.

The rainforest will be gone in less than 40 years (as determined from 3 independently-derived estimates made in the late 1980's).

Rainforest- Species Extinctions

Based on conservative calculations-the number of species doomed each year is 27,000 each day – 74 each hour -3

Human activity has increased extinction rates to between 1,000 and 10,000 times normal in the rainforest by destruction of habitat.

Introduction of New SpeciesBrown Tree Snake in Guam

Introduced to Guam during WWII Now have highest population

density of any terrestrial snake

30,000 per square mile In 40 years, the brown tree

snake has caused millions of dollar of damage to Guam's electrical lines and caused frequent power failures.

Snakes have eaten puppies and kittens and rabbits, attacked small infants, and sent children to hospitals with respiratory arrest. 

  The snakes have destroyed nine of Guam's thirteen native birds. 

Over Harvest of Bison

What’s going on in Texas?

Habitat Loss in Texas

50% loss of springs and spring runs due to groundwater pumping.

More than 60% loss of bottom land hardwoods.

More than 50% loss of our original 1.2 million acres of coastal wetlands (35% of the losses have been since 1950).

Since the 1920s 95% of the subtropical woodlands of the Lower Rio Grande Valley have been converted to agricultural and urban uses.

Most of Texas' tallgrass prairies have been lost to agriculture and urbanization.

Habitat Loss in Texas

Extinction in Texas

Extinction in Texas