Year 13 Evolutionary Processes Relating to Speciation

This page is devoted to helping you tell the stories of past and present species. Learning how to examine fossil evidence, explore biogeography, explain protein and DNA analysis, and compare homologous and analogous structures is the first step. 

When presented with observable evidence, past or present, you must explore the ways in which barriers to gene flow may have allowed for the allele frequencies for certain traits to become different across multiple groups of an ancestral species. Describing the different selection pressures affecting each group and explaining how specific phenotypes relate to each population's definition of fitness will allow you to discuss how natural and sexual selection have resulted in the production of new species over time.

You will show your comprehensive understanding of this topic when you are effectively linking biological ideas and/or scientific evidence to evolutionary processes leading to speciation.

Evolutionary processes involve the following biological ideas:

·  role of mutation

·  gene flow

·  the concept of natural selection including directional, stabilising, and disruptive selection and genetic diversity
·  modes of speciation (sympatric, allopatric)

·   barriers to gene flow that contribute to speciation (geographical, temporal, ecological, behavioural, structural barriers, polyploidy)

·  patterns such as divergence, convergence, adaptive radiation, co-evolution, punctuated equilibrium, and gradualism.
·  reproductive isolating mechanisms (the accumulation of genetic differences in two gene pools which prevent the production of fertile offspring - pre-zygotic, post-zygotic, hybrid breakdown and sterility)

Note (2018): The following points are important!

Evolutionary change at the population level which reflects underlying changes in allele frequencies for beneficial phenotypes must be explained

The application of molecular biology in terms of proteins and DNA analysis, which may include mtDNA and nuclear DNA and genetic distances, is used to determine relationships between species

Phylogeny and cladistics are ways of showing re.

In order to achieve at the highest levels, you must be able to relate these ideas to presented evidence, and create a story which provides a likely explanation for observed similarities between species and accounts for the events leading to their development as unique species.

Before we begin, let's check out this great video


MSN video - Rare fish developing hind quarters

An evolutionary biologist observing the Brachionichthys hirsutus (spotted walking fish) may ask these questions:

- In what ways are these fish adapted so that they may use their fins as walking tools? (which structures/genes show variation from other fish?)
- What environmental conditions would make this form of movement advantageous? (feeding, fighting, fooling around?) 
- how is the natural habitat of these fish different to that of its ancestors? (did the habitat change or have the fish migrated?)

We have no way of knowing exactly how these fish came to walk, nor can we guarantee what will become of these unique Aussies. Rather, we must look to the patterns that can be observed over time and examine how DNA and the environment interact to produce change, allowing new species to evolve.

Evidence for Evolution

Fossils provide a glimpse into the history of the Earth by showing the physical features and DNA that was present thousands and millions of years ago.

We look for relationships between present day organisms and historical species by comparing similar structures and relatedness in DNA sequences.

Homologous structures have similar form or shape, but may have different function. An example is the pentadactyl forearm of the mammal (human hand, cat paw, whale flipper, bat wing). The similarity in features indicates similar enzymes and proteins were responsible for their development; which in turn indicates similarities in the genetic code. The degree of relatedness of species can therefore be determined by the similarity in homologous structures, or % of shared DNA (% of similar proteins). The common mammal ancestor to all 4 named species had a pentadactyl forearm before speciation. When species share an immediate common ancestor they are said to be the result of divergent evolution.

Analagous structures have similar function, but very different evolutionary histories. They may be the result of very different gene expression which results in a similar phenotypic advantage to overcome the same selection pressure. The process producing similar phenotypes in unrelated species is called convergent evolution. An example would the wings of insects, birds and bats. Although each is able to fly, insect wings, bird feathers and bat skin are all different in structure and therefore are the result of unrelated gene expression. 

This video may assist you in understanding.




Variation and Selection

Variation is produced through mutation (new alleles = new phenotypes), and sexual reproduction: segregation (generational variation); independent assortment (sibling variation); and recombination (new combinations of alleles). 

Charles Darwin's theory of evolution by natural selection revolutionised the way in which biologist observe the natural world. Natural selection is a process which involves:
1) Variation exists in a population - phenotypic and genetic
2) Overproduction of offspring
3) Competition for resources
4) Survival of the fittest - differential reproductive success




In a relatively unchanging enviornment, the variation within a population would expect to be fairly stable (the frequcency of each allele ought to remian similar over time). This is called stabilizing selection. 

However, in an environment with changing/new environmental conditions, certain specific phenotypes may provide a slight advantage and increase in frequency over time as those "more fit" organisms produce a greater number of offspring who survive into the next generation.

Directional selection, such as with the giraffe, resulted in one end of the variation continuum being advantageous and so these once small necked antelopes adapted to have the longest neck in the animal kingdom.



Disruptive selection occurs when two distinct varieties are favoured and the intermediate phenotype is selected against.







Natural Selection and Sexual Selection

The following video relates both natural selection and sexual selection to fitness. The information is also relevant for explaining adaptive advantages for the development of different feeding behaviours, reproductive strategies and sexual dimorphism.



Sexual Selection

Here is another video series entitled "Why Sex" that talks about the process of sexual selection. This is part 2, but checkout the series for more information.




Speciation


For a catchy tune and animation highlighting the key processes involved check out this video.




Speciation is the process by which a population becomes unable to produce viable offspring with another population, even though the two populations shared a common ancestor.

Through selection processes, the groups may have adapted differences in one or more of the following:
- gaining nutrition (mouth parts/feeding strategies/diet)
- seeking protection (camouflage/shelter/activity pattern)
- mating behaviours (timing/attractiveness/fitness/courtship)

There are two main processes that lead to speciation
1) Allopatric Speciation - Populations are physically isolated from one another then accumulate differences as each adapts to a different set of selection pressures. 
2) Sympatric Speciation - Populations develop different behaviours and adapt to unique niches in the same general location.

In both cases, gene flow between the populations is interrupted meaning that changes in allele frequencies of one group do not affect the other group. Eventually, as a result of the accumulated changes, the populations become permanently reproductively isolated.   

Below is a fantastic video on speciation.




In the previous video, the central ridge and valley separated each of the 3 populations over long periods of time and each developed differences through mutation, recombination, natural selection and sexual selection. Thus, the mode of allopatric speciation was at play. 

However, had their been no valley geographically separating the Eastern population into two groups (later blue and green), disruptive selection could have resulted in the development of different species through sympatric speciation.

Niche specialisation through ecological isolation (adapting to different food supplies, stratification, zonation, biorhythm) or temporal isolation (altitude, temperature, photoperiod may produce natural variation in timing of fertility/flowering), behavioral isolation (sexual selection through disruptive reproductive strategies/courtship), or mechanical isolation (reproductive organs don't fit together) can result in divergence. This is the case for some species of Bird of Paradise in New Guinea.

As the result of accumulated differences in DNA, when two isolated groups are reunited after long periods of separation, the above mechanisms often serve as permanent barriers to gene flow. In rare cases, hybrids may form from the pairing of the odd male and female. These hybrids may be inviable (eggs don't hatch, seeds don't germinate, embryos fail to develop fully) or the hybrids are sterile/infertile (hybrid breakdown). 

Adaptive Radiation

In this series of videos, titled Galapagos Born of Fire, countless examples of how mainland species colonized and gave rise to numerous species adapted to the unique challenges and diverse niches of the Galapagos.

Part 1



Part 2



Part 3



Part 4



Part 5




Gradualism and Punctuated Equilibrium

   Test Yourself

Salamanders in California share a common evolutionary tale as Skinks in the South Island of New Zealand in that they have formed rare ring species. 

Watch this video on the Californian salamanders and answer the questions which follow

 
Questions

1) Identify the reproductive isolating mechanism/barrier which separated the two migratory routes for salamanders.
2) Identify how different selection pressures offered alternative ways for salamanders to evolve different protective mechanisms.
3) Justify the mode of speciation as allopatric or sympatric.
4) Explain how this example is related to each of the following patterns of evolution:
    i)  Divergent evolution
    ii) Gradualism
    iii) Adaptive radiation
5) Explain why the brightly coloured salamanders and spotted salamanders:
     i) are not yet considered different species.
     ii) show evidence of becoming different species.  


Speciation by Polyploidy


The unique process of instantaneous sympatric speciation by polyploidy gives rise to new species in the blink of an eye. With the exception of insects and a few reptiles, this form of speciation is usually only seen in plants, and is responsibile for nearly all the varieties of food that we eat today.

Autopolypolidy is the process by which a single organism can give rise to a new polyploid species.  It involves two steps.

i) Non-disjunction is the process by which homologous chromosomes fail to separate during meiosis.  This can result in the production of diploid gametes (2n).

ii) Self-pollination and self-fertilization can occur in plants which have flowers which have both male and female gametes. Self-fertilization of two diploid (2n) gametes from the same plant produce seeds with tetraploid (4n) embryos.

Allopolyploidy is the production of a new species as a result of the hybridization of two gametes from different plants. Normally, this would result in a sterile hybrid since there are no homologous chromosomes to pair up during meiosis. However, sometimes chromosomal doubling occurs. If the cells responsible for the development of reproductive organs undergo DNA replication, but fail to divide, this results in the production of homologous pairs of chromosomes in cells which previously had two sets of non-homologous chromosomes.  For example a AB (2n) hybrid would produce cells with AABB (4n). These cells could then produce AB gametes.

Fertilisation of AB gametes (2n) with either A pollen (n) or B pollen (n) may produce sterile triploid hybrids.  This is the case with some seedless varieties such as watermelon and banana. 



This is a process that you will need to practise describing to a study buddy.  Perhaps you would like to make your own video. Here's one for inspiration:


  
Coevolution

No one species lives in isolation from another species. Animal/plant interactions and predator/prey interactions are the most easily understood relationships.

This video explores how the reproductive success of both bees and orchids could be linked by a very unlikely relationship between the two. Watch the video and attempt the questions that follow to see how well you understand the concept of coevolution in this case.



(Questions to come)

Speciation and Evolution in New Zealand

It's difficult to imagine a New Zealand different to the one we live in today, but over the last 65 million years, since Zealandia first broke off of Australia, the land that is now New Zealand has experienced many geological and climate changes.

Species in New Zealand today have two main origins: 
1) They are directly descended from organisms that were on the original raft and share ancestry with Australian (Gondwana) species. These species have gradually adapted to life through the ups and downs of a changing NZ.
2) They arrived here by air or ocean currents more recently and have adapted to the unique niches available in New Zealand.

Geological History of NZ

Pushing the Boundaries - This video explores the early days of Zealandia




This video from the Auckland Museum describes more recent continental changes



Between 65mya and 26mya most of Zealandia was submerged below the Pacific Ocean.

A three unique features of New Zealand's geological history which have driven evolutionary processes are:

1) Volcanic activity (26mya - present) The appearance of new land, void of life has provided opportunity for new variation to thrive.

2) Mountain building - the formation of the Southern Alps 5 million years ago has led to speciation along the mountain ridge, and provided opportunity for adaptive radiation across diverse niches.


3) Ice Ages - The rising and falling sea levels have divided New Zealand in to a chain of isolated islands, and allowed for continual migration between the north and south island at different times. 

(shoreline during the last glaciation 20 thousand years ago shown to the left)





Unique Adaptation of New Zealand Birds

This video highlights some of the challenges faced by a new and isolated New Zealand and begins to explore how birds have evolved to occupy the niches of mammals found elsewhere in the world as a result of adaptive radiation.




The following BBC documentary "To Fly or Not to Fly" featuring David Attenborough is compulsory viewing for all New Zealanders, particularly those sitting the Speciation examination.
From 16:28 - Pay attention to ostrich adaptations as they relate to those of NZ ratites (Moa, Kiwi)
From 25:39 - The remainder of this video discusses the unique adaptations and evolutionary history of New Zealand's flightless birds. 

What does the Future Hold?


Check out this video which contemplates the future for Antarctica. In what ways has our understanding of New Zealand's history shaped the way in which we predict change in the future?