Darwin’s theory of evolution by natural selection is, in my opinion, the most awe-inspiring of scientific theories. Sure, it doesn’t quite have the indispensable quality of gravitational theory in literally holding us to the Earth… But, to look around at the endless forms most beautiful with which we share our planet, and know that every single one of them (and us) shares a common ancestor that lived approximately 4 billion years ago, usually has my brain going something a little like this:
What Darwin got wrong…
Or rather, what he didn’t get quite right.
The branches of Darwin’s classic tree notably extend outwards: simplicity giving rise to greater complexity: one ancestral species giving rise to multiple different species.
But we know that this is not always the case.
Hybrids can be formed through the mating between species (interspecific hybridisation), or between distinct populations/breeds within species (intraspecific hybridisation).
Life as a hybrid.
Hybridisation has the potential to impact evolution through a variety of ways. After all, the sudden mixing of two dissimilar sets of genetic material has repercussions not otherwise associated with conventional Darwinian evolution. The Nobel Prize winner Barbara McClintock coined the term genome shock, which aptly describes the multitude of potential challenges faced by a genome following hybridisation.
These potential challenges most notably include complications during the phase of cell division where chromosomes are required to pair up; the set of chromosomes the hybrid inherits from each parent may be too different from one another that it interferes with this pairing. In spite of the potential challenges they face, there must be some benefits to being a hybrid, as they can be found throughout the plant, animal and fungal kingdoms.
In fact, the prevalence of hybrid species within the plant kingdom means there’s a good chance that you ate one within the last week (and even the brewer’s yeast used in wine and beer making is a hybrid!)
Or, you may have even admired one in your garden without realising it.
Take the humble sunflower, for instance.
Sunflowers are a crop heavily reliant on hybrid breeding.
Can hybridisation act as an evolutionary stimulus?
A study published just earlier this year used an experimental evolution approach to investigate if naturally-occurring hybridisation can accelerate adaptive evolution in sunflowers.
Experimental evolution studies biological populations under defined environmental conditions and over multiple generations, to understand their evolutionary responses.
But how can someone measure evolution, you ask?
This study tracked 27 traits in a hybrid population, and closely-related non-hybrid control population, of sunflowers. The sunflowers were planted in open fields; the two populations separated by a dense copse of trees to prevent any cross-pollination.
Leaf samples were taken yearly and used in genetic analyses, to track the progression of trait evolution. Seed samples were also taken yearly and stored for usage in a common garden in the final stage of the experiment.
A common garden allows for the comparison of biological populations without the confounding effects of their corresponding environments.
As neither population was originally from the testing area, the change of the 27 traits over the course of the experiment (which I should add, was eight years long!) in the hybrids relative to the controls, when analysed at each time-point and in the final common garden, would give an indication of how well they were evolving in their new local environment.
If the traits were observed to evolve more rapidly in the hybrid population over the experimental period, this would suggest that hybridisation was accelerating evolution.
What did they find?
Hybrid fitness was shown to increase over time (dark blue and light blue lines), while control fitness (gold line) did not change overall. H. a. texanus (black horizontal line) is a locally-adapted wild sunflower hybrid used as a standard for comparison. The second hybrid population (and the reason for which there are two blue lines) was established to assess the generality of the results.
Fitness is a measure of reproductive success; how well a biological organism or population is adapted to a given environment in order to survive to a reproductive age and to produce offspring.
From the above figure, hybridisation does appear to have accelerated evolution!
So which of the 27 traits evolved most rapidly?
Some key traits associated with
• leaf area
• bud initiation time
• seed maturation time
• flower disk diameter
• herbivore resistance
evolved faster in the hybrids relative to the controls, which could have enabled the hybrids to acquire resources (e.g. sunlight), avoid pest-based damage, and produce viable offspring more effectively: all qualities that would promote the propagation and continuation of the population (which is, of course, the inherent goal of life itself!)
But was it adaptive evolution, or just evolution?
And how can you tell?
Although traits were observed to evolve in the hybrids, this evolution was not necessarily adaptive. To prove adaptive evolution, the researchers needed to show that the traits were under selection, otherwise the evolution could have been driven by additional forces, such as genetic drift or a genetic correlation with other traits under selection.
Selection is the preferential survival and reproduction (or, preferential elimination) of individuals with particular characteristics or features.
Genetic drift is a type of evolution that typically occurs in small populations, whereby certain genetic frequencies change over time due to chance or random processes.
The researchers performed some statistical analyses of the data, focusing on trait selection, and determined that some traits had indeed evolved adaptively.
In the above-left figure (3a), colour is used to differentiate whether trait values increased (blue) or decreased (pink), along the shading gradient that corresponds to values derived from the statistical analyses. Traits with an ‘a’ were determined to have evolved adaptively. In the above-right figure (3b), colour is instead used to differentiate the rate of trait evolution and whether it was steeper in the control (orange) or hybrid (blue) population, along the shading gradient. We see in Figure 3b, from the prominent blue shading, that most traits evolved more rapidly in the hybrid populations. In both figures, ‘LBJ’ and ‘BFL’ correspond to the two hybrid populations, and ‘control’ to the control population. A solid black outline of a square denotes a highly statistically significant result, a dashed black outline a statistically significant result, and no outline a non-significant result.
When considering Figure 3a, we see that all adaptively evolved traits are associated with high statistical significance, which gives us confidence in the results. However, sunflower disk diameter (‘DiskDiam’) and overall plant size (‘Volume’) are the only traits for which the hybrid populations both demonstrated adaptive evolution and the control population didn’t. Thus, the title ‘Hybridization speeds adaptive evolution in an eight-year field experiment’ is perhaps a bit of a stretch, and could have done without the inclusion of the word ‘adaptive’, in my opinion.
Some final thoughts.
It is important to remember that the researchers’ conclusions have been drawn from a single study using a single control and two hybrid populations, making it entirely possible that the observed effects are due to species-specific factors at play, or are not completely representative of reality.
Experimental evolution studies in higher organisms (e.g. plants and animals) are impeded by their long generation times; a barrier not encountered in studies of microorganisms such as E. coli, which has a generation time of 15-20 minutes in the lab and has subsequently been used to great effect in the E. coli long-term experimental evolution project. Field-based studies, such as that discussed in this blog post, encounter further barriers in ensuring that conditions are held constant for the duration of the experiment, according to the conditions defined at its outset. Aptly highlighting this was the need for this case study to be terminated after eight years due to a change in land usage at the host site. Conversely, the benefit of field experimental evolution over lab experimental evolution is the ability to study a population in its natural environment, and without the introduction of artifacts that may occur in lab settings.
An artifact, in biological science, is a misleading observation or misrepresentation in the data, introduced by the experimental equipment or techniques utilised.
Scientific knowledge, and its countless applications, would benefit significantly from further field-based experimental evolution studies; particularly those on higher organisms, as well as those on hybrid populations. Such studies are largely under-represented in the literature, as reviewed in Kawecki et al (2012). A follow-up to this case study would also help to clarify and consolidate its results.
As I touched on towards the beginning of this post, hybrid species are found throughout the plant, animal and fungal kingdoms. They are also imperative to the functioning of many industries, and to humanity itself. The more we can learn about hybrids and their evolution, therefore, the greater ability we have to make informed decisions to benefit our ever-increasing global population.
When we consider the data from this study, however, it does suggest that hybridisation accelerates evolution, at least in sunflowers. Thus it appears, in quite a circle-of-life type of way, that the evolutionary processes responsible for shaping sunflower populations able to hybridise with one another, are now being influenced by that very hybridisation.
And for that, I have just three words: science. is. awesome.
Case study paper:
Mitchell, N., Owens, G. L., Hovick, S. M., Rieseberg, L. H., & Whitney, K. D. (2019). Hybridization speeds adaptive evolution in an eight-year field experiment. Scientific Reports, 9(1), 6746. doi:10.1038/s41598-019-43119-4
Further readings on this topic:
Barton, N. H. (2001). The role of hybridization in evolution. Molecular Ecology, 10(3), 551-568. doi:10.1046/j.1365-294x.2001.01216.x
Dimitrijevic, A., & Horn, R. (2018). Sunflower hybrid breeding: from markers to genomic selection. Frontiers in Plant Science, 8, 2238. doi:10.3389/fpls.2017.02238
Kawecki, T. J., Lenski, R. E., Ebert, D., Hollis, B., Olivieri, I., & Whitlock, M. C. (2012). Experimental evolution. Trends in Ecology & Evolution, 27(10), 547-560.