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Cultural Evolution in Darwin’s Finches

Cultural Evolution in Darwin’s Finches

By Dominique Potvin

A new study is analysing the songs of Darwin’s finches to determine the role of cultural evolution in speciation.

When I write the word “evolution”, what comes to mind? Adaptations in the natural world? A controversial education subject in the US? The hit 1998 song by Pearl Jam?

Chances are, there is one historical figure that almost immediately pops into our heads when we think of evolution: Charles Darwin, the internationally renowned naturalist almost universally credited with being the father of modern-day evolutionary biology.

I say “almost”, because many of us evolutionary biologists and ecologists now also recognise Alfred Russell Wallace – a contemporary and colleague of Darwin – as a co-founder of the theory of evolution by natural selection. Both men undertook similar adventures in the 19th century that led them to think about plant and animal adaptations in a new way. Wallace travelled throughout Indonesia, for instance, and remarked on the biodiversity of the islands in South-East Asia. Darwin, on the other hand, was inspired by his travels west of South America, in the archipelago known as the Galapagos Islands.

The story goes that Darwin noticed that the different finch species on the islands all had differently shaped bills, and that each species tended to feed upon a particular food item. Form follows function: round, broad bills were much better at cracking open seeds to eat, while long, slender bills were better at picking up insects off leaves.

If you ever visit these small, isolated islands located 900 km from the west coast of Ecuador, you can see how he would be inspired. The wildlife is incredible. The series of islands is volcanic in origin, so all of the terrestrial species that live there now had to have colonised the islands from the mainland. After colonisation, they would have lived for many generations in isolation: an almost-perfect experiment demonstrating evolution by means of natural selection in isolation.

Darwin didn’t publish On the Origin of Species until 25 years after his initial voyage, but his observations of the birds, tortoises and other animals of the Galapagos were undeniable evidence supporting his theory.

Since Darwin and Wallace first published about natural selection, understanding the processes of evolution has been one of the major goals of biological science. The key to achieving this understanding is analysing how populations of organisms diverge just like the finches, leading to the emergence of new species.

For decades, studies have demonstrated how environmental factors such as food sources can shape the morphology of different finch populations, especially their body size and bill shape, thus leading to the evolution of new species. Over the past decades we have learned a huge amount not just from studying the body characteristics of these birds, but we have also been able to delve into the genetics of individuals, populations and species to understand how they are all related.

Only recently have we had the technology to map the genetics of the finches across species, populations and islands, thus obtaining a truer picture of this incredible radiation. We can do things like calculate how long a species has been around, with whom they share a common ancestor, which adaptations they have developed over time, and the environmental pressures that have pushed them in certain evolutionary directions.

In the group of Galapagos finches that now bear Darwin’s name, recent work has identified one of the genes that codes for bill shape. This allows scientists to study how that one particular gene differs from species to species, and how it contributed to their divergence based on the food preferences of different populations. By using this type of data, we can create wonderful images known as phylogenetic trees: sprawling branches that act as a map guiding us over the historic evolutionary pathways that have led to our present-day species.

Genetics, morphology, colour, physiology: it’s all there. At first glance, you wouldn’t think anything would be missing from the evolutionary picture.

But it is something that can be heard – not seen – that is missing: the role of culture. Animal culture is a behavioural trait that can be passed among individuals by teaching and/or learning. This definition is vague because we still know relatively little about culture. Birds are teaching us more every single day, and much of what we understand about it comes from studies of their songs.

Songbirds like finches learn songs – or parts of songs – from other individuals around them. They can, however, also invent new sounds, steal sounds they like from other sources, and can even tweak their songs according to what other individuals (especially females) seem to be attracted to. Because of this, individuals, populations and species can start to sound very different from one another, even if they look very similar.

The rules for the cultural evolution of new songs – or geographically distinct songs, dialects or accents – can be remarkably similar to those for the development of new body characteristics. Just as the environment might select for a particular physical trait, it may also select for a particular cultural trait.

For example, a dense forest might select for smaller wings. Birds that have smaller wings can more easily navigate through dense bush, and thus survive to pass on the genes for small wings to their offspring.

Likewise, dense forests may select for sounds that tend not to echo. In this case, animals that sing clear notes might be better heard by their friends and females than those that sing warbled notes, and thus the clear notes will used by the population over generations.

The big difference between these two processes is that one is predominantly genetically determined (the wings) while the other is predominantly learned (the song).

But how do these two processes of evolution – genetic and cultural – interact? Does a phylogenetic tree look the same once we add cultural factors? Can we begin to use non-genetic means to describe species?

Cultural evolution is further complicated by the fact that your ability to learn, your ability to sing and your capacity to change your song is restricted by genetically determined factors. Bill shape and body size, which are directly related to diet and are predominantly determined genetically, have direct implications for the sounds that a bird can produce. Birds with large, broad bills can’t sing broadband or wide frequency trills: their vocal apparatus just doesn’t allow it. We see these kinds of constraints all throughout the bird world: morphology and genetics both play a role in what a birds’ song sounds like.

So what comes first: the evolution of a new song or the genetic evolution of body shape and other adaptations? Research has found that the development of different song dialects often precedes genetic divergence. Because song is learned, it can evolve at a faster rate than other traits that rely on the slow process of random genetic mutation and then subsequent selection over many generations. Understanding the mechanisms underlying song divergence in birds therefore provides fundamental insights into the first steps leading to speciation.

These are questions I am currently tackling using the most elegant of models – Darwin’s finches on the Galapagos Islands. By obtaining vocalisations from each finch species on every island, my colleagues and I are going to produce a “cultural” tree similar to the phylogenetic tree that has recently been constructed. We will then compare the genetic and cultural data to determine how much culture shapes speciation. We’ve come up with three main scenarios that our data may reveal.

  1. Finch species maintain vocalisation similarity to one another due to innate preferences for certain sounds (i.e. cultural evolution slowing genetic evolution).
  2. Finches show a direct correlation between morphological, genetic and cultural changes, with cultural evolution occurring at the same rate as other divergence processes.
  3. Finches demonstrate higher divergence in culture than in genetics or morphology, indicating acoustic adaptation, with cultural divergence likely pre-empting or facilitating speciation.

Our results will give us a new framework for studying evolution and speciation, highlighting the role of cultural factors when determining the past or future evolutionary potential of animals.

Our study provides a new framework for incorporating culture as a significant factor in how new species evolve, or even be lost. This is not only interesting for evolutionary biologists, but is also applicable to conservation science as it can demonstrate how recent human-directed changes in our environment might be influencing genetic changes in animal populations. Furthermore, it may also even provide insights into how human dialects and languages evolve.

I have now completed one field season on the islands, but have some gaps in my dataset that still need to be filled: songs from the smallest and most remote islands can be difficult to obtain. Once we have a full song repertoire for all of the species and populations on the archipelago, we can begin our analysis.

To be consistent with other modern evolutionary analyses, we will be using the same techniques as those used by geneticists. Instead of sequencing the genomes of the finches, we will be sequencing their sounds.

We can even develop a tree using songs in the same way as using genetics. This involves a cultural application of bio­informatics software originally developed for aligning, sequencing and comparing genomic data.

This research not only provides insight into the mechanisms of evolution of Darwin’s finches from a cultural perspective, but also evidence of the potential usefulness of acoustic recordings in studying speciation. One more piece – culture – can then be added to Darwin’s amazing puzzle of evolution, continuing the legacy of the finches.

Dominique Potvin is a Lecturer in Animal Ecology at University of the Sunshine Coast.