What are the Parrots and Where Did They Come From?

The Evolutionary History of the Parrots

PHYLOGENY

CONTENTS

The Marvelous Diversity of Parrots 3

Reconstructing Evolutionary History 5
Fossils, Bones, and Genes 5

The Evolution of Parrots 8
Parrots' Ancestors and Closest
Relatives 8
The Most Primitive Parrot 13
The Most Basal Clade of Parrots 15
Other Major Groups of Parrots 16
Box 1. Ancient DNA Reveals the
Evolutionary Relationships of the
Carolina Parakeet 19
How and When the Parrots Diversified 25

Some Parrot Enigmas 29
What Is a Budgerigar? 29
How Have Different Body Shapes Evolved in
the Parrots? 32

THE MARVELOUS DIVERSITY OF PARROTS

The parrots are one of the most marvelously diverse groups of birds in the world. They dazzle the beholder with every color in the rainbow (figure 3). They range in size from tiny pygmy parrots weighing just over 10 grams to giant macaws weighing over a kilogram. They consume a wide variety of foods, including fruit, seeds, nectar, insects, and in a few cases, flesh. They produce large repertoires of sounds, ranging from grating squawks to cheery whistles to, more rarely, long melodious songs. They inhabit a broad array of habitats, from lowland tropical rainforest to high-altitude tundra to desert scrubland to urban jungle. They range over every continent but Antarctica, and inhabit some of the most far-flung islands on the planet. They include some of the most endangered species on earth and some of the most rapidly expanding and aggressive invaders of human-altered landscapes. Increasingly, research into the lives of wild parrots is revealing that they exhibit a corresponding variety of mating systems, communication signals, social organizations, mental capacities, and life spans. In a great many respects the 360 or so species of parrots represent a textbook illustration of how the process of evolution can, over much time, lead to the diversification of many species from a single ancestral population.

At the same time, parrots are one of the most physically homogeneous groups of birds. Anyone with a passing familiarity with birds can instantly recognize a parrot by its sharply curved upper beak topped by a fleshy cere, muscular prehensile tongue, relatively big head and stout body, and distinctive zygodactyl feet with two toes pointing forward and two pointing back (figure 4). This combination of anatomical features clearly sets parrots apart from other birds. There are other, less obvious, commonalities in physiology, behavior, and ecology that tend to distinguish parrots from most other birds. These shared features illustrate another principal feature of evolution: that it tinkers with the materials at hand rather than starting anew with each species. In other words, major innovations are rare. What more typically happens is that features already present in an ancestor are slowly modified through natural selection over many generations to produce a constrained range of variations on the basic template as different lineages adapt to changing and localized environments.

In the following chapters we will delve deep into what recent scientific investigations have revealed about the lives of wild parrots. We will discuss how parrots perceive the world around them, how individuals go about their daily lives and interact with others, and how populations are adapting to a world that is rapidly changing. Our focus will be both on what these investigations tell us about parrots in general, and on what can be learned from the interesting exceptions to these generalities. But before we start this exploration, we want to set the stage by summarizing the current state of knowledge of the evolutionary history of parrots: Where did they come from, how did they diversify, who among them is most closely related to whom, and what does this evolutionary history reveal about the process of evolution itself? To understand these topics, we must first understand how scientists explore what happened in the long-distant past.

RECONSTRUCTING EVOLUTIONARY HISTORY

Fossils, Bones, and Genes

Reconstructing the past history of life is both a historical exercise and a scientific one. Scientists typically illustrate evolutionary patterns as trees, with the common ancestor of a group of species placed at the root, and existing species at the tips of the branches. The branching points between the root and the tips represent points where a single lineage split to produce two new lineages, while the length of each branch represents the amount of time or evolutionary change between branching points. As an aside, this representation of evolutionary history in tree form was an innovation of Charles Darwin himself, appearing first in his scientific notebook and then popularized in his seminal work, On the Origin of Species. These trees, or phylogenies, as they are termed by evolutionary biologists, are best viewed as hypotheses of how evolution occurred in a particular group of species. As such, they represent a well-informed supposition as to who is more closely related to whom, and when and how current species diversified from a common ancestor. As we will see below, such phylogenies also furnish predictions as to what traits or attributes might be shared among which species. Like all scientific hypotheses, they are subject to a rigorous process involving the collection and analysis of data and a careful evaluation of whether these results support or contradict the particular hypothesis in question. If the data are consistent with the hypothesis, then it remains standing as our best estimate of how evolution proceeded, for now. But, like all hypotheses, it is always subject to further testing and investigation with new data, and such investigations may well lead to modifications of the hypothesis and a new understanding of the past.

What sort of data do evolutionary biologists use to reconstruct evolutionary history? There are three primary sources: fossils of ancient taxa, physical traits measured from the anatomy of current specimens, and genetic data sampled from living or preserved animals. Fossils have the great virtue of concretely demonstrating how specific lineages appeared in the past, including lineages that have become extinct. Importantly, the geologic layer in which fossils are...