Evolution in Isolation
Two evolutionary forces have combined to shape the extraordinary birds–of–paradise. The males owe their exotic plumes and dances to sexual selection – the subtle power of females choosing mates over millions of years. And the bewildering diversity of species is the result of geographic isolation. Learn more about these forces to understand how, why, and where the birds–of–paradise evolved.
Natural & Sexual Selection: An Illustrated Introduction
Why do some birds spend time and energy frantically dancing?
Why do some have oversized ornaments making it harder to fly and forage?
How can this be natural selection?
Well, natural selection isn’t actually the competition to survive: it’s the competition to pass on genes.
The battle to breed generates a particular kind of selection called sexual selection.
Sexual selection is about out-competing, often out-attracting competitors to get more and better mates, and that’s where the dancing and ornaments come in.
To understand how sexual selection can make an animal like this, we need to simplify things.
First we’ll show how natural selection favors some traits over others and how that results in changes to a population.
Then we’ll show how sexual selection can change the same population in unexpected ways. Let’s start with a representative everyday bird from a small population living on an island.
In every generation, due to recombination or mutation, some chicks are born with changes in their genes that cause them to grow up with traits that are a little different.
Here are three hypothetical new traits that might appear: Awkward, Armored Shield, and Long-bill. Let’s take Awkward first.
Like most new traits Awkward isn’t helpful. It’s hard to get around and forage.
A baby born with the Awkward trait won’t grow up as strong as other chicks and might even die before it has a chance to breed. So the genes that caused Awkward don’t make it into the next generation.
Armored Shield seems at first to be beneficial.
This chick is totally safe from predators and it lives twice as long as everyone else.
But suppose the shield gets in the way and makes it physically impossible to breed.
Even though the individual survives to a ripe old age, the Armored Shield genes die off. Every once in a while, a new trait pops up that provides an advantage.
Long-bill allows this bird to eat fruit that was too hard to handle before, enabling it to access a nutritious and plentiful resource. He grows up stronger and is able to gather more food and help raise more young.
These young, both male and female, inherit both the trait and the advantage, so everyone with Long-bill, generation after generation, is more successful until everyone on the island eventually has a long bill.
What we’ve just examined is classic adaptation by natural selection: the process by which traits become more for less common depending on an individual’s ability to survive and gather resources.
So what then is sexual selection? Sexual selection is the process by which traits become more or less common depending on an individual’s ability to mate with more or better partners.
To really understand how this works, let’s revisit our island population, now full of long billed birds. These new bills have improved their diets so much that it’s easier to raise young.
Females can raise chicks on their own, freeing up males for the mating game. And in this food rich environment, sexual selection can become a more dominant evolutionary force. How food is distributed, whether it’s clustered or dispersed, can have a dramatic effect on what kinds of traits will be favored by sexual selection. Let’s look at two situations.
One that leads to male-male competition, and one that leads to female choice. First male-male competition—which we’ll demonstrate with the trait Burly. When a primary food source like fruit trees are clustered, males can defend territories around them to gain exclusive access to the females who come there to eat.
A male who was born burly is better able to defend a territory and will be able to mate with more females. Thus burly males will have more offspring, and their genes will become more common. Each generation of males will get burlier and burlier to the point of impacting their survival. Females who mate with burly males benefit in two ways: they have access to the food they need and their sons are more likely to be burly and successful.
But females don’t get burly since they were already the right size for survival and there’s no advantage for them to carry this extra weight.
Natural selection and sexual selection are pushing females and males into very different forms and this difference between the sexes is a telltale indicator of sexual selection at work. Now let’s look at mate attraction, specifically female mate choice. This kind of selection happens most dramatically when bountiful resources like fruit are spread out and impossible for males to defend.
Instead males must try to attract females, convincing them that they are the best possible mates.
A male who was born Fancy might have an ornament that he uses to attract females. If the ornament works and more females choose to mate with him, the genes for fancy will be passed on. Fancy traits maybe innately appealing to females, or they may reveal something about the male’s underlying health.
In either case, these genes give rise to fancy males and females who prefer mates with those particular genes.
Each generation males become more elaborate until the traits significantly decrease their chances of survival.
And just like our first situation, the females don’t change because natural selection still favors their camouflage coloration.
To sum up, classic natural selection leads to adaptations for gathering resources and surviving.
Sexual selection leads to adaptations for gathering mates and breeding. these two kinds of selection may seem different, but the mechanism behind them is actually the same: the competition to leave more copies of the genes in the next generation. Evolution is never ending, with recombination and mutation creating new traits every generation.
Most of these will die out, but those traits that provide an advantage in the competition to breed inevitably become more common over time—sometimes transforming the drab into the magnificent.
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How does evolution happen? Through a gradual process called selection. Individuals that are better equipped to survive and reproduce pass those traits to their offspring. These “selected” changes accumulate over thousands of years. We tend to think of natural selection – “survival of the fittest” – but sexual selection works the same way and can be just as strong in shaping how species look and act.
Speciation: An Illustrated Introduction
There are almost 10,000 known bird species on the planet. How did they evolve into so many unique forms?
Take the Birds-of-Paradise for example, with their dizzying array of colors, shapes, and sizes. From genetic evidence, we know that the ancestor of all 39 species was a plain crow-like bird.
And the process that caused this one species to split into many- is called speciation. Before we explain speciation, let’s talk about what we mean when we use the word species. According to the classic definition, species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other groups.
Speciation begins when groups become separated in space or become different enough in form and behavior that individuals from one group no longer regularly mate with individuals outside the group.
One way for a new species to evolve starts when a few individuals colonize a new area that is outside their native range.
Take for example a volcanic island that rises out of the sea just off the coast of the mainland.
Over millions of years, as the island grows larger and less volcanically active, plants begin to grow but there are no birds here yet.
Then one day, birds from a common species on the mainland are blown over by a storm and start a new population. If these birds move between the island and the mainland only rarely, the conditions for reproductive isolation are set and the brand new island population is on its way to becoming a new species.
Over thousands of generations the two populations will be shaped by natural and sexual selection into separate forms, each with a unique genetic fingerprint.
But at what point are we confident that this island population has really changed enough to become a new species? Well let’s put our definition to the test. If we introduce a female from the mainland species to potential mate from the island after two hundred generations and isolation (about four hundred years in bird terms), will they mate? It appears that these two [chirp, chirp] still recognize each other as members of the same species.
How about after another thousand generations of isolation? Now the female finds this island male’s song a bit strange, [chirp, chirp] but she still chooses to meet with him. What if we fast-forward again to 10,000 generations of isolation? [tweetie, tweet]
This time the female doesn’t even recognize the male’s song and is completely uninterested. If this female’s mating preferences are widespread in the population, we now have two bona fide species: Mainland and Island.
But these kinds of dating match ups don’t happen in reality. Here because our birds avoid flights across the open ocean, Island birds do not regularly encounter Mainland birds.
Because genes from the two populations no longer intermix, genetic differences accumulate and the two populations become different species. So in practice, scientists classify species separated in space by their genetic differences, not by observing blind dates.
But there is another kind of speciation that can occur when a habitat becomes divided and a once widespread bird population becomes fragmented. Imagine that our new island grows larger over time and the volcano develops a central mountain ridge while a valley forms along the Eastern Coast.
Over time, the Eastern birds who have plentiful fruit resources and only need a single parent to successfully raise young evolve specialized mating systems. Males compete for multiple mates, and and to get noticed evolve fancy plumage and display behaviors. On one side of the valley the males evolve elaborate head plumes and on the other it’s the tales that get fancy.
At the same time Western birds are consistently dealing with harsher food conditions and do not evolve a specialized mating system or fancy feathers. Just as before early in the speciation process when individuals from isolated populations meet, our Eastern and Western birds readily mate. But as generations pass and Eastern males become fancier, the Western females begin to find the Eastern males’ habits foreign and surprising.
But these traits are clearly not deal breakers! Even after 10,000 generations of geographic isolation, some other Western females continue to find the male displays attractive.
At this stage the eggs she lays aren’t viable because the separated populations have evolved too many genetic differences to be compatible.
To review, in this kind of speciation, geographic barriers have split the habitat into three fragments and on each a unique species evolved.
What if the Southwestern slope erodes into a low peninsula, allowing Southeastern birds to mix with the Western birds?
By this time, their mating systems are completely different and the two species can’t create fertile offspring. Now related species can coexist, but because they no longer successfully interbreed, there’s no going back. So how did the 39 bird-of-paradise species evolve from one crow-like ancestor? New Guinea and the surrounding islands where birds- of-paradise evolved have changed dramatically throughout geologic time, repeatedly isolating populations.
Over thousands of generations in isolation, natural and sexual selection have morphed the fragmented populations into a wonderful array of colorful species. The process of speciation is ongoing and split after split new and unique organisms evolve. Incredibly, all those 10,000 bird species we see today evolved from one single bird ancestor. And it’s thanks to the evolutionary processes driving speciation that our planet is home to such impressive biodiversity.
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There is a dizzying diversity of species on our planet. From genetic evidence we know that all species evolved from a single ancient ancestor. But how does one species split in to many? Through the evolutionary process of speciation—which begins when populations become isolated so that they no longer interbreed. This video illustrates the speciation process in birds to help you understand the basis of earth’s biodiversity.
Female Choice: and the Birds-of-Paradise
Seven Features of Female Choice
Put a male and female bird–of–paradise side by side and it’s always the bright, glittering, fluttering, posing male that grabs our attention. But the females are just as important – if not more so – in determining what birds–of–paradise look like. Here’s why:
- It started with Darwin: Female choice is a form of sexual selection – Charles Darwin’s “other big idea,“ after natural selection. For years, Darwin wrestled to understand how ostentatious display traits – like a bird–of–paradise’s plumes or a peacock’s tail – could improve the survival of an individual. Eventually he grew so frustrated that he wrote to a friend, “The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!” But finally he hit on the elegant solution of sexual selection: traits that improve an individual’s mating success can be selected by female choice the same way as traits that improve survival. Sexual selection by female choice offered a solution to the otherwise baffling evolution of beauty in
- It puts females in the evolutionary drivers seat: In birds–of–paradise, males do the showing–off and females do the choosing. This makes females into “arbiters of evolution” – they shape the course of evolution through their choices. Over millions of years, their preferences for what’s attractive have trumped most other factors influencing how males look and act. Beauty has trumped flying ability, inconspicuousness from predators, thermoregulation, and so on. The male birds–of–paradise we see today are the physical manifestation of many generations of past female preferences.
- You can watch it happening: Take a look at how intently female birds–of–paradise examine displaying males. It’s pretty apparent that sexual selection by female choice is occurring as these birds crane their necks and nearly fall off their perches to check out males. They’re so choosy that it takes many visits to many males before anything happens. A long–term field study in the 1980s found that female Lawes’s Parotias visited displaying males up to six weeks before and up to six weeks after mating! Ed and Tim have seen other female birds–of–paradise spend 5–6 hours a day watching males display. Extreme female choice indeed.
- It’s a matter of taste, at first: How does an extravagant display begin to evolve? It starts with an innate, possibly random preference in a female for a showy male trait. In some animals, these display traits actually give females information about the relative health and condition (sometimes called “quality“) of potential mates. But evidence is accumulating that this doesn’t always have to be the case. Elaborate display traits can evolve simply because they are attractive – because they satisfy some innate preference that females have. When a female with such a preference chooses a male with a showy trait, their offspring inherit both the showy trait and the preference, reinforcing the pattern.
- Beauty creates diversity: Female preference for aesthetic traits is one element that has allowed so many strange birds–of–paradise to evolve. Unlike traits that indicate a mate’s “quality,“ a simple preference for aesthetics or “beauty“ opens up many possibilities. As an analogy, think of fashion items like shoes, phones and phone cases, or purses. Our preferences for them are only partly based on how well they function. They’re largely based on how they appeal to us. If we chose items based on their function (comfort, reliability, etc.), we wouldn’t expect to see the riot of colors, shapes, and styles that we see. All this diversity exists because we judge them as objects of beauty rather than utility. There are nearly infinite ways for them to be beautiful, but many fewer ways for them to be functional. In the same way, as the birds–of–paradise seem to show, there are many ways for animals to be aesthetically attractive.
- It’s not a level playing field: It may sound cruel, but lots of male birds–of–paradise will never get the chance to mate. This is why female choice is so powerful. Individual choices by females tend to converge on a handful of males – the ones perceived to be the most attractive. As a consequence, those few males father most of the offspring. The genetic makeup of the next generation is heavily influenced by those few chosen males. Similarly, the next generation of females reflects their mothers’ preference for those same male traits. The outcome at the end of a year is that the “genetic score“ is lopsided – skewed toward a handful of successful males – and pushes evolution of the display trait a little farther along each time.
- Females are as extraordinary as males: A male bird–of–paradise may be a squawking, yellow–and–red bird hanging upside down from a tree branch, but the female is a bird that can minutely observe this bizarre scene and make an informed decision. The “biology of female choice” is just as extreme (and extremely interesting) as the biology of male displays – and much less well understood.
Isolation & New Guinea
Learn how the complex geology of New Guinea combined with intense sexual selection has led to the stunning diversity of birds-of-paradise species we see today.