Ecological selection (or environmental selection or survival selection or individual selection or asexual selection) refers to natural selection without sexual selection, i.e. strictly ecological processes that operate on a species' inherited traits without reference to mating or secondary sex characteristics.[citation needed] The variant names describe varying circumstances where sexual selection is wholly suppressed as a mating factor.[citation needed]
Ecologists often study ecological selection when examining the abundance of individuals per population across regions, and what governs such abundances.[1]
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Transcription
Hi. It's Mr. Andersen and in this video I'm going to talk about selection. Most of us are familiar with natural selection and the work of Charles Darwin. We'll talk about that. But I'm going to talk about two other forms of selection, both artificial and sexual selection in this video. When you're trying to use the word selection sometimes students are confused. And so the best analogy I can come up with is thinking back to your days playing kick ball or playing baseball when you were young and they'd have everybody line up and then the two captains would get to choose who they want on their team. And hopefully you didn't get picked last. I hope they don't do this in elementary anymore, but that's selection. Selection is when someone is choosing or something is choosing who is the strongest, the fastest. And then those that aren't chosen, in kickball you just get chosen last, but in natural selection or artificial selection that means you don't get to pass your genes on to the next generation. And before you can have selection then you have to have variation. And so right here are two graphs of variation in human heights. And so this purple is going to be male and this yellow is going to be females. And so an average male in the US is going to be around 5 feet 8. The average female is going to be around 5 foot 3. But we have a few people that are much shorter than that and a few people that are going to be like 6'8", 7 feet tall. And so we got variation. And so in nature we see that same thing. So these are brassica rapa. It's a fast plant and we were playing with these in class. Basically you plant a bunch of them. Some come up in two days, some in three days, some in one and a half days, and so you get variation. And also as they grown you're going to get variation in size of the leaves, variation in the number of flowers, how tall they are. They ave little tricomes on the side, so how many of those they have. And so you have to start with variation, just like in that kickball example, then you're going to choose based on that variation. And so basically selection we could break down in this concept map. First of all selection can either be artificial selection. That's going to be when humans make the choice of who, what genes get passed to the next generation. And then natural selection, that's when the selection takes place in the environment. Natural selection we could break down into ecological natural selection. That's going to be that run of the mill survival of the fittest, differential reproductive success that you're all familiar with. And then another type of natural selection is sexual selection. That's when females are making the choice as far as what genes go on to the next generation. And we could break that down into inter and intra and so that's basically what rest of the podcast is going to be about. We'll start with artificial selection and we'll end with sexual selection. So let's start with artificial selection. So let's say this is the mom dog and she has a bunch a puppies. Well basically what we can do is we can choose which puppies we want to pass their genes on to the next generation. And so that's how we created a Chihuahua and St. Bernard. By just choosing the traits that we want in the dogs, breeding those to create the next generation and the next generation and the next generation. It's not that we're killing dogs that we don't like. Basically we're choosing the ones that we do like and we're allowing those to breed and pass their genes on to the next generation. Humans have been doing this from the beginning time. And so basically once agriculture starts then we start selecting traits that we want. And we've been doing this for thousands of years. This guy right here, Norman Borlaug, is somebody who most of you don't know but he's known as the father of the green revolution. In other words, we were under duress on our planet. We were running out of food. We weren't able to produce enough food to feed everyone and so this started 30s, 40s, 50s on. And so basically Norman Borlaug was a farmer and what he did is produce a number of different traits of wheat. And he started by working in Mexico. And so he produced wheat. First of all he was able to produce wheat two times a season. He produced wheat that had bigger grains on the top by doing artificial selection. He also, when they grew too tall they'd fall over, and so then you would hybridize them with a shorter kind of a mutant that had short stalks and so they could have a lot of seeds on that. And so as a result of that we were able to increase wheat yields on the planet. So look at this. By the work of Norman Borlaug and others, we took production of wheat on our planet, 1950 up to in 1990 and up to 2000 it's increased you know many fold. And that's through artificial selection and selecting the traits we want. And so when humans are making that choice based on variation we call that artificial selection. Ecological selection is a form of natural selection where nature makes the choice. And this was first described by Darwin, along with sexual selection. And basically we could break it down in to three things that can happen to a bell shaped curve. The first of those is going to be called directional selection. So imagine we have this variation. So we're going to have one end and one end. So we've got extremes, but most of the averages are going to be in the middle. And so let's say change takes place in the ecology. Change takes place in the ecosystem, how organisms are going to be selected for or against. And so an example could be these which are the glacier lilies. Glacier lilies come up right after the snow leaves. And so basically they'll start to come out right when the snow melts and then their going to do pollination and then make the next generation. But basically what's going on right now is that the climate is getting warmer and warmer and warmer. And so what's happening, well as it gets warmer and warmer and warmer, the glacier lilies that come out right here aren't going to do very well. But the glacier lilies that happen to come out a little earlier are going to do better. And so we're going to see directional selection. In other words, we're going to see this curve move in one direction. And so how does that work? Basically it works by, if you're a glacier lily that now comes out much later, you're going to die because it's going to be way too hot at that point. And so we get a push in this direction. That's directional selection. We also can have stabilizing selection. A great example of that, if we think of this red line as before, why do most babies weigh around 7 pounds? Well, if you were to be born and weigh like one pound back in the day, you'd die. You were premature and you didn't survive. If you were to be an 18 pound baby and you were to be born right here, you're going to get stuck in your mom and your genes are going to die and your mom's genes are going to die. And we're not going to pass that on to the next generation. And so basically what we have, we're having selection. We're killing on either side of this bell shaped curve. And so that's going to squeeze the bell shaped curve together. And then the last thing that can happen is if we think of this red being the bell shaped curve, maybe the ones in the middle aren't doing that well but the ones on the extremes are doing well. What's an example of that? Well remember those first finches on Galapagos arrived there and they started filling different niches. They started eating different foods. And so let's say one feeds on a small seed, one feeds on a big seed, basically we get disruptive or diversifying selection. Now what can eventually happen is that can split into two species. And so basically this is all natural selection. The organisms that are selected, the organisms that survive pass their genes on to the next generation. And so this made sense to everybody and the time of Darwin. But the things that was puzzling was sexual selection. So let me tell you the story of the whiptail lizard. Whiptail lizard lives in the desert Southwest. They are camouflaged well but what's interesting about them is that they don't have any males. They only have females in this certain species of whiptail lizard. So how do they reproduce you might think. Basically what they do is they make a copy of a cell, parthenogenesis we call this, and that cell is going to be just like every other cell in the lizard except it can spawn a clone of itself. And so every female lizard in that area is a clone of every other female lizard in the area. So they've basically gotten rid of males. So you might think, wow, especially if you're a female. That would be great. We could get rid of all the males and then life would be easy, but there's problems here. And since they're all clones than any disease that might target one of them is going to target all of them. And so basically there is no variation. There's no selection. And so the reason we have males is to provide new genes to our offspring. In other words, females that share their DNA with a male produce offspring that are not like them. And so that produces variation. And so that led to what is called sexual selection. And this was puzzling to Darwin for awhile. In fact, he's quoted as saying the site of a peacock just makes me ill because he didn't understand why you'd have these real elaborate things in nature if they're always going to be selected for ones that fit well in their environment. In other words, these feathers are going to slow them down, they're going to make them easier to be caught in nature. And so basically what he set along this idea of sexual selection. In sexual selection the females are choosing which traits are passed on and which genes go to the next generation. And so basically we could break that down into Inter and Intra sexual. Inter sexual, I remember this like an interstate goes from state to state, so it's between states. Inter sexual is when a female is choosing a male based on characteristics that they have. And so inter sexual the female peahen is choosing a male that has the brightest of feathers, because if he has genes that can make bright feathers he also has genes that are going to make offspring that are able to survive. And so she's mating with him, not just because she's impressed by the color, but she's impressed by his genes. And so humans do that as well. And so Angelina Jolie when she's choosing Brad Pitt basically she's looking at all of his characteristics on the outside and she's saying, oh he has good genes, so maybe he's going to be able to produce offspring that have genes. Intra sexual, intra means between, and so that's going to be fighting between, within an individual sex. And so an example of that would be like these elephant seals battling to control a harem of females or like bull elk in Montana will fight to control a Harem of females. They're fighting within that. And so basically what goes on there is that the females just kind of sit back, they wait until that one male dominates all the other males. If he can win that fight then he's going to have good genes. He's also going to be able to produce fertile offspring. And so in both these cases it's the female that's making the choice. It's not nature it's the female. And once we have a trait that really is a good indication of genetic health it has a tendency to go out of control. And so some scientists suggest that in humans it's the brain that's that. In other words females are choosing a mate based on how good their genes are. And a good indicator of that is your brain. And so when you ask females what they're attracted to, a lot of the time it's not physical characteristics, although those are very important, it's, does he make me laugh? Is he willing to take care of me? And these are really good judges of health. And so in review, what do we have? We've go three types of selection. Artificial is when humans choose. Natural, ecological natural selection is going to be when the environment chooses. And then sexual selection is when the females choose. And why do they get to choose? Well, they have more to lose. They carry the eggs. They have a limited number of eggs. Lots of times in nature they have to take care of the young and sperm is cheap. And so it's more important that the females make that choice. And so that's selection and I hope that's helpful.
Circumstances in which it occurs
Ecological selection can be said to be taking place in any circumstance where inheritance of specific traits is determined by ecology alone without direct sexual competition, when e.g. sexual competition is strictly ecological or economic, there is little or no mate choice, females do not resist any male who wishes to mate, all traits will be equally propagated regardless of mating, or the species is hermaphroditic or asexually reproducing, an ecological selection is taking place. For example, environmental pressures are largely responsible for the evolution of different life history strategies between the African honey bee, A. m. scutellata, and the European honey bee.[2]
In sexually reproducing species, it is applicable mostly to situations where ecological pressures prevent most competitors from reaching maturity, or where crowding or pair-bonding or an extreme suppression of sexual selection factors prevents the normal sexual competition rituals and selection from taking place, but which also prevent artificial selection from operating, e.g. arranged marriages, where parents rather than the young select the mate based on economic or even astrological factors, and where the sexual desires of the mated pair are often subordinated to these factors, are artificial unless wholly based on an ecological factor such as control of land which is held by their own force.
In forests, ecological selection can be witnessed involving many factors such as available sunlight, soil quality, and the surrounding biota. During forest growth, tree seedlings in particular, are ecosystem pioneers, and different tree seedlings can often react to a number of members in their ecological community in completely different ways, thus providing a spectrum of ecological occupations.[3] On the other hand, adult trees can heavily impact their ecological communities, reversing the roles of ecological selection.[4] Elements of the soil are an extremely influential selective factor in forest growth. Throughout time, every species of tree has evolved to grow under specific soil conditions, whether it is dependent on the pH levels, the mineral contents, or the drainage levels. Each of these is a vehicle for ecological selection to do its work in the course of evolution. However, ecological selection can be much more specific, not only working within species but within populations, even populations in the same region. For example, scientists in Quebec recently examined how tree seedlings react to different nitrate levels. What they found was that areas with higher nitrate levels contained plants that could much more efficiently metabolize nitrogen. Such plants could perform photosynthesis and respiration at a much faster rate than their nitrogen lacking peers, and also had longer root lengths on average, giving them an evolutionary advantage for their habitat. Nitrogen levels that are unexpectedly too high could harm some tree species, but these particular specimens created a niche for themselves, and could outcompete others around them.[3] A site of tree growth can also be influenced by slope, rockiness, climate, and available sunlight. Space is initially available to everything, but seedlings that can most quickly inhabit the soil and take advantage of the available nutrients are usually most successful. Generally, one of the first factors to control which species grow best in the soil is the amount of sunlight. Soil and water themselves are both very important (For instance, a dry hardwood such as a white oak will not grow in a swamp), but sunlight is the initial decider in forest succession.[5] Shade intolerant trees can immediately grow impressively. They need the sunlight that is offered by an open canopy found in a bare environment. Selection weeds out the seedlings that can not handle full sun, thus tall, straight trees will eventually grow and develop a full, lush canopy. However, these behaviors will soon be reversed. Seedlings that were once removed by ecological selection now become favored, because the shaded forest floor has become ideal for such shade tolerant species. This is a great example of how ecological selection can create niches for different species by performing the same function with different outcomes.
Vs. sexual selection
In cases where ecological and sexual selection factors are strongly at odds, simultaneously encouraging and discouraging the same traits, it may also be important to distinguish them as sub-processes within natural selection.
For instance, Ceratogaulus, the Oligocene horned gopher, left in the fossil record a series of individuals with successively longer and longer horns, that seemed to be unrelated or maladaptive to its ecological niche. Some modern scientists have theorized that the horns were useful or impressive in mating rituals among males (although other scientists dispute this theory, pointing out that the horns were not sexually dimorphic) and that it was an example of runaway evolution. The species seems to have suddenly died out when horns reached approximately the body length of the animal itself, possibly because it could no longer run or evade predators—thus ecological selection seems to have ultimately trumped sexual.[citation needed]
It is also important to distinguish ecological selection in cases of extreme ecological abundance, e.g. the human built environment, cities or zoos, where sexual selection must generally predominate, as there is no threat of the species or individuals losing their ecological niche. Even in these situations, however, where survival is not in question, the variety and the quality of food, e.g. as presented by male to female monkeys in exchange for sex in some species, still influences reproduction, however it becomes a sexual selection factor. Similar phenomena can be said to exist in humans e.g. the "mail order bride" who primarily mates for economic advantage.
Differentiating ecological selection from sexual is useful especially in such extreme cases; Above examples demonstrate exceptions rather than a typical selection in the wild. In general, ecological selection is assumed to be the dominant process in natural selection, except in highly cognitive species that do not, or do not always, pair bond, e.g. walrus, gorilla, human. But even in these species, one would distinguish cases where isolated populations had no real choice of mates, or where the vast majority of individuals died before sexual maturity, leaving only the ecologically selected survivor to mate—regardless of its sexual fitness under normal sexual selection processes for that species.
For example, if only a few closely related males survive a natural disaster, and all are able to mate very widely due to lack of males, sexual selection has been suppressed by an ecological selection (the disaster). Such situations are usually temporary, characteristic of populations under extreme stress, for relatively short terms. However, they can drastically affect populations in that short time, sometimes eliminating all individuals susceptible to a pathogen, and thereby rendering all survivors immune. A few such catastrophic events where ecological selection predominates can lead to a population with specific advantages, e.g. in colonization when invading populations from more crowded disease-prone conditions arrive with antibodies to diseases, and the diseases themselves, which proceed to wipe out natives, clearing the way for the colonists.
In humans, the intervention of artificial devices such as ships or blankets may be enough to make some consider this an example of artificial selection. However it is clearly observed in other species, it seems unreasonable to differentiate colonization by ship from colonization by walking, and even the word "colony" is not specific to humans but refers generically to an intrusion of one species on an ecology to which it has not wholly adapted. So, despite the potential controversy, it may be better to consider all examples of colonist-borne diseases to be ecological selection.
For another example, in a region devastated by nuclear radiation, such as the Bikini Atoll, capacity to survive gamma rays to sexual maturity and (for the female) to term is a key ecological selection factor, although it is neither "natural" nor sexual. Some would call this too artificial selection, not natural or ecological, as the radiation does not enter the ecology as a factor save due to man's effort. Ambiguous artificial-plus-ecological factors may reasonably be called "environmental", and the term environmental selection may be preferable in these cases.
See also
References
- ^ McCoughlin, Phillip D.; Morris, Douglas W.; Fortin, Daniel; Vander Wal, Eric; Contasti, Adrienne L. (January 1, 2010). "Considering ecological dynamics in resource selection functions". Journal of Animal Ecology. 79 (1): 4–12. doi:10.1111/j.1365-2656.2009.01613.x. PMID 19732211.[permanent dead link]
- ^ Fewell, Jennifer H.; Susan M. Bertram (2002). "Evidence for genetic variation in worker task performance by African and European honeybees". Behavioral Ecology and Sociobiology. 52 (4): 318–25. doi:10.1007/s00265-002-0501-3. S2CID 22128779.
- ^ a b Marks, C.O.; Conti, E. (2007). "The causes of variation in tree seedling traits: the roles of environmental selection versus chance". Evolution. 61 (2): 455–469. doi:10.1111/j.1742-4658.2007.00021.x. PMID 17348954. S2CID 23528064.
- ^ Barnes, B.V.; Spurr, S.H. (1980). Forest Ecology. New York: Wiley.
- ^ Park, A.; Van, B.M.; Ashton, M.S.; Wishnie, M.; Mariscal, E.; Deago, J.; Ibarra, D.; Hall, J.S. (2012). "Local and regional environmental variation influences the growth of tropical trees in selection trials in the Republic of Panama". Forest Ecology and Management. 260 (1): 12–21. doi:10.1016/j.foreco.2010.03.021. S2CID 86520815.
This page was last edited on 9 November 2023, at 00:46